Patent Publication Number: US-2020292803-A1

Title: Ir microscope

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims foreign priority under 35 U.S.C. § 119(a)-(d) to German Application No. 10 2019 203 560.4 filed on Mar. 15, 2019, the entire contents of which are hereby incorporated into the present application by reference. 
     FIELD OF THE INVENTION 
     The invention relates to an IR microscope comprising an IR light source configured to generate a collimated IR input beam, an effectively beam-limiting element in a stop plane in the collimated IR input beam, a sample position for recording a sample, an IR detector having an IR sensor, a detector stop having a detector stop opening, said detector stop being arranged upstream of the IR sensor, a first optical device configured to focus the collimated IR input beam emerging from the IR light source onto the sample position, and a second optical device configured to image the sample position onto the IR sensor, wherein the second optical device comprises an objective and an intermediate optical unit. IR microscopes of this type are known for example from [1], [2], [3], [4]. 
     BACKGROUND 
     Infrared (IR) microscopes can generate IR images and visual images of a sample to be examined, wherein the measurements can typically be carried out both in transmission and in reflection. For this purpose, the beam paths for infrared and visible light can be switched between transmission and reflection. 
     The IR microscopes under consideration here make it possible, in particular, to spectrally measure microscopic samples in a spatially resolved manner, i.e. a spectrum in the infrared spectral range is obtained for different regions on the microscopic sample. 
     For this purpose, the IR light passes through an interferometer, in which the lengths of the interferometer arms are varied with respect to one another by moving one or more mirrors and, as a result, the infrared light is modulated depending on its wavelength. In this case, the modulation frequency is dependent on the wavelength, wherein each wavelength can be uniquely assigned to a modulation frequency. The infrared light thus modulated is guided onto a sample to be examined and the light emanating from the sample is focused onto an infrared detector by way of a second optical device. 
     The modulated light received by the detector contains all spectral information of the sample. However, the detector is also sensitive to IR light from the surroundings, which is not modulated. If this light reaches the detector, it does not contribute to the information gain, but it does reduce the dynamic range of the detector. 
     Added to this there is a further portion of undesired detector signal, the dark current. The detector is therefore generally cooled in order to minimize undesired signals that can be caused by thermal excitations in the sensor material. The detector is intended to yield, where possible, only signals that are generated by the modulated IR radiation coming from the sample. 
     In order to suppress radiation from the surroundings as much as possible, the detector contains a detector stop. 
     In the prior art the size and position of the detector stop is always determined by way of the last imaging optical unit upstream of the detector. In this regard, in [3], for example, the detector stop is dimensioned such that only radiation from the upstream intermediate optical unit can pass through the detector stop. 
     With the use of a cooled IR sensor surrounded by a cooled housing (radiation shield), such that light passes to the sensor only through the likewise cooled detector stop (cold stop), the position and size of the detector stop cannot be dimensioned arbitrarily, however, since it has to be installed within a Dewar vessel. The position and size of the detector stop thus generally cannot be optimally adapted to the upstream intermediate optical unit, rather the upstream intermediate optical unit has to be designed such that the radiation desired for the measurement passes through the detector stop, but undesired radiation (e.g. radiation from the surroundings) cannot pass through the detector stop. For this purpose, the intermediate optical unit directs the IR radiation in a cone with half the opening angle α/2 in the direction of the detector and generates an image of the sample plane or of downstream intermediate image planes of the sample plane on the IR sensor. The ratio of the diameter of the input aperture to the distance d between the detector stop and the IR sensor thus corresponds to the F-number (F/#) of the intermediate optical unit. This procedure is entirely suitable for single-element detectors, such as are used e.g. in [4]. This procedure is not sufficient, however, for extensive 1- or 2-dimensional detectors having a multiplicity of sensor elements, such as are used e.g. in [1] and [3]: although the cooled input aperture can be set well for the central pixel on the sensor, infrared radiation leading to pixels that do not lie centrally on the sensor, in particular at the edge regions, is vignetted by the detector stop. If only the detector stop is enlarged in order to reduce the vignetting, then stray light, in particular radiation from the uncooled surroundings, which does not contain a measurement signal, also reaches the sensor of the infrared detector and corrupts the measurements. 
     [1] discloses an IR microscope comprising a multi-element detector, in which the detector stop is imaged onto a mirror of the Cassegrain objective of the second optical device. In this case, the mirror of the Cassegrain objective forms the beam-limiting element of the apparatus. On account of the beam limiting by the mirror of the second optical device, however, stray radiation occurs at the edges of the mirror, which adversely influences the quality of the measurement. 
     SUMMARY 
     It is an object of the invention to provide an IR microscope comprising a multi-element IR detector in which all sensor elements (pixels) of the IR detector pick up only modulated infrared radiation that comes from a collimated IR source and illuminates a user-defined region in the sample plane. It is a further object of the invention to provide an IR microscope that avoids disturbing effects such as vignetting and stray radiation. 
     Description of the Invention 
     According to one formulation of the invention, the effectively beam-limiting element is situated in the collimated IR input beam prior to entry into the first optical device. The first optical device and the second optical device image the detector stop opening of the detector stop into an input beam plane, wherein the following holds true for the area A1 of the image of the detector stop opening in the input beam plane and the area A2 of the cross section of the collimated IR input beam in the input beam plane: 
       0 &lt;A 1 /A 2≤1.
 
     An element is “effectively beam-limiting” if within the device no further element exists which limits the cross section of the IR beam to a greater extent than the effectively beam-limiting element. The effectively beam-limiting element defines the stop plane (perpendicular to the optical axis of the IR radiation). The stop plane is thus situated at the location of the beam limiting by the effectively beam-limiting element, e.g. at the location of an effectively beam-limiting stop or an effectively beam-limiting mirror. 
     The light from the IR light source prior to entry into the first optical device is designated as IR input beam. In the region in which the IR input beam is collimated, it preferably has a divergence of less than 2°. 
     The limitation according to the invention of the ratio of detector stop opening and cross section of the IR input beam in the input beam plane prevents light from the external regions (e.g. on account of thermal radiation from the surroundings) from passing into the detector. It is thus ensured that exclusively infrared light which leaves the beam-limiting element and illuminates a defined region in the sample plane reaches the detector and all pixels on the detector are illuminated equally, without vignetting. In this case, the indication A1/A2≤1 should be understood to mean that small deviations therefrom that do not significantly impair the result of the measurement are concomitantly encompassed under this condition. The measurement is significantly impaired for example if the useful signal available for the measurement were to fall to below 70% of the useful signal with the optimum setting. In this case, the measurement time would already have to be at least doubled in order to obtain a comparable signal-to-noise ratio (70%×sqrt(2)=99%) compared with the optimum setting. 
     With the use of conventional IR radiation sources, an IR microscope in which A1/A2=1 holds true is preferably used. This firstly prevents ambient light from passing into the detector, and secondly ensures a maximum luminous efficiency. This is the optimum setting if the detector is not saturated by the radiation source. However, it may also be advantageous to provide a ratio of A1/A2&lt;1, particularly if a radiation source of very high light intensity (e.g. QC laser) is used, since, in the event of an excessively large detector stop opening being used, there is a risk of the detector being operated in saturation. Furthermore, the invention provides for the effectively beam-limiting element and also the input beam plane to be situated in the collimated IR input beam (that is to say prior to entry into the first optical device). The relevant beam cross section to be imaged onto the detector stop is thus not defined by the intermediate optical unit, but rather already upstream of the first optical device. By virtue of the fact that the beam-limiting element is situated in the collimated input beam, that is to say within a beam having only a very low divergence, there is a certain flexibility concerning the position of the input beam plane onto which the detector stop is imaged relative to the effectively beam-limiting element. Furthermore, by virtue of the fact that the effectively beam-limiting element is situated in the collimated input beam, the influence of stray effects occurring at the edges of the effectively beam-limiting element will be minimized. 
     Preferably, the input beam plane is situated in the effectively limited input beam, that is to say in the stop plane or between stop plane and first optical device. 
     If the microscope is intended to be used in transmission, the first optical device comprises a further objective (condenser) for focusing the IR input beam onto the sample position. That is to say that the objective for focusing the IR input beam onto the sample position is not the same objective as the one for imaging the sample position after the irradiation of the sample. 
     For using the IR microscope in reflection, the first optical device comprises the objective configured to focus the IR input beam onto the sample position, and a beam splitter optical unit configured to couple the IR input beam emanating from the IR light source into the objective. In this case, the objective serves both for focusing the IR input beam onto the sample and for generating an intermediate image with the radiation reflected from the sample. In this case, the objective for focusing the IR input beam onto the sample position can be the same objective as the one for imaging the sample position after the irradiation of the sample. 
     In the simplest case, the first optical device can be an infinite-corrected objective. If the objective used is a finite-corrected objective, the first optical device furthermore comprises mirrors that image the collimated input beam onto an intermediate focus, which is then imaged onto the sample by the finite-corrected objective. 
     Both objectives can be mirror objectives, refractive objectives or catadioptric objectives. Preferably, the objectives are embodied as Cassegrain objectives comprising two spherical mirrors. 
     The detector is preferably a 2-dimensional detector having a multiplicity of detector elements (pixels) in a sensor plane. Alternatively, a linear array detector can also be used, in which the detector elements are arranged along a straight line. 
     In one particularly preferred embodiment of the IR microscope according to the invention, the detector stop and the IR sensor are situated within a common detector housing and are arranged at a distance d away from one another, wherein the distance d is preferably a maximum of 50 mm. The imaging of the sample and of the input beam plane is then effected according to the invention such that the distance between the image of the sample that is generated by the intermediate optical unit and the image of the input beam plane that is generated by the intermediate optical unit comprises a distance d. 
     The intermediate optical unit has an effective focal length f for which the following preferably holds true: 
     
       
         
           
             f 
             = 
             
               
                 d 
                 · 
                 x 
               
               
                 
                   x 
                   · 
                   m 
                 
                 - 
                 
                   d 
                   / 
                   m 
                 
               
             
           
         
       
     
     where
         f: effective focal length of the intermediate optical unit   x: distance between the image of the input beam plane that is created by the first optical device and by the objective of the second optical device and the image of the sample that is created by the objective of the second optical device   d: distance between detector stop ( 19   b ) and IR sensor ( 19   a )   m: magnification factor of the intermediate optical unit.       

     The intermediate optical unit can comprise a plurality of imaging optical elements, in particular mirrors, which together have the effective focal length f. 
     The distance d between detector stop and IR sensor is preferably chosen as large as possible, however, the distance d is limited by the space available for the arrangement. Since the detector stop/cold stop is preferably arranged in an evacuated dewar vessel, the distance d is limited by the dimensions of the evacuated dewar vessel. The distance d between detector stop and IR sensor is therefore preferably between 2 mm and 50 mm, in particular between 15 mm and 35 mm. 
     The magnification factor m is dependent on the desired total magnification of the objective of second optical device and the intermediate optical unit, the lateral resolution of the objective of the second optical device and the pixel size of the sensor elements. Preferably, the magnification factor m is selected in the range between 0.1 and 10. 
     The first optical device is configured so that the IR radiation is focused on the sample position. The objective is configured to generate an image of the sample in the intermediate image plane. The first optical device and the objective designed in this way together also produce an image of the input beam plane behind the objective. The image of the input beam plane behind the objective and the image of the sample behind the objective have a distance x to each other. Typically, the distance x is between 1 mm and 250 mm, especially 70 mm. 
     What is achieved by choosing the effective focal length according to the invention is that an intermediate image plane is imaged with a magnification m onto the sensor plane, and the input beam plane is simultaneously imaged onto the detector stop. The magnification m makes it possible to adapt the size of the image of the sample on the detector. The size of the image of the input beam plane at the location of the detector stop is thus influenced as well. The size of the detector stop is then adapted to the size of the image of the input beam plane at the location of the detector stop. 
     Ideally, both the detector and the detector stop are cooled in order to minimize thermal radiation from surfaces onto the detector. The IR sensor and the detector stop are then preferably accommodated in a cooled detector housing, which provides for a great reduction of infrared light which does not come from the interferometer and from the sample. In the case of such joint accommodation of the IR sensor and the detector stop in a detector housing, a corresponding adaptation of the imaging ratios by the intermediate optical unit is necessary since the distance between detector stop and detector housing cannot be chosen to be arbitrarily large. 
     In one specific embodiment of the IR microscope according to the invention, the magnification factor m of the intermediate optical unit is 1. The intermediate optical unit thus brings about a 1:1 imaging ( 2   f  imaging). 
     One preferred embodiment of the IR microscope according to the invention provides for the effectively beam-limiting element to be an output aperture of the IR light source. The input beam plane is then preferably chosen at the output, that is to say in the vicinity of the output aperture of the IR light source, e.g. at the exit pupil of a Michelson interferometer. 
     In another embodiment, the effectively beam-limiting element and the input beam plane are arranged within the IR light source. The effectively beam-limiting element can be e.g. a stop or mirror within an interferometer which serves as IR light source. In particular, the input beam plane can lie between input and output openings of an interferometer. 
     In one preferred embodiment, the input beam plane is the stop plane. The detector stop opening is thus imaged in the stop plane. As a result, a simple adaptation of the detector stop, the intermediate optical unit and the beam-limiting element to one another can be effected since this can be effected independently of any divergence of the collimated input beam that is possibly present. 
     The intermediate optical unit can comprise an Offner objective, for example. An Offner objective comprises two spherical mirrors, which produce an astigmatism-free imaging of the sample and of the input beam plane within the intermediate optical unit. The intermediate optical unit can comprise further spherical mirrors, aspherical mirrors or toroidal mirrors in addition to the Offner objective. In particular, the image generated by the Offner objective can be imaged onto the detector or the detector stop by an aspherical mirror or toroidal mirror; or the image generated by an aspherical mirror or toroidal mirror can be imaged onto the detector or the detector stop by the Offner objective. A combination with refractive optical elements (lenses) is likewise conceivable. 
     The IR light source of the IR microscope according to the invention can comprise an interferometer, a quantum cascade laser (in this case the IR radiation source is identical to the IR light source) or a Fourier-transform infrared (FTIR) spectrometer. 
     Further advantages of the invention are evident from the description and the drawing. Likewise, the features mentioned above and the features that will be explained still further can be used according to the invention in each case individually by themselves or as a plurality in any desired combinations. The embodiments shown and described should not be understood as an exhaustive enumeration, but rather are of exemplary character for outlining the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE INVENTION AND DRAWING 
         FIG. 1A  shows a microscope which can be operated both in transmission and in reflection, comprising an interferometer as IR light source and the output aperture of the interferometer as effectively beam-limiting element. 
         FIG. 1B  shows a microscope which can be operated both in transmission and in reflection, comprising an interferometer as IR light source, wherein the effectively beam-limiting element is the collimating element. 
         FIG. 2A  shows the adaptation of the detector stop size according to the prior art under the assumption that the intermediate optical unit is beam-limiting. The setting is optimized only for the central pixel. 
         FIG. 2B  shows the vignetting of the edge pixels that occurs if the detector stop is optimized only for the central pixel and the stop plane image  25  does not coincide with the detector stop  19   b.    
         FIG. 2C  shows the reduction of the vignetting of the edge pixels such as was illustrated in  FIG. 2B , by the enlargement of the detector stop  19   b  with increased ambient light being accepted. 
         FIG. 2D  shows, for the arrangement from  FIG. 2C , the radiation cone of the IR radiation from the radiation source and a radiation cone of the disturbing radiation from the surroundings. 
         FIG. 2E  shows beam paths of the IR radiation between the intermediate optical unit and a central and respectively an outer sensor element of the IR sensor of the IR microscope, wherein the beam-limiting element is imaged at the location of the detector stop opening and A1=A2. 
         FIG. 3A  shows the dependence of the total signal detected by the IR detector, of the useful signal and of the surroundings signal on the ratio A1/A2. 
         FIG. 3B  shows the ratio of useful signal to total signal multiplied by the useful signal as a function of the ratio A1/A2. 
         FIG. 4  shows beam paths of the IR radiation between the second optical device and a central and respectively an outer sensor element of the IR sensor of the IR microscope. 
         FIG. 5  shows the IR microscope from  FIG. 1  with a detailed illustration of the intermediate optical unit. 
     
    
    
     DETAILED DESCRIPTION 
     A typical construction of an IR microscope which can be used both in transmission and in reflection is illustrated in  FIG. 1A . 
     In order to generate the visual image of the sample in reflection, the light from a visual light source  20   b  is guided via various optical elements  21   b,    12   b  and  13   b  into a microscope objective  16 , which focuses the light onto a sample position  15 . The reflected light from the sample is then captured again by the objective  16  and the sample (or a selected region of the sample) is imaged onto a first intermediate image plane  17 . Said intermediate image plane  17  is then in turn imaged onto a CCD camera  24  via a dichroic mirror  22  and an imaging optical unit  23 . 
     In transmission, the visual image of the sample is generated by the visible light from a light source  20   a  being guided via the optical elements  21   a,    12   a  and  13   a  into a condenser (further objective  14 ), which focuses the light onto the sample position  15 . The transmitted light through the sample is then imaged onto the first intermediate image plane  17  by the objective  16 . Said intermediate image plane is then in turn imaged onto the CCD camera  24  via a dichroic mirror  22  and the imaging optical unit  23 . 
     For the spectral examination of the sample with infrared radiation, the IR light from an IR radiation source  2  is modulated by an interferometer  1 . The light from the radiation source  2  leaves the radiation source  2 , is collimated via a mirror  4  and is guided into the interferometer  1 . Here the light impinges on a beam splitter  7  and ideally 50% of said light is transmitted and 50% is reflected. The transmitted infrared light then impinges on a fixed mirror  6  and is reflected back again from the latter in the direction of the beam splitter  7 . The light previously reflected at the beam splitter  7  impinges on a movable mirror  5  and is likewise reflected back again to the beam splitter  7 . Both partial beams are recombined at the beam splitter  7  and leave the interferometer  1  through an output aperture  3  of the interferometer  1 . The movement of the movable mirror  5  ensures that the infrared light is modulated. Besides the traditional Michelson interferometer shown in  FIG. 1A , other types of interferometers can also be used as IR light source  1 , for example interferometers comprising two movable mirrors having the form of retroflectors, as known from [6], for example. The modulated infrared light leaves the interferometer  1  through the exit aperture  3  thereof and is focused onto a sample in the sample position  15  with a first optical device and thus illuminates a region of the sample. 
     The IR beam passing between the radiation source  2  and the first optical device is referred to as “input beam”. In the present example, the input beam is collimated by the mirror  4  and passes as collimated input beam  26  in the interferometer  1  and also after emerging from the interferometer  1 . 
     The IR microscope shown in  FIG. 1A  can be used both in transmission and in reflection. The beam path for infrared light can be switched between transmission and reflection for this purpose. 
     In the reflection mode (IR-R), the first optical device comprises the objective  16  (here: Cassegrain objective having two spherical mirrors  16   a  and  16   b ) and also mirrors  9 ,  10   b,    11   b ,  12   b  and  13   b.  The mirrors  9 ,  10   b,    11   b,    12   b  and  13   b  guide the infrared light modulated by the interferometer  1  into the objective  16 , which focuses the infrared light onto the sample position  15  and illuminates a region of the sample. 
     In the transmission mode (IR-T), the first optical device comprises the further objective  14  and also mirrors  9 ,  10   a,    11   a,    12   a  and  13   a.  The mirrors  9 ,  10   a,    11   a,    12   a  and  13   a  guide the light emerging from the interferometer into the further objective  14 , which focuses the infrared light onto the sample position  15  and illuminates a region of the sample. The further objective  14  can likewise be embodied as a Cassegrain objective having two spherical mirrors  14   a  and  14   b.    
     It is also possible to realize the invention in IR microscopes which are provided either only for transmission measurements (in this case, the components designated with b are absent) or only for reflection measurements (in this case, the components designated with a are absent). 
     Light (reflected or transmitted) emanating from the sample is imaged onto an infrared detector  19  using a second optical device. For this purpose, the second optical device comprises the objective  16  and also an intermediate optical unit  18 . The objective  16  images the light emanating from the sample onto the first intermediate image plane  17 . A field stop can be introduced in the intermediate image plane  17 , said field stop transmitting only light from a selected region of the sample position  15  and thus making it possible to select a region on the sample which is intended to be examined. 
     The intermediate optical unit  18  then images the first intermediate image plane  17  onto the infrared detector  19 . The infrared detector  19  is preferably a two-dimensional detector comprising an IR sensor  19   a  and having a multiplicity of detector elements (pixels) in a sensor plane of the IR sensor  19   a.  Alternatively, a linear array detector can also be used, in which the sensor elements are arranged along a straight line. The IR sensor  19   a  is ideally cooled in order to minimize thermal excitations in the detector elements. In the example shown, the IR sensor  19   a  is incorporated into a detector housing  19   c,  which is likewise cooled. The light to be analyzed is incident in the detector housing  19   c  through a cooled detector stop  19   b  of the detector  19  and generates an image of the sample position  15  on the sensor  19   a.    
     The size and position of the detector stop  19   b  determine the regions from which radiation can reach the sensor  19   a  of the detector  19 ; they thus determine the field of view of the detector  19 . 
     The IR beam is restricted by an effectively limiting element  8 . This can involve optical elements (e.g. of the interferometer). According to the invention, the effectively beam-limiting element  8  is situated in the collimated input beam  26  and defines a stop plane  27 . The opening/aperture (in the case of a stop) or the effective aperture (in the case of a curved mirror) of the beam-limiting element  8  determines the cross section of the input beam in the stop plane  27 . In  FIG. 1A , the output aperture  3  forms the effectively beam-limiting element  8 . 
       FIG. 1B  shows another embodiment of an IR microscope according to the invention, in which the collimating mirror  4 , which defines the beginning of the collimated input beam and is thus situated in the collimated input beam, is the effectively beam-limiting element  8 ; that is to say that, in the case shown in  FIG. 1B , beam limiting no longer takes place at the output aperture  3  of the interferometer  1 . 
     Furthermore, the effectively beam-limiting element  8  can e.g. also be defined by the fixed mirror  7 , the movable mirror  5  or the beam splitter  7  or it can also be situated outside the interferometer  1  between interferometer and first optical device (not illustrated). 
     The quality of the signal detected by the IR detector depends on the illumination of the IR sensor with the IR light emerging from the interferometer and on the ambient light impinging on the sensor. According to the invention, the detector stop  19   b  is imaged onto an input beam plane  29  in the collimated input beam  26 . The relationship between the image quality, the ratio A1/A2 and also the positions of the imaging of the detector stop is described below: 
       FIG. 2A  shows the infrared detector  19  and the intermediate optical unit  18 . By way of the intermediate optical unit  18 , an image of the sample is generated (by imaging of the sample plane or of downstream intermediate image planes) on the IR sensor  19   a.  In the prior art, the size and position of the detector stop  19   b  are dimensioned such that only the radiation cone coming from the intermediate optical unit  18  upstream of the detector  19  is transmitted through the detector stop  19   b.    
       FIG. 2B  shows beam paths of the IR radiation between the intermediate optical unit  18  and the central and respectively an outer sensor element of the IR sensor  19   b  of the IR microscope for the case where the image of the cross section of the input beam in the stop plane (stop plane image  25 ) does not correspond to the size and position of the detector stop  19   b.  In the case of  FIG. 2B , this has the consequence that radiation is vignetted on an edge pixel by the detector stop  19   b  since the detector stop  19   b  is too small to capture for all pixels the entire IR radiation emanating from the stop plane image  25 . All radiation emanating from the stop plane image  25  also originates from the collimated IR source. 
       FIG. 2C  shows the same case as in  FIG. 2B , except that the detector stop  19   b  has been enlarged in such a way as to preclude vignetting of the IR radiation that emanates from the stop plane image  25  and reaches the edge pixels. However, the enlargement of the detector stop  19   b  also has disadvantages since now IR radiation that does not originate from the stop plane image  25  and thus from the collimated IR source can also impinge on the pixels. Unusable IR radiation from the surroundings can now also be incident on the detector. 
       FIG. 2D  illustrates this substantive matter. Only the IR radiation  30  of the inner white cone passes completely through the stop plane image  25  and thus comes from the collimated IR source. The radiation in the hatched regions  31  does not come from the collimated IR source but nevertheless reaches the sensor  19   a.    
       FIG. 2C  and  FIG. 2D  show that there is no optimum size for the detector stop  19   b  as long as the stop plane image  25  does not coincide with the detector stop  19   b.    
       FIG. 2E  shows the case in which the stop plane image  25  coincides with the detector stop  19   b.  In this case, exclusively light that originates from the collimated IR source and thus passes through the stop plane image  25  reaches the detector. At the same time, the modulated IR light from the collimated IR source is not vignetted for any pixel on the sensor  13   a.    
     For optimum illumination without vignetting and simultaneous suppression of ambient light, therefore, the stop plane image must correspond to the detector stop. Argued conversely, the intermediate optical unit and the size of the detector stop must be adapted such that the image of the detector stop corresponds to the cross section of the input beam in the stop plane. 
     The dependence of the total signal detected by the IR detector, of the useful signal and of the surroundings signal on the ratio of the area A1 of the image of the detector stop opening in the input beam plane to the area A2 of the cross section of the collimated IR input beam in the input beam plane is illustrated graphically in  FIG. 3A . In the case of an optimum setting, care must be taken to ensure that the detector does not become saturated. Ideally, therefore, the ratio of useful signal to total signal is as high as possible. In the range of 0≤A1/A2≤1, however, the ratio of useful signal to total signal is equal to 1 in a constant way. However, it is advantageous at the same time if the absolute useful signal is high.  FIG. 3B  therefore shows the ratio of useful signal to total signal multiplied by the useful signal as a function of A1/A2. It is clearly evident that the ratio of useful signal to total signal multiplied by the useful signal is maximal for the ratio A1/A2=1, wherein the fall in the case of smaller ratios A1/A2&lt;1 is attributable to the shading on account of a detector stop smaller than the beam cross section to be imaged, and the fall in the case of larger ratios A1/A2&gt;1 is attributable to the contribution of stray light from the surroundings (surroundings signal). 
     According to the present invention, the detector stop  19   b  is imaged by the first and second optical devices onto an input beam plane situated in the collimated input beam, specifically such that the area A1 of the image of the detector stop  19   b  in the input beam plane is maximally equal in magnitude to the area of the cross section of the collimated input beam in the input beam plane. This prevents ambient light from passing into the detector  19 . According to the invention, therefore, the opening of the detector stop  19   b  is coordinated with the beam cross section of the collimated input beam in the input plane. In the ideal case, the detector stop  19   b  is imaged onto the stop plane (that is to say onto the effectively beam-limiting element  8 ). Stop plane and input beam plane thus coincide in this case. Conversely, this means that the opening of the beam-limiting element  8  and thus the beam cross section of the collimated input beam in the stop plane are imaged onto the detector stop  19   b  (stop plane image  25  is then situated in the plane of the detector stop  19   b ), as shown in  FIG. 2E . The detector stop  19   b,  the second optical device  16 ,  18  and the effectively beam-limiting element  8  can then be coordinated with one another independently of the divergence of the input beam, such that the desired cross section of the input beam passes into the detector  19 . In combination with the condition that the area A1 of the image of the opening of the detector stops  19   b  in the input beam plane is less than or equal to the area A2 of the cross section of the collimated IR input beam  26  in the input beam plane, it is ensured that exclusively the IR light of the collimated input beam  26  passes through the detector stop  19   b.  In the example shown in  FIG. 2E , A1=A2, such that all light of the IR beam of the input beam plane reaches the IR sensor  19   a.    
     According to the invention, input beam plane  29  and stop plane  27  need not necessarily correspond as long as both input beam plane  29  and stop plane  27  are situated in the collimated input beam  26  and the condition A1≤A2 is satisfied. Since the cross section of the input beam  26  in the collimated region does not change or changes only slightly, in the case of the device according to the invention the beam cross section imaged onto the detector stop  19   b  is always one which is equal or almost equal to the beam cross section in the stop plane  27 , such that to a first approximation the beam cross section prevailing at the location of the beam-limiting element can be assumed as beam cross section in the input beam plane  27  and can be used for determining the size of the detector stop  19   b.  The positioning of the beam-limiting element  8  can therefore be chosen relatively freely within the collimated input beam  26 , without significantly influencing the quality of the measurement. This also simplifies the design of the intermediate optical unit  18 . At the same time, however, there is also the possibility of realizing a desired shading by choosing the ratio of opening of the detector stop  19   b  and beam limiting, e.g. if a light source  2  of very high light intensity is used (e.g. a quantum cascade laser), which would saturate the IR sensor  19   a  without shading and thereby render the measurements unusable. 
     The device according to the invention therefore offers a high flexibility in the type and positioning of the beam limiting with maximum luminous efficiency and minimization of disturbing influences. 
     For the design of the intermediate optical unit  18 , the effective focal length f of the intermediate optical unit  18  is relevant, in particular. Said effective focal length is dependent on the distance d between detector stop  19   b  and IR sensor  19   a  and the distance x between the image  28  of the input beam plane (which is created by the first optical device and by the objective  16  of the second optical device) and the image of the sample in the intermediate image plane  17  which is created by the objective  16  of the second optical device ( FIG. 4 ): 
     
       
         
           
             
               f 
               = 
               
                 
                   d 
                   · 
                   x 
                 
                 
                   
                     x 
                     · 
                     m 
                   
                   - 
                   
                     d 
                     / 
                     m 
                   
                 
               
             
             , 
           
         
       
     
     such that, firstly, the sample arranged in the sample position  15  is imaged onto the IR sensor  19   a  and, secondly, the input beam plane situated in the collimated input beam is imaged onto the detector stop  19   b.    
       FIG. 5  shows how the intermediate optical unit  18  can be embodied for example. The intermediate optical unit  18  here comprises a 1× objective according to Offner [5], which produces an astigmatism-free imaging  18   d  of the intermediate image  17  by way of two spherical mirrors  18   a  and  18   b.  A plane mirror  18   c  serves merely for beam deflection. The 1× objective according to Offner serves to generate an image of the sample at a location in the microscope in the vicinity of which there is enough space for the cooled area detector  19 . The IR detector  19  is usually cooled with liquid nitrogen by way of a Dewar vessel. Alternative systems have, for example, Stirling coolers for producing the low temperatures required at the sensor element. All these detectors need a comparatively large amount of space. In addition, in the case of the Dewar vessel, the filling opening for the liquid nitrogen has to be accessible from the outside. Besides the image of the sample  18   d,  the 1× objective according to Offner also generates an image  18   g  of the input beam plane  29 . A downstream optical unit comprising plane mirror  18   e  for beam deflection and mirror  18   f  is configured such that an image of the sample image  18   d  is generated on the sensor  19   a  of the detector  19  and at the same time an image of the input beam plane image  18   g  is generated on the cold stop  19   b  of the detector  19 . For this purpose, the focal length f of the mirror  18   f  is firstly determined by way of the above-described equation 
     
       
         
           
             
               f 
               = 
               
                 
                   d 
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                   x 
                 
                 
                   
                     x 
                     · 
                     m 
                   
                   - 
                   
                     d 
                     / 
                     m 
                   
                 
               
             
             . 
           
         
       
     
     Since the mirror  18   f  effects an off-axis imaging, firstly it is advantageous to minimize the deflection angle; secondly, the use of a toroidal mirror having focal lengths f and f′ instead of a spherical mirror having the focal length f determined is advantageous for the image quality. f′ can be determined from f and the deflection angle. Said toroidal mirror can subsequently still be altered easily in terms of its shape. Optimum surface shapes that result in an optimized image quality on the sensor of the detector can be determined here with ray tracing programs (e.g. Zemax). The surface of the mirror  18   f  then deviates slightly from the ideal toroidal shape; a toroid-like mirror arises. The light coming from the mirror  18   f  passes through a window  19   d  of the detector, generates an image of the input beam plane  29  at the detector stop  19   b  and an image of the sample on the sensor  19   a  of the detector  19 . 
     LITERATURE LIST 
     [1] U.S. Pat. No. 7,440,095 
     [2] DE 10 2012 200 851 B3 
     [3] U.S. Pat. No. 7,378,657 B2 
     [4] Bruker Optik GmbH “HYPERION Series: FTIR Microscopes” 
     [5] DE 2 230 002 C2 
     [6] DE 19 704 598 C1 
     LIST OF REFERENCE SIGNS 
       1  IR light source/interferometer 
       2  Radiation source 
       3  Output aperture of the interferometer  1   
       4  Mirror of the interferometer  1   
       5  movable mirror of the interferometer  1   
       6  fixed mirror of the interferometer  1   
       7  Beam splitter of the interferometer  1   
       8  effectively beam-limiting element 
       9  Mirror 
       10   a  Mirror for measurement in transmission 
       10   b  Mirror for measurement in reflection 
       11   a  Mirror for measurement in transmission 
       11   b  Mirror for measurement in reflection 
       12   a  optical element for measurement in transmission 
       12   b  optical element for measurement in reflection 
       13   a  optical element for measurement in transmission 
       13   b  Beam splitter optical unit 
       14  Condenser/further objective 
       15  Sample position 
       16  Objective 
       17  first intermediate image plane 
       18  Intermediate optical unit 
       18   a,    18   b  Offner objective/spherical mirrors 
       18   c  Plane mirror 
       18   d  Image of the sample in intermediate optical unit  18   
       18   e  Plane mirror 
       18   f  Mirror 
       18   g  Image of the sample in input beam plane in intermediate optical unit  18   
       19  IR detector 
       19   a  IR sensor 
       19   b  Detector stop 
       19   c  Detector housing 
       20  visual light source for measurement in transmission 
       20   b  visual light source for measurement in reflection 
       21   a  optical element for measurement in transmission 
       21   b  optical element for measurement in reflection 
       22  dichroic mirror 
       23  Imaging optical unit 
       24  CCD camera 
       25  Image of the stop plane 
       26  collimated input beam 
       27  Stop plane 
       28  Image of the input beam plane upstream of intermediate optical unit  18   
       29  Input beam plane