Note: Descriptions are shown in the official language in which they were submitted.
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Detector Unit and a Method for Detecting
an Optical Detection Signal
The present invention relates to a detector unit and a method for detecting an
optical detection signal, and in particular, to a laser warning device and a
muzzle
flash detector.
Background
Both for the detection of pulsed laser sources (for example, for laser
controlled
missiles) and also for muzzle flashes very short localized optical flashes are
to be
detected, namely often in front of a strong background which is irradiated by
sunlight. The typical flash duration of light flashes may be in the range of
nano-
seconds for pulsed lasers, and in the range from about loo ps for detecting a
muzzle flash.
A primary limiting factor for detecting such signals during daytime is the
shot
noise of the existing solar background. Typical reference background light sig-
nals are a beach irradiated by the sun or a cloud irradiated by the sun
(having
albedo values of o.6 to 0.8), or even snowfields which are irradiated by the
sun
(having an albedo value of up to 1), for example.
Previously known systems for the detection of pulsed lasers and hostile fire
are
significantly limited regarding the background light, and are often not suited
for
state-of-the-art applications. However, also acoustic detectors are used for
fire
detection, but they often generate false alarms (for example, in helicopter
appli-
cations). The also used radar systems are complex, and in addition have to ac-
tively emit signals, which is often not desired.
Thus, there is a need for detectors which are capable to detect very short
flashes
of light of a very low intensity against a background signal of high
intensity, as
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for example objects irradiated by the sun having albedo values of up to 0.8
(cloud, beach) or more. In addition, there is a need for detectors of laser
warning
systems or detectors for hostile fire warning systems, which may achieve high
positional solution of for example < 1 .
Summary
At least a part of the problems mentioned above is solved by a detector unit
ac-
cording to claim 1, a camera according to claim 15, and a method according to
claim 19. The dependent claims relate to the advantageous developments of
subjects of the independent claims.
The present invention relates to a detector unit for detecting an optical
detection
signal from a localized optical source in front of a background, and in
particular
to a detector unit for detecting short optical detecting signals. Thus, the
detect-
ing unit may in particular be used for laser warning systems and for the detec-
tion of hostile fire, and includes: an image sensor and a signal processing
unit.
The image sensor comprises a plurality of pixels in order to detect the
detection
signal and the background, wherein the image sensor is designed to generate an
image signal separately for each pixel depending on the detection signal and
the
background. The signal processing unit is designed to compare the image signal
with a threshold for each pixel, and to output event data, if the threshold is
ex-
ceeded, within a time duration. Optionally the signal processing unit is
integrat-
ed with the image sensor.
Here, the threshold comparison may be performed by an analog threshold com-
parator per pixel, thus the time duration may result from the time constant of
said analog threshold comparator. Hereby it will be achieved that detection is
not confined to a fixed time pattern defined by the time duration, but event
data
may be generated every time, when the threshold will be exceeded within the
duration specified by the time constant. The threshold and the time constant
are
configurable, wherein the threshold may be optionally set individually for
each
pixel. This way it is possible to compensate the different sensitivities of
the indi-
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vidual pixels caused by manufacturing tolerances. Thus, with an appropriate
threshold setting, the same sensitivity may be achieved for all pixels. The
event
data depend on the detection signal (for example, related to only those pixels
which detect optical signals from the localized source).
The (electric) image signal may for example represent an electrical voltage
value
or a number of charges (for example, generated by a CCD sensor). It is also
pos-
sible that the image signal indicates a current intensity, as generated by a
photo-
diode, for example. The background also represents an optical signal which may
change during the course of time or according the location. Generally, during
the
considered time interval of nanoseconds or microseconds, there are very few
changes in the natural background light, and are dominated by the shot noise
which is determined by the photon statistics of the background signal. In
order
to detect a source, which is localized in a limited region, the signal
processing
unit comprises a respective signal processing component for each pixel, which
performs an evaluation for each pixel and determines a potential event.
In further exemplary embodiments, the signal processing unit is further de-
signed to adjust the time period and/or the threshold in a way that a number
(rate) of event data output in a predetermined time unit (e. g., per second)
is in a
predetermined range. Hereby, it may optionally be achieved that the average
number of event data per time unit becomes independent of the background.
This way it is achieved that the detector unit is always operated with the
maxi-
mum sensitivity which is limited by the background radiation.
The present invention is not limited to a specific predetermined range.
Rather,
the determined range may be freely chosen and adapted to the respective hard-
ware (for example, may only be defined by a maximum value). Thus, for exam-
ple, the predetermined range is selected in a way that an optimization of the
detection is achieved, and only the number of events (for example, from one
source) will be detected, which can be processed. For example, said rate may
be
about 10 to loo/s for a very simple signal processing hardware; if the best
possi-
ble sensitivity is desired, a more powerful signal processing hardware may be
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used, which then also allows to process rates of loo,000/s.
Optionally, the image sensor comprises a CMOS image sensor, and the time
period is an integration time for summing up the detection signal.
Optionally, only the signal processing may be realized in a CMOS readout elec-
tronics, on which the light-sensitive pixels may be contacted on the backside
thereof. This may for example be performed by a so-called "flip chip bonding",
where pixel-specific signal components of the readout electronics are
contacted
on the backside by the image sensor.
In further exemplary embodiments, the signal processing unit is further de-
signed to cause a readout of a pixel value for the respective pixel during
output of
the event data, so that a readout of the pixel is performed in an event-driven
and
asynchronous way. In particular, no complete frames have to be generated dur-
ing readout, instead the pixel values may be processed independently from each
another. In order to trigger a readout for said pixel (and only for said
pixel), it is
sufficient that one single pixel value is exceeded. The pixel value comprises,
for
example, the summed signal, wherein the summed signal is given by the com-
plete number of charges, which have been generated within the time constant of
the threshold comparator, for example, and which corresponds to the image
value of the respective pixel. Thus, exemplary embodiments also define a
signal
processing unit, which outputs signal event data separately for each pixel.
In further exemplary embodiments, the detector unit comprises a warning unit,
which is designed to generate a warning signal as warning of a detection
signal
based on the event data. Thus, event data are not necessarily a warning
signal.
Only after other criteria are met, a warning signal may be generated
therefrom,
for example.
As the event data also includes the identification of the pixel, for which the
event
has been triggered, the warning unit may also determine the direction of the
optical source. Hereby, a respective calibration of the image sensor may be
used
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in order to assign a direction in the target region to each pixel, and thus to
de-
termine the direction of the localized source.
The warning unit optionally comprises a filter which is designed to filter a
peri-
odic signal by using a first threshold and/or a non-periodic signal by using a
second threshold, wherein the first threshold is smaller than the second
thresh-
old. Hereto, the filter may comprise a digital filter, for example, and the
warning
unit may comprise a feedback to the signal processing unit in order to cause a
change of the threshold.
Optionally, the signal processing unit and/or the warning unit is designed to
adjust the threshold in a way that the signal-to-noise-ratio is at a value of
13
(also a smaller or larger ratio may be chosen).
The invention also relates to a camera including a detector unit which has
been
described above. Optionally, the camera comprises a lens, a spectral filter
and/or
an interference filter.
The invention also relates to a method for detecting a detection signal of a
local-
ized optical source in front of a background, wherein the detection signal and
the
background are detected by an image sensor including a plurality of pixels.
The
method comprises the steps: Generating of an image signal, wherein the image
signal is generated independently for each pixel, comparing the image signal
with a threshold; and outputting event data, if the threshold is exceeded
within a
time period. Comparing is performed separately for each pixel in an
independent
way, and the threshold and the time period may be set. Optionally, an
individual
threshold and/or an individual time period may be set for each pixel.
Exemplary embodiments solve at least some of the technical issues mentioned
above by using a CMOS image sensor, in which an analog comparator is inte-
grated in each pixel, which compares the detected image signal with the thresh-
old and triggers an event, if the threshold is exceeded. Said event indicates
for
example, that an absolute speed of change of the number of photo electrons has
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been exceeded. The content of the respective pixel may then be read out in an
asynchronous, event-driven way - not necessarily frame-based.
Here, the threshold may be controlled and set in a way that constant (having a
tolerance range of +/- io%, for example) and easily to process event rate is
oh-
tamed. Said event rate may lie in a range of 10 to 100,000 events/second, for
example. However the invention is not to be limited to these values. The
actual
value to which the event rate is adjusted, may be freely chosen, and may be
adapted to the respective situation (for example, computing performance).
Optionally, also event data for each pixel may be counted in signal processing
1 o during fixed time intervals. The thus received obtained number of
events per
pixel will then be readout at the end of the time intervals for all pixels.
According to further exemplary embodiments, the detector may be formed as a
camera, wherein a respective bright lens including an adequate field of vision
of
900*900, for example, and a suitable spectral filtering is used.
Short Description of the Figures
The exemplary embodiments of the present invention will be better understood
by referring to the following detailed specification and the appended figures
of
the various exemplary embodiments, however it is not intended to limit the dis-
closure to the specific embodiments, but they shall only provide a description
and facilitate understanding.
Fig. 1 shows a detector according to an exemplary embodiment
of the pre-
sent invention.
Fig. 2 shows an example of a signal waveform for a signal
which is detected
by one of the pixels of the image sensor.
Fig. 3 shows the detector including further optional components according
to further exemplary embodiments.
,
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Fig. 4 shows a flow diagram for a method for detecting a
detection signal
from a localized optical source according to an exemplary embodi-
ment of the present invention.
Detailed Description
The underlying detector concept, as it is realized in exemplary embodiments of
the present invention, may be described as follows.
For an opto-electronic detector, the smallest detectable signal is limited by
the
intrinsic noise of the detector and by the noise of the background signal
against
which the useful signal is to be detected. For very short useful signals, the
back-
ground signal is generally hardly changing during the duration of the useful
signal, so that the shot noise generated by the background intensity becomes
decisive for the noise contribution of the background. This is calculated from
the
photon statistics from the root of the photons received at the pixel during
detec-
tion time.
For an daylight application, as it is the case during usage of a laser warning
sys-
tem or for muzzle flash detection, the noise ratio, which is generated by the
shot
noise, is the significantly prevailing ratio, which thus physically limits the
detec-
tion limit of the detector.
In order to obtain a very low detection limit, the detector is designed such
that
the number of background photons detected during the detection period be-
comes as small as possible, wherein as many photons of the useful signal as
pos-
sible are to be detected at the same time.
Here, a first step is to use an imaging detector including as many pixels as
possi-
ble. When the number of pixels is increasing, the field of vision of a single
pixel
becomes smaller and smaller, thus the light intensity of the background light
is
reduced. However, as the useful signal to be detected comes from a point-type
light source, it remains approximately the same for the hit pixel,
irrespective of
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the number of pixels - a high number of pixels thus reduces the background
signal using the same optics without impacting the useful signal.
A second step is the reduction of the detection period. The shorter the
detection
period is, the less photons of the background light are detected. As long as
the
detection period is larger than the duration of the useful signal, it will be
detect-
ed in the complete range thereof. Ideally, the detection period should be the
same as the duration of the useful signal. In conventional CMOS cameras, the
image frequency is about 50 to 200 Hz, the detection period is between 5 and
20
ms and thus by a factor of o.5-2*1o6 longer than a duration of a laser pulse
of for
example 10 ns. Even very sophisticated high-speed cameras achieve image fre-
quencies of only up to about loo kHz, whereby the detection period corresponds
to 10 ms and is thus by a factor of 1,000 longer than the duration of the
above-
mentioned laser pulse. Thus the known imaging sensors, where the complete
image is read out of after integration time, respectively, are not suited very
well
for the detection of smallest impulses of light against very bright background
light. This limitation is circumvented by the proposed signal processing for
each
pixel. Here, threshold comparators of up to 10 ns time constants may be
achieved in CMOS technology. The effective detection time corresponds to this
time constant and thus may ideally be realized at the same length as the laser
pulse which is to be detected.
The third step is a spectral limitation of the detected wavelength range by
means
of a respective interference filter. As long as a useful signal lies within
the detec-
tion band, it is not impacted thereof, however the background signal decreases
corresponding to the band width. However, this spectral limitation may only be
used insofar as the wavelength of the useful signal is known. In addition, a
min-
imum bandwidth is limited by the desired viewing angle of the camera, as inter-
ference filters comprise a wavelength shift with the incidence angle. This
limita-
tion may be reduced by using dedicated lenses, as for example the one shown in
Fig. 3.
In order to limit the triggering number of false alarms, a relatively high
detection
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threshold of for example S/N = 13 may be used (S/N = signal to noise ratio).
If
the useful signal is periodic, also significantly smaller detection thresholds
may
be used. These first generate a relatively high background signal rate, from
which then the existence of the periodic signal may be detected by using a
digital
comb filter, for example, by thus triggering only a few false alarms.
In the following an actual detector and a camera which realize said detector
concept will be described.
Fig. 1 shows an exemplary embodiment of a detector which is capable of detect-
ing a detection signal of a localized optical source (not shown in Fig. 1) in
front of
a background. The detector comprises an image sensor no, a signal processing
unit 120, and a warning unit 130.
The image sensor no comprises a plurality of pixel 111, 112,... for optically
de-
tecting the detection signal and the background. The image sensor no is de-
signed to generate electric image signals for each individual pixel 111,
112,... and
is designed as a pixel array of photodiodes based on silicon or InGaAs (or
other
III-V semiconductor materials), for example, which is coupled to the CMOS
signal processing unit 120 on the back side. The image signals are electrical
sig-
nals corresponding to the incident light intensity and are readout by the CMOS
signal processing components (121, 122, 123) of the signal processing unit
120.
The signal processing unit 120 is designed to continuously compare the image
signal with a threshold for each pixel, and to trigger event data 125, if the
thresh-
old is exceeded within a time period, which are then output via the output
unit
129 to the warning unit 130.
The event data 125 indicate an event which potentially has to be detected, and
the output unit 129 may then also output image data (for example, intensity)
together with said event data 125 and also other information regarding the re-
spective pixel (for example, point of time, when the threshold is exceeded).
The
warning unit 130 generates a warning or alarm signal 135, in case
predetermined
criteria are met. These criteria may for example include that a predetermined
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minimum number or maximum number of pixels has been detected, when the
threshold has been exceeded, and thus a localized optical source has been de-
tected, and not only a random noise or a sudden brightening of the complete
background has occurred.
Fig. 2 shows by way of example a signal waveform for an image signal S which
is
detected for one of the pixels by the image sensor no. The image signal S may
represent a photoelectric current (from a photodiode) or a CCD pixel in the im-
age sensor no, for example. The exemplary signal waveform shows the back-
ground signal given by the background light and also two short superimposed
useful signals Si and S2. The signal K is the resulting signal at the
threshold
comparator with the time constant At without the middle background intensity
(which may be subtracted, for example). The dotted line represents the thresh-
old. At the times ti and t2 the threshold comparator is triggered,
respectively, as
the integrated signal, which is located in the interval At before ti or t2, is
exceed-
ing the threshold. By using an analog signal processing, detection is not
bound to
a time pattern. That means, distances between ti and t2 do not have to be an
integer multiple of At. For each time ti and t2 the event signal 125 is
triggered
for the pixel and transferred to the warning unit. Optionally, also the level
of the
signal or the time of triggering in relation to a reference counter may be
part of
the event signal.
If said image signal is only detected by a limited number of pixels, it is to
be
assumed, that it was generated by a localized source and does not represent a
random noise signal and a sudden brightening of the background.
The time duration/integration time At may be selected in a variable way, and
may be modified by the signal processing unit 120 or the alerting unit 130 de-
pending on predetermined criteria, for example. Accordingly, the signal S is
not
necessarily summed over a predetermined constant period of time. Rather, the
range At may be flexibly adapted. In addition to or instead of the integration
time At, the threshold SW may also be adjusted. The threshold may be set by
means of a comparator, for example, which may be present in each pixel.
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In addition, warning unit 130 may comprise an optional filter in order to
locate
periodic signals of a very weak repeating source. Hereto, a digital filter can
be
used, for example. For example, such periodic sources are controlled laser for
laser guided missiles (Beamrider missiles). An advantage of these filters is
that
very weak signals can be detected. For example, for this application
thresholds
SW1 may be used, which are smaller than S/N = 13. For non-periodic sources,
the warning unit is capable of looking for signal intensities of pixels and
would
thus apply a higher threshold SW2 (for example, a value of S/N = 13 or more).
This allows to generate an alarm for a single event with a low false alarm
rate.
1 o Said changes or adjustments of the threshold SW (or SWi for periodic,
and SW2
for non-periodic sources) may also be performed dynamically (for example,
during operation), and may ensure that predetermined criteria are met. One of
the criteria is, for example, the event rate, that is how many events are
detected
within a predetermined time (for example, per second). Thus, it may be advan-
tageous to select the magnitude of a rate in a way (for example, by setting
the
threshold) that the detected event rate may be processed by the downstream
electronics, but not higher. If the threshold SW is continuously (dynamically)
adjusted, an approximately constant event rate may be achieved irrespective of
the actual background signal. By setting the integration time At, the detector
may be optimized for the expected useful signals by setting an integration
time
corresponding to the expected length of the useful signal.
It is also possible to control the event rate by the threshold SW, so that the
event
rate does not exceed a maximum value. The maximum value may, for example,
depend on the computing power of the downstream evaluation electronics, and
may ensure that events will not be ignored or may be evaluated at a point of
time
which is not acceptable for the specific application (for example, laser
warning
system or hostile fire detector).
In further exemplary embodiments, the detector unit is integrated in a camera,
which for example, has a large aperture, a lens with spectral filtering and a
cus-
tom CMOS sensor focal plane array. In particular, the applicable wavelength
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range for this concept covers the UV range, the visible range, and the near
infra-
red spectrum (NIR), up to a wavelength of for example 1064 nm. In turn, the
detector has a threshold detector which is integrated in each pixel. The
threshold
detector re-triggers, when an configurable threshold signal (e. g., SW) is
reached,
and namely within its integration time (At). Said event triggers an
asynchronous
readout of the pixel value, wherein the pixel coordinate, and the pixel
intensity
may also be provided together with a time value, in order to obtain access to
the
detected event. The asynchronous readout is thus event-driven and independent
of any readouts of other pixels.
In the exemplary embodiment shown in Fig. 1, wherein the pixel array is
coupled
to a CCD evaluation unit on the back side thereof, also a wavelength of up to
about 1.9 microns may be supported by using a pixel array in InGaAs
technology.
Fig. 3 shows an exemplary embodiment according to the present invention for
said detector/camera containing the further optional components.
The exemplary embodiment shown represents, for example, a camera is, where-
in the image sensor no and the processing unit 120 constitute a unit, wherein
an
optical signal from a localized optical source 50 is projected on a first
position Pi
on a sensor surface (image sensor pixel no) by means of one or more optical
components 140. In addition to the optical signal, the background H will also
be
projected on the sensor surface of the image sensor no after passing through
the
optical components 140. By way of example, a part of the background H is pro-
jected on a second position P2 of the sensor surface. It is understood that
each
position Pi, P2, ... on the sensor surface detects another direction of the
back-
ground, and it is determined by a calibration, which position on the sensor
sur-
face (e. g., which pixel) "is facing in which direction". Thus, the direction
of the
detected optical source may be determined by the identification of the pixels.
The image sensor no and the processing unit 120 generate event data 125 based
on the detected optical signals, which are forwarded to the warning unit 130.
The warning unit 130 includes a warning processor to generate a warning signal
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or an alarm 135 based on the event data and other criteria, for example. For
example, the event data 125 comprise the image values, which have been detect-
ed by the individual pixels and which correspond to the respective intensity
of
the incident optical radiation. Optionally, also the detected time and the
position
of the pixel may be transferred as part of the event data 125 in order to
identify
the direction of the optical source based thereon, as described above.
In order to perform a narrow spectral filtering and thus achieve a desired
sensi-
tivity, the detector may include a special lens design. For example, the
optical
component(s) 140 comprise(s) a lens system of an inverted Galilean telescope
141 for an angular spread, an interference filter 142 and a primary lens 143.
The
interference filter 142 is thus inserted at a position having a low beam diver-
gence, so that the wavelength shift of the transmission is reduced by the
angle of
incidence. A suitable lens design would allow the use of a 40 nm filter width
for
a 900 nm laser detection in a 90 -FOV camera, for example.
In particular, embodiments enable a laser warning system with only minor re-
curring costs. In addition, different individual line filter sensors may be
used for
the laser warner in order to improve the sensitivity and to provide wavelength
information in addition to the warning. Detection sensitivities are achieved,
which could not be realized with previously known laser detectors having a
high
positional resolution.
For any values specified in this application, it is to be understood that any
varia-
tions within a tolerance range may also fall within the defined objects and
are
considered as disclosed. Thus, an upper tolerance limit may deviate by + 10%
or
+ 50% or + 100 %, and the lower tolerance limit may deviate by -10% or 30% or -
50%) of the specified value.
The features of the invention disclosed in the specification, the claims and
the
figures may each be individually or in any combination substantial for
realizing
the invention.
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List of reference numbers
50 Optical source
no Image sensor
111, 112, 113, ... .. Pixel
120 Signal processing unit
125 Event data
130 Warning unit
135 Warning signal
140 Optical parts
141 Galilei filter
142 Interference filter
143 Primary lenses
Background
SW, SWi, SW2 Threshold
P1, P2 Positions on the sensor surface
