Note: Descriptions are shown in the official language in which they were submitted.
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A SPECTROMETER INCORPORATING SIGNAL MATCHED
FILTERING
FIELD OF THE INVENTION
This invention relates to the field of optical detectors, in particular with
regard to
spectrometers.
BACKGROUND OF THE 'INVENTION
The are a number of spectrophotometers or spectrometers that are used to
detect the
spectral characteristics of a test sample. For example, U.S. Patent No.
4,330,207
discloses a fluorescence spectrophotometer comprising a light source, an
excitation side
monochromator which makes light from the light source be subjected to
spectroscopic
analysis for illuminating as actinic light a sample, a fluorescence side
monochromator
which makes fluorescence light from the sample be subjected to spectroscopic
analysis,
a detector which detects light from the fluorescence side monochromator, and a
scanning means which adjusts both the monochromators to the wavelengths of the
actinic light and the fluorescence light to be scanned. These monochromators
are
arranged in such a way that one of them is automatically set to the location
of the peak
wavelength value which is detected by itself through a simple and automatic
wavelength
scanning operation and then the other is wavelength-scanned for excitation
spectrum or
fluorescence spectrum measurement. This device is designed to detect a range
of
wavelengths of photonic radiation and saves in a memory means the wavelength
having
the higher peak value.
In addition, U.S. Patent No. 5,194,916 describes a fluorescence
spectrophotometer
which comprises an excitation monochromatic light generating means for
irradiating
excitation monochromatic light onto a sample to be measured, an emission
monochromator for selecting monochromatic light from fluorescent light emitted
from
the sample, an emission photometer for generating a primary output signal
corresponding to the strength of the monochromatic light selected by the
emission
monochromator, a filtering means for eliminating noises from the primary
output and
for generating a secondary output, the filtering means being characterized by
a response
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value; determination means for determining a content of the sample based on
the
secondary output and a response setting means for setting the response value
of the
filtering means based on the primary output. A table includes the strength
data of the
primary signal and the corresponding response value, which are determined
before hand
through experiments and in some cases this correlation can be determined by a
mathematical formula.
A spectrophotometer including a light source operative to emit a beam of
light, an
optical system for directing the light beam to a sample to be analyzed, and a
detector
which detects the intensity of the light beam after the beam interacts with
the sample is
disclosed in U.S. Patent No. 6,002,477. The light source is operative to emit
bursts of
light separated by an interval during which no light is emitted. By way of
example, a
xenon tube may be used for that purpose. The spectrophotometer measures the
intensity
of the light beam generated by each burst of light after that beam interacts
with the
sample. Each such light beam may be divided into first and second parts prior
to
interaction with the sample, and the optical system is arranged to direct the
first part to
the sample and to direct the second part to a second detector for conducting a
reference
measurement. A dark signal measurement may be conducted immediately before or
after each burst of light. Thus by having a reference signal determining the
noise within
the system provides a means for isolating the received signal. However, the
determination of the reference signal at the time of scanning of the test
sample is critical
to the functionality of this system, since "dark noise" can change
dramatically over very
small periods of time.
This background information is provided for the purpose of making known
information
believed by the applicant to be of possible relevance to the present
invention. No
admission is necessarily intended, nor should be construed, that any of the
preceding
information constitutes prior art against the present invention.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a spectrometer incorporating
signal
matched filtering. In accordance with an aspect of the present invention,
there is
provided an optical system for performing a spectral analysis of test samples
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comprising: a photonic energy source for emitting electromagnetic radiation,
wherein
said photonic energy source is controlled by a digital signal processing
means; an
optical emission processing means for receiving electromagnetic radiation from
the
photonic energy source and transmitting one or more illumination wavelengths
to a test
sample, wherein the optical emission processing means is controlled by the
digital signal
processing means; a received light optical processing means for collecting and
isolating
one or more wavelengths of received electromagnetic radiation from the test
sample and
transmitting the isolated one or more wavelengths of received electromagnetic
radiation
to an optical detector, wherein said received light optical processing means
is controlled
by the digital signal processing means; an optical detector for sensing and
converting the
isolated one or more wavelengths of received electromagnetic radiation into an
electrical
signal; and digital signal processing means for performing matched filtering
of the
electrical signal received from the optical detector and for controlling the
functionality
of the photonic energy source, the optical emission processing means and the
received
light optical processing means.
In accordance with another aspect of the invention, there is provided a system
for
performing an optical analysis of a fluid comprising: an optical probe
including an
illumination system including a photonic energy source for emitting
electromagnetic
radiation and optical devices for directing said electromagnetic radiation
towards the test
sample, said optical probe further including detector optics for collecting
and directing
electromagnetic radiation emitted by the test sample towards a photodetector,
wherein
said optical probe is inserted into a fluid flow or a sample chamber
containing a fluid
sample; a control means for activating the photonic energy source; a
photodetector for
sensing and converting the electromagnetic radiation emitted by the test
sample into an
electrical signal; and a digital signal processing means for performing
matched filtering
of the electrical signal received from the photodetector, said digital signal
processing
means further controlling the activation of the photonic energy source and
encoding the
electromagnetic radiation directed towards the test sample.
BRIEF DESCRIPTION OF THE FIGURES
Figure 1 is a schematic diagram of the optical system components corresponding
to one
embodiment of the present invention.
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Figure 2 is a schematic diagram of the optical system according to another
embodiment
of the present invention.
Figure 3 is a schematic diagram of a scanning spectrometer system according to
a
further embodiment of the present invention incorporating a Matched Filter
Receiver.
Figure 4 is a schematic diagram of a digital signal processing means light
pulse
processing system.
Figure 5 demonstrates On-Off keyed signal with a 0 dB signal to noise ratio,
using pulse
amplitude modulation detection.
Figure 6 demonstrates signal detection using frequency domain detection.
Figure 7 demonstrates the results of the time domain correlation output from
binary
pulse coding signal detection.
Figure 8 is a schematic representation of a pulse coding channel model.
Figure 9 depicts the detector output using a linear FM Chirp, which is a 125
msec wide
rect function, swept from 500 Hz to 3500 Hz and sampled at 8000 samples/sec.
Figure 10 demonstrates the use of a linear FM pulse coding technique where the
pulse
duration was left at 0.125 seconds and the bandwidth was 1600 Hz for a time
bandwidth
product (TBP) of 200. A log scale of the detector was calculated as; P = 20 X
log s,
where s is the time domain output of the matched filter.
Figure 11 demonstrates the use of a linear FM pulse coding technique as in
Figure 10
for a TBP of 800.
Figure 12 demonstrates the use of a linear FM pulse coding technique as in
Figure 10
for a TBP of 2250.
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Figure 13 is a time domain plot for the case of a TBP of 2250, where the
detector
amplitude was plotted.
Figure 14 is a schematic representation of a spectrometer that incorporates a
matched
filter receiver.
Figure 15 shows a detector output, recorded for ~.; and 7~e, from the
spectrometer of
Figure 14 and plotted for display.
Figure 16 illustrates an optical system that can be used for the testing of
water quality,
according to one embodiment of the present invention.
Figure 17 is a plot of the turbidity readings for a water test site that were
obtained using
the optical system illustrated in Figure 16.
Figure 18 is a plot of the biomass readings for a water test site that were
obtained using
the optical system illustrated in Figure 16.
DETAILED DESCRIPTION OF THE INVENTION
Definitions
The term "electronic light modulator" means an acousto-optic modulator,
mechanical
light chopper, hologram or electrically driven opto-electonics, or similar
devices.
The term "illumination light source" means a light emitting diode (LED),
incandescent,
laser, gas discharge lamp, laser diode, arc lamp, x-ray source or similar
devices.
The term "monochromator" means a light-dispersing instrument which is used to
obtain
light of substantially one wavelength, or at least of a very narrow band of
the spectrum
and may be for example an interference filter, cutoff filter, diffraction
prism, diffraction
grating, interferometer, hologram or similar devices.
The term "photodetector device" means a light detection device or optical
detector and
includes a photodiode, photomultiplier, charge couple device (CCD) or similar
devices.
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The term "resultant radiation" refers to each or all of the reflected,
transmitted, absorbed
and fluoresced light that result when a subject is exposed to an illuminating
radiation.
The phrase "weak signal detection" refers to techniques used to enable
measurement of
low intensity emission radiation from a sample. For any given signal to noise
ratio, the
information error rate can be lowered by increasing the bandwidth used to
transfer the
information. The signal bandwidths are spread prior to transmission in the
noisy
channel, and then despread upon reception. This process results in what is
called
Processing Gain.
The term "signal spreading" refers to a number of means of spreading the
signal,
including Linear Frequency Modulation (sometimes called Chirp Modulation) and
Direct Sequence methods and other techniques.
The term "signal despreading" refers to a process that is accomplished by
correlating the
received signal with a similar local reference signal using a Correlation
Receiver or
Matched Filter receiver technique. When the two signals are matched, the
spread signal
is collapsed to its original bandwidth before spreading, whereas any unmatched
signal is
spread by the local reference to essentially the transmission bandwidth. This
filter then
rejects all but desired signals. Thus, in order to optimize a desired signal
within its
interference (thermal noise in the detection system, ambient light induced
noise, AC line
noise, for example), a matched filter receiver enhances the signal while
suppressing the
effects of all other inputs, including noise.
Unless defined otherwise, all technical and scientific terms used herein have
the same
meaning as commonly understood by one of ordinary skill in the art to which
this
invention belongs.
The various aspects of this invention will become more readily appreciated and
better
understood by reference to the following detailed description.
The system according to the present invention provides an optical scanning
system
incorporating optical signal encoding and matched filtering, enabling the
detection of
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the response of a test sample to its illumination, wherein this response can
include
reflection, fluorescence, transmission and/or absorption. Due to the enhanced
signal-to-
noise ratio provided by this system, this invention can detect subtle optical
changes in a
test sample.
With reference to Figure 1, the optical system of the present invention
comprises a
spectrometer and a digital signal processing means 5, comprising: a photonic
energy
source 15 which is controlled by the digital signal processing means 5
(specifically the
emitter control electronics 10), to emit electromagnetic radiation which can
range from
ultraviolet to far infrared (or a bandwidth from 150 nm to 3000 nm) and
optical
emission processing means 20 which is controlled by the digital signal
processing
means 5 (specifically the emitter control electronics 10) to receive light
from the
photonic energy source 15 and to deliver one or more illumination wavelengths
in a
pulsed sequence to a test sample 25. The optical emission processing means 20
can
comprise a means for isolating one or more illumination wavelengths and
emitter optics
that orient and focus the illumination wavelengths) onto the test sample 25.
The system
further comprises received light optical processing means 30 which is
controlled by the
digital signal processing means 5 (specifically the emitter control
electronics 10) to
collect and isolate one or more wavelengths of received light due to the
illumination of a
test sample 25. The received light optical processing means 30 can comprise
detector
optics for collecting the received light from the test sample 25 and a means
for isolating
one or more of the wavelengths of the received light. Additionally, the system
comprises an optical detector 35 to sense and convert to an electrical signal,
the received
light which has been transmitted by the received light optical processing
means 30 and a
DSP received signal processing means 40, which is a component of the digital
signal
processing means 5, to perform the match filtering on the output of the
optical detector
35. The match filtering of the received signal is performed based on the
received
electrical signals from the optical detector 35 and control parameters from
the emitter
control electronics 10.
There are various locations for noise or interference to enter the system
according to the
present invention, with this interference decreasing the ability to detect
signals received
from the test sample due to its illumination. For example and with further
reference to
Figure 1, ambient light can enter the system through the received light
optical
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processing means 30 and electrical noise can enter the system through the DSP
received
signal processing means 40. The incorporation of a digital signal processing
means can
provide a means for the encoding of the illumination signal and the matched
filtering of
the received signal in relation to the encoded illumination signal. As such,
the digital
signal processing means can enable improved detection of the received signals
resulting
from the illumination of the test sample.
In order to describe how the components operate together, an overview of one
embodiment of a system in accordance with this invention is presented in
Figure 2. In
this embodiment an illumination light source 100 is controlled by digital
signal
processing means 300 to emit a radiation bandwidth ranging from, for example,
250 nm
to 1000 nm. A collimator 110 linearises the illumination light and directs it
to the light
modulator 200, wherein a collimator 110 may be, for example, a long narrow
tube in
which strongly absorbing or reflecting walls permit only radiation travelling
parallel to
the tube axis to traverse the entire length. The light modulator 200 which
could be an
encoding disc (as shown in Figure 2), acousto-optic modulator, or electronic
modulator
such that it may enable amplitude or phase modulation, for example,
essentially
spreading the optical signal. An illumination monochromator 120 is controlled
by the
digital signal processing means 300 to receive light from the illumination
light source
100 and to deliver the N~" wavelength in a pulsed sequence to an optical probe
which
delivers the N'h wavelength to the test sample 140, for example, a fluid
sample. The
resultant radiation, due to the illumination of the test sample, is collected
and delivered
to the emission monochromator 160. Radiation signals detected from the subject
are
still encoded with the spread function coding and the intensity is
proportional to the
reflection coefficient and the fluorescence coefficient. The detection
monochromator
160, which is controlled by digital signal processing means 300, separates the
reflection
and fluorescence spectra optically, by performing specific digital processing
tasks to
pass the Nt" wavelength reactive characteristics for a specific illumination
wavelength,
so that each of these encoded optical signals can be detected by the photo
detector 170.
The photo detector 170 detects the optical signal and converts it to an
electrical signal,
which is then processed by the bandpass filter 180, (essentially an Analog to
Digital
Converter) and transmits it to the digital signal processing means 300. The
digital
signal processing means 300 performs matched filtering in order to identify
and isolate
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the response of the test sample to the illumination radiation from the noise
that enters
the optical system from various sources.
In an alternate embodiment the optical system can be configured with the
components as
illustrated in Figure 3. A light source 100 generates photonic energy which is
directed
towards an encoding disc 200 by a collimator 110. The directed encoded
photonic
energy passes through an illumination prism 520 for separating the various
wavelengths
of the illumination radiation. The separated illumination radiation is
directed towards a
slit 530 oriented in a manner such that the desired wavelength or band of
wavelengths
are transmitted to the test sample 140. The radiation emitted by the test
sample 140 as a
result of its illumination, is collected by a lens 150 and transmitted to an
emission prism
540 wherein the emitted radiation is separated into the various wavelength.
The
emission prism 540 directs the emitted radiation to a slit 550 oriented in a
manner such
that the desired wavelength or band of wavelengths is directed towards the
detector 170.
The detector 170 converts the emission radiation into an electrical signal
which is
directed to the matched filter receiver 560 for processing the gathered
information
relating to the illumination of the test sample.
There are a number of embodiments of this optical system, comprising
variations of
particular components. Each embodiment, however, has a form of each of these
components. Some criteria for choosing which component should be included in a
particular embodiment is described below. .
A Photonic Energy Source
Each embodiment includes a photonic energy source that is controlled by the
digital
signal processing means to emit electromagnetic radiation which can range from
ultraviolet to far infrared (or a bandwidth from 150 nm to 3000 nm).
A photonic energy source which can be used in conjunction with the present
invention
can be selected from the group comprising: a laser, laser diode, light
emitting diode
(LED), arc flashlamp, a continuous wave bulb, an electronically controlled
flashlamp,
any gas discharge lamp or any other photonic energy source as would be known
to a
worker skilled in the art. The selection of the photonic energy source that is
to be used
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in a particular embodiment of the present invention, can be determined by the
required
spectral analysis of the test sample. The functionality of the device may
require a broad
spectral analysis of a test sample or may require the spectral characteristics
over a
narrow bandwidth or even specific wavelength, for example.
For example, a laser has a very narrow spectrum (a highly coherent "single"
wavelength), narrow spatial beam and a high pulsed power. An incandescent
light bulb
has a broad spectrum, wide beam and continuous transmission.
In one embodiment of the present invention, the electromagnetic radiation
generated by
the photonic energy source may be in the form of pulsed electromagnetic
radiation.
Optical Emission Processing Means
The optical emission processing means receives light from the photonic energy
source
and may deliver one or more illumination wavelengths in a pulsed sequence to a
test
sample, wherein the optical emission processing means can comprise a means for
isolating one or more illumination wavelengths and emitter optics that orient
and focus
the illumination wavelengths) onto the test sample. The optical emission
processing
means is controlled by the emitter control electronics contained in the
digital signal
processing means, wherein the emitter control electronics may perform
functions
comprising pulse coding and pulse shaping, for example enabling the modulation
of the
illumination energy.
In order to distinguish the light wavelengths due to reflection and
fluorescence, which
are received from the test sample, from ambient light noise, the illumination
of the test
sample should be performed using narrowband illumination.
In one embodiment of the present invention a generic device may require the
ability to
easily vary the illumination spectral characteristics, such that spectral
characteristics of
the test sample can be determined for a range of illumination wavelengths.
This can be
accomplished by using a broadband light source, such as a halogen bulb or a
Xenon tube
and subsequently using wavelength separation optics to filter the emitted
light thereby
isolating narrow portions of the spectrum for illuminating the test sample. An
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approach is to use an array of multiple narrowband or mediumband light sources
(eg.
laser diodes and/or various coloured LED's), each having particular desired
spectral
characteristics, and subsequently activating them one at a time, thereby
effectively
traversing a broad spectrum of light and isolating particular illumination
wavelengths
during the sequence of illumination of these devices.
The optical control processing means further comprises a light control device
that
provides a means for modulating the light, which is to illuminate the test
sample, for
example producing a pulsed sequence of light emissions. A light control device
can be
an indirect light modulator, for example, a light chopper, shutter, liquid
crystal filter,
galvanometric scanner or acousto-optic device. In addition, light modulation
can be
performed in a direct manner using an amplitude modulator circuit or a
frequency
modulator circuit. A worker skilled in the art would understand alternate
methods of
modulating the illumination light emissions.
The wavelength separation optics can be selected from fixed light conditioning
optics
including optical filters, refractive optics, diffractive optics and a
variable light
conditioning subsystem including a refractive or diffractive optical system
whereby the
optical centre wavelength is chosen by the use of a position controlled
reflective surface
after the light has passed fixed light conditioning optics. The fixed light
conditioning
optics may also be a refractive or diffractive optical system whereby the
optical centre
wavelength is chosen by use of a position controller to move fixed light
conditioning
optics. An example of a wavelength separation optic device is a monochromator.
Other
forms of wavelength separation optic devices would be known to a worker
skilled in the
art.
Emitter optics can be used to transmit the photonic energy between the
components of
the optical emission processing means and also to transmit the illumination
light to the .
test sample. The emitter optics can be selected from the group comprising,
condensers,
focusing devices, fibre optics, apertures and other devices as would be known
to a
worker skilled in the art.
In one embodiment of the invention wherein the analysis of a fluid is desired,
for
example, a water tight optical probe may be incorporated into the optical
system,
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wherein this optical probe can be inserted into a fluid test chamber. This
optical probe
comprises components for illuminating the test sample and for collecting the
light
emitted by the test sample due to its illumination. Since the optical probe is
being
inserted into the fluid sample, a reduction in the reflection effects of the
surface
boundary layers of the sample will be realised. In this embodiment, the
optical probe
comprises the photonic energy source, for example a LED or a laser diode, the
illumination optics for directing the illumination radiation to the fluid
sample and the
detection optics for collecting the reflected and fluoresced light. The
optical probe is
interconnected with the further components of the optical system, for example
the DSP
means and the photodetector for the transmission of instructions and collected
data
therebetween. The illumination optics and the light detection optics can be
oriented
within the optical probe such that the fluid's interaction with the
illumination light can
be sufficiently detected. In one embodiment of the invention, the optical
probe further
comprises a self cleaning feature, for example a wiper mechanism, wherein
residue from
1 S the fluid which adheres to the surface of the probe can be removed.
Received Light Optical Processing Means
The received light optical processing means collects and isolates one or more
wavelengths of received light from the test sample, with this received light
being related
to the illumination of the test sample as described above. The received light
optical
processing means can comprise detector optics for collecting the received
light from the
test sample and a means for isolating one or more of the wavelengths of the
received
light for detection by the optical detector. The received light optical
processixig means
is controlled by the emitter control electronics contained in the digital
signal processing
means and thus its function can be correlated with the optical emission
processing
means, which can provide a means for the efficient analysis of the received
spectral
emissions.
In one embodiment of the present invention, the received light optical
processing means
can isolate particular wavelengths of received light by using wavelength
separation
optics, which provides a means for isolating one or more wavelengths of
received light
thus allowing the received light to be correlated to the illumination
wavelength.
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The wavelength separation optics can be selected from fixed light conditioning
optics
including optical filters, refractive optics, diffractive optics and a
variable light
conditioning subsystem including a refractive or diffractive optical system
whereby the
optical centre wavelength is chosen by the use of a position controlled
reflective surface
after the light has passed fixed light conditioning optics. The fixed light
conditioning
optics may also be a refractive or diffractive optical system whereby the
optical centre
wavelength is chosen by use of a position controller to move fixed light
conditioning
optics. An example of a wavelength separation optic device is a monochromator.
Other
forms of wavelength separation optic devices would be known to a worker
skilled in the
art.
In a further embodiment of the present invention, the received optical
processing means
may be required to isolate one selected wavelength. For example, if the test
specimen is
illuminated by a particular wavelength of light and the level of reflection of
this
illumination photonic energy by the test sample, is required, the received
optical
processing means can have a fixed light separation means. In this manner only
a
particular received light wavelength is being evaluated.
Detector optics can be used to transmit the photonic energy between the
components of
the received light optical processing means and also to transmit the received
light to the
optical detector. The detector optics can be selected from the group
comprising,
condensers, focusing devices, lenses, fibre optics and apertures. In one
embodiment of
the invention an optical filter may provide this functionality, wherein the
optical filter
may include a low pass, high pass band filters or other compatible filters as
would be
known to a worker skilled in the art.
Optical Detector
Each embodiment includes an optical detector which can sense the light
transmitted by
the received light optical processing means and convert this received light
into an
electrical signal for processing by the digital signal processing means and in
particular
the DSP received signal processing means.
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An suitable optical detector can be a diode, photomultiplier, or a charge-
coupled device
(CCD) arranged in a linear array or an area array, for example. A specific
example of
an optical detector is a blue enhanced Gallium-Arsenide photodiode, a Cadmium
Sulfide
(CdS) photodiode or a silicon avalanche diode.
Digital Signal Processing Means
Digital Signal Processing (DSP) means can be used to control the photonic
energy
source, the optical emission processing means and the received light optical
processing
means in order to be able to detect one or more wavelengths of the resultant
radiation in
relation to one or more wavelengths of illumination radiation, wherein this
detection is
being performed in the presence of noise introduced into the system. The
digital signal
processing means comprises emitter control electronics, which provide a means
for
controlling the illumination radiation (optical emission processing system)
and the
received light optical processing system. In addition, the DSP means comprises
a
received signal processing means which enables the DSP to correlate the
received light
radiation with the illumination radiation, which can provide a means for
identifying
reflectance, fluorescence and absorption from a test sample due to its
illumination.
The emitter control electronics which control the illumination radiation
performs tasks
including: supplying electrical power and driving circuitry to convert
electrical energy
into light energy, controlling the amplitude and timing of light source
pulses, controlling
optical devices which filter, focus, or mechanically pulse the illumination
radiation, for
example, a light filter, monochromator, collimator and/or a chopper. In
addition, the
emitter control electronics provide a means for controlling the received light
optical
processing means enabling the isolation of reflectance and fluorescence light
wavelengths from the test sample due to its illumination. For example, the
incorporation of a monochromator into the received light optical processing
means can
provide a means for isolating desired wavelengths and the functionality of the
monochromator is controlled by the received light optical processing system.
The coding function which is employed by the emitter control electronics in
order to
encode the illumination signal prior to interaction with the test sample can
be provided
by any number of signal modulation techniques. For example, pulse code
software can
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be used to create a synchronous pulse for direct modulation of the light
control device
frequency (pulse frequency modulation, PFM). With PFM the frequency of the
pulses is
modulated in order to encode the desired information. Pulse code software can
be used
to create a synchronous pulse for direct modulation of the light control
device amplitude
(pulse amplitude modulation, PAM), wherein with PAM the amplitude of the
pulses is
modulated in order to encode the desired information. In addition, pulse code
software
can be used to create synchronous pulse for direct modulation of the light
control device
pulse width (pulse width modulation, PWM). With PWM the width of the pulses is
modulated in order to encode the desired modulation. Finally the illumination
signal
may be encoded using a function generator to create a fixed synchronous pulse
enabling
pulse rate and amplitude modulation, in addition to a mechanical encoder
driver to
create a synchronous pulse for an indirect light modulator, for example a
chopper,
shutter, galvomirror etc.
In one embodiment of the invention the coding function which is employed by
the
emitter control electronics is binary phase shift keying (BPSK) which is a
digital
modulation format. BPSK is a modulation technique that can be extremely
effective for
the reception of weak signals. Using BPSK modulation, the phase of the carrier
signal
is shifted 180° in accordance with a digital bit stream. The digital
coding scheme of
BPSK is as follows, a "1" causes a phase transition of the carrier signal
(180°) and a "0"
does not does not produce a phase transition. Using this modulation technique
a
receiver performs a differentially coherent detection process in which the
phase of each
bit is compared to the phase of the preceding bit. Using BPSK modulation may
produce
an improved signal-to-noise advantage when compared other modulation
techniques, for
example on-off keying.
The DSP received signal processing means enables matched filter correlation
between
electrical signals received from the optical detector and the corresponding
time period as
defined by the emission control electronics. This correlation between
transmitted and
received signals can provide a means for enhanced identification of received
signals
over the noise (ambient light or electrical noise, for example) which may
enter the
optical system of the present invention. Filtering and time averaging of
received
signals, synchronized and matched with the emitted pulse sequence, enhances
the
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signal-to-noise ratio (SNR) and improves the confidence in the measurement of
the
sample response at a wavelength or wavelengths of interest.
A matched filter is an exact copy of the signal of interest. The filter is
correlated with
the input signal, with this procedure basically being a sum of the products of
the signal
multiplied by the filter over the total duration of the filter. Upon the
matching of the
filter and the signal of interest, the correlation (convolution) sum typically
peaks relative
to the non-matched sums providing a means for identifying the signal over the
external
noise within the optical system. In one embodiment of the present invention, a
bank of
narrowband filters centered at intervals of the pulse rate can capture more
lines from the
pulse spectrum, and thus may provide a means for improved light pulse energy
estimation and subsequent identification of the detected wavelength.
In one embodiment of the present invention, if the time domain spreading
function is
represented by F(w) and the received signal is represented by H(w), then the
output of
the matched filter receiver can be obtained using the digital signal
processor:
s~t)= f Fpv)Il~tr~)e'~'df where: w = 2~tf
In this equation F(w) is the Fourier Transform of the input signal f(t) and
H(ca) is the
Fourier Transform of the receiver linear filter h(t). In a matched filter, the
receiver
linear filter H(w) is adjusted to optimise the peak signal-to-noise ratio of
the receiver
output s(t) for a specific input signal f(t). When the receiver linear filter
response H(w)
is given by:
H~~) = KF' (~~-i~~~
then the output signal-to-noise ratio is maximised and the receiver filter
response H(w)
is matched to the input signal f(t), wherein f(t) has the Fourier Transform
F(w). The two
above equations are taken from "Information Transmission, Modulation and
Noise,_A
Unified Approach to Communication Systems"; Schwartz, Mischa; Third Edition. A
matched filter receiver enables one to potentially maximise the signal-to-
noise ratio of
the output signal s(t), the detection of which is desired. Thus a matched
filter receiver
may provide optimum detection of the output signal. Since a matched filter
receiver is a
linear system, s(t) is directly proportional to the intensity of the
reflectance and
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fluorescence illumination on the detector. The use of a matched filter can
enable one to
detect weak signals in the presence of noise (external and internal noise of
the optical
system), which may not be detectable using other optical systems.
In one embodiment of the invention, the signal processing system involves both
analog
front-end and digital back-end tasks. In general the analog processing tasks
are
concerned with recovering the small sensor signals and applying highly
selective .
filtering operations. The digital domain tasks are concerned with further
signal filtering
as well as analysis functions, in relation to energy detection and data
output. To
minimize the interference and to provide immunity against shot noise, the
illumination
signal is modulated by a frequency of typically a few hundred Hz. The analog
section is
designed to high gain amplify and prefilter the photodiode output and recover
the
modulation frequency. Utilizing these signals, a narrowband tracking filter
can provide
the very high selectivity for modulated signal recovery. The output of the
narrowband
filter, after amplification, is analog/digital converted and input into a DSP
(digital signal
processor) which in real time performs the back-end tasks of filtering, energy
detection,
averaging and converting the results into usable data. The filtering will
further enhance
the rejection of a/c noise and harmonic distortion, which may have been
introduced in
the final stages of analog processing. The filtering is followed by an
averaging energy
detector, which outputs the values proportional to the energy of the sensor
signal. These
values are sent to the host computer in short intervals, where they can be
stored and
processed for further analysis.
In another embodiment of the present invention, the digital signal processing
means can
be designed as illustrated in Figure 4. Initially, a pulse sequence generator
450
transmits a pulse period counter to the pulse period buffer 440 and further
transmits a
digital signal defining the generated sequence to a digital to analog
converter 460. The
resulting analog pulses are sent to the light source upon passing through an
analog low
pass filter 470 and the light source subsequently illuminates the test sample
based on
these pulses. Upon the collection and detection of the emitted radiation from
the test
sample due to its illumination, the pulses generated by the photodetector as a
result of
photonic radiation detection are transmitted to an analog low pass filter
(LPF) 400,
which transmits the filtered information to a analog to digital converter
(ADC) 410. The
analog LPF can suppress frequencies over 10 kHz, for example, thereby
providing anti-
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abasing. This digitized information is sent to a bank of narrowband finite
impulse
response (FIR) filters 420, wherein each filter is matched to one of the lines
in the pulse
sequence spectrum (input signal pulse). This provides a means for matching the
pulse
spectrum in order to identify the signal over the external noise within the
system (match
filtering). The sums of the filter - input signal correlation 430 are
transmitted to the
peak detector through pulse period buffers 440 and 480 and the average light
measured
is then sent to the control logic 500 of the DSP. The control logic 500
provides a means
to perform scheduling control and configuration control of the digital signal
processing
(DSP) means. The averaged measured light signals are subsequently transmitted
to a
computing device located on a personal computer, for example, via a RS232
serial port
510, in order to be organised into a usable and presentable format, for
example
generating a graphical representation of the collected information.
In one embodiment of the invention, the functionality of the DSP means may
further
comprise the ability of establishing an alarm setting, wherein one or more
actions are
taken upon the activation of an alarm setting. For example, the DSP means may
constantly correlate and perform statistical analyses on the processed data
and once a
predetermined level of change in the received light is reached, the DSP means
will
activate the alarm setting. The activation of an alarm setting may result in a
message
being sent to personnel which are monitoring the optical system, for example
in the
form of a warning light or noise. In one embodiment, wherein the test sample
is a
flowing fluid sample, the activation of an alarm setting can result in a fluid
sample being
extracted from the fluid flow, through the use of a valve to transfer fluid
from the flow
to a collection container, for example. This fluid sample may subsequently be
subjected
to a detailed analysis for evaluation of its contents at a laboratory, for
example. In the
example of the monitoring of a flowing fluid, the incorporation of an alarm
setting may
enable the capturing of significant changes in the fluid contents by the
sampling of the
fluid upon the detection of a particular level of change in fluid's reaction
to light
illumination. This procedure can provide an improved evaluation of the changes
in a
fluid's content as opposed to periodic, time based, sampling of the fluid.
The utilization of advanced signal processing techniques, enables the
detection of
optical reflectance and fluorescence emissions that would normally not be able
to be
detected
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Moreover, the signal processing algorithms can be implemented in standard
digital
signal processing chips, enabling the overall cost of devices based on this
technology to
be relatively low.
The DSP means can be incorporated into a computer system in the form of a
circuit
board that can be installed in a computer, wherein the computer can provide a
means for
manipulating and organising the received information after matched filtering
into a
format that is easy to interpret by the operators of the system, for example.
Alternately,
the DSP may comprise stand alone hardware providing a means for the DSP to
function
independently.
Stand Alone DSP System
In one embodiment of the present invention, the digital signal processing
means together
with its sub-systems is designed in a stand-alone configuration. In this type
of stand-
alone configuration, the DSP system can further include the capability of
interconnecting with a global communication system, for example the Internet
or for
networking within a local area network (LAN). This type of interconnection
with a
communication network can enable the collection of information from a
plurality of test
sites by a central station, thereby potentially reducing the personnel
required for the
collection of this test data.
As would be known to a worker skilled in the art, depending on the
communication
system (LAN, WAN, Internet) by which the information from the optical systems
is
transmitted and the desired level of security desired for the information,
varying levels
of encryption of the data may be required.
In this embodiment the stand alone DSP comprises a transmitter and receiver
block, a
micro-controller block (MCU), a networking block and a digital and analog
power
supply block.
In this embodiment the DSP block comprises a digital signal processing chip
and an
additional external static random access memory (SRAM). The DSP block performs
the
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computation algorithms for fast, real-time processing of spectral data being
transferred
from the optical detector(s). This block also generates signals that are
capable of
modulating the photon energy source, wherein this modulation signal can be
multiplexed to multiple photon energy sources if required. However, each
detector, if
there is more than one, has a separate channel into the DSP block for the
transmission of
information relating to the received light. In addition, the DSP block can
control the
optical devices) that mechanically pulse the illumination radiation, for
example, a
chopper. As would be known to a worker skilled in the art, the required
processing
speed of the DSP chip can be determined by the estimated amount and frequency
of the
incoming data that is to be processed, for example. In this manner an
appropriate chip
can be determined based on its processing speed for example the number of
hertz that
the DSP operates, 40 Hz, 60 Hz and so on.
According to this embodiment, the transmitter and receiver block comprise
analog-to-
digital converters) (ADC), digital-to-analog converters) (DAC) and low-pass
filters,
wherein these filters enable anti-aliasing of the received signal. If light
emitting diodes
(LEDs) or laser diodes are used as the photon energy source for the optical
system, this
block further comprises a multiplexer and high current amplifiers. The
multiplexer
enables the transmission of signals for the activation of the multiple photon
energy
sources independently and the high current amplifiers provide a means for
providing
sufficient energy in order to activate these photon energy sources such that
their
maximum spectral power output is obtained.
The networking block of the stand alone DSP means comprises a networking card,
for
example, an ethernet chip or a wireless network chip, which enables the
interconnection
of thwstand alone DSP system to a communication network, for example a local
area
network (LAN), a wide area network (WAN) or a wireless network (for example
BluetoothTM or IEEE 802.11 ). A worker skilled in the art would understand the
format
and type of chip or card that is required for the desired network connection.
In addition
the network block further comprises a serial interface chip, for example a. RS-
232 port
which can provide a serial interface to another component or system, for
example a
computer or a serial modem, for example dial-up or wireless type modem or a
serial
connection to a monochromator.
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Furthermore, in this stand alone embodiment, the micro-controller unit (MCU)
block
comprises a MCU chip, which may be an 8-bit, 16-bit or 32-bit chip, for
example, an
external SRAM and an external FLASH unit. The MCU block manages the DSP block
and the networking block, wherein the MCU block collects processed data from
the
DSP block and forwards this information to the networking block. Optical
devices
which filter and/or focus the illumination radiation and received light, for
example light
filters or monochromators, can be controlled by the MCU block. The MCU block
may
additionally perform statistical analyses on the data and may possibly
activate an alarm
setting. For example, an alarm setting may be activated if the level of
fluorescence of
the test sample exceeds a predetermined level, wherein this alarm activation
may
comprise the collecting of a sample for a more detailed analysis or the
notification of
personnel of the alarm activation. In the case where software updates to the
DSP block
are required, for example the modification of the match filtering procedure,
the MCU
block can manage the remote software updates of the DSP code, for example. The
type
of MCU chip incorporated into the MCU block may vary depending on the volume
of
information that is to be processed for example, as would be known to a worker
skilled
in the art.
The digital and analog power supply block of the stand alone DSP system can
provide
regulated DC power at a variety of levels depending on that required by the
components
of the stand alone DSP system. In one example, the input power to this stand
alone
system may be supplied by an unregulated or varying power supply, for example
a wall
plug. The digital and analog power supply block comprises elements which can
regulate the input power and,subsequently generate the required analog and
digital
voltage levels for each component of the stand alone DSP system. As examples,
elements which enable the adjustment of the input power comprises
transformers, AC-
DC converters or any other power regulation element as would be known to a
worker
skilled in the art.
Test Sample Housing
Optionally, the optical system comprises a test sample housing which provides
a means
for orienting a test sample such that the illumination of the sample and the
detection of
the samples resultant emissions can be performed. For example, as would be
used with
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a microscope, the test sample housing may comprise a cover slip and a glass
plate for
sec~:ring the test sample and a set of clips may be used to orient and
restrain the
movement of this glass plate and cover slip during testing.
In one embodiment of the present invention wherein the test sample is a
flowing fluid, a
test sample housing may be a tube inserted and appropriately oriented within
the fluid
flow wherein this housing provides a means for an optical probe to be oriented
therein.
This sample housing can be designed such that it minimises the effects on the
flow of
the fluid thereby potentially reducing its affects on the detected response of
the fluid to
its illumination. The size, in particular the cross sectional area, of the
test sample
housing can be designed such that the surface area of the housing is outside
of the
optical detector's field of view. In this manner, the detection of reflectance
from the
housing may be minimised. In order to potentially further reduce the test
sample
housing's affect of the response, the surface are of the housing can be
fabricated using
or painted with a non-reflective light absorbing material. Furthermore, the in
this
embodiment, the test sample housing can be fabricated such that the optical
probe can
be removed for cleaning, if desired and subsequently replaced in the same
orientation.
A form of indexing may be used in order to facilitate the realignment of the
optical
probe upon replacement with in the test sample housing.
Considerations
Iri one embodiment, the requirements of the optical system are that: 1) it is
able to
resolve optical spectra over the range 250 nm to 800 nm; 2) the spectral
resolution is on
the order of 5 nm of better; and 3) that it has a stray light suppression of
10-5 or better,
for both the illumination and emission units. In addition, a spectral
resolution of 5 to 10
nm can allow reasonable sampling of the fluorescence peaks, which appear to be
the
order of 30 to 50 nm. However, finer resolution may be useful in some
applications.
The stray light suppression factor required depends on how small an area of
received
light one wishes to detect. Stray light essentially determines the optical
noise floor for
the system, and sets the limit of optical detectability.
In choosing the illumination wavelength, the factors that should be balanced
are overall
scanning time for the area of interest or test sample and the resolution of
the scan. The
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total number of steps N required to sweep out the illumination light and
received light
spectrums is:
N = n; ~ nd/2
where:
n; = number of steps for the illumination monochromator
nd = number of steps for the received light monochromator
The factor 1/2 determines that only the diagonal terms of the illumination
light/received
light matrix are of interest in addition to the terms on one side of the
diagonal only.
Moreover, N is proportional to ~~./2, where ~~, is the spectral resolution of
a
monochromator. Since the scanning time is proportional to N, then there is a
trade-off
between 07~ and the scanning time.
Weak Signal Detection
In one embodiment, the tone encoded method is used for signal encoding due to
its basic
simplicity and the fact that it yields a reasonable degree of noise
suppression relative to
the complexity. In this embodiment, the key consideration is the amount of
time
required to take one measurement. This is determined by: 1) the amount of time
required to acquire the samples for a frequency domain transfer, which is
essentially the
number of samples required divided by the sample rate and 2) the filter
bandwidth in the
case of a bandpass filter technique, which is essentially the reciprocal of
the bandwidth
of the filter.
The trade-off with the electrical signal bandwidth is observation time versus
noise. As
the bandwidth is increased and the observation time is decreased, the noise
power
increases in proportion to the bandwidth. Any increase in noise reduces the
detector
sensitivity. The total processing time to scan the area of interest can be
determined by
T = Ni, where i is the time for one measurement at one wavelength. The two key
variables in the observation time are the optical filter bandwidth and the
electrical filter
bandwidth.
A rough first order calculation of T can be made by making the following
assumptions:
1) resolve optical spectra over the range 250 nm to 800 nm; 2) use an optical
resolution
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bandwidth of 10 nm; and 3) use an electrical bandpass filter BW of 10 Hz,
therefore T =
0.10 sec. By using these assumptions, the scanning time is 151.25 seconds, or
about 2.5
minutes.
When a test sample to be examined is exposed to illuminating radiation, the
detection of
reactive radiation characteristics is the goal. In general, the fluorescent
light will be
much weaker than the reflected light. The spectral resolution required is
determined by
the ability of the optical spectrometer system to discriminate between
reflected and
fluorescent wavelengths. This may be achieved through the use of a prism
and/or
grating monochromators with variable apertures, which suppress stray
radiation.
For optical signatures to be adequately resolved, the system must be able to
detect very
weak electrical signals, which result from the optical radiation being
detected by the
photodiode. Ultimately, the goal is to detect a very weak signal in a
background of
noise due to electrical noise, optical background radiation and out of band
emissions
from the test sample (due to the spectrometer spectral resolution).
Other variables in the measurement of spectral signatures comprise: a) time
duration the
test sample is illuminated; b) the amplitude of the illumination at the test
sample first
surface; c) the amplitude of the noise variables; d) spectral shifts in the
illuminators over
time; and e) the decay of the fluorescence emitted by a test sample after the
illumination
of the test sample has been discontinued. These variables need to be addressed
to
compare the performance of various detection schemes.
In one embodiment of the present invention, adaptive filtering of the received
light may
enable the detection of the decaying intensity of fluorescence emitted from a
test sample
upon the discontinuation of the illumination of the test sample. The
discontinuation of
the illumination may be complete termination of transmission of photonic
energy or the
discontinuation of a particular illumination wavelength. The measurement of
the decay
of fluorescence emitted by a test sample using a device according to the
present
invention may provide a means for the identification of a test sample.
Pulse amplitude modulation techniques as applied to this situation may be On-
Off
keying of the illumination. The detection is based on the ability to detect
the presence
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of the signal in an ambient noise. The signal detectability depends on the
ability to
discriminate the signal from the noise and generally requires a signal power
much
greater than the noise (> 10 dB typically). An example of an On-Off keyed
signal is
shown in Figure 5. The signal to noise ratio (SNR) in this case is 0 dB and it
is not
possible to distinguish the noise portion of the signal from that consisting
the signal plus
noise.
The frequency domain detection mechanism is a detection means based on
frequency
modulation of the signal with a constant frequency modulation. This has great
advantages over time domain detection means such as On-Off keying. Even though
the
RMS amplitudes of the signal and the noise can be equal (SNR = 0 dB), the
power
spectral density of the modulated signal is usually much greater than the
power spectral
density of the broadband noise. The carrier can be isolated from the noise by
a number
of means, including: a) spectral measurement techniques, such as a DFT or FFT;
and b)
I 5 narrow band filtering with the centre frequency of the filter located at
the modulation
frequency.
An example of this is shown in Figure 6. In this case, the RMS amplitudes of
the first
signal and the noise are equal (SNR = 0 dB). Two other signals were added
which had
magnitudes relative to the first signal of 0.50 and 0.1 respectively. The time
domain
signal happens to look exactly like that shown in Figure S. In the frequency
domain
however, the spectral peaks for the first and second signals are apparent. The
spectral
signature for this signal is buried in the noise and cannot be resolved. This
detection
technique is relatively simple to implement in practice and can be suitable
for use in an
optical spectrometer.
The pulse coding techniques (binary, linear, enhanced) are an alternative
means of
detection. Pulse coding techniques are often used to detect very weak signals
in the
presence of noise. They may be more complex than traditional techniques such
as tone
detection and pulse amplitude detection, however they are sometimes the only
choice
when amplitude of the signal to be detected is weak relative to the noise and
there are no
means available to increase the signal to noise ratio other than pulse coding.
Two
exemplary pulse coding techniques are Binary Pulse Coding and Linear Frequency
Modulation (FM) Coding. Both of these techniques fall into the realm of pulse
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compression and spread spectrum and they are adequately described in numerous
references (Barton, DK (1978) Radars Volume 3: Pulse Compression, Artech House
Inc.).
Binary Pulse Coding, as an example, uses a 1000-bit synchword, which can be
created
by using a uniform random number generator and constructing a binary sequence
from
that data. Pulses are generated at specific locations in the time domain and
the relative
amplitudes are measured. Results of a time domain correlation output are shown
in
Figure 7. In an amplitude plot, all three pulses can be detected. The third
and smallest
signal pulse is just distinguishable from the noise.
Linear FM Pulse Compression schemes have traditionally been used in radar
systems to
reduce the overall peak power of transmitted signals while still achieving
large detection
ranges. They also figure prominently in Synthetic Aperture Radar processing
for
airborne and spaceborne imaging radars. This form of coding is achieved by
linearly
sweeping a carrier signal from f~ to f2 (for a swept bandwidth of nf) for a
duration i. In
general, the "output power" of a linear FM coded signal is increased by the
Time
Bandwidth Product (TBP) 4fT, which is the product of the pulse duration in
seconds and
the swept bandwidth in Hertz. The detection process is essentially a matched
filter
detector, which is matched to the linear FM transmitted pulse. The overall
process is
shown in Figure 8. The signal s(t) is usually a Dirac Delta,function, which in
reality is
simply the trigger pulse for the encoder h(t) which generates the transmitted
signal
U(i,OfJ which is the linear FM coded pulse (or Chirp) which has a duration i
and a
bandwidth Of. This is the signal that would drive an optical emitter to
illuminate a test
sample. Noise n(t) is added to the coded signal in both the optics and the
electronics.
This optical signal is detected by a photodetector, whose electrical output
signal is
comprised of the actual optical signal of interest, optical background noise
and electrical
noise in the photodetector and electronics. The matched filter detector then
processes
this electrical signal. Since the optical signal of interest is the only one
of the three
components of the signal which is matched to the matched filter, it is the
only
component which experiences gain due to the linear FM pulse coding. The
optical and
electrical noise components are essentially suppressed relative to the coded
signal. This
is an advantage of such a scheme. A linear FM Chirp output is shown in Figure
9. In
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the amplitude plot, only the largest two pulses can be detected, with the
third being
essentially buried in the noise. This example graphically demonstrates the
coding gain
offered by a linear FM Pulse Compression Technique.
Enhanced Pulse Coding Techniques take advantage of the fact that by increasing
the
Time Bandwidth Product, greater coding gain can be achieved. Using this
technique the
weakest of the time domain pulses was just visible.
A plot of the original case with a TBP of 200 is shown in Figure 10 and the
new case
with a TBP of 800 is shown in Figure 11. The increase of the time bandwidth
product
has increased the coding gain sufficiently enough that the third and weakest
pulse is
now visible above the noise floor. The coding gain was increased from 23.0 dB
to 29.0
dB or an overall increase 6.0 dB. In both plots, the power has been normalised
to the
peak located at sample 100. The drop in the noise floor in going from a TBP of
200 to
800 is readily apparent.
To further make this point, a plot for the case of a TBP of 2250 is shown in
Figure 12.
In order to compare this high time bandwidth product detection scheme to the
other
coding techniques, a time domain magnitude plot where the detector amplitude
has been
plotted is shown in Figure 13. The noise amplitude should be suppressed by
X2250, or
about 47.4. The peak amplitude of pulse 1 is 2505, pulse 2 is 1252 and pulse 3
is 250.
The noise magnitude was the same as that for the signal for peak l, therefore
the noise
magnitude should be suppressed to a level of approximately 52. As seen from
Figure
13, this is more or less the case. Due to the high level of noise suppression
achieved, the
signal for pulse 3 is quite visible relative to the noise background. This is
readily
apparent when the TBP=375 case in Figure 9 where pulse 3 is not visible, is
compared
with the TBP = 2250 case in Figure 13 where pulse 3 is visible.
Higher Time Bandwidth Products can be used to achieve higher coding gains,
however
these may be limited depending on the means used to achieve the signal coding.
A
mechanical chopper would be limited by the ability to replicate the linear FM
code onto
the chopper wheel, whereas acoustic-optic modulators could achieve much higher
TBP's but at much higher expense.
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Scanning Methodologies
For manual scanning, the probe is moved manually across the surface to be
analyzed,
and analyzes only the area immediately under observation. The spectral
characteristics
are observed at a fixed point in space (xo, yo) . Thus, one obtains a one-
dimensional plot
of the spectral response for each point (xo, yo). This mode of operation is
useful if the
fluorescing material is diffusely distributed throughout the medium to be
observed, or if
localized analysis is required.
For two-dimensional scanning, the fluid flows through a chamber and spectral
responses
are obtained for each point in time (t, ~,). This mode of operation is useful
if the
fluorescing material is periodic as in an open system such as a municipal
water system
or a closed system for fluid processing.
For three-dimensional scanning, quantitative and qualitative data can be
obtained for
closed loop feedback control and detection of physical and optical
characteristics in
subject matter. As the probe is scanned across a two-dimensional surface,
spectral
responses are obtained for each point (x;, y;, z;, ~,).
To gain a better understanding of the invention described herein, the
following examples
are set forth. It should be understood that these examples are for
illustrative purposes
only. Therefore; they should not limit the scope of this invention in any way.
EXAMPLES
EXAMPLE I: OPTICAL SYSTEM FOR ANALYZING FL UID SAMPLES
In one embodiment of the present invention, the optical system can be designed
to
perform the analysis of fluid samples, for example for the detection of
turbidity and/or
bio-mass in a flowing water sample. This form of the invention may also be
used for
the analysis of a petroleum sample, for example. In this embodiment of the
optical
system the changes) in the spectral properties of a test sample are detected
and
evaluated.
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An embodiment of the invention a system used for this purpose is illustrated
in Figure
16. This example is directed towards water analysis, however it may equally
well be
used for the analysis of other fluids. The optical system comprises a digital
signal
processor 600, a LED control system 610, an illumination system 620, a sample
chamber or water flow 630 into which the optical probe 700 may be placed,
detector
optics 640, a photodetector 650, photodetector electronics 660 and a network
670 to
which the DSP 600 is connected. The optical probe 700 which can be inserted
into the
flowing water comprises both the illumination system 620 and the detector
optics 640.
which can be aligned in order to maximise the detection of radiation emitted
by the
water sample upon it illumination.
The digital signal processor 600 comprises software and hardware integrated
together to
enable it to perform tasks including signal processing, data processing,
system control
through the use of control logic and communication with an external network
for
example the Internet or a local area network (LAN). The signal processing
performed
by the DSP includes the operation of the signal generator enabling the
encoding of the
illumination signal (radiation). In addition, the signal processing enabled by
the DSP
includes the FIR matched filtering and the correlation filtering of the
received light
signal (detected radiation emitted by the water sample). The data processing
performed
by the DSP can include the collection, processing and analysis of the
collected data. A
statistical analysis of the data may also be performed by the DSP in order to
determine
for example return periods of particular levels of detected radiation. The
control of a
valve for withdrawing a sample from a water flow into a sample chamber and the
control of the LED switch thereby controlling the activation of a LED, are
both provided
by the control logic incorporated into the DSP. The control logic may
additionally
control an optical wiper that can be used to remove bio-fouling which may
collect and
grow on the optical probe. The inclusion of an optical wiper may reduce the
frequency
of the removal of the probe from the sample chamber or water flow, for
cleaning. The
DSP further comprises a communication system which enables it to connect with
a
network thereby enabling the transmission of the collected information to
other sites
without the need for personnel to visit the test site for data retrieval. In
the present
embodiment, this communication is provided by software and hardware which
enables
an ethernet link to be created.
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The LED control 610 includes the LED switch and a high current amplifier,
wherein the
LED switch activates the desired LED and the high current amplifier transforms
the
available power supply to a level which is compatible with the activation of a
LED to a
desired intensity level.
The optical probe 700 comprises both the illumination system 620 and the
detector
optics 640 wherein this probe can be inserted into the water flow directly or
into a
sample chamber containing water extracted from the water flow. If the probe is
inserted
into the moving water flow, the shape of the probe should be designed for
minimal
disruption of the water flow. The illumination system comprises a LED array
and LED
optics for focusing the photonic radiation generated by the LED array. The LED
array
may be a single diode or may be a collection of diodes thereby spanning a
predetermined band of wavelengths. In one embodiment, a green and blue light
emitting diode is used in the optical system. The detector optics comprise
lens for
collecting the radiation emitted by the water sample in addition to an.optical
bandpass
filter for pre-filtering the collected radiation before it is detected by the
photodetector
650 for conversion of the detected radiation into an electronic signal.
The photodetector electronics 660 comprise a collection of filters which pre-
filter the
collected information prior to its processing by the DSP, for example the
match filtering
of the collected information relating to the water samples illumination.
In this embodiment of the invention, the DSP is a stand-alone system which may
include an internal power supply or a power converter in order for the DSP to
be
interconnected to a standard AC power source, for example a wall socket. In
addition,
this-stand-alone type of system enables the deployment of this optical system
at a
plurality of sites, for example at various locations in a water supply system.
Through
the interconnection of this collection of optical systems to a communication
network, for
example the Internet or a local area network, the information which is
collected and
processed by these optical systems can be transmitted to a central site,
without the need
for personnel to visit each test site to collect the information. This type of
system may
provide a means for efficiently and cost effectively evaluating a water supply
system.
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This optical system is capable of detecting reflectance and fluorescence,
wherein
reflectance is indicative of the turbidity of the water sample and the
fluorescence is
indicative of the bio-matter contained within the water sample. It is known to
a worker
skilled in the art that bio-matter, upon its illumination will fluoresce and
the detection of
the intensity of fluorescence can potentially enable the determination of the
level of bio-
matter within a water system. This embodiment of the optical system evaluates
the
changes in the reflectance and the fluorescence within the water flow, thereby
potentially being able to identify situations which may be of particular
relevance. In this
manner, upon the detection of a particular level of change in the optical
signature of the
water flow, the DSP may be able to activate a sampling procedure, wherein a
sample of
the water flow is collected for laboratory analysis. This type of almost
constant testing
and selective laboratory analysis can potentially reduce the cost of
monitoring a water
supply system and increase the identification of a potential problem.
For example, Figure 17 illustrates in a graphical format, the turbidity
readings which are
collected over the course of a day as collected by this optical system. Figure
18
illustrates the bio-mass reading which are detected by the optical system also
over the
course of a day.
EXAMPLE IL~ SPECTROMETER INCORPORATING A MATCHED FILTER
RECEIVER
One embodiment is shown in Figure 14 comprising a light source, for example, a
miniature Xenon bulb that has an emission spectrum approximately equal to that
of a
6000 °K Blackbody with a few discrete spectral lines. The light is
collimated and
modulated by a chopper wheel, which provides a 500 Hz On-Off modulation to the
light
entering the illumination monochromator. The illumination monochromator
operating
under the control of the CPU sweeps the illumination wavelength from 250 nm to
800
nm in steps of 10 nm. This illumination is focused onto the area of interest.
The
received light monochromator operating under the control of the CPU sweeps the
received light wavelength from 250 nm to 800 nm in steps of 10 nm. It is
controlled in
such a way that for every illumination wavelength sample ~,; , it sweeps over
the range
of wavelengths greater than or equal to ~,; . A Ga-As photodiode is used as
the optical
detector, with the signal from the photodiode being amplified by a low noise
amplifier
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(LNA). The output of the LNA can be filtered using an analog filter, or it can
be
digitized using an analog to digital converter (ADC) and processed digitally
using an
IIR or FIR digital filter. The detector output is recorded for each 7~; and
7~e, and can be
plotted for display as shown in Figure 15.
One important issue to be dealt with is the magnitude versus wavelength
calibration of
the system, since the Xenon Light source is not spectrally flat. This can be
done using a
standard diffuse reflection source, which is spectrally flat in an optical
wavelength
sense. The calibration factor can then be applied to the collected data such
that the
spectral colouring of the illumination source can be removed from the data.
This
process is essentially spectral equalization of the data.
Once the data is equalized, it can be displayed in a number of ways such as
contour
plots, surface plots, for example, enabling easy visualization of the
illumination/emission spectra. This may require some normalization to say the
strongest
spectral peak of some response at a fixed wavelength location. This will have
to be
determined experimentally. An example of a surface type of plot is shown in
Figure 15.
EXAMPLE III
One embodiment of this invention comprises an optical system comprises: a) a
light
emitting diode (LED), as the illumination light source, which is controlled by
the digital
signal processing means to emit a radiation bandwidth ranging from 380 to 500
nanometers; b) a stepper motor controlled, grating illumination monochromator
which
is controlled by the digital signal processing means to receive light from the
illumination
device and to deliver the N'h wavelength in a pulse sequence; c) an optical
fibre probe
that is coupled to the monochromator with collimating and focusing elements
that
delivers the N'~' wavelength to a subject area. This optical probe is located
in an
assembly that orients the illumination optics with that of the detecting
optics such that
they are at a constant angle to each other; d) collecting means for gathering
the resultant
radiation of the N'h wavelength and delivering the information via light
collection
lenses and fibre coupled to a stepper motor controlled, grating detection
monochromator; and e) a photodetector such as a Ga-As integrated photodiode
and
amplifier. The stepper motor controlled, grating monochromators (both the
illumination
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and detection monochromators) are controlled by the digital signal processing
means to
perform tasks to pass the Nt~' wavelength reactive characteristics at a
specific time.
Typically a Ga-As integrated photodiode and amplifier is made up of stock
electronic
components that consist of a photodiode and transimpedance amplifier on the
same chip.
This is sampled by the digital signal processing means to sense the radiation
at a specific
time. A photodiode is used as an optical detector, with the signal from the
photodiode
being amplified by a low noise amplifier (LNA). The output of the LNA is
filtered using
an analog filter to condition the signal from the photodetector with an op
amp,
amplifying the signal to a specific range and is digitised using an analog to
digital
converter (ADC) and processed digitally using an FIR digital filter and a
digital signal
coding software technique such that a time/bandwidth product can be measured
using a
correlation receiver.
The system further comprises a DSPS device where an illumination modulation
coding
signal is created using a 32 bit linear FM pulse coding technique for pulse
compression,
the detection pulse coding is resolving the time bandwidth product with a
matched
correlation receiver, and the detection of specific amplitudes of irradiance
can prompt
the DSPS to run a specific routine to test for specific signal response
characteristics in
this case fluorescence and reflectance can be measured depending on the
limitations of
the wavelengths of illumination. The monochromator gratings can operate
through the
visible spectrum and can be substituted for other wavelengths into the UV or
IR ; and a
digital signal processing technique such that software that recognises the
peaks of data
and their rule based weighted relevance can control the illuminator and
detector
monochromators.
EXAMPLE IV
In one embodiment of the present invention an optical system can be designed
with the
ability to control the wavelength of the scan (illumination radiation)
including
modulation techniques. This type of optical system can provide maximum optical
flexibility, which can be required for applications including research and
diagnostics.
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An embodiment of the optical system designed for this scenario comprises: a
digital
signal processing means which is integrated into a computing device with the
emitter
control electronics comprising pulse code software to create a synchronous
pulse for
direct modulation of the optical emission processing means frequency and the
received
signal processing means incorporating a signal correlation match filter; a
flashlamp
providing the photonic energy source; optical emission processing means
incorporating
a frequency modulator circuit for modulating the illumination radiation, a
refractive or
diffractive optical system whereby the optical centre wavelength is chosen by
the use of
a position controller to move the fixed light conditioning optics of the
emitter optical
system; received light optical processing means incorporating a refractive or
diffractive
optical system whereby the optical centre wavelength is chosen by the use of a
position
controller to move the fixed light conditioning optics of the detector optical
system; and
a silicon APD photodetector acting as the optical detector.
In this embodiment, correction of the emitted spectrum for a flashlamp may
require
more than simply comparing it to a standard diffuse reflection source as in
Example II.
Highly specific wavelengths of peak emissions may require adjustment of
apertures and
detection system gains in order to maintain accurate signal to noise ratio
normalisation.
These adjustments are ideally made automatically and recorded as a data
variable in the
normalisation process. This is especially the case where sample absorption and
reflection is highly variable at the peak emitter wavelengths. A system that
is set for
maximum fluorescence detection at non-peak wavelengths could easily be over
saturated by the response by the peak wavelength.
EXAMPLE V
In one embodiment of the present invention an optical system can be designed
for
maximum sensitivity of resultant radiation resulting from the illumination of
a test
sample by a known wavelength of light. This type of optical system can be
useful for
fluorescence analysis, especially if a spectral probe is attached to the
subject of interest
and has know spectral properties such that detection of a specific wavelength
of
fluorescence, absorption or reflection can be measured.
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An embodiment of the optical system designed for this scenario comprises: a
digital
signal processing means which is integrated into a computing device with the
emitter
control electronics comprising pulse code software to create a synchronous
pulse for
direct modulation of the optical emission processing means frequency and the
received
signal processing means incorporating a signal correlation match filter; a
laser providing
the photonic energy source; optical emission processing means incorporating an
acousto-optic scanner and a fixed emitter optical system; received light
optical
processing means incorporating fixed light conditioning optics; and a
photomultiplier
acting as the optical detector. This embodiment of the system can be used for
direct
sensing of specific fluorescence response from biomolecules within a test
sample or
subject matter.
EXAMPLE VI
In a further embodiment of the present invention an optical system, which can
be used
in the lowest cost applications of fluorometry and reflectometry including for
example
particle measurement such as water turbidity. In this example, the optical
device can be
designed to illuminate a test sample with a specific wavelength of light and
can detect
the reflection of the same or a different wavelength or waveband reflected, or
emitted by
stimulation of the particles in the fluid.
An embodiment of the optical system associated with this type of device
comprises: a
stand alone digital signal processing means with the emitter control
electronics
comprising a pulse code software to create a synchronous pulse for the direct
modulation of the optical emission processing means amplitude and the received
signal
processing means comprising a signal correlation matched filter; a light
emitting diode
(LED) acting as the photonic energy source; optical emission processing means
comprising an amplitude modulator circuit and a fixed light conditioning
emitter optical
system; received light optical processing means incorporating a fixed light
conditioning
detector optical system and a silicon photodiode acting as the optical
detector.
In this embodiment, the optical system emits directly into the fluid and the
detector
receives light from within the fluid. This is achieved by means of an optical
fibre or by
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direct insertion of the optical system into the subject fluid. This type of
procedure will
reduce the reflection effects of the surface boundary layers, thereby
potentially
improving the signal to noise characteristics of the optical system.
The embodiments of the invention being thus described, it will be obvious that
the same
may be varied in many ways. Such variations are not to be regarded as a
departure from
the spirit and scope of the invention, and all such modifications as would be
obvious to
one skilled in the art are intended to be included within the scope of the
following
claims.
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