Abstract:
Instruments (as well as related components and methods) to measure the angular distribution of light produced by an illumination system of a microlithographic projection exposure apparatus are disclosed. Such apparatusus are used for the production of integrated circuits and other microstructured components.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority under 35 U.S.C. section 119 to European patent application 06020464.1, filed Sep. 28, 2006, which is hereby incorporated by reference. 
       FIELD 
       [0002]    The disclosure relates generally to instruments (as well as related components and methods) to measure the angular distribution of light produced by an illumination system of a microlithographic projection exposure apparatus. Such apparatusus are used for the production of integrated circuits and other microstructured components. 
       BACKGROUND 
       [0003]    Microlithography (also called photolithography or simply lithography) is a technology for the fabrication of integrated circuits, liquid crystal displays and other microstructured devices. The process of microlithography, in conjunction with the process of etching, is used to pattern features in thin film stacks that have been formed on a substrate, for example a silicon wafer. At each layer of the fabrication, the wafer is first coated with a photoresist which is a material that is sensitive to radiation, such as deep ultraviolet (DUV) light. Next, the wafer with the photoresist on top is exposed to projection light through a mask in a projection exposure apparatus. The mask contains a circuit pattern to be projected onto the photoresist. After exposure the photoresist is developed to produce an image corresponding to the circuit pattern contained in the mask. Then an etch process transfers the circuit pattern into the thin film stacks on the wafer. Finally, the photoresist is removed. Repetition of this process with different masks results in a multi-layered microstructured component. 
         [0004]    A projection exposure apparatus typically includes an illumination system, a mask stage for a aligning the mask, a projection lens and a wafer alignment stage for aligning the wafer coated with the photoresist. The illumination system illuminates a field on the mask that often has the shape of an (elongated) rectangle or a ring segment. 
         [0005]    In current projection exposure apparatus a distinction can be made between two different types of apparatus. In one type each target portion on the wafer is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper. In the other type of apparatus, which is commonly referred to as a step-and-scan apparatus or scanner, each target portion is irradiated by progressively scanning the mask pattern under the projection light beam in a given reference direction while synchronously scanning the substrate parallel or anti-parallel to this direction. The ratio of the velocity of the wafer and the velocity of the mask is equal to the magnification of the projection lens, which is usually smaller than 1, for example 1:4. 
         [0006]    The angular distribution of the projection light impinging on the mask is usually adapted to the kind of pattern to be projected onto the photoresist. For example, relatively large sized features may require a different angular distribution than small sized features. The most commonly used angular distributions of projection light are referred to as conventional, annular, dipole and quadrupole illumination settings. These terms refer to the irradiance distribution in a pupil plane of the illumination system. With an annular illumination setting, for example, only an annular region is illuminated in the pupil plane. Thus there is only a small range of angles present in the angular distribution of the projection light so that all light rays impinge obliquely with similar angles onto the mask. 
       SUMMARY 
       [0007]    In some embodiments, the disclosure provides an instrument configured to measure the angular distribution of light produced by an illumination system of a microlithographic projection exposure apparatus which has a reduced sensitivity to the state of polarization. 
         [0008]    In one aspect, the disclosure generally features an instrument configured to measure an angular distribution of light produced by an illumination system of a microlithographic projection exposure apparatus. The instrument includes a pinhole, a polarization conversion unit and an irradiance sensor. The polarization conversion unit is configured to convert at least one state of polarization of the light into a circular state of polarization. The irradiance sensor is configured to detect the spatial distribution of light having passed the pinhole and the polarization conversion unit. 
         [0009]    In another aspect, the disclosure generally features an instrument configured to measure an angular distribution of light produced by an illumination system of a microlithographic projection exposure apparatus. The instrument includes a pinhole, a polarizer, and an irradiance sensor. The polarizer is configured to selectively pass light in one of two distinct linear states (P 1 , P 2 ) of polarization, and the irradiance sensor configured to detect the spatial distribution of light passing the pinhole and being in either of the two linear states (P 1 , P 2 ) of polarization. 
         [0010]    In a further aspect, the disclosure generally feature an instrument configured to measure an angular distribution of light produced by an illumination system of a microlithographic projection exposure apparatus. The instrument includes a pinhole, an aspherical lens, and an irradiance sensor that detects the spatial distribution of light after it has passed the pinhole and the lens. 
         [0011]    In an additional aspect, the disclosure generally features a projection exposure apparatus. The apparatus includes an illumination system and an instrument as described in any of the preceding three paragraphs. The pinhole is arranged at least substantially in an illumination plane of the illumination system where a mask is positioned during exposure operation. 
         [0012]    In another aspect, the disclosure generally features a method of measuring an angular distribution of light produced by an illumination system of a micro-lithographic projection exposure apparatus. The method includes: a) positioning a pinhole in an illumination plane of the illumination system; b) providing an irradiance sensor that detects a spatial distribution of light having passed the pinhole, wherein the irradiance sensor displays a maximum sensibility difference for light which is in a first linear state of polarization (P 1 ) and light which is in a second linear state of polarization (P 2 ); c) measuring the sensibility of the irradiance sensor for light impinging in the first linear state of polarization (P 1 ); d) measuring the sensibility of the irradiance sensor for light impinging in the second linear state of polarization (P 2 ); e) providing a polarizer that passes light in the first linear states of polarization (P 1 ); f) detecting the spatial distribution of light passing the pinhole and being in the first linear state of polarization (P 1 ); g) providing a polarizer that passes light in the second linear states of polarization (P 2 ); h) detecting the spatial distribution of light passing the pinhole and being in the second linear state of polarization (P 2 ); i) determining the total irradiances for the light impinging on the irradiance sensor in both linear states of polarizations (P 1 , P 2 ), thereby taking into account the sensibility measured in c) and d). 
         [0013]    It has been discovered that converting at least one state of polarization into a circular state of polarization is can be a good way to achieve an insensitivity to the state of polarization. Producing completely unpolarized light having a rapidly varying succession of different polarization states is difficult because it requires the use of depolarizers. However, such depolarizers destroy the direct relationship between the angular distribution at the mask plane and the spatial distribution at the sensor plane. A conversion to a circular state of polarization is possible with a conversion unit that does not destroy this relationship. 
         [0014]    It is usually not necessary to convert all states of polarization to a circular state of polarization. There may be linear states of polarization with a polarization direction that can be decomposed into two equal linear components for which the sensor has the maximum and the minimum sensitivity. For such linear states of polarization the same averaging effect occurs as with a circular state of polarization. 
         [0015]    The conversion of a state of polarization having a dominant direction (i.e. linear or elliptical state of polarization) to a circular state of polarization involves the introduction of a phase shift between two orthogonal components. For example, if the light in the mask plane is elliptically polarized, it suffices to introduce an additional phase shift β with 0°&lt;β&lt;45° to convert the elliptical state of polarization to a circular state of polarization. 
         [0016]    Usually, however, the light in the mask plane is linearly polarized with polarization directions depending on the structures contained in the mask. For converting a linear state of polarization into a circular state of polarization, the polarization conversion unit has to introduce a phase shift of (2m+1)·90° with m=0, 1, 2, . . . , between two orthogonal states of polarization. Such a phase shift may be produced by different optical effects. For example, a phase shift of 45° is obtained at optical surfaces where total internal reflection occurs. The polarization conversion unit may then be configured as a prism that deflects the light by total internal reflection. 
         [0017]    Another approach to introduce a phase shift is to exploit the optical effect of birefringence. The use of optical retarders comprising a birefringent crystal is a well-known technology for introducing phase shifts. Alternatively, form-birefringent structures having dimensions smaller than the wavelength of the projection light may be used for introducing the desired phase shift. 
         [0018]    Often, the simplest way to convert a linear state of polarization into a circular state of polarization is to use a quarter-wave plate as a polarization conversion unit. If the optic axis of the quarter-wave plate forms an angle of 45° to a linear polarization direction, circularly polarized light is obtained. 
         [0019]    However, the polarization direction of the light in the mask plane is often not (exactly) known when the measurement is carried out. It should then be possible to carry out the measurement without determining beforehand the polarization distribution of the light. 
         [0020]    In such a case it is possible to exploit the fact that the irradiance sensors usually display a maximum sensibility difference for light which is linearly polarized along a first direction and light which is linearly polarized along a second direction which is orthogonal to the first direction. In the case of CCD sensors, these directions correspond to the orthogonal edges of the quadratic or rectangular sensor chip. When the quarter-wave plate comprises a birefringent material having an optic axis that is arranged at 45° to the first direction, a circular state of polarization is obtained for light which is linearly polarized along the first or second direction. Light being linearly polarized along the optic axis or perpendicular thereto is in an eigenstate of the quarter-wave plate and is thus transmitted without any change of the state of polarization. Since the eigen-states comprise two equal components along the first and second directions, there is no dominant direction which could result in measurement errors due to polarization sensitivities of the irradiance sensor. 
         [0021]    All other linear states of polarization may be decomposed into components along the first or second direction and the direction of the optic axis. Therefore these states of polarization are converted to elliptical states of polarization. However, these states do not have a dominant component along the first or second direction, and thus the measured irradiance does again not depend on the polarization direction. 
         [0022]    The polarization conversion unit may be arranged anywhere between the illumination system and the irradiance sensor. If the polarization conversion unit is arranged in front of or behind the pinhole where the light bundle is not collimated, it should have a low sensibility to the angle of incidence. In the case of a quarter-wave plate a low order, or preferably a zero-order, plate should be used. A zero-order quarter-wave plate is characterized by introducing a phase shift of exactly 45° (and not 360°+45°, for example). Such low-order or zero-order wave plates display a low sensitivity to oblique incidence. 
         [0023]    If a lens having a positive refractive power is arranged between the pinhole and the irradiance sensor, the polarization conversion unit is preferably arranged between the lens and the irradiance sensor. This has the advantage that the positive lens reduces the maximum angles of incidence occurring at the wave plate. If the lens is a collimator lens that converts a diverging light bundle emerging from the pinhole into an at least substantially collimated light bundle, normal incidence on a wave plate may be obtained. Then there is no need to use low-order or even zero-order wave plates. 
         [0024]    In many illumination systems the light in the mask plane has a numerical aperture in the order of 0.3. With a spherical positive lens and a pinhole having a finite diameter, it is not possible to accurately transform the angular distribution in the mask plane into a spatial distribution in the sensor plane. This is because spherical lenses produce significant distortion and field curvature for large numerical apertures. As a result, parallel light rays passing through the pinhole with the same (large) angle do not converge on the same point in the sensor plane. Instead, a blurred spot is produced that makes it impossible to accurately determine the angle of the light rays. 
         [0025]    In order to maintain the desired relationship between the angular distribution in the mask plane and the spatial distribution in the sensor plane even for larger numerical apertures, it is proposed to use an aspherical lens instead of a spherical lens. Only with an aspherical lens it is possible to prevent the occurrence of blurred spots observed for larger numerical apertures. This concept may also be applied to those instruments that are not polarization insensitive in the way as has been described above. 
         [0026]    The design of the instrument can be developed in view of being able to carry out a calibration of the irradiance sensor before the measurement commences. Usually the irradiance sensor displays a maximum sensibility difference for light which is in one of the two linear states of polarization and light which is in the other of the two linear states of polarization. 
         [0027]    In a calibration measurement, which may be carried out without an illumination system, the sensitivity for light having these two linear states of polarization is independently determined. The difference of the sensitivities are recorded and used for subsequent measurements of the angular distribution. 
         [0028]    Since the state of polarization is usually not or at least not exactly known when the angular distribution is measured, it is necessary to use the polarizer which passes only light being in one of the two distinct linear states of polarization. After a first measurement with the first polarization direction a second measurement is carried out with the second polarization direction. The results of the measurements are then computationally modified by taking into account the different sensitivities of the sensor which has been determined during the calibration measurements. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    Various features and advantages of the present disclosure may be more readily understood with reference to the following detailed description taken in conjunction with the accompanying drawing in which: 
           [0030]      FIG. 1  is a perspective and considerably simplified view of a projection exposure apparatus; 
           [0031]      FIG. 2  is a meridional section through an illumination system contained in the projection exposure apparatus shown in  FIG. 1 ; 
           [0032]      FIG. 3  is an enlarged meridional section through an instrument; 
           [0033]      FIG. 4  is a top view on a CCD image sensor showing different polarization directions; 
           [0034]      FIG. 5  is a top view on a CCD image sensor similar to  FIG. 4 , but showing other polarization directions; 
           [0035]      FIG. 6  is an enlarged meridional section through an instrument in a first operational state; and 
           [0036]      FIG. 7  is an enlarged meridional section through the instrument of  FIG. 7  in a second operational state. 
       
    
    
     DETAILED DESCRIPTION 
       [0037]    By way of introduction, the general structure of a microlithographic projection exposure apparatus, and in particular of its illumination system, will be explained. This is because it is believed that certain aspects of the disclosure can be fully appreciated only if the technological environment where the instrument is used is thoroughly understood. 
         [0038]    1. General Structure of Projection Exposure Apparatus 
         [0039]      FIG. 1  is a perspective and highly simplified view of a projection exposure apparatus  10  that comprises an illumination system  12  for producing a projection light bundle. The projection light bundle illuminates a field  14  on a mask  16  containing minute structures  18 . In this embodiment, the illuminated field  14  has approximately the shape of a ring segment. However, other, for example rectangular, shapes of the illuminated field  14  are contemplated as well. 
         [0040]    A projection objective  20  images the structures  18  within the illuminated field  14  onto a light sensitive layer  22 , for example a photoresist, which is deposited on a substrate  24 . The substrate  24 , which may formed by a silicon wafer, is arranged on a wafer stage (not shown) such that a top surface of the light sensitive layer  22  is precisely located in the image plane of the projection objective  20 . The mask  16  is positioned via a mask stage (not shown) in an object plane of the projection objective  20 . Since the latter has a magnification of less than  1 , a reduced image  14 ′ of the structures  18  within the illuminated field  14  is formed on the light sensitive layer  22 . 
         [0041]    2. General Structure of Illumination System 
         [0042]      FIG. 2  is a more detailed meridional section through the illumination system  12  shown in  FIG. 1 . For the sake of clarity, the illustration of  FIG. 2  is also considerably simplified and not to scale. This particularly implies that different optical units are represented by very few optical elements only. In reality, these units may comprise significantly more lenses and other optical elements. 
         [0043]    The illumination system  12  comprises a housing  28  and a light source that is, in the embodiment shown, realized as an excimer laser  30 . The excimer laser  30  emits projection light that has a wavelength of about 193 nm. Other types of light sources and other wavelengths, for example 248 nm or 157 nm, are also contemplated. 
         [0044]    In the embodiment shown, the projection light emitted by the excimer laser  30  enters a beam expansion unit  32  in which the light bundle is expanded. After passing through the beam expansion unit  32 , the projection light impinges on a first optical raster element  34 . The first optical raster element  34  slightly increases the geometrical optical flux and modifies the local irradiance distribution in a subsequent pupil plane. 
         [0045]    The first optical raster element  34  is positioned in an object plane  42  of an objective  44  that comprises a zoom lens group  46  and a pair  48  of axicon elements  50 ,  52  having opposing conical faces. If both axicon elements  50 ,  52  are in contact, as is shown in  FIG. 2 , the axicon pair  48  has the effect of a plate having parallel plane surfaces. If both elements  50 ,  52  are moved apart, the spacing between the axicon elements  50 ,  52  results in a shift of light energy radially outward. Since axicon elements are known as such in the art, these will not be explained here in further detail. 
         [0046]    Reference numeral  54  denotes an exit pupil plane of the objective  44 . An optical integrator  56  producing secondary light sources is positioned in or in close proximity to the exit pupil plane  54  of the objective  44 . The optical integrator  56  modifies the angular distribution in the pupil plane  54 . The angular distribution in the pupil plane  54  directly translates into an irradiance distribution in a subsequent field plane. Thus the design of the optical integrator  56  has a strong influence on the irradiance distribution and the geometry of the illuminated field  14  on the mask  16 . The angular distribution of the light on the mask  16 , however, is directly related to the spatial irradiance distribution of the light in the pupil plane  54 . This spatial irradiance distribution is mainly determined by the first optical raster element  34  and the positions of the elements contained in the objective  44 . 
         [0047]    The projection light emerging from the secondary light sources enters a condenser  62  that is represented in  FIG. 2  by a single lens element for the sake of simplicity. The entrance pupil plane of the condenser  62  coincides with the exit pupil plane  54  of the objective  44 . The condenser  62  superposes the light bundles emitted by the secondary light sources in a field stop plane  64  of the condenser  62  in which a field stop  66  is positioned. A field stop objective  68  images the field stop  66  onto a mask plane  70  in which the mask  16  is positioned during exposure operation, as it has been shown in  FIG. 1 . 
         [0048]    3. Angular Distribution of Light 
         [0049]    At the bottom of  FIG. 2  three light bundles are indicated each converging to one of three points  721 ,  722  and  723  which are arranged in the mask plane  70  within the illuminated field  14 . For the sake of simplicity it is assumed that the light bundles are formed only by the light rays that are represented by the (broken) lines shown in  FIG. 2 . Each light ray impinges on the mask plane  70  with a certain angle of incidence. The totality of light rays contained in a single bundle defines an angular distribution which is associated to the point to which the light rays converge. In this schematic illustration it is assumed that the angular distribution is the same for all points  721 ,  722 ,  723 . 
         [0050]    4. Measuring Instrument 
         [0051]    In order to measure the angular distribution of light converging to one of the points  721 ,  722  and  723 , an instrument  80  is provided which has a small pinhole  82 . During the measurement the pinhole  82  is arranged at the point in the mask plane  70  where the angular distribution is to be measured. In  FIG. 2  this is assumed to be point  721 . 
         [0052]      FIG. 3  shows an enlarged meridional section through the instrument  80 . The pinhole  82  is formed by a small circular aperture formed in an opaque lid  84 . The lid  84  covers a housing  86  which is completely opaque as well. Therefore light can enter into the inside of the instrument  80  only through the pinhole  82 . 
         [0053]    Along an optical axis  88  of the instrument  80  running through the pinhole  82  a positive aspherical lens  90 , a quarter-wave plate  92  supporting an attenuation layer  94  and an irradiance sensor  96  are arranged. Since the pinhole  82  is located in the mask plane  70 , the light bundle converging to the point  721  diverges inside the instrument  80 . The aspherical surface of the positive lens  90  is determined such that the light bundle is, even for the rays  110   a,    110   d  having larger angles of incidence, perfectly collimated before it impinges on the quarter-wave plate  92 . As the quarter-wave plate  92  has plane and parallel surfaces that are aligned perpendicular to the optical axis  88 , the light impinging on the irradiance sensor  96  is still collimated. The attenuating layer  94  causes an irradiance offset and ensures that the irradiance sensor  96  does not run into saturation. 
         [0054]    The irradiance sensor  96  is, in the embodiment shown, realized as a CCD image chip that has a square base surface  98 , which is mounted to a support  104 , and four edges. Two of these edges, which will be referred to in the following as side edges, are denoted in  FIG. 3  by  100 ,  102 . The irradiance sensor  96  contains a grid of pixels each being sensitive to radiation. The signal produced by each pixel is a measure for the light energy impinging on the pixel during a certain (short) time interval. From this signal it is possible to determine the irradiance (light energy per unit area per unit time) at the respective pixel. As will be explained further below, the pixels of the irradiance sensor  96  have a polarization dependent sensitivity. 
         [0055]    Since angles in the mask plane  70  of the light rays translate into positions in the sensor plane where the pixels are arranged, the irradiance distribution detected by the sensor  96  is used to determine the angular distribution of the light at point  721  in the mask plane  70 . This determination is known in the art as such. 
         [0056]    A high-precision stage (not shown) may be provided for moving the instrument  80  with its pinhole  82  to different positions within the illuminated field  14  in the mask plane  70 . The movement is indicated in  FIG. 2  by a double arrow  106 . This makes it possible to measure, for a large plurality of different points in the mask plane  70 , the angular distribution of the light produced by the illumination system  12  without the necessity to adjust the position of the instrument  80  for each individual measurement. 
         [0057]    In the following it is assumed that two out of four light rays  110   a,    110   b,    110   c  and  110   d  entering the instrument  80  through the pinhole  82  are linearly polarized with a first polarization direction being parallel along the front and rear edges of the sensor  96 , i.e. parallel to the extension of the sheet of  FIG. 3 . The polarization direction of these two light bundles  110   a,    110   d  is indicated in  FIG. 3  by double arrows indicated by P 1 . The other two rays  110   b,    110   c  are in an orthogonal state of polarization, i.e. with a polarization direction parallel to the side edges  100 ,  102  of the irradiance sensor  96 . This polarization direction is indicated in  FIG. 3  by dots P 2 . 
         [0058]      FIG. 4  shows a top view on the irradiance sensor  96  in which the orientation of the optic axis  112  of the quarter-wave plate  92  is indicated by a dot-dash line. The optic axis  112  forms an angle of 45° with respect to the side edges  100 ,  102  of the irradiance sensor  96 . As a result of this orientation of the optic axis, the quarter-wave plate  92  converts both linear states of polarization P 1 , P 2  of the light rays  110   a  to  110   d  into circular states of polarization, as is indicated by circles P c  in  FIGS. 3 and 4 . Thus the light rays  110   a  to  110   d  impinge on the irradiance sensor  96  without having a dominant polarization direction. 
         [0059]    This ensures that light rays having the same intensity but a different linear state of polarization produce the same irradiance signal at the pixels of the irradiance sensor  96  where the light rays impinge. Without the quarter-wave plate  92 , the light rays  110   a  to  110   d  would impinge on the irradiance sensor  96  with different linear states of polarization. Since the pixels of the irradiance sensor  96  have a polarization dependent sensitivity, this would result in different values for the measured irradiance even if the intensity of the light is identical. Usually the pixels have a sensitivity which is different for light being linearly polarized along the direction P 1 , which is parallel to the side edges  100 ,  102  on the one hand, and for light being linearly polarized along the direction P 2  which is parallel to the front and rear edges  101 ,  103  (see  FIG. 4 ) of the irradiance sensor  96 . Thus different irradiances would be measured for the light rays  110   a,    110   d  on the one hand and the light rays  110   b,    110   c  on the other hand if all rays have the same intensity. Due to the provision of the quarter-wave plate  92 , however, the light impinging on the irradiance sensor  96  is circularly polarized, and thus the different sensitivities of the pixels for the two orthogonal polarization directions P 1 , P 2  do not matter. 
         [0060]    If light rays are impinging on the quarter-wave plate  92  that are linearly polarized along the optic axis  112  or perpendicular thereto, as is shown in the top view of  FIG. 5 , the quarter-wave plate  92  does not have any effect on the polarization, because these are eigenstates of the quarter-wave plate  92 . Although such light rays have a dominant polarization direction when they impinge on the irradiance sensor  96 , there is no dominant polarization component along the directions P 1  and P 2  where the maximum sensitivity difference occurs. Thus the different sensitivities of the irradiance sensor  96  for the different states of polarization do not matter. 
         [0061]    Apparently the quarter-wave plate  90  does not produce light having a dominant polarization component along the edges  100 ,  101 ,  102 ,  103  irrespective of the polarization direction of the light impinging thereon. This may be easily understood if the polarization direction of the impinging light is decomposed into a component being parallel to the direction P 1  and a component parallel to the optic axis  112 . The component being parallel to the direction P 1  is converted by the quarter-wave plate  92  into a circular state of polarization, whereas the component parallel to the optic axis  112  is not changed. A superposition of these two states of polarization results in an elliptical state of polarization in which the longer main axis (i.e. dominant direction) is parallel to the optic axis  112 . As a result, the pixels of the irradiance sensor  96  will measure the same irradiance irrespective of the polarization direction of the light passing through the pinhole  82 . 
         [0062]      FIG. 6  shows a meridional section through an instrument  80 ′. For like parts the same reference numerals as in  FIG. 3  are used. The instrument  80 ′ differs from the instrument  80  shown in  FIG. 3  mainly in that the quarter-wave plate  92  is replaced by a polarizer  130  that linearly polarizes light propagating therethrough. The polarizer  130  is mounted in this embodiment so as to be rotatable around the optical axis  88 , as is indicated in  FIG. 6  by a double arrow  132 . The polarizer  130  may also be provided with an attenuating layer  94  attached to one of its surfaces. 
         [0063]    In  FIG. 6  the polarizer  130  is shown in a first angular position in which only the light rays  110   a,    110   d  being polarized along the direction P 1  are allowed to pass through the polarizer  130 . Thus the irradiance sensor  96  detects only those components of the impinging light which have a linear polarization direction parallel to the front and rear edges  101 ,  103 . 
         [0064]    If the polarizer  130  is rotated by 90°, or replaced by another polarizer having an orthogonal polarization direction, the light rays  110   a,    110   d  are blocked, and now the light rays  110   b,    110   c  being polarized along the direction P 2  are allowed to pass through the polarizer  130 . This is shown in  FIG. 7 . Now the irradiance sensor  96  detects only those components of the light that have a polarization direction parallel to the side edges  100 ,  102 . 
         [0065]    If the sensitivity of the irradiance sensor  96  is different for both polarization directions, it is possible to take this computationally into account if a calibration measurement has been carried out beforehand. During the calibration measurement, the different sensitivities are measured with a similar setup as shown in  FIGS. 6 and 7 . The only difference is that the light intensity of the rays has to be known. In the simplest case unpolarized light enters through the pinhole  82 . The different measurement values obtained with the two different polarization directions obtained with by the polarizer  130  result in correction parameters that are computationally accounted for during subsequent measurements of the angular distribution. For example, the irradiance values measured in the configuration shown in  FIG. 7  will be multiplied with a correction parameter that accounts for the different sensitivity of the irradiance sensor  96  for light being linearly polarized along the side edges  100 ,  102 . 
         [0066]    Other embodiments are covered by the claims.