A device for measuring the rotation of the plane of vibration of linearly polarized electromagnetic radiation includes a beam splitter that divides the radiation into beams following a measurement beam path and a reference beam path, and each of the beam paths are analyzed after interaction with a reference element and a sample to be measured. The beams impinge upon photosensitive sensors and the output signal from such sensors is processed to determine the characteristics of the sample that are desired to be analyzed.

DESCRIPTION 
The present invention relates to a device for the measurement of the 
rotation of the plane of vibration of linearly polarized radiation caused 
by optically active substances. 
In an arrangement for measuring in accordance with the invention, each of 
the measured values of the rotation for the sample is always standardized 
against the optical activity of a reference substance determined in a 
reference beam of radiation. In this way the result of the measurement is 
always free from the influences of the wave length used for the 
measurement and/or from the temperature. The determination of the plane of 
vibration under these conditions is effected in real time. 
A measuring device in accordance with the characterizing features of the 
patent claims may, for example, be utilized in all those situations where 
the circular double refraction or birefringence or its dynamic changes are 
required to be measured rapidly with great accuracy. Thus, for example, it 
may be used in polarimetry for the determination of the concentration, the 
layer depth, or the specific rotation of optically active substances. 
PRESENT STATE OF THE ART 
A multi-beam measuring device for polarimetric investigations of test 
samples in real-time procedures is described in the PCT-Application 
PCT/EP84/00050 (SCHMIDT DISTL). 
The inventive teachings of the above-named PCT/EP84/00050 rest upon, 
amongst other things, the fact that the direction of vibration of the 
light beam may be calculated from the determination of the ratio of the 
relative intensity of the light beam to be analyzed, after it emerges from 
an analyzer, to its absolute intensity before entering the analyzer. For 
this purpose, the beam of light after passing through the test sample is 
divided up by means of a beam divider, preferably a diffraction grating, 
into a reference beam and at least one test beam in the path of which an 
analyzer with fixed direction of transmission is located. The intensities 
of the part beams are determined by means of the photosensitive sensor 
which is allocated to each of the beams. The signal outputs of the 
photosensitive sensors are connected to the inputs of a measuring circuit 
for the determination of the polarimetric values assignable to the test 
sample. 
The measuring circuit is constructed essentially as follows: By means of a 
temporary storage which is coupled in after each of the photosensitive 
sensors, the output signals from the photosensitive sensors are stored 
synchronously and for a short time. A control circuit which is allocated 
to the temporary storages assumes control of the temporary storage. The 
formation of the ratio of the relative intensity to the absolute intensity 
is effected by means of a delay-free operating division circuit, which is 
connected on its input side to the output of the temporary storage. For 
the calculation and the output of the polarimetric values assignable to 
the test sample, in particular the optical activity, a digital data 
processing device is provided, which is connected on its input side to the 
output from the division circuit. In addition to this, the measuring 
circuit is provided with at least one A/D-converter which converts the 
analog signals to digital form for further processing. 
In the above-named multi-beam measuring device, the method of operating of 
the beam divider assumes decisive importance with respect to the accuracy 
of measurement. 
Dielectric beam dividers, for example, are not suitable because their 
dividing ratio is a function of the direction of vibration of the incident 
light beam. Consequently, the falsifying mode of operating of this beam 
divider also enters into the measurement results. 
In the afore-mentioned PCT/EP84/00050, a proposal is made in this regard to 
use a diffraction grating as the beam divider element. Under these 
conditions, it is of primary importance to ensure that the beam of light 
incident on the diffraction grating always strikes the grating structure 
in exactly the same way, because the diffraction efficiency is a function 
of the angle of incidence. 
Consequently, an adequate concentration of the light source at a point must 
be striven for, which can only be achieved by an expensive system for 
guiding the path of the beam. A further disadvantage resides in the fact 
that considerable losses of light are involved in this procedure. Since 
the direction of vibration of the reference beam is not defined, the 
measurement result is further falsified by the vector sensitivity of the 
photosensitive sensor allocated to the reference beam. 
It is already known from the European Patent Application No. 80106584.8 
(MULLER) that the beam divider and the analyzer within a polarimeter may 
be incorporated together into a plane-parallel plate of a glass prism, for 
example. 
These elements can, most certainly, with certain limitations, as intended 
in this Application, serve as demodulators but they are, however, as will 
be described at a later stage, not suitable as a beam divider within a 
measuring device for the quantitative determination of the polarization 
state of electromagnetic radiation. 
The transmitted portion of the radiation is only partly polarized by these 
types of elements, that is to say, the desired function of the analyzer is 
very substantially inhibited. This fact is of importance in so far as the 
measuring accuracy of a measuring device having the elements as described 
is directly limited thereby. Furthermore, the degree of polarization of 
the reflected radiation substantially depends upon the angle of incidence 
of the incident light beam. This applies particularly in the region of the 
Brewster angle. In relation to this it should be noted that, for example, 
during the investigation of test samples, the measuring beam will rarely 
impinge on the divider element at a constant angle, for the simple reason 
that the molecular structure or the inhomogeneity of the test sample leads 
to an unavoidable partial deflection of the measuring beam. This means 
that the degree of polarization of the reflected radiation is altered to 
an indeterminate extent. 
Accordingly, the measured values determined by means of a measuring device, 
with the elements as described in the foregoing section for the purpose of 
beam division and simultaneous use as an analyzer, are encumbered with two 
systematic errors of measurement 
A polarimetric measuring device is described in the German 
Offenlegungsschrift No. P 22 61 875.3 (SIEMENS AG) in which a polarizing 
double prism is employed as beam divider and analyzer. 
Here the great disadvantage is the fact that, with a double prism--for 
example a Wollaston prism--the angle of divergence of the emergent part 
beams is dependent upon the wave length of the incident radiation. Because 
the photosensitive sensors which are provided to detect the intensity of 
the part beams are, as a general rule, mounted in a stationary position, 
their utilization for variable or indeterminate wave lengths of the 
radiation to be analyzed is accordingly not possible. 
Furthermore, in the case of a double prism, the two radiation components do 
not emerge from the crystal in a direction perpendicular to its surface of 
emission, which means that different reflection and absorption losses 
occur, and these falsify the measurement results. 
A procedure for measuring the optical activity of a test sample is 
described in the European Patent Application No. EPA 0 087 535 (American 
Crystal Sugar Company). In this case, the radiation emitted from an 
IR-light source is linearly polarized and passes through the sample 
chamber subsequently to impinge upon a beam dividing analyzer. This splits 
up the radiation into its polarized components which are perpendicular to 
one another, following which they are picked up by separate 
photo-receivers. The signals are transmitted, by way of an amplifier with 
adjustable offset and amplification, to a divider. The division signal is 
fed into an evaluation unit, which determines the result of the 
measurement by reference to empirical Tables. 
The temperature of the test sample is determined by means of a sensor. If 
no data are available concerning the temperature dependence of the optical 
activity, it is not possible to use any compensation for temperature. The 
basic Tables used with the measurement results for compensation of 
non-linearity are different for individual measuring instruments because, 
for example, the degree of polarization of the totally-reflected 
components in the beam dividing analyzer is not a physical constant. 
The adjustment of the offset and amplification of the operations amplifiers 
is effected by means of variable resistances. With the sought-after 
accuracy of the 16-bit resolution, the result cannot be stable for long 
periods of time, because there are no structural components available 
whose drift and thermal behavior lie within the tolerances which are 
required for it. The absorption by the test sample is therefore 
compensated for by means of intensity control of the light source. This 
signifies that the dynamic range of the measuring system is limited by the 
dynamic range of the light source. 
In all of the previously discussed procedures for the measurement of the 
optical activity of a test sample, any deviation of the measurement 
temperature and the measurement wave length from the reference 
specification has a direct influence upon the results of the measurements. 
A knowledge of the actual conditions of these various parameters--the 
ORD-(optical rotatory dispersion)-spectrum, as well as the temperature 
dependence--is then, however, essential for the determination of values 
derivable from the optical activity.

DISCLOSURE OF THE INVENTION 
The teachings of the present invention are concerned essentially with the 
further development of the measuring arrangement of the type known from 
the PCT/EP84/00050, with respect to both function and accuracy of 
measurement, in which case it is particularly the problems which were 
initially referred to in the said PCT/EP84/00050 that are to be solved in 
a technically effectual and simple manner. 
The problems which have been posed are solved in accordance with the patent 
claims made for the invention. 
The electromagnetic radiation emanating from the light source is split up 
into a measuring beam path and a reference beam path before its 
interaction with the substance under investigation. Under these 
circumstances, the reference beam, after interaction with a reference 
element, impinges on a first beam-dividing analyzer, and the measuring 
beam, after interaction with the test sample under investigation, impinges 
on a second beam-dividing analyzer. 
The beam-dividing analyzers consist of a birefringent material, which means 
that the two part beams emerge at different places from this element. Each 
one of these part beams has a photosensitive sensor allocated to it, under 
which conditions these are connected on their output side with the inputs 
to a measurement circuit. This measurement circuit possesses temporary 
storages coupled in after each photosensitive sensor, and these store the 
output signals from the photosensitive sensors synchronously and for a 
short period of time only. In this way, identical measurement conditions 
are guaranteed during the course of the measurement. Furthermore, for 
determination of the absolute intensity, the measurement circuit is 
furnished with addition-circuits which are coupled-in before the 
beam-dividing analyzers, as well as a division-circuit allocated to each 
beam path, and this is connected at its input side with the output from 
the addition-circuit allocated to the relevant beam path, and to the 
output of one of the two temporary storages to which the addition-circuit 
is allocated. This supplies a standardized output signal which, in 
particular, is independent of the output intensity of the light source and 
the absorption of the test sample under investigation or that of the 
reference element. The output signals from the two division-elements are 
selectively supplied to a digital data-processing device by way of an 
analog circuit. In addition to this, there is at least one analog/digital 
converter for this purpose coupled-in before the digital data-processing 
device. As the result of this, a further processing of the measurement 
signals is possible in real time. 
The teachings of the invention describe a measuring arrangement which 
allows for the application of the dual-beam principle which is already 
known from other domains of measurement techniques, for example, 
spectroscopy and photometry, within the field of polarimetry. Here too it 
is fundamentally a matter of eliminating the influence of certain 
measurement-result-falsifying parameters, which are inherent in the 
system, from affecting the measurement results. 
The optical activity or the values derivable from it, such as those to be 
determined by polarimetry are, amongst other things, functions of the 
temperature of the substance under investigation and the wave length of 
the radiation which is to be passed through the substance. These two 
parameters can only be kept stable and constant at a specified value, 
within the strict tolerances demanded by precision polarimetry 
(+/-0.01.degree. C. +/-0.1 mm deviation from the effective wave length) 
during the period of measurement, at very considerable expense. In many 
applications it is quite impossible to keep within such narrow tolerance 
limits. This is the case, for example, when using transmitter diodes whose 
peak wave lengths vary greatly because of thermal effects and dependence 
upon the power output. 
The dependence upon wave length and temperature, as referred to previously, 
is eliminated by the dual-beam measurement procedure in accordance with 
the invention by having the optical activity of the substance being 
analyzed--that is to say, the rotation of the plane of vibration caused by 
the substance--standardized during every measuring cycle, with respect to 
the reference wave length and the temperature, by means of a correction 
factor which is determined in the reference beam. Considered functionally, 
the reference beam path and the measurement beam path are similar in 
configuration. In this situation, the reference beam and the measurement 
beam are produced by splitting of the electromagnetic radiation beam, 
emanating from the light source, by means of a beam divider. With this 
arrangement, a linear polarizer is assigned with a fixed orientation in 
relation to a beam-dividing analyzer. In other words, the direction(s) of 
transmittance of the linear polarizer(s) has/have a fixed-angle 
relationship to the direction of vibration of the ordinary and of the 
extraordinary components of the beam which are produced by means of the 
beam-dividing analyzers. Subsequently, the ordinary and extraordinary 
components of the beam are detected by means of the photosensitive sensors 
allocated to them. 
The reference and measurement paths differ in accordance with the invention 
only in so far that, in the latter, the test sample is located between the 
linear polarizer and the beam-dividing analyzer, whereas, in the reference 
beam path, this position is occupied by an optically-active substance, 
hereinafter named the reference element, which has a known specific 
optical activity. Now, if the optical rotatory dispersion (ORD), as well 
as the temperature dependence of the optical activity of both the 
reference element and the substance to be analyzed, are qualitatively 
equal--or not capable of being differentiated within the limits of 
accuracy of the measurement--then the following relationship may be 
utilized: 
EQU [.alpha.].sub..lambda..sbsb.M.sup..theta..sbsp.M 
=[.alpha.].sub..lambda..sbsb.B.sup..theta..sbsp.B 
.multidot.C.sub..DELTA..lambda. .multidot.C.sub..DELTA..theta. 
where: 
[.alpha.].sub..lambda..spsb.M.sup..theta..sbsp.M =the specific optical 
activity at the momentary temperature and wave length 
[.alpha.].sub..lambda..sbsb.B.sup..theta..sbsp.B =the specific optical 
activity at the reference temperature (generally 293.degree. K.) and the 
reference wave length 
C.sub..DELTA..lambda. =correction factor determined by the deviation of the 
actual wave length from the reference wave length 
c.sub..DELTA..theta. =correction factor determined by the deviation of the 
actual temperature from the reference temperature 
The product of c.sub..DELTA..lambda. and c.sub..DELTA..theta. is 
determined in the reference beam path and it is used to correct the 
current measurement value. Accordingly, the measurement value is free from 
the influences occasioned by changes in the temperature and the wave 
length. This advantage is also shared by the saccharimeter which is 
chiefly used in the sugar industry. In this instrument, the rotation of 
the direction of vibration due to the test sample is compensated for by 
means of a quartz wedge. Since quartz and most sugars have practically the 
same dependence of their optical activity on the ORD and the temperature 
(compare, for example, Icsuma Proceedings, 17th Session, Montreal 1978), 
the compensation location of the quartz wedge is a direct measure of the 
optical activity of the test sample, independent of fluctuations of 
temperature and wave length. This property is the foundation of the 
reliability of the saccharimeter, even when the environmental conditions 
are unfavorable. 
In accordance with this present invention, the afore-named advantages are 
now attainable in other fields of application. This is because of the 
possibility of using any desired reference element, that is to say, it is 
always possible to select the ideal reference standard for the measurement 
arrangement. 
The beam-dividing analyzers consist of a birefringent material, in which 
case the two part beams emerge from the element at different locations. 
For this purpose, use is made preferentially of a Foster beam divider 
because, with this divider, the angle of divergence between the two 
emergent radiation components is independent of the wave length of the 
incident radiation. The portion of radiation which passes straight through 
such a double prism is better than 99.99995% polarized, but the 
totally-reflected portion of the radiation is only partly polarized 
because of crosstalk effects. 
If a source of radiation with a wave length in the visible spectrum is 
used, then the degree of polarization of the radiation component passing 
straight through, at least within the range of +/-4 angular degrees, is 
independent of the angle of incidence of the radiation impinging on the 
birefringent element. 
In the determination of the plane of vibration of the linearly polarized 
light, the teachings of the invention, as are those of the 
generically-similar PCT/EP84/00050, are based upon the Malus Law (compare 
Ellipsometry and polarized light, R. M. A. Asam, N. M. Bashara, 1977, page 
110). From this it is known that the intensity following an ideal analyzer 
may be determined from the following relationship: 
EQU A=A.sub.o .multidot.cos.sup.2 .alpha..multidot.C 
where: 
A=the measured intensity after the analyzer 
A.sub.o =the incident intensity 
.alpha.=the angle between the incident linearly-polarized radiation and the 
transmittance direction of the analyzer 
C=constant factor determined by the reflection losses on entry and 
emergence of the radiation as well as the absorption of the analyzer 
material. 
By solving of the above equality for .alpha., the following relationship is 
established: 
##EQU1## 
The majority of beam-dividing crystal polarizers are, within the range of 
the measurement accuracy, ideal polarizers for both radiation components. 
Therefore it is possible to use the Malus Law for both radiation 
components. As already discussed, in the case of a Foster beam divider, it 
is only the radiation component passing straight through which is 
polarized to an adequate extent. Accordingly, it is only its output 
intensity which follows the Malus Law and may be used to determine the 
value of A. 
For the further processing of the output signal from the photosensitive 
sensor and the output of the physical values assignable to the 
polarization state of the electromagnetic radiation, a measuring circuit 
is coupled-in after the photosensitive sensors. 
The teachings of this present invention are now based upon the fact that 
the value of the product C.multidot.A.sub.o may be determined from the 
following relationship: 
EQU C.multidot.A.sub.o =A.sub.T +A.sub.D 
where: 
A.sub.o =the energy of the light before the birefringent element 
A.sub.T =the energy of the totally-reflected radiation component 
A.sub.D =the energy of the transmitted radiation component 
C=constant factor. 
The above relationship also applies for the Foster beam divider, because 
the sum of the intensities of the two part beams is always independent of 
the direction of polarization of the incident radiation. In accordance 
with the above relationship, the sum of A.sub.T and A.sub.D is first of 
all determined by means of an addition-circuit which is coupled-in after 
the temporary storages. The output value from the addition-circuit is 
accordingly directly proportional to the energy of the light from the 
birefringent element. 
Subsequently, with the aid of a division-circuit, which is connected on its 
input side to the output of the addition circuit and the output of one of 
the two temporary storages, the ratio of the intensity of the part beam 
which is allocated to the relevant temporary storage, that is to say, the 
value A, to the output value from the addition-circuit, that is to say, 
the value A.sub.o .multidot.C, is determined. The standardized value 
obtained in this way is then fed into the digital data-processing device 
for further processing in real time. 
In a preferred form of embodiment of the measurement arrangement in 
accordance with the invention, there is an amplifier coupled-in before 
each of the temporary storages and it is programmable with respect to 
amplification. For this purpose, use is made preferably of the so-called 
instrumentation amplifiers, in which the resistance determining the 
amplification may be altered by means of a resistance- and 
switching-network. Under these conditions, the switching networks are 
controlled by way of a second control circuit. Differing absorptions of 
the test samples may be compensated for in this way in specified steps 
which are defined by the values of the resistances in the resistance 
network. 
The advantage achieved thereby resides in the fact that it is only the 
analog components before the instrumentation amplifier, that is to say, 
the photosensitive sensors and their input-operations-amplifiers, which 
must suffice for the desired resolution and the accuracy multiplied by the 
required dynamic range, which arises from the various absorptions of the 
test samples. In contrast to this, all of the analog components coming 
after the instrumentation amplifier have to be sufficient only for the 
desired measurement accuracy. 
If, for example, it is required to have a resolution and accuracy of the 
measurement results of 0.01 angular degrees, and if substances are to be 
able to be investigated which have an absorption up to log. density 2, 
then the analog components coming after the instrumentation amplifier 
should have a maximal non-linearity of 15 ppm, and the value for those 
coming in front of the instrumentation amplifier should be 0.15 ppm. This 
fact is of very considerable importance, because there are practically no 
sample/hold circuits available (here as temporary storages) which have an 
accuracy of 0.15 ppm. Without the measurement arrangement in accordance 
with the invention having the programmable instrumentation amplifier, 
there would accordingly be a very considerable worsening of the 
measurement accuracy which would have to be accepted, at least in the case 
of higher absorptions by the test samples. 
It must be emphasized at this juncture that the instrumentation amplifiers 
which are programmable with respect to amplification are only necessary 
for the purpose of obtaining the best possible modulation of the analog 
components which come after them, with the objective of preserving 
linearity characteristics at higher test sample absorptions. However, 
because of the division circuit, the measurement results are fundamentally 
uninfluenced by the alterations of the output intensity of the light 
source and differing absorptions of the test samples. 
On the basis of the unavoidable resistance tolerances in the resistance 
network of the programmable instrumentation amplifier, inequalities in the 
degree of amplification rise between the instrumentation amplifiers. 
In order to be able to determine these inequalities, in accordance with the 
invention switching-circuit elements are provided, before each 
instrumentation amplifier, with which it is possible to provide a 
calibrated voltage to the inputs of the instrumentation amplifiers. These 
switching-circuit elements are controlled by means of a third control 
circuit allocated to them. 
The calibration process functions in essentially the following manner: 
First of all, the amplification of the instrumentation amplifier is 
programmed to the value 1 by means of the second control circuit. 
Following this, the switching-circuit elements supply a calibrated voltage 
to the inputs of the instrument amplifier with such a value that the 
following components can be fully modulated over the whole of their 
dynamic range. The signal arriving at the analog-digital converter is 
thereupon digitalized and the information so obtained is put into a 
storage device. In the following steps, the amplification is set to the 
next higher value and the foregoing procedure is repeated until all of the 
amplification stages have been encompassed. 
The stored values, correlating to each of the amplification stages, which 
have been obtained in this manner are then calculated as correction 
factors with the measured value currently under consideration, 
corresponding to the immediately adjusted amplification stage. 
Furthermore, the offset may be determined by having the calibrating voltage 
set at the value of 0 volt. 
The calibration procedure which has been described in the foregoing may 
also be effected while the measurement program is running. Accordingly, 
all of the drifts are detected and correspondingly the correction factors 
are actualized. 
In an especially preferred form of embodiment of the measurement 
arrangement in accordance with the invention, the required calibration 
voltages are produced by means of digital/analog converter. Associated 
with this, there is the additional advantage that a plurality of different 
voltage values may be applied to the inputs of the instrumentation 
amplifier, by which means the linearity behavior of the analog components 
may be determined. Thus, for example, it is possible, after the switching 
on of the measurement system, to detect hidden errors within the analog 
circuitry by means of a subsequent systems-test. 
In another preferred form of embodiment of the measurement arrangement in 
accordance with the invention, the voltage at the outputs from the 
photosensitive sensors is utilized to generate the calibration voltage. 
Under these conditions, there is then only a calibration cycle when--and 
this is dictated by the absorption due to the test sample--the instrument 
amplifier has to be switched over to a different amplification stage, that 
is to say, actually only in the situation where a calibration voltage is 
available. In order to be able to determine here the offset of the 
components following the instrumentation amplifier, the switching-circuit 
elements are designed in such a way that the inputs of the instrumentation 
amplifier can also be connected with the analog ground potential. 
The dynamic range of the measurement system, that is to say, the maximum 
admissible optical density of the test sample, for which the accuracy of 
the measurement results still lies within the tolerance limits determined 
by the electronic components is, in accordance with the invention as 
described previously, considerably widened by the use of the 
instrumentation amplifiers. According to patent claim 7, a utility supply 
unit is provided for the control of the optical output voltage of the 
source of radiation, which is connected on the input side to the digital 
data-processing device and on the output side to the source of radiation. 
In this way, there is achieved an additional widening of the dynamic 
range, which is then able to cope with even the most extreme demands, for 
example in the analysis of dark-colored sugar juices in the sugar 
industry. 
In the investigation of optically-active substances, it is possible, for 
example, to use an impulse radiation source for generating the measurement 
beam. This has the advantage of being able to supply very high output 
voltages over short periods of time. In order to be able to guarantee the 
best possible exploitation of the working mode of the impulse radiation 
source in combination with the measurement arrangement in accordance with 
the invention, the synchronisation- or trigger-input of the utility supply 
unit is connected to the output of the control circuit allocated to the 
temporary storages. This measure makes possible a simultaneous, 
synchronous working mode of the impulse radiation source and the 
measurement circuitry, under which conditions it is possible to achieve 
measurement times in the submicrosecond range. 
Yet another embodiment of the invention enables photometric analyses to be 
carried out. For this purpose, it is necessary to be able to establish the 
absolute intensity of the light beam which is to be analyzed, in 
particular quite independently of the polarization state. As already 
described, the output value from the addition-circuit of the measurement 
branch is a direct measure of the intensity of the measurement beam. 
For the evaluation of this signal, provision is made in accordance with the 
invention to have a device for the selectable direct connection of the 
output of this addition-circuit to the measurement input to the 
analog/digital converter with the masking of the remaining signals from 
the measurement input to the analog/digital converter. 
WAYS IN WHICH THE INVENTION MAY BE IMPLEMENTED 
FIG. 1 depicts an embodiment of the measurement arrangement in accordance 
with the invention for the optical section, and this will be described in 
more detail in what follows. 
The radiation source 2, for example a xenon photo-flash lamp or an 
impulseIR-diode, which is connected to the utility supply unit 1, 
generates a main beam H. This impinges upon a 45-degree Foster beam 
divider (beam-dividing polarizer) 3 which separates the ordinary radiation 
component (totally-reflected portion) R from the extraordinary radiation 
component (portion transmitted straight through) M. 
The reference beam R is subsequently polarized, for example by means of a 
Glan polarizer 4, before it passes through a quartz crystal 5 which is 
appropriately cut in relation to the axis, and which represents the 
optically-active reference element. In a second 45-degree Foster beam 
divider (first beam-dividing analyzer) 6, the reference beam R is then 
split into a so-called additive beam (totally-reflected portion) `a` and a 
so-called test beam (portion passing straight through) `b`. Both these 
beam components a, b are then picked-up quantitatively by means of a 
photosensitive sensor 7, 8 respectively. 
The measurement beam M which is produced by the beam-dividing polarizer 3 
passes through test-sample chamber 9 and this likewise impinges upon a 
third 45-degree Foster beam divider (second beam-dividing analyzer) 10. 
Again, this splits the measurement beam M into the additive beam `c` and 
the test beam `d`. Here too, there is a large-surface photosensitive 
sensor 11, 12 allocated to each of the radiation components c, d 
respectively. 
An example of embodiment of the measurement circuit is shown in FIG. 2. 
All the photosensitive sensors 7, 8, 11, 12 are connected to the 
input-operations amplifiers 14, which are preferably fabricated by means 
of FET-technology (FET=field-effect transistor). Voltages are present at 
their outputs, the values of which are directly proportional to the 
intensities of the radiation incident upon the photosensitive sensors 7, 
8, 11, 12 which are respectively allocated to the relevant 
input-operations amplifier 14. Each output of the input-operations 
amplifiers 14 referred to above passes by way of a switching-circuit 
element 15 to the input of one of the instrumentation amplifiers 17 which 
has been programmed for amplification. The switching-circuit elements of 
each channel are connected to the outputs of the third control circuit 16, 
and thus they are programmed by way of the digital data-processing device. 
The instrumentation amplifiers 17 themselves are controlled in their 
amplification by means of the second control circuit 18. The outputs of 
the instrumentation amplifiers 17 are connected to the inputs of the 
sample/hold circuits 19 allocated to them, which here assume the function 
of temporary storages. Their synchronous control is taken over by the 
first control circuit 20. 
The sample/hold circuits 19 which are allocated to the photosensitive 
sensors 7 and 8 are connected at their output side with the inputs of a 
first addition-circuit 21. The output signal of the first addition-circuit 
21 is further fed into a first division-circuit 23. Its second input is 
coupled to sample/hold circuit 19 which is allocated to the photosensitive 
sensor 8. Accordingly, there is a voltage signal present on this line 
which is directly proportional to the intensity of the part beam `b` which 
passes straight through the first beam-dividing analyzer 6. In other 
words, a signal is available at the second input of the division circuit 
23, with said signal following the Malus Law and therefore it may be used 
for determining the value of A. 
In an analogous manner, the output signals derived from the sample/hold 
circuits 19 allocated to the photosensitive sensors 11, 12 are supplied to 
a second division-circuit 24 by way of a second addition-circuit 22. 
The outputs of the two division-circuits 23, 24 lead to the inputs of a 
CMOS-circuit 25, the output of which, in its turn, is connected to the 
measurement input of an analog/digital converter 26. Furthermore, the 
control input of the CMOS-circuit is connected to the digital system bus. 
In this way it is selectively possible to switch the first 
division-circuit 23 or else the second division-circuit 24 through to the 
analog-digital converter. 
The output signals from the two division-elements are selectively supplied 
to a digital data processing device 28 through at least one analog-digital 
converter 26. The digital data processor 28 permits further processing of 
the measurement signals in real time. That is, the digital data processor 
28 provides for computation and output of the polarization state of the 
physical magnitudes assignable to the electromagnetic radiation, and has 
its input connected to both of the division circuits 23 and 24 through the 
analog-digital converter 26. 
By means of an digital to analog converter 27, it is possible to supply 
defined voltages for calibration of the whole of the circuitry to the 
inputs of the instrumentation amplifier 17 by way of the switching 
elements 15 which have already been described. This calibration takes 
place automatically at certain specified intervals of time, in order to be 
able to detect any possible drift of the constructional components and to 
compensate for them in the computations. 
In order to be able to control the light source 2 synchronously with the 
temporary storages (19) (sample/hold circuits), the trigger input of the 
utility supply unit 1 of the light source 2 is likewise connected to the 
control outputs of the first control circuit 20. Furthermore, for control 
of output voltage of the light source 2, its utility supply unit 1 is 
directly connected to the digital data-processor.