Note: Descriptions are shown in the official language in which they were submitted.
~E___a ~ chromator for the near
The invention relates to an optical analyser with a
grating-monochromator for the near infrared range.
Optical analysers are known (eOg. the products of
Technicon and Neotec) which focus the wide-band energy of
a light source emitting in the near infrared range through
a monochromator - the wavelength-selector of which is
formed by a narrow-band interference fil~er - to the
surface of a cuvette containing a prepared sample. With
the aid of the monochromator the surface of the sample is
lighted with a narrow-band light bundle with different
wavelengths. The intensity of the reflected energy
diffused from the sample is detected. Optical analysers
for the near infrared range are based on a property of
subs~ances, namely ~hat the most important organic
components~ e.g. protein, oil, moisture, etc. absorb light
at wavelengths characteristic for them. Accordingly there
is a correlation between the reflected energy and the
component characteristics of the substance tested.
Although the absorption maxima of the components differ
depending on the wavelength, within the near infrared
range they are rela~ively close to each other. Thus at
the selected wavelength their absorption interaction
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becomes effective. Elimination takes place by technical
and mathema~ical measuring means, in general, with the aid
of a microcomputer incorporated into the ins~rument.
However, the known optical analysers for the near
infrared range can be operated only in wavelength ranges
limited by ~he interference fil~ers. This restriction
involves the disadvantage that there is no possibility of
adapting methods that require adjustment of the wavelength
to one differing from that proposed by the prod~cers. The
forms of equipment are not all provided with a linear
wavelength scale. ~oreover, in certain cases it is not
even possible to calibrate directly in wavelengths, e.g.
with the Neotec apparatus.
Hitherto monochromators ensuring continuous selection
of the wavelength (e.g. the grating-monochromators~ cannot
be used in optical analysers for the near infrared range,
since monochromators of this type - on a cost-level
compatable with a system with an interference filter - can
be constructed with about a tenth the light intensity.
The aim of the invention is to eliminate these
drawbacks of the known systems and to develop an optical
analyser for the near infrared range that contains an
optical grating-monochromator, a detector system and a
processing system that ensures a sensitivity corresponding
to that of constructions having an inter~erence filter.
Simultaneously the invention should show the same
advantageous features as the apparatus using a
grating-monochromator. These are, as follows:
- a continuous spectrum can be produced in nanometer-stages;
- the apparatus is provided with a linear wavelength scale;
- the spectral band width is approximately constant within
the wavelength range.
To this end the invention consists of an optical analyser
for the near infrared range comprising: a light source
having a filament, a spherical mirror for focussing an image
of said filament along a path of light from the light source,
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a monochromator arranged in said light path and having an
optical grating connected to a sine-mechanism, a light
beam interrupting mechanism interposed between said light
source and said monochromator, an optical system in said
light path after said monochromator and including a mirror
operable for deflecting the light path for selecting
between measurement by light transmission or light
reflection, transmission and reflection detectors
respectively provided with at leas~ one light sensor,
means for thermostatically controlling the temperature of
said sensors, each detector containing an amplifier
connected to the output of a light sensor, and a measuring
system having an input connected to the output of a said
amplifiers, said measuring system having in series a
band-filter, a demodulator and an A/D converter, said A/D
converter being connected to a micro-computer.
The field of application of the apparatus is large; it
is suitable for displaying organic components of solid,
liquid and gaseous substances.
The simultaneous determination of a maximum o~ 5iX
components requires an analysis time of less than one
minute. The apparatus can be principally used for testing
materials such as fodders, mixed fodders, foodstuffs and
provisions.
The apparatus will be ~alibrated using the usual
wet-chemical method.
The advantageous features of the pre~erred form of
optical analyser according to ~he invention - in
comparison to the traditional methods - are, as follows:
- operation does not require special qualification;
- chemicals are not needed;
- it is harmless to health;
- the apparatus operates quickly;
- operation at low costs becomes possible;
- accuracy corresponds to the accuracy of the traditional
methods;
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- measuring results can be recorded by a printer;
- the software-system of the equipment enables the
development of independent methods for the user;
- optical density is formed via the software.
A preferred embodiment o the invention is illustrated
in drawings wherein;
Figure l shows the block schematic of the apparatus;
Figure 2 illustrates a monochromator and optical
arrangement;
Figure 3 shows a sine-mechanism; and
Figure 4 shows a control and measuring system.
In Figure l the c~mplete block schematic of the
apparatus may be seen.
A microcomputer lO0 operated by a microprocessor
controls a monochromator 104 and an optical system~ as
well as a detector 105 and a sampling system. The
apparatus is operated from keys lOl. Measuring results
are displayed by a display 102, which - cooperating with a
printer 103 - enables communication between the user and
the instrumentO
The arrangement of the monochromator and the optics is
seen in Figure 2.
A light source 1 is a wide-band halogen lamp with high
energy. A spherical mirror 6 produces an image of the
filament of the light source l above the filament. A
focussing lens 2 produces an image of the filament in the
plane of an admission slit 8 via a diaphragm 3, a high
pass edge filter 4, a flat mirror 5 and a focussing lens
7. The focussing lens 7 produces on the surface of a
spherical mirror lO an image of the surface of the
approximately uniformly illuminated focussing lens 2
revealed by the diaphragm 3. The spherical mirror lO
illuminates with a parallel light bundle an optical
grating ll by which the radiation is diffracted. From the
diffracted radiation the spherical mirror lO, produces
after reflection by a flat mirror 16, in the plane
~4~
of an exit slit 17, a spectrum with the given linear
dispersion~ The size of the admission slit 8 and the exit
slit 18 define the spectral band-width of the light passing
forward.
The high-pass edge filter 4 cuts undesired scattered
light. Adjacent the admission slit 8 a light~interrupting
wheel 9 is located, which is driven by a quartz-controlled
synchronous motor 15. Simultaneously the light-interrupting
wheel interrupts an optical coupling circuit 39 (Figure 3)
which delivers a signal proportional with the momentary
number of revolutions to the control unit of the detector
system,
In this arrangement the angular position of the optical
grating ll defines the frequency of the light passing
through the exit slit 18. By using a sine-mechanism 12 the
possibility is provided that a linear correlation can be
established between the number of steps - reckoned from the
start position of a stepping motor 13 - and the wavelength
of the light passing through the exit slit 17~
The sine-mechanism 12 is illustrated in detail in ~igure
3~ The stepping motor 13 rotates a threaded spindle 27
supported in bearings 38 via a clutch 26 with transverse
articulation. A threaded nut 28 (secured rotationally~ is
displaced together with a plate 29 and a switch arm 30 in a
direction depending on the sense of rotation of the motor
13, ~o an extent proportional to the number of stepsO Under
the influence of this movement the axis 35 of the optical
grating ll suppor~ed in bearings 36 is turned by the lever
33 and the spring 34 in a play-free manner. The sine of the
angular displacement is proportional to the number of steps
of the motor 13.
This mechanism including the feature of the optical
diffraction grating ll that with a fixed direction oE
illumination and observation the wavelength will be
proportional to the sine of the angle of the optical
grating 11 measured in a suitably selected direction, offers
a solution or obtaining a linear wavelength scale.
The designation of the basic position is perfor~ed by
the switch arm 30 and a switch 31.
Lenses 18, 20 (Figure 2) and a diaphragm 19 provide for
uniform illumination of the sample contained in a cuvette 22
or 24~ By means of the mirrox 21 selecting the detector,
the light bundle leaving ~he monochromator can be ro~ated by
+ 90~ Accordingly, one can choose between the trans-
lQ mission or reflection measuring methods.
A switch 51 (Figure 4) on the axis of the mirror 21
signals the position of the mirror 21 to the electronics~
~igure 4 also shows a ~emperature control and measuring
system. Both the transmission detector 52 and the reflection
detector 53 are similarly constructed, light sensors 23, 25
being photoresistors.
Taking the sensitivity of the photoresistors and
temperature~dependence of the dark current into
consideration~ thermal stabilization is imperative. In
the course of measuring there is a requirement that the
threshold-sensitivity should be equal to 50 D (optical
densîty~. The prerequisite of realization lies in that the
temperature change of the light sensors 23, 25 should not be
higher than l/100C/minute.
The temperature of the light sensors 23, 25 having been
controlled by a thermostat 49 is observed by a temperature
sensor 48. A circuit 45 of a temperature control unit 50
generating an error signal delivers a signal proportional to
the temperature difference.
A complementary capacity - end stage 46 feeds a
Peltier-cell 47 in such a manner that, in compliance with
the sign of the error signal, the Peltier-cell 47 either
heats or cools.
The light sensors 23, 25 deliver to the input of
amplifier 40 or 41 an alternating voltage proportional to
the light modulated by the light-interrupting wheel 9.
The amplifying factor of the amplifiers 40, 41 can be
changed by the microcomputer 100 (ACC). Depending on the
position of the switch 51 the output of the amplifier 40
or 41 is connec~ed to a measuring system 54. The input of
the measuring system 54 is formed by a band-filter 42. The
task of the band-filter 42 lies in suppressing the total
noise-output resulting from the detector 52 or 53 in a
proportion corresponding to the bandwidth of the band-
filter 42. As a consequence - depending on the width of
the band the sensitivity can be increased. The signal,
being proportional to the phase difference of the input
signal and the output signal of the band filter 42r keeps
the frequency always at a predetermined value.
The alternating voltage signal appearing at the output
of the band-filter 42 is transformed in a demodulator 43
into a direct voltage.
Analogue/digital conversion is performed by an A/D
converter 44 connected directly to the bus-system of the
2Q microcomputer 100.
Compared to the apparatus hitherto known, the
following new features appear:
- with the aid of the light-interrupting system, the
interrupting frequency can be adjusted to the optimum
value and the sensitivity of the detectors increased by
two orders of magnitude;
- the stability of the thermostat detectors can be
increased by one order of magnitude (0.01 C/min) in
comparison to known apparatus. The Peltier-elements are
either heating or cooling and the required temperature
stability can be achieved within some minutes instead of
the heating time of 1-2 hours required up to now.
