Determining polarization rotation characteristics of a sample taking into consideration a transmission dispersion

Abstract

Optical measuring system for determining polarization-optical properties of a sample, which comprises a polarization state generator (PSG) which is configured for preparing a measuring light which is propagating along an analysis beam path with a defined polarization state; a sample receptacle which is arranged downstream of the PSG in the analysis beam path and which is adapted for receiving the sample; a polarization state analyzer (PSA) which is arranged downstream of the sample receptacle in the analysis beam path; a detector which is arranged downstream of the PSA in the analysis beam path for detecting the measuring light, wherein the PSA and the detector are configured for capturing a polarization rotation .sub.P(.sub.eff) of the measuring light which is caused by the sample; and an evaluation and control unit for evaluating measuring signals from the detector and/or PSA and/or PSG, wherein a wavelength-spectrum of the measuring light contains at least a first wavelength .sub.1 and a second wavelength .sub.2, wherein the detector is configured for detecting measuring light with the first wavelength separated from measuring light with the second wavelength, and wherein the evaluation and control unit is configured for calculating a polarization rotation .sub.P(.sub.0) of the measuring light which is caused by the sample at a standardized wavelength .sub.0 in dependency from (a) a first polarization rotation .sub.P(.sub.1) at the first wavelength .sub.1, (b) a second polarization rotation .sub.P(.sub.2) at the second wavelength .sub.2, (c) a first transmission T(.sub.1) at the first wavelength .sub.1, and (d) a second transmission T(.sub.2) at the second wavelength .sub.2.

Claims

1. An optical measuring system for determining polarization-optical properties of a sample, the optical measuring system comprising: a polarization state generator which is configured for preparing a measuring light which is propagating along an analysis beam path with a defined polarization state; a sample receptacle which is arranged downstream of the polarization state generator in the analysis beam path and which is adapted for receiving the sample; a polarization state analyzer which is arranged downstream of the sample receptacle in the analysis beam path; a detector which is arranged downstream of the polarization state analyzer in the analysis beam path, for detecting an intensity of the measuring light, wherein the polarization state analyzer and the detector are configured for capturing a polarization rotation .sub.P(.sub.eff) of the measuring light which is caused by the sample; and a processor for evaluating measuring signals from the detector, wherein a wavelength-spectrum of the measuring light contains a first wavelength .sub.1 and a second wavelength .sub.2, wherein the detector is configured for detecting measuring light with the first wavelength .sub.1 separated from measuring light with the second wavelength .sub.2, and wherein the processor is configured for calculating a polarization rotation .sub.P(.sub.0) of the measuring light which is caused by the sample at a standardized wavelength .sub.0 in dependency from (a) a first polarization rotation .sub.P(.sub.1) at the first wavelength .sub.1, (b) a second polarization rotation .sub.P(.sub.2) at the second wavelength .sub.2, (c) a first transmission characteristic T(.sub.1) of the sample at the first wavelength .sub.1, and (d) a second transmission characteristic T(.sub.2) of the sample at the second wavelength .sub.2.

2. The optical measuring system of claim 1, wherein the wavelength-spectrum of the measuring light further contains at least a third wavelength, wherein the detector is configured for detecting measuring light with the third wavelength separated from measuring light with the second wavelength .sub.2 and from measuring light with the first wavelength .sub.1, and wherein the processor is further configured for calculating the polarization rotation .sub.P(.sub.0) of the measuring light which is caused by the sample at the standardized wavelength .sub.0, further in dependency from a third polarization rotation at the third wavelength, and a third transmission of the sample at the third wavelength.

3. The optical measuring system according to claim 1, further comprising: a light source which is adapted for sending the measuring light along the analysis beam path.

4. The optical measuring system according to claim 1, wherein a wavelength difference between the first wavelength .sub.1 and the second wavelength .sub.2 is smaller than 30 nm.

5. The optical measuring system according to claim 1, wherein the processor is further configured for calculating the polarization rotation .sub.P(.sub.0) of the measuring light which is caused by the sample at the standardized wavelength .sub.0 based on a first shift .sub.P(T) of a center-of-gravity-wavelength .sub.eff of the measuring light due to a wavelength-dependency of the transmission of measuring light through the sample, a second shift .sub.G of the center-of-gravity-wavelength .sub.eff of the measuring light due to a wavelength-dependency of a transmission of measuring light through an entirety of the optical components of the optical measuring system and the first polarization rotation .sub.P(.sub.1) and the second polarization rotation .sub.P(.sub.2).

6. The optical measuring system according to claim 1, wherein the processor is further configured for calculating the polarization rotation .sub.P(.sub.0) of the measuring light which is caused by the sample at the standardized wavelength .sub.0, further based on a polarization rotation .sub.P(.sub.eff) which is caused by the sample at an effective wavelength .sub.eff which is determined by optical properties of the entirety of the optical components of the optical measuring system and by the optical properties of the sample, and an optical rotation dispersion .sub.P(.sub.0) which is caused by the sample and which is pre-known, at the standardized wavelength .sub.0.

7. The optical measuring system according to claim 5, wherein the optical rotation dispersion .sub.P(.sub.0) which is caused by the sample at the standardized wavelength .sub.0 is determined by the quotient of (a) a difference between the first polarization rotation .sub.P(.sub.1) and the second polarization rotation .sub.P(.sub.2) and (b) a wavelength difference between the first wavelength .sub.1 and the second wavelength .sub.2.

8. The optical measuring system according to claim 5, wherein the processor is further configured for determining the first shift .sub.P(T) based on a relative transmission dispersion T/T of the sample and a proportionality factor , wherein the relative transmission dispersion T/T of the sample is given by a quotient of the transmission dispersion T of the sample and the transmission T of the sample and the proportionality factor is specific for the transmission dispersion of the entirety of the optical components and is determinable by an optical calibration of the optical measuring system using a reference sample.

9. The optical measuring system of claim 8, wherein the relative transmission dispersion T/T of the sample is determined by the quotient of (a) a difference between the first transmission characteristic T(.sub.1) and the second transmission characteristic T(.sub.2) and (b) a product of (b1) a sum of the first transmission characteristic T(.sub.1) and the second transmission characteristic T(.sub.2) and (b2) a wavelength difference between the first wavelength .sub.1 and the second wavelength .sub.2.

10. The optical measuring system according to claim 1, further comprising: a switchable optical filter device which is located in the analysis beam path and which is adapted for determining an operational state of the optical measuring system to the effect that in a first operational state only measuring light with the first wavelength .sub.1 and in a second operational state only measuring light with the second wavelength .sub.2 reaches the detector.

11. The optical measuring system according to claim 10, wherein the switchable optical filter device comprises a first optical filter which is assigned to the first wavelength 1 and a second optical filter which is assigned to the second wavelength 2, and wherein the switchable optical filter device is configured for arranging the first optical filter in the analysis beam path in the first operational state and for arranging the second optical filter in the analysis beam path in the second operational state.

12. The optical measuring system according to claim 10, wherein the switchable optical filter device comprises an optical filter which is arranged in the analysis beam path, and an actuator which is adapted for varying an angular position of the optical filter between a first angle which is assigned to the first operational state and a second angle which is assigned to the second operational state.

13. The optical measuring system according to claim 1, further comprising: a beam splitter which is arranged downstream of the sample in the analysis beam path and which is configured for splitting the measuring light into a first partial beam and into a second partial beam, wherein the first partial beam is assigned to the first wavelength .sub.1 and the second partial beam is assigned to the second wavelength .sub.2, wherein the detector comprises two detector elements, wherein a first detector element is assigned to the first wavelength .sub.1 and the second detector element is assigned to the second wavelength .sub.2.

14. The optical measuring system according to claim 1, wherein the detector forms at least a part of a spectrometer.

15. A method of determining polarization-optical properties of a sample, the method comprising: preparing, by a polarization state generator, a measuring light which is propagating along an analysis beam path with a defined polarization state; directing the measuring light to a sample which is located downstream of the polarization state generator in the analysis beam path; capturing a polarization rotation .sub.P(.sub.eff) of the measuring light which is caused by the sample (i) by a polarization state analyzer which is arranged downstream of the sample in the analysis beam path, and (ii) by a detector which is arranged downstream of the polarization state analyzer in the analysis beam path, wherein a wavelength-spectrum of the measuring light contains at least a first wavelength .sub.1 and a second wavelength .sub.2 and wherein the detector is detecting measuring light with the first wavelength .sub.1 separated from measuring light with the second wavelength .sub.2; and evaluating measuring signals from the detector by a processor, wherein a polarization rotation .sub.P(.sub.0) of the measuring light which is caused by the sample at a standardized wavelength .sub.0 is calculated in dependency from (a) a first polarization rotation .sub.P(.sub.1) and a first wavelength .sub.1, (b) a second polarization rotation .sub.P(.sub.2) at the second wavelength .sub.2, (c) a first transmission characteristic T(.sub.1) of the sample at the first wavelength .sub.1, and (d) a second transmission characteristic T(.sub.2) of the sampling at the second wavelength .sub.2.

16. The method of claim 15, further comprising: determining the first transmission characteristic T(.sub.1) by a comparison of a first intensity which is measured by the detector and a further first intensity which is measured by the detector, wherein the first intensity results from a measurement without a sample and the further first intensity results from a measurement with the sample; and determining the second transmission characteristic T(.sub.2) by a comparison of a second intensity which is measured by the detector and a further second intensity which is measured by the detector, wherein the second intensity results from a measurement without a sample and the further second intensity results from a measurement with the sample.

17. The method of claim 16, further comprising: determining the first polarization rotation .sub.P(.sub.1) by a comparison between a captured first polarization state and a captured further first polarization state, wherein the first polarization state results from a measurement without a sample and the further first polarization state results from a measurement with the sample; and determining the second polarization rotation .sub.P(.sub.2) by a comparison between a captured second polarization state and a captured further second polarization state, wherein the second polarization state results from a measurement without a sample and the further second polarization state results from a measurement with the sample.

18. A non-transitory computer program for determining polarization-optical properties of a sample, wherein the computer program, when it is executed by a processor (P), in connection with a polarization state generator (PSG), a sample receptacle (PT), a polarization state analyzer (PSA) and a detector (Det) is adapted for generating a measuring light with a defined polarization state; directing the measuring light to a sample located downstream of the polarization state generator in an analysis beam path; capturing a polarization rotation of the measuring light with a polarization state analyzer arranged downstream of the sample in the analysis beam path, and a detector arranged downstream of the polarization state analyzer in the analysis beam path, wherein a wavelength-spectrum of the measuring light contains at least a first wavelength and a second wavelength, wherein the detector is detecting measuring light with the first wavelength separated from measuring light with the second wavelength; and evaluating signals from the detector with the processor, wherein a polarization rotation of the measuring light caused by the sample at a standardized wavelength is determined from (a) a first polarization rotation at the first wavelength, (b) a second polarization rotation at the second wavelength, (c) a first transmission characteristic of the sample at the first wavelength, and (d) a second transmission characteristic of the sample at the second wavelength.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows an optical measuring system which is configured as a polarimeter, wherein two measuring channels are realized by a rotatable wheel to which two interference filters which are configured as bandpass filters are attached.

(2) FIG. 2 shows a polarimeter, in which two measuring channels are realized by a tiltable interference filter.

(3) FIG. 3 shows a polarimeter, in which two measuring channels are spatially separated by a beam splitter.

(4) FIG. 4 shows a polarimeter, in which multiple measuring channels are realized by a spectrometer.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

(5) It should be noted that in the following detailed description, features and components, respectively, of different embodiments which are equal or at least functionally equal to the respective features and components, respectively, of another embodiment, are provided with the same reference signs or with a reference sign which only differs in the first digit from the reference sign of the equal or at least functionally equal features and components, respectively. In order to avoid unnecessary repetitions, features and components, respectively, which have been already described by means of a previously described embodiment, shall not be described in detail later.

(6) Further, it should be noted that the subsequently described embodiments merely constitute a restricted selection of possible variants of the embodiments of the invention. In particular, it is possible to combine the features of the single embodiments in a suitable manner, such that with the variants of the embodiments which are explicitly shown here, a multiplicity of different embodiments are to be considered as obviously disclosed for those skilled in the art.

(7) For sake of a better understanding of the embodiments of the invention, some technical and/or mathematical basics are described below on which embodiments of the present invention are based.

(8) Ideally, polarimetric measurements should be performed with monochromatic light at a standardized wavelength .sub.0. However, the most commercial polarimeters use broadband incoherent light sources, such as tungsten halogen lamps or LEDs. The wavelength which may also be denoted as measuring wavelength is then adjusted by suitable wavelength selectors. Real wavelength selectors have to comprise a finite bandwidth, in order that enough light is available for the measurement. Preferred spectral bandwidths are in a range from 5 to 10 nm (full width at half maximum). Within this restricted spectral range, the relevant effective wavelength .sub.eff results.

(9) The effective wavelength .sub.eff is the center-of-gravity wavelength of the effective spectrum W() of the polarimetric measurement. The effective spectrum W() summarizes the consequence of all wavelength-dependent effects which are influencing the measurement. These typically include the emission spectrum of the light source, the spectrum of the sensitivity of the detector, the transmission spectrum of the wavelength selector, and other effects which are specific for the measuring principle of the polarimeter, such as the wavelength-dependent modulation amplitude of a Faraday-modulator.

(10) The above mentioned contributions to the effective spectrum W() are device-specific and can also be summarized in a so-called device function G(). However, in general, the effective spectrum W() is also influenced by a wavelength-transmission or sample transmission T() of a sample which is introduced in the beam path of the polarimeter. Thus, the effective spectrum W() is the product of the device function G() and the sample transmission T(). The effective wavelength therefore is:

(11) $\begin{array}{cc}{}_{\mathrm{eff}}=\frac{{}_{{}_1}^{{}_2}G\left(\right)T\left(\right)d}{{}_{{}_1}^{{}_2}G\left(\right)T\left(\right)d}& \left(1\right)\end{array}$
.sub.1 and .sub.2 specify the limits of the spectral range which is restricted by the wavelength selective elements.

(12) In samples whose transmission is constant within a restricted spectral range (T()=const., no transmission dispersion), the sample does not have an influence on the center-of-gravity wavelength, and the effective wavelength .sub.eff is equal to the center-of-gravity wavelength of the device function G() and is denoted as wavelength of the device .sub.G in the following:

(13) $\begin{array}{cc}{}_G=\frac{{}_{{}_1}^{{}_2}G\left(\right)d}{{}_{{}_1}^{{}_2}G\left(\right)d}& \left(2\right)\end{array}$

(14) This in particular applies for transparent samples (T()=1) and thus also for quartz-control plates which are usually used for calibration purposes as polarization-optically active reference samples. When using stable wavelength selectors, a setting of the wavelength of the device can be performed in a known manner by an adjustment using a quartz-control plate. The adjusted wavelength then is maintained between subsequent measurements.

(15) In colored samples, i.e. in samples whose transmission T() changes due to absorption or scattering within the restricted spectral range (the transmission dispersion T()=dT/d is unequal to zero), the sample conversely has, according to equation (1), an influence on the effective wavelength .sub.eff which is therefore no longer equal to the wavelength .sub.G of the device. This means that the effective wavelength .sub.eff may change from sample to sample, namely due to a wavelength error which is generated by the sample itself due to its transmission dispersion T().

(16) In general, the error .sub.P(T) by colored samples can be defined only as difference between the effective wavelength .sub.eff with the sample and the wavelength .sub.G of the device (without the sample):
.sub.P(T)=.sub.eff.sub.G

(17) When the relevant wavelength interval .sub.1 . . . .sub.2 is narrow enough, the course of the sample transmission can be linearly approximated.

(18) For the general device function G(), there is a proportionality to the relative transmission dispersion T/T of the sample:
.sub.P(T)=*T/T(3)

(19) The proportionality factor (kappa) is determined by the exact shape of the device function G(), wherein in particular the effective width of the device function G() is important.

(20) It can be seen that the absolute value of the transmission of the sample at the effective wavelength .sub.eff is not the relevant point for the wavelength change, but the relative slope of the transmission. A maximum influence of the sample transmission on the wavelength occurs when the wavelength of the device is located on an absorption edge of the sample.

(21) The wavelength error which is relevant for the practical measurement is the deviation of the effective wavelength .sub.eff with the sample with respect to the standardized wavelength .sub.0. It applies:
=.sub.eff.sub.0=.sub.G+.sub.P(T)

(22) In this respect, .sub.G=.sub.G.sub.0 is the wavelength error of the device (without the sample), such as it results from a typical calibration with a quartz-plate.

(23) The measuring value of a sample is the value of the optical rotation (=alpha) at the effective wavelength of the combination of the device and the sample, therefore the value .sub.P(.sub.eff). However, the value .sub.P(.sub.0) at the standardized wavelength .sub.0 is searched. If the wavelength error is small enough, a linear approximation leads to the following equation:
.sub.P(.sub.eff)=.sub.P(.sub.0)+.sub.P(.sub.0)*

(24) In this respect, .sub.P(.sub.0)=d.sub.P/d is the optical rotation dispersion of the sample, therefore the wavelength-dependent deviation of the optical rotation of the sample.

(25) The measuring error of the optical rotation
=.sub.P(.sub.eff).sub.P(.sub.0)
is proportional to both the rotation dispersion of the sample and also to the wavelength error during the measurement. In summary, therefore the following equation results:
.sub.P(.sub.0)=.sub.P(.sub.eff).sub.P(.sub.0)*[.sub.G+.sub.P(T)](4)

(26) As can be taken from the following description of embodiments of the invention, by means of embodiments of the invention which is described in this document, errors in polarimetric measurements can be corrected and/or compensated which are caused by a wavelength-dependent absorption and/or transmission of the sample.

(27) In this document it is now described how the above described analysis can be used for correcting and compensating, respectively, the wavelength error of the optical rotation by means of a direct measurement, which measuring error results from both a wavelength-dependent transmission of the sample and also from a rotation dispersion of the sample.

(28) A combination of the equations (3) and (4) results to:
.sub.P(.sub.0)=.sub.P(.sub.eff).sub.P(.sub.0)*[.sub.G+*T/T](5)

(29) In order to correct the error by means of this combination, the following quantities therefore have to be known: 1. The (statistical) wavelength error of the device .sub.G. This can be determined by means of a conventional quartz-calibration. 2. The proportionality factor . This can be determined by means of a calibration. 3. The relative transmission dispersion T/T of the sample to be measured. This transmission dispersion T/T has to be measured. 4. The optical rotation dispersion .sub.P(.sub.0) of the sample: this either has to be known or it has to be measured.

(30) The quantities 1 and 2 are calibrations of the device. They can be performed previously to sample measurements. The calibration of can be performed by a colored optically active reference sample.

(31) The quantity 3 (relative transmission dispersion T/T) has to be determined for each sample. For this purpose, according to the here described embodiment, the transmission is measured at at least two wavelengths. For this purpose, the used polarimeter has to comprise two measuring channels which are different on the wavelength scale.

(32) The quantity 4 (optical rotation dispersion .sub.P(.sub.0)) may be known in known sample types, such as sugar, and does not have to be measured for each single sample. In unknown samples, the optical rotation dispersion has to be measured by measuring at least two wavelengths. However, since the determination of the quantity 3 (relative transmission dispersion T/T) anyway requires measurements at at least two wavelengths, the quantity 4 (optical rotation dispersion .sub.P(.sub.0)) in many embodiments of the invention can be advantageously measured without additional effort.

(33) In the following, it is now described in detail how, according to the here described embodiment, the quantity 3 (relative transmission dispersion T/T) and the quantity 4 (optical rotation dispersion .sub.P(.sub.0)) are measured in a simple manner.

(34) Both in the calibration of the proportionality factor (kappa) and also in measuring a sample, the relative transmission dispersion T/T and generally also the optical rotation dispersion .sub.P(.sub.0) have to be measured. Both quantities are defined as derivations with respect to the wavelength. However, according to the here described embodiment, they are determined as difference quotient from measurements at at least two wavelengths. The following description for the sake of simplicity is restricted to two wavelengths.

(35) At each wavelength, both the sample transmission, thus T(1) and T(2), and also the optical rotation of the sample, thus .sub.P(.sub.1) and .sub.P(.sub.2), are measured and the quantities 3 and 4 are calculated therefrom.

(36) For the quantity 3 (relative transmission dispersion T/T) results:

(37) $\begin{array}{cc}\frac{T^{}}{T}=2*\frac{T\left({}_1\right)-T\left({}_2\right)}{\left(T\left({}_1\right)+T\left({}_2\right)\right)*\left({}_1-{}_2\right)}& \left(6\right)\end{array}$

(38) For the quantity 4 (optical rotation dispersion .sub.P(.sub.0)) results:

(39) $\begin{array}{cc}{}_P^{}\left({}_0\right)=\frac{{}_P\left({}_1\right)-{}_P\left({}_2\right)}{{}_1-{}_2}& \left(7\right)\end{array}$

(40) The exact values of the both wavelengths .sub.1 and .sub.2 are not known, since they are subject to the sample-dependent wavelength error themselves. However, in this context it should be considered that the both wavelengths .sub.1 and .sub.2 are located so closely together, that the relative transmission dispersion T/T in this range can be considered as being constant. The device function G() of the wavelength selectors and thus the respective statistical wavelength error .sub.G and the proportionality factor 1 for the wavelength .sub.1 and the proportionality factor .sub.2 for the wavelength .sub.2 may be different for the both wavelengths .sub.1 and .sub.2. Therefore, preferably both proportionality factors .sub.i (i=1, 2; for both wavelengths .sub.1 and .sub.2) can be calibrated with a colored optically active reference sample (as described below) and further, the both statistical wavelength errors .sub.Gi (i=1, 2; for both wavelengths .sub.1 and .sub.2) can be determined by a normal calibration with a quartz-plate.

(41) The exact values of the wavelengths .sub.1 and .sub.2 can then be iteratively determined as follows: assuming undisturbed wavelengths, by means of equation (6), a first approximation of the relative transmission dispersion T/T is determined. Thereby, by means of equation (3), the sample-dependent wavelength errors for the both wavelengths .sub.1 and .sub.2 are determined. Based on these wavelength errors, the wavelengths .sub.1 and .sub.2 are corrected and by means of equation (6), the relative transmission dispersions T/T in turn are determined more accurately. This mathematical procedure may be repeated, if necessary. Then, by means of the thus corrected wavelengths .sub.1 and .sub.2, also the optical rotation dispersion .sub.P(.sub.0) is determined which in turn is constantly approximated for both wavelengths.

(42) In the following it is now described in detail how, according to the here described embodiment, the corrected measuring value .sub.P(.sub.0) for the polarization rotation of the sample to be measured at the standardized wavelength .sub.0 is determined.

(43) From the single measurements for both wavelengths .sub.1 and .sub.2, by means of equation (5), respectively one measuring value of the sample can be calculated. It is reasonable to average both measuring values.

(44) In this respect, it should be noted that the above ascertained values for the relative transmission dispersion T/T and the optical rotation dispersion .sub.P(.sub.0) apply for both wavelengths .sub.1 and .sub.2, but the statistical wavelength error .sub.Gi (i=1, 2; for both wavelengths .sub.1 and .sub.2) and the respective proportionality factor .sub.i can be different for both wavelengths .sub.1 and .sub.2. Thus, from equation (5) explicitly results:

(45) ${}_P\left({}_0\right)=\frac{1}{2}*\left\{{}_P\left({}_1\right)+{}_P\left({}_2\right)-{}_P^{}\left({}_0\right)*\left[{}_{G1}+{}_{G2}+\left({}_1+{}_2\right)*\frac{T^{}}{T}\right]\right\}$

(46) In the following it is now described in detail how, according to the here described embodiment, the proportionality factor and/or the proportionality factors .sub.i are determined.

(47) For this purpose, a colored optically active reference sample is used whose optical rotation and optical rotation dispersion and whose relative transmission dispersion at the standardized wavelength .sub.0 are known. Such a reference sample may be simply manufactured by a combination of a normal quartz-plate and a suitable filter, for example. The optical rotation of the quartz-plate is determined separately, thus without the colored filter, as in a normal calibration in a polarimeter. The optical rotation dispersion of quartz is known from literature (see for example http://www.icumsa.org/ or http://www.oiml.org/fr). The relative transmission dispersion of the colored filter is determined separately, thus without the quartz-plate, in a spectrometer. For this purpose, the optical rotation of the colored filter (without a quartz-plate) is determined.

(48) A measurement of such a reference sample in a polarimeter to be calibrated results in a deviation of the measured optical rotation from the known reference value. The optical rotation dispersion of the reference sample and its relative transmission dispersion are known. The wavelength error of the device .sub.G is determined by a previous normal quartz-calibration. Then, by means of equation (5), the proportionality factor and the both proportionality factors .sub.i, respectively, can be determined.

(49) In this context, it is assumed that the reference sample at the wavelengths of interest comprises an optical rotation dispersion and causes a change in absorption. Alternatively to the above-mentioned example, the reference sample may comprise a quartz-plate and a colored solution, an optically active solution (colored sugar solution) and/or a quartz-plate with a vapor-deposited absorption layer.

(50) In the following by means of the FIGS. 1 to 4 multiple optical measuring systems which are configured as polarimeter are described, by which a polarization rotation .sub.P(.sub.0) of the measuring light which is caused by the sample at a standardized wavelength .sub.0 can be determined, wherein systematic measuring errors are corrected and/or compensated which are caused by a wavelength-dependent absorption and/or transmission of the sample.

(51) The described polarimeters respectively measure the optical activity of a substance, wherein the optical activity is the property of chemical compounds, in a solid state or in solution, to rotate the plane of polarized light when passing, by an amount which is characteristic for the respective compound (rotation value of the polarization rotation). For determining the rotation value , the sample to be examined is brought between two polarization filters (so-called Nicol prisms, Glan-Thompson polarizers, Tourmaline plates or foil polarizers, Glan Taylor polarizers, dielectric polarization beam splitters, glass-based polarization filters, metal grating-polarizers etc.). The light which is sent out from a light source is polarized by a first polarization state generator (PSG), for example a polarization filter. If the polarization state analyzer (PSA), for example a polarization filter, is standing rotated by 90 with respect to the PSG, no light impinges on a detector. If the optically active substance is now brought between the PSG and the PSA, the optically active substance rotates the polarization direction of the passing light and it is required to rotate the PSA and/or the PSG by an angle, in order to achieve again that no light impinges on the detector. This rotation angle is proportional to the polarization-optical ability for rotation of the sample and/or substance to be examined and its concentration. Dependent on the embodiment, the light source may be integrated in the PSG and the detector or the detectors may be integrated in the PSA. The transferability of the invention from the subsequently as exemplary described concrete embodiments of a polarimeter to other types of polarimeters, for example such with rotating elements or multiple detectors is obvious for those skilled in the art.

(52) FIG. 1 shows an optical measuring system which is configured as polarimeter 100 with two optical measuring channels. The polarimeter 100 comprises a light source L which sends out a measuring light ML along an analysis beam path. A polarization state generator PSG generates the polarized measuring beam and is radiating through the sample P to be examined which is located in the analysis beam path. After passing through the sample, the measuring light ML impinges on a polarization state analyzer PSA which is configured in a known manner such that it only allows that portion of measuring light ML to pass which comprises a certain polarization. The intensity of the measuring light ML which was let through by the polarization state analyzer PSA is captured by a detector Det.

(53) It should be noted that an optical monochromator (not shown in the drawing) may be arranged in the analysis beam path, which serves for the spectrum of the measuring light ML which impinges on the sample P having a smaller bandwidth than the light which is directly emitted from the light source L.

(54) According to the here illustrated embodiment, the polarization state generator PSG and/or the polarization state analyzer PSA may be rotated by a motor M1 and M2, respectively, until an optical rotation is compensated by an optically active sample P. The position of the polarization state generator PSG and/or the polarization state analyzer PSA is measured by at least one not illustrated angle measuring device (encoder) which is assigned to the at least one of the both motors M1, M2. The difference of a comparison angle with and without a sample P results in the optical rotation of the sample P.

(55) Optionally, the polarization plane of the measuring light ML may be frequency-varied by a Faraday modulator FM and at the detector Det only signals with the same frequency may be detected and/or processed. Thereby, in a known manner disturbing influences, such as in particular scattering light, can be reduced or eliminated.

(56) Typically, the sample P is located in a sample carrier PT which is exchangeably arranged in a sample container of the polarimeter 100. According to the illustrated embodiment, the sample P is a liquid sample and the sample carrier PT is a cuvette. The cuvette PT is transparent for the measuring light at least at its end faces which are perpendicular with respect to the optical axis of the measuring light ML. The cuvette may be also realized as flow-through cuvette.

(57) In addition, according to the illustrated embodiment, also a temperature measurement of the sample P is performed by a temperature sensor TS which is principally known.

(58) The polarization state of the measuring light ML which is rotated by the sample P is examined by the polarization state analyzer PSA which is arranged at an outlet of the cuvette and by the detector Det which ascertains at least an intensity of the measuring light ML. The results are supplied to an evaluation and control unit P. The analysis may be performed for example by a defined rotation of the polarization state analyzer PSA with the motor M2. The regulation is performed on basis of the intensity value of the measuring light ML which is transmitted by the detector Det by a specification, for example of steps for a motor M2 which is configured as a step motor, for example. The evaluations of the evaluation and control unit P are displayed on a display unit Dis.

(59) It should be noted that diverse variants of this basic measuring principle are known in which the order of the elements which are radiated through may also be changed, where applicable.

(60) For performing the analysis which is described in this document, in order to, by a direct measurement, correct the wavelength error of the optical rotation which results both from a wavelength-dependent transmission of the sample and from a rotation dispersion of the sample, the polarimeter 100 which is shown in FIG. 1 comprises two measuring channels which are assigned to two different wavelengths of measuring light ML. According to the here described embodiment, the respective wavelength is selected by a filter wheel 110 at which two interference filters 112 and 114 are mounted. For a fine adjustment of the interference filters 112 and 114, these may be also tilted in a not illustrated manner, such that the optical thickness of the respective interference filter 112, 114 is changing.

(61) The filter wheel 110 may be rotated by an actuating motor 116, such that the interference filter 112 or 114, which is respectively assigned to the desired wavelength is located in the analysis beam path. Therefore, the polarimeter 100 is operated alternately with respectively one active measuring channel of the both measuring channels.

(62) According to the here illustrated embodiment, the both measuring channels on the wavelength scale have a distance which is in a range between 0.1 nm and 20 nm. Within this range, small distances of, for example approximately 1 nm may be especially suitable. In contrast to known polarimeters, wavelength errors here are compensated. Therefore, it is not required anymore to meet the pre-given standardized wavelength as exactly as possible with an interference filter. Therefore, the interference filters 112, 114 do not mandatorily have to be adjusted by suitable tilting. Thereby, a significantly simpler construction of the filter wheel 110 without adjustment device for the tilting position of the interference filters 112, 114 is possible.

(63) It should be noted that instead of the filter wheel 110, also a linearly slidable frame may be used at and/or in which the both interference filters 112, 114 are located.

(64) The evaluation and control unit P is adapted for performing the inventive method, in particular based on the above described explanation of the physical basics of the operation of a polarimeter with different wavelengths of measuring light.

(65) FIG. 2 shows a polarimeter 200 which differs from the polarimeter 100 only in that the both measuring channels are realized by a tiltable filter holder 220 at which an interference filter 222 is attached. The filter holder 220 can be tilted from a first position which is shown dashed, which is exactly defined by a stop 228, into a second position whose exact angular position is determined by a stop 229. When tilting, the optical thickness of the interference filter 222 which is relevant for the measuring light ML is changing in a known manner, which in turn determines the wavelength which is let through by the interference filter 222.

(66) It should be noted that the filter principles which are used for the both polarimeters 100 and 200 may also be combined. This in particular applies for the case that the respective polarimeter has more than two measuring channels. In this case, namely a rotatable filter wheel may be used in a simple manner, for example, which can be folded to two different stops by an actuator.

(67) FIG. 3 shows a polarimeter 300 in which two measuring channels are spatially separated by means of a beam splitter 330. This means that the measuring light ML after passing through the sample P is split in two partial beams. To each partial beam, an own wavelength selector 332 and 334, respectively, and an own detector Det is assigned. In this embodiment, the both measurements which are respectively assigned to one measuring channel can be performed simultaneously.

(68) The beam splitter 330 may also be a polarization-dependent beam splitter. In this case, the polarization direction in the polarization state analyzer PSA has to be re-adjusted, if necessary, and the both measurements have to be performed subsequently to each other.

(69) In further not illustrated embodiments, two wavelengths are realized by which the interference filters are radiated through by two partial beams in different angles simultaneously. Thereby, the measurements at both wavelengths can be performed simultaneously. These embodiments have the advantage that per standardized wavelength only one interference filter is required. They may be used both for one single standardized wavelength with a fixedly mounted interference filter and also for a polarimeter with multiple standardized wavelengths with exchangeable interference filters.

(70) For example, a Wollaston prism can be used as polarization beam splitter, such that the beam is split in two partial beams which are divergent with respect to each other. At the outlet of the Wollaston prism, the partial beams are still so close together, that they both can commonly pass through an interference filter which is attached in close proximity. In a larger distance, the partial beams are separated so far that they can be mapped on two detectors. Here, multiple interference filters on a filter wheel are conceivable again. It is important that the interference filter is oriented such that both partial beams radiate through the interference filter with different angles, such that the partial beams have different wavelengths.

(71) Furthermore, a normal beam splitter may be used and the one partial beam may be subsequently redirected, such that it intersects the first partial beam in the interference filter. Here, a variant with a single fixed interference filter and with two detectors or with two light-sensitive regions on an area detector is conceivable. When the beam splitter is polarization-sensitive, both measurements can be performed simultaneously.

(72) FIG. 4 shows a polarimeter 400 in which multiple measuring channels are realized by a spectrometer 450. A high measuring stability of the polarimeter 400 may be achieved if the spectrometer does not comprise movable components. Such a spectrometer may be a known monolithic array-spectrometer, for example, which allows a simultaneous measurement in different measuring channels which respectively are assigned to different wavelengths.

(73) Suitable spectrometers 450 may be placed directly behind the polarization state analyzer PSA in the polarimeter 400, as illustrated in FIG. 4. This embodiment can be realized in an especially simple manner. It should only be noted that each wavelength channel and measuring channel, respectively, has an individual effective wavelength, an individual device function G() and an individual proportionality factor . The above described measurements of the relative transmission dispersion T/T and the optical rotation dispersion (.sub.0) may be performed for each desired standardized wavelength in the spectral range which is covered by the spectrometer with two wavelengths-channels whose effective wavelengths are adjacent to the standardized wavelength .sub.0.

(74) If the measuring principle which is implemented in the polarimeter is reliant on an adjustment of the polarization state analyzer PSA and/or of the polarization state generator PSG, which adjustment is dependent on the value of the optical rotation (.sub.0) of the sample (for example in a comparison of a polarizing filter to a dark position), the measurements for the selected standardized wavelengths have to be performed subsequently to each other. If the measuring principle which is implemented in the polarimeter is not reliant on an adjustment of the polarization state analyzer PSA and/or of the polarization state generator PSG, which adjustment is dependent on the value of the optical rotation (.sub.0) of the sample, the measurements for the selected standardized wavelengths, or if desired, also for a complete spectrum can be performed simultaneously.

(75) Subsequently, some components are described in more detail which can be used for embodiments of the invention in an advantageous manner.

(76) As suitable light sources, besides thermal light sources (light bulbs), also light-emitting diodes, laser diodes, superluminescence-diodes, laser, broadband discharge lamps, narrowband discharge lamps, such as hollow-cathode lamps and in particular low-pressure-spectral lamps, can be used. If required, also wavelength converters can be used. Moreover, multiple light sources may be automatically or manually exchangeable or may be permanently (for example by wavelengths-selective elements) combined to a measuring beam. The measuring beam may further be prepared by diffusers or homogenizers and may be spatially guided by lenses or mirrors.

(77) The polarization state generator (PSG) and the polarization state analyzer (PSA) may be configured according to diverse principles. PSGs and PSAs with fix, rotating or modulated polarization filters, retardation plates, polarization compensators and/or beam splitters may be used.

(78) Dependent on the embodiment of the polarization state generator PSG, of the polarization state analyzer PSA and of the used evaluation algorithm, all or some elements of the so-called Mllermatrix of the sample can be determined as sample property. An example is the optical activity of the sample which is measured by the rotation of the polarization direction of linearly polarized light which is caused by the sample.

(79) A simple possibility to compensate the polarization direction of the measuring light which is changed by the substance and to achieve the initial intensity values at the detector unit (the polarization state generator and the polarization state analyzer, after inserting the rotating substance, are preferably brought in a crossed position again which leads to minimum or no light passing) is a rotation of the polarization state generator or polarization state analyzer by means of a motor or a step motor.

(80) A temperature from a temperature sensor and a light intensity which is received at the detector may be processed in the control and evaluation unit and the polarimeter may be regulated with it. The measurement of the rotational angle is regularly performed by an angle measuring device, typically an optical encoder, which is connected to the rotated optical element in a rigid manner. Alternatively, for lower accuracies, the steps which are moved by a step motor may be used for the angular measurement.

(81) According to further embodiments, the rotation of the polarization plane which is caused by the optical activity of the sample can be compensated not by a mechanically movable element, but by a purely optical element, for example a Faraday-rotator. In a Faraday-rotator, the rotation of the polarization plane is proportional to the current through the coil of the Faraday-rotator. The current which is required for compensating the rotation which is caused by the sample is proportional to the ability for rotation of the substance and its concentration.

(82) Regularly, the polarization properties of samples are dependent on the sample temperature as well. Thereby, the sample temperature can be measured by temperature sensors which immerse in the sample or are attached to the cuvette. In particular, photo multipliers, photodiodes, Avalanche-diodes, CCD-detectors, NMOS-detectors, CMOS-detectors and spectrometers are possible as detectors.

REFERENCE SIGNS

(83) 100 polarimeter 110 filter wheel 112 interference filter 114 interference filter 116 actuating motor 200 polarimeter 220 tiltable filter holder 222 interference filter 228 stop 229 stop 300 polarimeter 330 beam splitter 332 wavelength selector 334 wavelength selector 400 polarimeter 450 spectrometer L light source ML measuring light PSG polarization state generator FM Faraday modulator PSA polarization state analyzer P evaluation and control unit Det detector M1 motor+angle measuring device M2 motor+angle measuring device TS temperature sensor P sample PT sample carrier/cuvette Dis display unit