IR Microscope

Abstract

IR microscope with an IR light source (1) for generating a collimated IR input beam (26), an effective beam-limiting element (8) in an aperture plane (27), a sample position (15) for receiving a sample, an IR detector (19) with an IR sensor (19a), a detector aperture (19b) arranged in front of the IR sensor (19a) with a detector aperture opening, a first optical device for focusing the collimated IR input beam (26) emerging from the IR light source (1) onto the sample position (15), and a second optical device for imaging the sample position (15) onto the IR sensor (19a), wherein the second optical device comprises a lens (16) and an intermediate optic (18). characterized by that the effective beam-limiting element (8) is located in the collimated IR input beam (26) before entering the first optical device, and that the first optical device and the second optical device image the detector aperture of the detector aperture (19b) into an input beam plane, wherein the area A1 of the image of the detector aperture in the input beam plane and the area A2 of the cross-section of the collimated IR input beam (26) in the input beam plane are given by: 0 < A 1 / A 2 ≤ 1.

Description

Background of the invention

[0001] The invention relates to an IR microscope with an IR light source for generating a collimated IR input beam, an effectively beam-limiting element in a diaphragm plane in the collimated input beam, a sample position for receiving a sample, an IR detector with an IR sensor, a detector diaphragm arranged in front of the IR sensor with a detector diaphragm opening, a first optical device for focusing the collimated IR input beam emerging from the IR light source onto the sample position, and a second optical device for imaging the sample position onto the IR sensor, wherein the second optical device comprises an objective and an intermediate optic.

[0002] Such IR microscopes are known, for example, from [1], [2], [3], [4].

[0003] Infrared (IR) microscopes can produce IR and visual images of a sample under investigation, with measurements typically performed in both transmission and reflection. For this purpose, the beam paths for infrared and visible light can be switched between transmission and reflection.

[0004] The IR microscopes considered here make it possible, in particular, to carry out spatially resolved spectral measurements of microscopic samples, ie a spectrum in the infrared spectral range is obtained for different areas on the microscopic sample.

[0005] The IR light passes through an interferometer, where the relative length of the interferometer arms is varied by moving one or more mirrors, thereby modulating the infrared light depending on its wavelength. The modulation frequency depends on the wavelength, with each wavelength being uniquely assigned to a modulation frequency. The thus modulated infrared light is directed onto a sample to be examined, and the light emitted from the sample is focused onto an infrared detector via a second optical device.

[0006] The modulated light received by the detector contains all the spectral information of the sample. However, the detector is also sensitive to ambient IR light that 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.

[0007] Added to this is another component of unwanted detector signal, the dark current. The detector is therefore generally cooled to minimize noise caused by thermal excitations in the sensor material. The detector should, if possible, only deliver signals caused by the modulated IR radiation emanating from the sample.

[0008] In order to suppress radiation from the environment as much as possible, the detector contains a detector aperture.

[0009] In the current state of the art, the size and position of the detector aperture is always determined by the last imaging optics upstream of the detector. For example, in [3], the detector aperture is dimensioned so that only radiation from the upstream intermediate optics can pass through the detector aperture.

[0010] When using a cooled IR sensor surrounded by a cooled housing (radiation shield) so that light only reaches the sensor through the likewise cooled detector aperture (cold aperture), the position and size of the detector aperture cannot be arbitrarily dimensioned because it has to be installed inside a Dewar. The position and size of the detector aperture cannot generally be optimally adapted to the upstream intermediate optics, but the upstream intermediate optics must be designed so that the radiation required for the measurement passes through the detector aperture, but unwanted radiation (e.g. ambient radiation) cannot pass through the detector aperture. The intermediate optics directs the IR radiation in a cone with half the aperture angle α / 2 towards the detector and creates an image of the sample plane or downstream intermediate image planes of the sample plane on the IR sensor.The ratio of the diameter of the entrance aperture to the distance d between the detector aperture and the IR sensor corresponds to the F-number (F / #) of the intermediate optics. This approach is certainly suitable for single-element detectors, as used, for example, in [4]. However, for extended 1- or 2-dimensional detectors with a large number of sensor elements, as used, for example, in [1] and [3], this approach is not sufficient: Although the cooled entrance aperture can be well adjusted for the central image point on the sensor, infrared radiation that leads to image points that are not centrally located on the sensor, especially at the edge areas, is vignetted by the detector aperture. If only the detector aperture is enlarged to reduce vignetting, stray light, especially radiation from the uncooled environment, which does not contain a measurement signal, also reaches the sensor of the infrared detector and falsifies the measurements.

[0011] [1] discloses an IR microscope with a multi-element detector in which the detector aperture is imaged onto a mirror of the Cassegrain objective of the second optical device. The mirror of the Cassegrain objective forms the beam-limiting element of the apparatus. However, due to the beam limitation by the mirror of the second optical device, scattered radiation occurs at the edges of the mirror, which negatively affects the quality of the measurement. Object of the invention

[0012] The object of the invention is to propose an IR microscope with a multi-element IR detector in which all sensor elements (pixels) of the IR detector only capture modulated infrared radiation coming from a collimated IR source and illuminating a user-defined area in the sample plane. At the same time, disruptive effects such as vignetting and scattered radiation are avoided. Description of the invention

[0013] This object is achieved according to the invention by an IR microscope according to claim 1.

[0014] According to the invention, the effective beam-limiting element is located in the collimated IR input beam before entering the first optical device. According to the invention, the first and second optical devices image the detector aperture of the detector aperture into an input beam plane, wherein the following applies to the area A1 of the image of the detector aperture 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 < A1 / A2 ≤ 1.

[0015] An element is "effectively beam-limiting" if there is no other element within the device that limits the cross-section of the IR beam more than the effectively beam-limiting element. The effectively beam-limiting element defines the aperture plane (perpendicular to the optical axis of the IR radiation). The aperture plane is therefore located at the point of beam limitation by the effectively beam-limiting element, e.g., at the location of an effectively beam-limiting aperture or an effectively beam-limiting mirror.

[0016] The IR input beam is the light from the IR light source before entering the first optical device. In the region where the IR input beam is collimated, it preferably has a divergence of less than 2°.

[0017] By limiting the ratio of detector aperture and cross-section of the IR input beam in the input beam plane according to the invention, light from the outer areas (e.g. due to thermal radiation from the environment) is prevented from entering the detector. This ensures that only infrared light that leaves the beam-limiting element and illuminates a defined area in the sample plane reaches the detector and that all pixels on the detector are illuminated equally, without vignetting. The specification A1 / A2 ≤ 1 should be understood to mean that small deviations from this that do not significantly affect the measurement result are to be understood as included under this condition. The measurement is significantly impaired, for example, if the useful signal available for the measurement falls below 70% of the useful signal with the optimal setting.In this case, the measurement time would have to be at least doubled to achieve a comparable signal-to-noise ratio (70% x sqrt(2) = 99%) as with the optimal setting.

[0018] When using conventional IR radiation sources, an IR microscope is preferably used in which A1 / A2 = 1. This prevents ambient light from entering the detector and ensures maximum light yield. This is the optimal setting if the detector is not saturated by the radiation source. However, it can also be advantageous to provide a ratio of A1 / A2 < 1, particularly when using a very intense radiation source (e.g. QC laser), since using an excessively large detector aperture risks operating the detector in saturation. Furthermore, the invention provides that the effectively beam-limiting element and the input beam plane are located in the collimated IR input beam (i.e. before entering the first optical device).The relevant beam cross-section to be imaged onto the detector aperture is therefore not defined by the intermediate optics, but rather before the first optical device. Because the beam-limiting element is located within the collimated input beam, i.e., within a beam with only a very low divergence, there is a certain degree of flexibility regarding the position of the input beam plane onto which the detector aperture is imaged relative to the effective beam-limiting element. Furthermore, because the effective beam-limiting element is located within the collimated input beam, the influence of scattering effects occurring at the edges of the effective beam-limiting element is minimized.

[0019] Preferably, the input beam plane is located in the effectively limited input beam, i.e. in the aperture plane or between the aperture plane and the first optical device.

[0020] If the microscope is to be used in transmission mode, the first optical device includes an additional lens (condenser) for focusing the IR input beam onto the sample position. This means that the lens for focusing the IR input beam onto the sample position is not the same lens as the one used to image the sample position after irradiation.

[0021] For use of the IR microscope in reflection mode, the first optical device comprises the objective lens, which is configured to focus the IR input beam onto the sample position, and a beam splitter optics, which is configured to couple the IR input beam emanating from the IR light source into the objective lens. The objective lens serves both to focus the IR input beam onto the sample and to generate an intermediate image using the radiation reflected from the sample. In this case, the objective lens for focusing the IR input beam onto the sample position can be the same objective lens as the one used to image the sample position after irradiation of the sample.

[0022] In the simplest case, the first optical device can be an infinity-corrected objective. If a finite-corrected objective is used, the first optical device also includes mirrors that image the collimated input beam onto an intermediate focus, which is then imaged onto the sample by the finite-corrected objective.

[0023] Both lenses can be mirror lenses, refractive lenses, or catadioptic lenses. The lenses are preferably designed as Cassegrain lenses with two spherical mirrors.

[0024] The detector is preferably a 2-dimensional detector with a plurality of detector elements (pixels) in a sensor plane. Alternatively, a line detector can be used, in which the detector elements are arranged along a straight line.

[0025] In a particularly preferred embodiment of the IR microscope according to the invention, the detector aperture and the IR sensor are located within a common detector housing and are arranged at a distance d from each other, wherein the distance d is preferably a maximum of 50 mm. The imaging of the sample and the input beam plane is then carried out according to the invention such that the distance between the image of the sample generated by the intermediate optics and the image of the input beam plane generated by the intermediate optics is a distance d.

[0026] The intermediate optics has an effective focal length f for which preferably applies: f=d⋅xx⋅m−d / m with f: effective focal length of the intermediate optics x: Distance between the image of the input beam plane produced by the first optical device and by the objective of the second optical device and the image of the sample produced by the objective of the second optical device. d: Distance between detector aperture (19b) and IR sensor (19a) m: Magnification factor of the intermediate optics

[0027] The intermediate optics may comprise several imaging optical elements, in particular mirrors, which together have the effective focal length f.

[0028] The inventive selection of the effective focal length ensures that an intermediate image plane is imaged onto the sensor plane with a magnification m, and simultaneously, the input beam plane is imaged onto the detector aperture. The magnification m can be used to adjust the size of the sample image on the detector. This also influences the size of the image of the input beam plane at the location of the detector aperture. The size of the detector aperture is then adjusted to the size of the image of the input beam plane at the location of the detector aperture.

[0029] Ideally, both the detector and the detector aperture are cooled to minimize thermal radiation from surfaces onto the detector. The IR sensor and detector aperture are then preferably housed in a cooled detector housing, which ensures a significant reduction of infrared light that does not originate from the interferometer and the sample. When the IR sensor and detector aperture are housed together in a detector housing, a corresponding adjustment of the imaging ratios is necessary using the intermediate optics, since the distance between the detector aperture and the detector housing cannot be chosen arbitrarily.

[0030] In a special embodiment of the IR microscope according to the invention, the magnification factor m of the intermediate optics is 1. The intermediate optics therefore produces a 1:1 image (2f image).

[0031] A particularly preferred embodiment of the IR microscope according to the invention provides that the effective beam-limiting element is an exit aperture of the IR light source. The entrance beam plane is then preferably selected at the exit, i.e., in the vicinity of the exit aperture of the IR light source, e.g., at the exit pupil of a Michelson interferometer.

[0032] In another embodiment, the effective beam-limiting element and the input beam plane are arranged within the IR light source. The effective beam-limiting element can, for example, be a diaphragm or a mirror within an interferometer serving as an IR light source. In particular, the input beam plane can be located between the input and output apertures of an interferometer.

[0033] In a particularly preferred embodiment, the input beam plane is the aperture plane. The detector aperture is thus imaged in the aperture plane. This allows for easy alignment of the detector aperture, the intermediate optics, and the beam-limiting element, since this can be done independently of any divergence of the collimated input beam.

[0034] Intermediate optics can, for example, include an Offner lens. An Offner lens comprises two spherical mirrors that produce an astigmation-free image of the sample and the input beam plane within the intermediate optics. In addition to the Offner lens, the intermediate optics can comprise further spherical mirrors, aspherical mirrors, or toroidal mirrors. In particular, the image generated by the Offner lens can be imaged onto the detector or detector aperture 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 detector aperture by the Offner lens. A combination with refractive optical elements (lenses) is also conceivable.

[0035] 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 equal to the IR light source) or an FTIR spectrometer.

[0036] Further advantages of the invention will become apparent from the description and the drawings. Likewise, the above-mentioned and further-described features can be used individually or in combination in any desired manner. The embodiments shown and described are not intended to be exhaustive, but rather are exemplary in nature for describing the invention. Detailed description of the invention and drawing Fig. Figure 1a shows a microscope that can be operated in both transmission and reflection, with an interferometer as the IR light source and the output aperture of the interferometer as the effective beam-limiting element. Fig. Figure 1b shows a microscope that can be operated in both transmission and reflection, with an interferometer as the IR light source, where the effective beam-limiting element is the collimating element. Fig. Figure 2a shows the state-of-the-art detector aperture size adjustment, assuming the intermediate optics are beam-limiting. The setting is optimized only for the central pixel. Fig. Figure 2b shows the vignetting of the edge pixels that occurs when the detector aperture is optimized only for the central pixel and the aperture plane image 25 does not coincide with the detector aperture 19b. Fig. Figure 2c shows the reduction of vignetting of the edge pixels as seen in Fig. 2b, by enlarging the detector aperture 19b at the expense of increased ambient light. Fig. 2d shows for the arrangement of Fig. 2c the radiation cone of the IR radiation from the radiation source and a radiation cone of the interference radiation from the environment. Fig. Figure 2e shows beam paths of the IR radiation between the intermediate optics and a central or an outer sensor element of the IR sensor of the IR microscope, whereby the beam limiting element is imaged at the location of the detector aperture and A1=A2. Fig. Figure 3a shows the dependence of the total signal detected by the IR detector, the useful signal and the ambient signal on the ratio A1 / A2. Fig. Figure 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 or an outer sensor element of the IR sensor of the IR microscope. Fig. 5 shows the IR microscope from Fig. 1 with a detailed representation of the intermediate optics.

[0037] A typical setup of an IR microscope, which can be used in both transmission and reflection, is shown in Fig. 1a.

[0038] To generate the visual image of the sample in reflection, the light from a visual light source 20b is directed via various optical elements 21b, 12b, and 13b into a microscope objective 16, which focuses the light onto a sample position 15. The reflected light from the sample is then collected again by the objective 16, and the sample (or a selected area of ​​the sample) is imaged onto a first intermediate image plane 17. This intermediate image plane 17 is then imaged onto a CCD camera 24 via a dichroic mirror 22 and imaging optics 23.

[0039] In transmission, the visual image of the sample is generated by directing the visible light from a light source 20a through the optical elements 21a, 12a, and 13a into a condenser (another objective lens 14), which focuses the light onto the sample position 15. The transmitted light through the sample is then imaged by the objective lens 16 onto the first intermediate image plane 17. This intermediate image plane is then imaged onto the CCD camera 24 via a dichroic mirror 22 and the imaging optics 23.

[0040] 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 by a mirror 4, and guided into the interferometer 1. Here, the light strikes a beam splitter 7 and is ideally transmitted at a ratio of 50% to 50%. The transmitted infrared light then strikes a fixed mirror 6 and is reflected back towards the beam splitter 7. The light previously reflected by the beam splitter 7 strikes a movable mirror 5 and is also reflected back to the beam splitter 7. Both partial beams are recombined at the beam splitter 7 and exit the interferometer 1 through an exit aperture 3 of the interferometer 1. The movement of the movable mirror 5 ensures that the infrared light is modulated. In addition to the Fig. In addition to the classic Michelson interferometer shown in Figure 1a, other types of interferometers can also be used as the IR light source 1, for example, interferometers with two movable mirrors in the form of retroreflectors, as known, for example, from [6]. The modulated infrared light exits the interferometer 1 through its exit aperture 3 and is focused onto a sample in the sample position 15 by a first optical device, thus illuminating a region of the sample.

[0041] The IR beam passing between the radiation source 2 and the first optical device is referred to as the "input beam." In this example, the input beam is collimated by the mirror 4 and passes through the interferometer 1 and exits the interferometer 1 as the collimated input beam 26.

[0042] The Fig. The IR microscope shown in Figure 1a can be used in both transmission and reflection modes. The infrared light beam path can be switched between transmission and reflection.

[0043] In reflection mode (IR-R), the first optical device comprises objective 16 (here: a Cassegrain objective with two spherical mirrors 16a and 16b) and mirrors 9, 10b, 11b, 12b, and 13b. Mirrors 9, 10b, 11b, 12b, and 13b guide the infrared light modulated by interferometer 1 into objective 16, which focuses the infrared light onto sample position 15 and illuminates an area of ​​the sample.

[0044] In transmission mode (IR-T), the first optical device comprises the additional objective 14 and mirrors 9, 10a, 11a, 12a, and 13a. Mirrors 9, 10a, 11a, 12a, and 13a guide the light emerging from the interferometer into the additional objective 14, which focuses the infrared light onto the sample position 15 and illuminates an area of ​​the sample. The additional objective 14 can also be designed as a Cassegrain objective with two spherical mirrors 14a and 14b.

[0045] It is also possible to implement the invention in IR microscopes designed either for transmission measurements only (in this case the components labeled b are missing) or for reflection measurements only (in this case the components labeled a are missing).

[0046] Light emanating from the sample (reflected or transmitted) is imaged onto an infrared detector 19 by a second optical device. The second optical device comprises the objective lens 16 and an intermediate lens 18. The objective lens 16 images the light emanating from the sample onto the first intermediate image plane 17. A field diaphragm can be inserted into the intermediate image plane 17, which only transmits light from a selected area of ​​the sample position 15, thus making it possible to select an area on the sample to be examined.

[0047] The intermediate optics 18 then images the first intermediate image plane 17 onto the infrared detector 19. The infrared detector 19 is preferably a 2-dimensional detector with an IR sensor 19a having a plurality of detector elements (pixels) in a sensor plane of the IR sensor 19a. Alternatively, a line detector can be used in which the sensor elements are arranged along a straight line. Ideally, the IR sensor 19a is cooled to keep thermal excitations in the detector elements as small as possible. In the example shown, the IR sensor 19a is installed in a detector housing 19c, which is also cooled. The light to be analyzed enters the detector housing 19c through a cooled detector aperture 19b of the detector 19 and generates an image of the sample position 15 on the sensor 19a.

[0048] The size and position of the detector aperture 19b determines from which areas radiation can reach the sensor 19a of the detector 19; thus, it determines the field of view of the detector 19.

[0049] The IR beam is limited by an effective limiting element 8. This can be an optical element (e.g., an interferometer). According to the invention, the effective beam limiting element 8 is located in the collimated input beam 26 and defines a diaphragm plane 27. The opening / aperture (in the case of a diaphragm) 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 diaphragm plane 27. Fig. 1a, the output aperture 3 forms the effective beam-limiting element 8.

[0050] Fig. Fig. 1b shows another embodiment of an IR microscope according to the invention, in which the collimating mirror 4, which determines the beginning of the collimated input beam and is thus located in the collimated input beam, is the effective beam-limiting element 8; ie in the Fig. In the case shown in Figure 1b, beam limitation no longer occurs at the output aperture 3 of the interferometer 1.

[0051] In addition, the effective beam-limiting element 8 can also be defined, for example, by the fixed mirror 7, the movable mirror 5 or the beam splitter 7 or can also be located outside the interferometer 1 between the interferometer and the first optical device (not shown).

[0052] 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 incident on the sensor. According to the invention, the detector aperture 19b is imaged onto an input beam plane 29 in the collimated input beam 26. The relationship between image quality, the ratio A1 / A2, and the positions of the detector aperture image is described below: Fig. Figure 2a shows the infrared detector 19 and the intermediate optics 18. An image of the sample (by imaging the sample plane or subsequent intermediate image planes) is generated on the IR sensor 19a via the intermediate optics 18. In the prior art, the size and position of the detector aperture 19b are dimensioned such that only the radiation cone coming from the intermediate optics 18 in front of the detector 19 is allowed to pass through the detector aperture 19b. Fig. Figure 2b shows the IR radiation beam paths between the intermediate optics 18 and the central or an outer sensor element of the IR sensor 19b of the IR microscope in the case where the image of the cross-section of the input beam in the aperture plane (aperture plane image 25) does not match the size and position of the detector aperture 19b. In the case of Fig. 2b, the result is that radiation on an edge pixel is vignetting from the detector aperture 19b, since the detector aperture 19b is too small to capture all the IR radiation emanating from the aperture plane image 25 for all pixels. All radiation emanating from the aperture plane image 25 also originates from the collimated IR source. Fig. 2c shows the same case as in Fig. 2b, except that the detector aperture 19b has been enlarged to such an extent that vignetting of the IR radiation emanating from the aperture plane image 25 and reaching the edge pixels no longer occurs. However, the enlargement of the detector aperture 19b also has disadvantages, since IR radiation that does not originate from the aperture plane image 25 and thus from the collimated IR source can now also reach the pixels. Unusable IR radiation from the environment can also now fall onto the detector. Fig. Figure 2d illustrates this situation. Only the IR radiation 30 from the inner white cone passes completely through the aperture plane image 25 and thus originates from the collimated IR source. The radiation in the hatched areas 31 does not originate from the collimated IR source, but still reaches the sensor 19a. Fig. 2c and Fig. 2d show that there is no optimal size for the detector aperture 19b as long as the aperture plane image 25 does not coincide with the detector aperture 19a. Fig. Figure 2e shows the case where the aperture plane image 25 coincides with the detector aperture 19a. In this case, only light originating from the collimated IR source, which thus passes through the aperture plane image 25, reaches the detector. At the same time, the modulated IR light from the collimated IR source is not vignetting for any pixel on the sensor 13a.

[0053] For optimal illumination without vignetting and simultaneous suppression of ambient light, the aperture plane image must match the detector aperture. Conversely, the intermediate optics and the size of the detector aperture must be adjusted so that the detector aperture image matches the cross-section of the input beam at the aperture plane.

[0054] The dependence of the total signal, useful signal and ambient signal detected by the IR detector on the ratio of the area A1 of the image of the detector aperture 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 shown in Fig. 3a. For optimal adjustment, care must be taken to ensure that the detector does not saturate. Ideally, the ratio of useful signal to total signal should be as high as possible. However, in the range 0≤A1 / A2≤1, the ratio of useful signal to total signal is constantly equal to 1. At the same time, however, it is advantageous if the absolute useful signal is high. Therefore, Fig. 3b shows the ratio of useful signal to total signal multiplied by the useful signal as a function of A1 / A2. It can be clearly seen that the ratio of useful signal to total signal multiplied by the useful signal is maximum for the ratio A1 / A2=1, with the decrease at lower ratios A1 / A2<1 being due to shadowing due to a detector aperture smaller than the beam cross-section to be imaged, and the decrease at higher ratios A1 / A2>1 being due to the contribution of scattered light from the environment (ambient signal).

[0055] According to the present invention, the detector aperture 19b is imaged by the first and second optical devices onto an input beam plane located in the collimated input beam, such that the area A1 of the image of the detector aperture 19b in the input beam plane is at most as large as the area of ​​the cross-section of the collimated input beam in the input beam plane. This prevents ambient light from entering the detector 19. According to the invention, the opening of the detector aperture 19b is thus matched to the beam cross-section of the collimated input beam in the input plane. Ideally, the detector aperture 19b is imaged onto the aperture plane (i.e., onto the effectively beam-limiting element 8). In this case, the aperture plane and the input beam plane coincide.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 aperture plane is imaged onto the detector aperture 19b (aperture plane image 25 is then located in the plane of the detector aperture 19b), as shown in . Fig. 2e. The detector aperture 19b, 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, so that the desired cross-section of the input beam enters the detector 19. In combination with the condition that the area A1 of the image of the opening of the detector apertures 19b 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 only the IR light of the collimated input beam 26 passes through the detector aperture 19b. In the Fig. In the example shown in Figure 2e, A1=A2, so that all light of the IR beam of the input beam plane reaches the IR sensor 19a.

[0056] According to the invention, the input beam plane 29 and the aperture plane 27 do not necessarily have to coincide, as long as both the input beam plane 29 and the aperture plane 27 are located in the collimated input beam 26 and the condition A1≤A2 is met. Since the cross-section of the input beam 26 does not change or changes only slightly in the collimated region, the device according to the invention always images a beam cross-section onto the detector aperture 19b that is equal to or almost equal to the beam cross-section in the aperture plane 27, so that, as a first approximation, the beam cross-section prevailing at the location of the beam-limiting element can be assumed to be the beam cross-section in the input beam plane 27 and can be used to determine the size of the detector aperture 19b. The positioning of the beam-limiting element 8 can therefore be selected relatively freely within the collimated input beam 26 without significantly influencing the quality of the measurement.This also simplifies the design of the intermediate optics 18. At the same time, however, it is also possible to realize a desired shadowing by selecting the ratio of the opening of the detector aperture 19b and the beam limitation, for example when a very intense light source 2 is used (e.g. a quantum cascade laser), which would saturate the IR sensor 19a without shadowing and thus make the measurements unusable.

[0057] The device according to the invention therefore offers a high degree of flexibility in the type and positioning of the beam limitation with maximum light output and minimization of interference.

[0058] For the design of the intermediate optics 18, the effective focal length f of the intermediate optics 18 is particularly relevant. This depends on the distance d between the detector aperture 19b and the IR sensor 19a and the distance x between the image 28 of the input beam plane (which is produced 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 produced by the objective 16 of the second optical device ( Fig. 4): f=d⋅xx⋅m−d / m., so that on the one hand the sample arranged in the sample position 15 is imaged onto the IR sensor 19a and on the other hand the input beam plane located in the collimated input beam is imaged onto the detector aperture 19b.

[0059] Fig. Figure 5 shows how the intermediate optics 18 can be designed, for example. The intermediate optics 18 here comprises a 1x lens according to Offner [5], which provides an astigmation-free Fig.The intermediate image 17 is generated via two spherical mirrors 18a and 18b. A plane mirror 18c serves only to deflect the beam. The 1x Offner objective serves to generate an image of the sample at a location in the microscope where there is sufficient space for the cooled area detector 19. The IR detector 19 is usually cooled by a Dewar containing liquid nitrogen. Alternative systems, for example, use Stirling coolers to generate the required low temperatures at the sensor element. All of these detectors require comparatively large amounts of space. Furthermore, in the case of the Dewar, the filling opening for the liquid nitrogen must be accessible from the outside. The 1x Offner objective generates not only the image of the sample 18d but also an image 18g of the input beam plane 29.A downstream optics system with plane mirror 18e for beam deflection and mirror 18f is configured to generate an image of the sample image 18d on the sensor 19a of the detector 19 and, simultaneously, generate an image of the input beam plane image 18g on the cold aperture 19b of the detector 19. For this purpose, the focal length f of the mirror 18f is first determined using the equation described above. f=d⋅xx⋅m−d / m determined. Since the mirror 18f creates an off-axis image, it is advantageous to keep the deflection angle as small as possible. On the other hand, the use of a toroidal mirror with focal lengths f and f' instead of a spherical mirror with a determined focal length f is advantageous for image quality. f' can be determined from f and the deflection angle. The shape of this toroidal mirror can then be slightly modified. Ray tracing programs (e.g., Zemax) can be used to determine optimal surface shapes, leading to optimized image quality on the detector sensor. The surface of the mirror 18f then deviates slightly from the ideal toroidal shape, resulting in a toroid-like mirror. The light coming from the mirror 18f passes through a window 19d of the detector, creating an image of the input beam plane 29 at the detector aperture 19b and an image of the sample on the sensor 19a of the detector 19. Literature list [1] US 7 440 095 B2 [2] DE 10 2012 200 851 B3 [3] US 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 symbols 1 IR light source / interferometer 2 Radiation source 3 Output aperture of the interferometer 1 4 mirrors of interferometer 1 5 movable mirror of the interferometer 1 6 fixed mirror of interferometer 1 7 Beam splitter of the interferometer 1 8 effective beam limiting element 9 mirrors 10a Mirror for measurement in transmission 10b Mirror for measurement in reflection 11a Mirror for measurement in transmission 11b Mirror for measurement in reflection 12a optical element for measurement in transmission 12b optical element for measurement in reflection 13a optical element for measurement in transmission 13b Beam splitter optics 14 Condenser / additional lens 15 sample position 16 lens 17 first intermediate image plane 18 Intermediate optics 18a, 18b Offner lens / spherical mirrors 18c plane mirror 18d Image of the sample in intermediate optics 18 18e Plane mirror 18f mirror 18g Image of the sample in the input beam plane in intermediate optics 18 19 IR detector 19a IR sensor 19b Detector aperture 19c detector housing 20 visual light source for measurement in transmission 20b Visual light source for measurement in reflection 21a optical element for measurement in transmission 21b optical element for measurement in reflection 22 dichroic mirrors 23 Imaging optics 24 CCD camera 25 Image of the aperture plane 26 collimated input beam 27 aperture level 28 Image of the input beam plane before intermediate optics 18 29 Input beam plane

Claims

[1] IR microscope with an IR light source (1) for generating a collimated IR input beam (26), an effective beam-limiting element (8) in an aperture plane (27), a sample position (15) for receiving a sample, an IR detector (19) with an IR sensor (19a), a detector aperture (19b) arranged in front of the IR sensor (19a) with a detector aperture opening, a first optical device for focusing the collimated IR input beam (26) emerging from the IR light source (1) onto the sample position (15), and a second optical device for imaging the sample position (15) onto the IR sensor (19a), wherein the second optical device comprises an objective lens (16) and an intermediate lens (18). characterized by , that the effective beam-limiting element (8) is located in the collimated IR input beam (26) before entering the first optical device, and that the first optical device and the second optical device image the detector aperture of the detector aperture (19b) into an input beam plane, wherein the area A1 of the image of the detector aperture in the input beam plane and the area A2 of the cross section of the collimated IR input beam (26) in the input beam plane apply: 0 <A1 / A2≤1. [2] IR microscope according to claim 1, characterized by that the first optical device comprises a further lens (14) for focusing the IR input beam onto the sample position. [3] IR microscope according to claim 1, characterized byin that the first optical device comprises the objective (16) which is configured to focus the IR input beam (26) onto the sample position (15), and a beam splitter optic (13b) which is configured to couple the IR input beam (26) emanating from the IR light source (1) into the objective (16). [4] IR microscope according to one of the preceding claims, characterized by that the detector aperture (19b) and the IR sensor (19a) are located within a common detector housing (19c) and are arranged at a distance d from each other, wherein the distance d is preferably a maximum of 50 mm. [5] IR microscope according to one of the preceding claims, characterized by that the intermediate optics (18) has an effective focal length f for which the following applies: f=d⋅xx⋅m−d / m with f: effective focal length of the intermediate optics (18) x: Distance between the image of the input beam plane produced by the first optical device and by the objective (16) of the second optical device and the image of the sample produced by the objective (16) of the second optical device. d: Distance between detector aperture (19b) and IR sensor (19a) m: Magnification factor of the intermediate optics (18) [6] IR microscope according to claim 5, characterized by that the magnification factor m of the intermediate optics (18) is m=1. [7] IR microscope according to one of claims 1 to 6, characterized by that the effective beam-limiting element (8) is an output aperture (3) of the IR light source (1). [8] IR microscope according to one of claims 1 to 6, characterized by that the effective beam-limiting element (8) and the input beam plane are arranged within the collimated IR light source (1). [9] IR microscope according to one of the preceding claims, characterized by that the input beam plane is the aperture plane (27). [10] IR microscope according to one of the preceding claims, characterized by that the intermediate optics (18) comprises an Offner lens (18a, 18b). [11] IR microscope according to one of the preceding claims, characterized by that the IR light source comprises an interferometer (1), a quantum cascade laser or an FTIR spectrometer.

Images