Ftir spectrometer

Abstract

The present invention relates to an FTIR spectrometer having an infrared radiation source, an interferometer having at least one arm which is variable in length, a reference laser, a measuring cell with a sample interface, preferably an ATR crystal, which can be brought into contact with a sample, an infrared detector, a control system which is configured to change the length of the at least one arm of the interferometer, and a mirror arrangement outside the interferometer with at least two mirrors, each with a reflective surface, and a main body which comprises the reflective surface, wherein the mirror arrangement is at least configured to direct a light beam from the interferometer onto the sample interface and to direct the light beam from the sample interface onto the infrared detector, wherein the main body of at least one mirror or all mirrors of the mirror arrangement is made of a plastics material and / or of 3D printed metal, or the main body of at least one mirror or all mirrors comprises plastics material and / or 3D printed metal.

Description

[0001] FTIR spectrometer

[0002] The invention relates to an FTIR spectrometer with a mirror made of a plastic material. The subject matter of the invention is defined in the appended claims.

[0003] FTIR (Fourier Transform Infrared) spectrometers are a special type of spectrometer that can record infrared spectra using a specialized measurement setup. In FTIR spectroscopy, a signal generated by an interferometer is converted into a spectrum using Fourier transformation. This spectrum contains information about the sample being measured. For example, the chemical composition of food, materials, chemicals, hazardous substances, medications, and / or plastics can be analyzed non-destructively. This makes FTIR spectrometers particularly suitable for the determination and quality control of starting materials for the production of medications.

[0004] For the optical analysis of samples by recording spectra, it is generally necessary to achieve the greatest possible interaction between light and sample material for the entire radiation spectrum used, but especially when using infrared radiation. At the same time, it is desirable to support a wide range of sample types (solids, liquids, powders, etc.). In spectroscopy and analytics, the use of so-called ATR crystals (attenuated total reflection) in FTIR spectrometers has proven successful. With the help of an ATR crystal, an evanescent wave can couple into the sample material or sample in contact with the ATR crystal. This effect is also called the optical tunneling effect. The remaining light carries information about the interaction with the sample, is guided back out of the ATR crystal by means of total internal reflection, and can then, for example,be directed to an infrared detector by reflection.

[0005] In the simplest case, an FTIR spectrometer comprises a collimated infrared radiation source, an interferometer, a reference laser, a measuring cell with a sample interface, which includes, for example, an ATR crystal, as well as an infrared detector and a control system.

[0006] The interferometer comprises a beam splitter that splits incoming light into two individual beams. The individual beams are each reflected by one (or possibly several) mirror(s) of the interferometer and recombined in the beam splitter, where they interfere with each other. The path of an individual beam in the interferometer from the beam splitter to the (last) reflecting mirror and back, or the structure associated with this path in the interferometer, is usually referred to as an arm. One or both of the interferometer's arms are usually variable in length. This is achieved by moving at least one mirror of one or both arms relative to the beam splitter. The length of the arm(s) (and thus the mirror movement(s)) is / are controlled by the control system. This makes the interference of the reflected individual beams variable or adjustable.With the help of the reference laser, the lengths of the arms or the distances traveled by the individual beams in the arm and / or the difference in distance in the interferometer are determined.

[0007] For example, the control system can adjust the mirror offset of a mirror in one of the interferometer's two arms, which is movable along a linear axis. This changes the distance of the movable mirror from the beam splitter in the arm and thus the distance traveled by the light, also called the path length, in the arm.

[0008] Alternatively, an interferometer with a rocker arm that can rotate in a plane is known in the prior art. The rocker arm is designed in such a way that it includes, in particular, the interferometer mirrors necessary for reflecting both individual beams coming from the beam splitter. The rocker arm thus forms both arms of the interferometer. The control system regulates the rotation of the rocker arm such that the rocker arm performs a pendulum motion relative to the stationary beam splitter between two endpoints. During the pendulum motion, one arm of the interferometer is alternately shortened while the other arm is simultaneously lengthened relative to the beam splitter. In this case, the path lengths of both arms traveled by the light are changed. This also allows the interference of the reflected individual beams to be adjusted.

[0009] The intensity of the light beam resulting from the interference of the individual beams is measured by the infrared detector, for example, after passing through the measuring cell with the sample interface. The absorption spectrum of the sample can then be calculated from the intensity measured at the infrared detector and the path length in the interferometer determined with the reference laser. This provides information about the type, composition, and state of the sample and thus represents a kind of chemical fingerprint of the sample or sample material.

[0010] Especially when using ATR crystals in FTIR spectrometers, a good signal-to-noise ratio (SNR) is crucial for a meaningful measurement with high measurement speed and the sensitivity required for a meaningful spectrum. The SNR is largely determined by the amount of light coupled into the ATR crystal over the widest possible wavelength range.

[0011] The coupling of light into the ATR crystal is typically achieved through technically complex and cost-intensive optical setups. For example, the state of the art uses beam splitters, which typically suffer high losses of more than 50% of the incident light when passing through the beam splitter twice. Alternative approaches to maximizing the sample signal on ATR crystals involve multiple reflections within the ATR crystal. This requires comparatively large ATR crystals compared to single-reflection ATR-FTIR spectrometers. However, large ATR crystals are associated with high production effort and high production costs for the ATR crystals. In addition, the materials used, such as diamond, are often very expensive.Other solutions use elaborately coated refractive optics, fiber optics or Schwarzschild lenses, which are complex, expensive and costly to manufacture or time-consuming to adjust the optical system, or even do not transmit the light from a broadband light or infrared source independently of the wavelength.

[0012] In addition, the FTIR spectrometers available in the state of the art generally incorporate technically complex and costly optical elements, particularly mirrors. A typical example of mirrors used with the aforementioned disadvantages are metal precision mirrors. Metal precision mirrors are typically milled from a solid metal block using complex CNC milling. The milling tools of a CNC machine are subjected to significant stress and wear during this manufacturing process. Furthermore, this type of mirror production is very resource-intensive due to the necessary very fine adjustment of the chip removal during the CNC milling process in order to achieve the desired mirror shape without grooves or score lines with optical surface roughness.The use of such mirrors therefore leads to a significant increase in the cost of the overall construction of a state-of-the-art FTIR spectrometer due to the complex manufacturing process, which does not become cheaper even with large quantities. Alternative, technically easier to manufacture mirror variants for use in FTIR spectrometers are not available in the state of the art. A metal precision mirror or precision metal mirror is thus a mirror known in the state of the art with a high manufacturing effort and thus a high price, which at the same time exhibits outstandingly advantageous optical properties. A metal precision mirror is an example of a precision mirror, i.e., an optic with high optical quality. Metal precision mirrors are generally the only alternative in the construction of high-precision interferometers in known FTIR spectrometers.

[0013] The consequence of using the aforementioned optical elements is a generally high technical manufacturing effort for an FTIR spectrometer as well as high acquisition costs, even for "entry-level" FTIR spectrometers. Due to the wide range of possible applications, simplifying the manufacturing effort of the optical setup and reducing both manufacturing and acquisition costs is particularly desirable. Furthermore, more sustainable production of at least some of the optical components is desirable. By eliminating these disadvantages, FTIR spectrometers for optical analysis will be made accessible to companies, government agencies, schools and universities, start-ups, physicians and pharmacists, as well as private individuals with limited budgets.

[0014] The object of the present invention is therefore to provide an FTIR spectrometer with a simplified optical design that is easier to manufacture, more reliable, more cost-effective, and more sustainable, while eliminating the disadvantages of the prior art. This object is achieved by the FTIR spectrometer described in claim 1. Preferred embodiments of the invention are set out in the subclaims and the following statements.

[0015] The problem is solved by an FTIR spectrometer according to claim 1.The FTIR spectrometer according to the invention comprises an infrared radiation source, an interferometer with at least one arm variable in length, a reference laser, a measuring cell with a sample interface, preferably an ATR crystal, which can be brought into contact with a sample, an infrared detector, a control system which is configured to change the length of the at least one arm of the interferometer, and a mirror arrangement outside the interferometer with at least two mirrors, each with a reflective surface and a base body which comprises the reflective surface, wherein the mirror arrangement is at least configured to direct a light beam from the interferometer onto the sample interface and to direct the light beam from the sample interface onto the infrared detector, wherein the base body of at least one mirror or all of the mirrors of the mirror arrangement is made of a plastic material and / or of 3D printed metal.or the base body of at least one mirror or all mirrors comprises plastic material and / or 3D printed metal.

[0016] The object is achieved in particular by the following FTIR spectrometer according to the invention, the FTIR spectrometer according to the invention comprises an infrared radiation source, an interferometer with at least one arm variable in length, a reference laser, a measuring cell with a sample interface, preferably an ATR crystal, which can be brought into contact with a sample, an infrared detector, a control system configured to change the length of at least one arm of the interferometer, and a mirror arrangement outside the interferometer with at least two mirrors, each with a reflective surface and a base body comprising the reflective surface, wherein the mirror arrangement outside the interferometer is at least configured to direct a light beam from the interferometer to the sample interface and to direct the light beam from the sample interface to the infrared detector,wherein the base body of at least one mirror of the mirror arrangement outside the interferometer or of all mirrors of the mirror arrangement outside the interferometer is or are made of a plastic material and / or 3D printed metal, or the base body of at least one mirror of the mirror arrangement outside the interferometer or of all mirrors of the mirror arrangement outside the interferometer comprises or comprise plastic material and / or 3D printed metal.

[0017] The core of the invention relates to the surprising discovery that the precision mirrors used in the prior art outside the interferometer, such as metal precision mirrors or precision mirrors made of other materials, can be partially or completely replaced by the mirrors of the mirror arrangement according to the invention. In other words, the surprising discovery lies in the fact that high-quality optics must be used within the interferometer of the FTIR spectrometer to obtain the necessary signal quality or constructive interference. Examples of such optics are the known precision mirrors already described above, such as metal precision mirrors. However, precision mirrors made of other materials are also conceivable.

[0018] Outside the interferometer of the FTIR spectrometer, however, it is surprisingly sufficient to use optics with a low optical quality or lower optical quality than conventional precision mirrors. These optics with a low optical quality or lower optical quality than precision mirrors can, in particular, exhibit a high wavefront error. These optics with a low or lower optical quality outside the interferometer correspond to the mirrors of the mirror arrangement outside the interferometer of the FTIR spectrometer according to the invention described in the context of this invention.

[0019] For the purposes of the invention, an optic with high optical quality describes an optic, in particular a mirror or a mirror arrangement, in which the wavefront error is significantly smaller than one wavelength of the reflected light. Those skilled in the art know from optical contexts regarding the interference of light waves that the wavefront error of the mirrors within the interferometer of FTIR spectrometers must be significantly smaller than one wavelength to achieve constructive interference with a usable intensity. Therefore, the use of optics with high optical quality is required within the interferometer of FTIR spectrometers.

[0020] For the purposes of the invention, an optic with low optical quality describes an optic, in particular a mirror or a mirror arrangement, in which the wavefront error is larger, preferably significantly larger, than a wavelength of the reflected light. Such low-quality optics enable no or only minimal constructive interference with usable intensity. Therefore, such optics with low optical quality are not suitable for use in interferometers of FTIR spectrometers. Surprisingly, however, optics with low optical quality are suitable for use outside the interferometer of an FTIR spectrometer, since there the wavelength error surprisingly has a smaller influence on the measured intensity.

[0021] The advantage of this surprising discovery is that it greatly simplifies the manufacturing process of an FTIR spectrometer while maintaining virtually consistent measurement quality. Furthermore, the costs and resources required for manufacturing an FTIR spectrometer are significantly reduced while maintaining virtually consistent measurement quality. Further advantages are described below. The terms "light" and "light beam" or "light rays" are used synonymously within the scope of this invention and describe electromagnetic radiation, preferably in the infrared and / or optical wavelength range, that follow a beam path. Within the scope of this invention, a beam path describes a trajectory of the light or light rays through or along optical elements and components in the FTIR spectrometer according to the invention, in particular the mirrors of the mirror arrangement of the FTIR spectrometer described within the scope of this invention.

[0022] The infrared radiation source can, for example, emit at least light in the near and / or mid-infrared wavelength range. For example, the infrared radiation source can emit at least light in the wavelength range from 1 pm to 50 pm. However, it is also conceivable for the infrared radiation source to additionally emit light in the visible spectrum. The infrared radiation source can, for example, be a heated element made of silicon carbide, which can be heated to a temperature in the range of approximately 1200 K. It is also conceivable for the infrared radiation source to be a tungsten-halogen lamp, a mercury discharge lamp, or a plasma light source. The infrared radiation source can be spatially extended, for example in at least one spatial direction in the range of up to 30 mm.

[0023] The light generated by such an extended infrared radiation source can be collimated using suitable optical means before entering the interferometer. In this context, the following components or arrangements are examples of suitable means: lenses and / or mirrors or mirror arrangements, e.g., comprising parabolic mirrors, off-axis parabolic mirrors, and / or so-called compound parabolic concentrator mirrors (CPCs). The light emitted by the infrared radiation source is preferably collimated using a parabolic mirror, an off-axis parabolic mirror, or a CPC. Such mirrors have the advantage that they collimate the incident light particularly efficiently. In addition, losses of the reflected light due to absorption or dispersion, which would otherwise occur with transmissive optical elements such as lenses, can be advantageously avoided.This can significantly improve the signal-to-noise ratio (SNR).

[0024] Within the interferometer of the FTIR spectrometer according to the invention, the quality of the optics is critical, as any errors on the scale of fractions of the wavelength directly lead to the destruction of the interference and thus to signal loss. Accordingly, the interferometer preferably comprises exclusively planar mirrors and a beam splitter with a planarity in the range of a fraction of the wavelengths to be measured.

[0025] Preferably, the beam splitter comprises the same material as a window of the infrared detector or is made of this material. This introduces only one source into the FTIR spectrometer instead of two different sources for dispersion and absorption. Ultimately, this significantly improves the signal reaching the detector. KBr, Csl, ZnSe, diamond, KRS-5, Ge, and Si are particularly preferred as materials for the window of the infrared detector and the beam splitter. These materials have a very broadband transmission of infrared radiation, making them well suited for simultaneous use in a beam splitter and an infrared detector window.

[0026] The interferometer comprises a beam splitter that splits incoming light into two individual beams. The path of an individual beam in the interferometer from the beam splitter, for example, along one or more mirrors, to the corresponding mirror at which the individual beam is reflected back to the beam splitter, or the structure associated with this path in the interferometer, is referred to as an "arm" in the context of the invention. The individual beams are reflected back to the beam splitter by one or more mirrors of the arms in the interferometer and recombined in the beam splitter, where they interfere with each other. One or both arms of the interferometer are variable in length. This can be achieved, for example, by moving at least one mirror of one or both arms relative to the beam splitter. The length of the arm(s) (and thus the mirror movement orThe movement of the mirrors can be controlled by the control system. This allows the interference of the reflected individual beams to be changed or adjusted.

[0027] For example, the control system can regulate the mirror offset of a mirror in the first of two arms of the interferometer that is movable along a linear axis. This changes the distance of the movable mirror from the beam splitter in the first arm and thus the distance traveled by the light, also called the path length, in the first arm.

[0028] Alternatively, the interferometer can comprise a rocker that is rotatably mounted in a plane relative to the stationary beam splitter. The rocker is designed such that it includes, in particular, the interferometer mirrors necessary for reflecting both individual beams coming from the beam splitter. The rocker thus forms the first and second arm of the interferometer. For example, the rocker can be designed as shown in J. Kauppinen et al., Appl. Spectrosc. Rev. 39, 99 (2004), Fig. 20. The control system regulates a rotary movement, for example with the aid of a drive of the rocker, such that the rocker executes a pendulum movement relative to the stationary beam splitter between two endpoints. During the pendulum movement, one arm of the interferometer is alternately shortened while the other arm is simultaneously lengthened relative to the beam splitter. In this case, the path lengths of both arms to be traveled by the light are changed.This also allows the interference of the reflected individual beams to be adjusted.

[0029] The rotating rocker can be mounted for virtually frictionless rotation, for example, via a flexure joint or a rolling bearing, such as a ball or roller bearing. The rotating rocker can be driven to rotate by the drive. The drive can be, for example, a voice coil. The voice coil has the advantage of having few or no mechanical parts compared to typical electric motors and / or drives, and thus introduces no or only negligible unwanted additional mechanical disturbances into the interferometer during operation. Furthermore, such a drive is durable and robust.

[0030] Alternatively, the interferometer can be any other suitable interferometer in which the path length difference within one or both arms can be changed during a measurement.

[0031] The two individual beams interfere with each other depending on the path length difference, which is caused by the movement of the movable mirror(s) in the interferometer. As a result, a strong constructive maximum (center burst) with flat tails (wings) is created in the wavelength range where the mirrors are equidistant from the beam splitter.

[0032] Preferably, one or both mirrors in the interferometer are held by a mirror holder as described below.

[0033] The mirror holder may comprise a base body that is connectable to a portion of the interferometer or the FTIR spectrometer, e.g., a housing portion.

[0034] The mirror holder may additionally have a first part. The first part may be connected to the base body.

[0035] The first part can comprise or be formed from a first spring steel sheet. In the first case, the base body can be connected to the first part by means of the first spring steel sheet. Spring steel sheets are cost-effective, easy to process, and have particularly advantageous spring properties.

[0036] In this case, the first part can be designed as a plate. Plate-like components are easy to manufacture.

[0037] The mirror holder can comprise a first screw rotatably mounted in the base body, which spaced the first part from the base body against a spring force of the first spring steel sheet. The first screw can then only have a force-locking connection with the first part.

[0038] The first spring steel sheet can exert a spring force so that the first part is prestressed in the direction of the base body and the first screw, or an end of the first screw facing the first part, forms an abutment to the spring force of the first part.

[0039] A mirror can be accommodated or provided on the first part. Such a mirror holder comprising a base body and a first part has the advantage that, when the first screw is screwed in or out, a distance between the base body and the first part can be adjusted with virtually no hysteresis due to the work carried out by the first spring steel sheet against the spring tension. Changing this distance, in turn, results in a change in the angle between the base body and the first part with appropriate positioning of the first screw. Thus, a first change in angle can be made with the mirror holder with no hysteresis, which is particularly advantageous for mirror adjustment.

[0040] The force-locking connection of the screw end of the first screw can, for example, be directly with the first spring steel sheet of the first part or with a separate material.

[0041] The separate material can preferably be abrasion-resistant and withstand the forces exerted by the screw end of the first screw on the first part through the frictional connection, especially during frequent rotational movements. This can extend the service life of the mirror mount.

[0042] The mirror holder can preferably have a second part, wherein the second part can be connected to the first part. The second part can have a second spring steel sheet or be formed therefrom. In the first case, the second part can be connected to the first part by means of the second spring steel sheet.

[0043] In this case, the second part can be designed like a plate.

[0044] A second screw rotatably mounted in the base body can space the second part from the first part and / or the base body. The second screw can have a force-locking connection only with the second part. The second spring steel sheet can exert a spring force such that the second part is preloaded toward the first part and the second screw, or an end of the second screw facing the second part, forms a counterforce to the spring force of the first part. In this case, the mirror can be accommodated or provided on the second part.

[0045] Such a mirror holder comprising a base body, a first part and an additional second part has the additional advantage, compared to the above-described structure comprising a base body and only the first part, that when the second screw is screwed in or out, a distance between the second part and the first part can be adjusted almost hysteresis-free due to the work against the spring tension by the second spring steel sheet. The change in this distance in turn results in a change in the angle between the second part and the first part when the second screw is arranged accordingly. Thus, with the mirror holder, a further, second change in the angle can be made hysteresis-free in a direction different from the first change in the angle, which is particularly advantageous for mirror adjustment. In addition, the structure is easy to manufacture because only simple components are used.

[0046] The first part and the second part can preferably be arranged substantially parallel to each other in an initial state. This allows the initial state to be easily defined.

[0047] The first and / or second part can preferably have a cuboid shape. Such shapes are easy to manufacture.

[0048] The second spring steel sheet can preferably be arranged on one of the side surfaces of the second part that is perpendicular or transverse to the surface that accommodates or provides the mirror. This enables easy assembly or fastening of the second spring steel sheet.

[0049] The first spring steel sheet can preferably be arranged on one of the side surfaces of the first part that is perpendicular or transverse to the surface that accommodates or provides the mirror. In this case, the first spring steel sheet can also be arranged non-parallel to the second spring steel sheet. In other words, the surface normals of the first and second spring steel sheets can be orthogonal and almost orthogonal to each other. This enables simple assembly or fastening of the second spring steel sheet. This also has the advantage that adjustment can occur in two (almost) mutually perpendicular spatial directions. In other words, the first and second angular changes can be decoupled from each other. This considerably simplifies the adjustment of the mirror included in the described mirror holder.

[0050] The reference laser has a known wavelength and is preferably actively current-stabilized and / or temperature-stabilized. For example, the reference laser can be a helium-neon laser. Alternatively, the reference laser can be a cost-effective and easily procured diode laser. In addition to the reference laser, a reference interferometer can also be provided. Such a reference interferometer serves to determine the position of the length change in the interferometer and is not another FTIR interferometer for reference and calibration purposes. The following applies analogously to the reference interferometer. With the help of the reference laser, the location and an inclination angle of a mirror of one arm or of the mirrors of both arms of the interferometer can be determined, or a relative path length difference between the mirrors of the first and second arm of the interferometer can be determined.In this context, the tilt angle can describe an angle between the mirror of an arm or between a surface normal of the mirror and, for example, the incident reference laser beam. The reference laser can emit light in the red, green, or orange range, for example. Typical wavelengths in the visible light range are between 730 nm and 543 nm.

[0051] The reference laser can alternatively or additionally emit light in the infrared range, preferably in the range from 900 nm to 1100 nm, particularly preferably in the range from 960 to 1000 nm, e.g. 980 nm. This has the advantage that the benefits of optics optimized for the reflection of infrared radiation can be utilized in the FITR spectrometer according to the invention. In addition, the use of infrared light in the reference laser makes the interference signal easier to measure due to the longer wavelength of infrared light compared to light from the visible range. The longer wavelength leads to a slower movement of the interference pattern when the arm(s) of the interferometer are moved compared to light from the visible range. This reduces the requirements for the measurement speed of the infrared detector or the control system.In particular, this reduces the requirements for an analog-to-digital converter of the microcontroller or for the microcontroller itself.

[0052] The reference laser can particularly preferably emit in the range of 960 to 1000 nm, e.g., 980 nm. The reference laser is preferably a diode laser with a wavelength in the range of 960 to 1000 nm, e.g., 980 nm. This wavelength range, in particular the 980 nm wavelength, represents an optimal compromise between the accuracy of determining the above-mentioned parameters and the required measurement speed of the infrared detector or an associated analog-to-digital converter. Furthermore, diode lasers are particularly cost-effective and easy to manufacture.

[0053] Alternatively, the reference laser can preferably emit in the range from 600 nm to 1600 nm. Reference lasers, especially diode lasers, that emit in this range are particularly easy to manufacture and cost-effective.

[0054] Preferably, by varying the temperature of the reference laser, the wavelength of the reference laser can be varied such that, when the wavelength is varied, known absorption lines of gas molecules, preferably of oxygen, inside or outside the FTIR spectrometer according to the invention are exceeded. In this case, the reference laser is preferably a diode laser. This reference laser particularly preferably emits in the range from 600 nm to 1600 nm.

[0055] It is also conceivable that by varying the temperature of the reference laser, the wavelength of the reference laser is varied such that, when the wavelength is varied, known absorption lines of gas molecules, preferably of oxygen, inside or outside of FTIR spectrometers or other FTIR spectrometers known in the prior art are exceeded. In this case, too, the reference laser is preferably a diode laser. This reference laser particularly preferably emits in the range from 600 nm to 1600 nm. A corresponding calibration method of the FTIR spectrometer according to the invention or of FTIR spectrometers or other FTIR spectrometers known in the prior art can preferably comprise the following steps:

[0056] 1. Varying the temperature of the reference laser, which is preferably a diode laser, in such a way that the wavelength of the reference laser changes, whereby the measurement is carried out without a sample in the measuring cell,

[0057] 2. Measuring a signal, preferably an intensity, of the reference laser by the infrared detector,

[0058] 3. Preferably, the temperature of the reference laser is controlled so that the measured signal of a known absorption line, preferably of a gas, particularly preferably of oxygen, is maximized.

[0059] As an alternative to or in addition to varying the temperature of the diode laser, the current of the diode laser can also be controlled analogously to steps 1 and 3 of the aforementioned calibration procedure. As an alternative to or in addition to varying the temperature of the diode laser, the current of the diode laser can also be controlled analogously to steps 1 or 3 of the aforementioned calibration procedure.

[0060] Particularly preferably, the calibration method can be carried out automatically, for example, by the control system of the FTIR spectrometer according to the invention. Automatic calibration can be performed at regular or irregular intervals. Automatic calibration can be performed before a measurement and / or as a step during a measurement sequence.

[0061] It is also conceivable for the calibration process to be performed automatically, for example, by the control system of FTIR spectrometers known in the art or other FTIR spectrometers. Automatic calibration can be performed at regular or irregular intervals. Automatic calibration can be performed before a measurement and / or as a step during a measurement sequence.

[0062] Oxygen has an absorption line at approximately 850 nm. This makes oxygen ideal for calibrating the wavelength of the reference laser.

[0063] The calibration method represents a fast, fail-safe, and robust method for absolute wavelength calibration. Furthermore, the above-described method of wavelength calibration has the advantage that calibration using additional samples, such as a polystyrene film, can be avoided. This is particularly advantageous when determining the quality of a sample containing pharmaceutical substances, as can be done, for example, in pharmacies or by pharmacists, since the aforementioned calibration method can be used to demonstrate sufficient resolution of an FTIR spectrometer, preferably the FTIR spectrometer according to the invention.

[0064] The interaction of the light coming from the interferometer and guided to the measuring cell with the sample material takes place in the measuring cell or at and / or in the sample interface contained therein. The sample interface provides an interface at which the infrared light can be coupled into and out of the sample. For example, the sample interface can be or comprise a fiber optic. Alternatively, the sample interface can also be a device that enables measurement using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Alternatively, the sample interface can also be a device that enables the recording of infrared spectra using the transmission method. Preferably, the light is coupled into the sample interface via a free beam.Particularly preferably, the light is coupled into the sample interface as a free beam after prior focusing by means of a parabolic mirror or an off-axis parabolic mirror.

[0065] The sample interface is preferably an ATR crystal. The ATR crystal can be made, for example, of ZnSe, Ge, thallium bromoiodide (KRS-5), Si, AMTIR (amorphous material transmitting infrared radiation, e.g., GeAsSe = AMTIR-1), or diamond. The ATR crystal can have a surface that can be brought into contact with the sample or a sample material. For example, a sample can be pressed onto the surface of the ATR crystal by suitable means. One suitable means could be a clamping or screw device that applies pressure to the sample.

[0066] The infrared detector has a sensitivity in the wavelength range in which the infrared spectra are to be measured. The sensitivity of the infrared detector can, for example, be in the entire range from 1 pm to greater than 50 pm or in one or more of the following subranges: 1 to 2.5 pm (near infrared), 2 to 25 pm (mid-infrared), or greater than 50 pm (far infrared). Preferably, the infrared detector has a sensitivity in the near and mid-infrared range, i.e., in the range from 1 pm to 25 pm.

[0067] The infrared detector can be or comprise, for example, a photodiode.

[0068] The infrared detector can preferably be a pyroelectric sensor or comprise a pyroelectric sensor. Additionally, the infrared detector can have a window made of a material permeable to infrared radiation. Suitable materials have already been mentioned above in connection with the material selection of the window of the infrared detector and the beam splitter of the interferometer. In the context of this invention, a pyroelectric sensor is a component in which, due to its pyroelectric properties, a temperature difference causes a change in the electrical voltage of the component. Pyroelectric sensors have the advantage of having a wide optical detection bandwidth during measurement. In other words, pyroelectric sensors have the advantage of being able to measure a wide wavelength range compared to other known sensors.

[0069] The control system configured to change the length of the at least one arm, e.g. the first and / or the second arm, of the interferometer can be designed in various ways. For example, the control system can be or comprise a microprocessor, microcontroller or a computer. The control system can, for example, be configured to control one or more electromechanical actuating elements, e.g. an electric motor or a voice coil, or one or more piezoelectric actuating elements. In this case, the one actuating element or the multiple actuating elements can be coupled to the at least one arm or both arms and / or the mirrors contained therein in such a way that upon actuation of an actuating element, the length of one arm or both arms can be changed.

[0070] Alternatively, it is conceivable that the control system is or includes an electronic or electrical circuit that controls the aforementioned control elements. For example, the control system can simply provide a periodically changing voltage, such as an alternating voltage, that causes the electric motor or the voice coil, and thus the rotating rocker, to move in a pendulum motion. Alternatively, the control system can provide a direct voltage or other voltage that is periodically or irregularly switched on and off, thus causing the electric motor or the voice coil, and thus the rotating rocker, to move in a pendulum motion.

[0071] The control system can be configured to control the length of the at least one arm, e.g. of the first and / or the second arm, or the actuating elements autonomously, ie without additional external control signals from outside the interferometer or the FTIR spectrometer according to the invention. Alternatively, the control system can be configured to control the length of the at least one arm, e.g. of the first and / or the second arm, or the actuating elements as a function of or in response to external control signals from outside the interferometer or the FTIR spectrometer. Within the scope of this invention, controlling orControlling the movement of the movable mirrors can be described as follows: switching the aforementioned control elements on, off, or switching them over; regulating the movement of at least one arm or both arms by means of the aforementioned control elements with a closed or open control loop known in the art; or any other suitable method by which one or more of the aforementioned control elements change the length of at least one arm of the interferometer in the desired manner. Additionally, the control system can be designed and configured to control the infrared detector and / or to control and / or perform data acquisition. For the purposes of the invention, the term "mirror arrangement" refers to the arrangement of those mirrors within the FTIR spectrometer according to the invention that are not included in the structure of the interferometer of the FTIR spectrometer.In other words, within the meaning of this invention, the mirror arrangement of the FTIR spectrometer comprises all mirrors within the FTIR spectrometer outside the interferometer. In other words, the mirrors described within the scope of the invention (sometimes referred to as "inventive mirrors") relate exclusively to at least one mirror outside the interferometer of the FTIR spectrometer. The mirrors within the interferometer of the FTIR spectrometer are not the subject of this invention. Within the meaning of the invention, the steering or alternatively the guiding of the light, for example, from the interferometer to the sample interface and further to the infrared detector, comprises a reflection of the light and optionally a beam shaping of the light beam. The steering can preferably be carried out by means of the reflective surfaces of the mirrors described within the scope of the invention.Beam shaping may include, for example, focusing, collimating or any other advantageous modification of the light beam.

[0072] The mirror arrangement outside the interferometer comprises at least two mirrors, each with a reflective surface and a base body comprising the reflective surface, wherein the mirror arrangement is at least configured to direct a light beam from the interferometer to the sample interface and to direct the light beam from the sample interface to the infrared detector. Preferably, the mirror arrangement outside the interferometer can comprise at least two mirrors, each with a reflective surface and a base body comprising the reflective surface, wherein the mirror arrangement is at least configured to direct a light beam from the infrared radiation source to the interferometer and / or from the interferometer to the sample interface and to direct the light beam from the sample interface to the infrared detector.The reflective surface of one of the at least two or all mirrors is preferably concave in sections or designed as a concave mirror. In the context of this invention, the base body of a mirror is any structure or body that comprises or holds the reflective surface or to which the reflective surface is applied in sections directly or indirectly, e.g. via intermediate layers, and thus makes the reflective surface connectable to other parts of the FTIR spectrometer according to the invention via the base body. If the reflective surface is or comprises, for example, a metal coating, the reflective surface can, for example, be applied directly to a section of the surface of the base body. Alternatively, the metal coating can be applied to intermediate layers. The intermediate layers (or at least one of them) can in turn be applied directly to the section of the surface of the base body.

[0073] The base body can have block-like sections at least in sections or consist entirely of one or more block-like sections. Block-like in this context means that it is not plate-like. A section is plate-like if it is thin and follows a plane or a curved surface. Block-like sections can, for example, be constructed according to one or more of the following basic geometric shapes: cuboid, cube, cylinder, pyramid, cone, or sphere.

[0074] The following parts are not part of the base body within the meaning of the invention: partial or complete external coatings of the base body, e.g. paints, varnishes, powder coatings, protective coatings, and / or other coatings. The following parts are also not part of the base body within the meaning of the invention: devices that are intended for a user to operate, hold or mount the mirror to the base body and / or have a decorative function. The following parts are also not part of the base body within the meaning of the invention: partial or complete coatings of the reflective surface, which, for example, provide a protective function for the reflective surface and / or influence the optical properties of the reflective surface.

[0075] The base body can be configured as a single piece together with the reflective surface. In this case, the reflective surface can be applied directly to a surface section of the base body. For example, the reflective surface can be applied directly to a block-like section of the base body. Alternatively, it is also conceivable for the reflective surface to be applied indirectly, i.e., for example, to an intermediate layer on the surface section of the, for example, block-like base body. The base body can, for example, be connectable to a part, e.g., a part of a housing or a base plate of the FTIR interferometer according to the invention, or more correctly, of the FTIR spectrometer according to the invention.

[0076] As an alternative to the one-piece design, the base body can be designed in multiple pieces with at least a first and a second part (and optionally further parts such as spacers or the like). In this context, it is conceivable, for example, that the first part of the base body, as in the one-piece case, comprises the reflective surface directly on a surface section or comprises the reflective surface indirectly via an intermediate layer. For example, a block-like section of the base body can comprise the intermediate layer and thereon the reflective surface. The first part of the base body can then be connectable to the second part of the base body (and optionally further parts of the base body) with a part of the housing or the base plate of the FTIR interferometer according to the invention or, more correctly, of the FTIR spectrometer according to the invention.

[0077] According to the invention, the base body of at least one mirror or all of the mirrors of the mirror arrangement are made from a plastic material. Alternatively, the base body of at least one mirror or all of the mirrors of the mirror arrangement comprise plastic material. Alternatively, the base body of at least one mirror or all of the mirrors of the mirror arrangement is made from 3D-printed metal. Alternatively, the base body of at least one mirror or all of the mirrors of the mirror arrangement comprise 3D-printed metal. In particular, one or all of the mirrors of the mirror arrangement can be made at least partially from a plastic material.

[0078] For the purposes of the invention, a plastic material refers to a thermoplastic, in particular a semi-crystalline thermoplastic or an amorphous thermoplastic. The plastic material is preferably a semi-crystalline or amorphous thermoplastic. Semi-crystalline and amorphous thermoplastics have the advantage of being easy to process, widely available, and cost-effective. Alternatively, the plastic material can also be a thermoset.

[0079] For the purposes of this invention, a 3D printing process for metal encompasses any suitable 3D printing process for printing metal known in the prior art. An example of a suitable material for 3D printing from metal is stainless steel, aluminum, or titanium. Preferably, the 3D printing process for metal has a maximum print resolution per layer of 230 pm. In combination with an optional subsequent polishing step, a smooth surface of high quality can be provided on the 3D-printed material. Particularly preferably, the 3D printing process for metal has a maximum print resolution of 30 pm per layer. A preferred material for 3D printing is stainless steel.

[0080] The entire FTIR spectrometer is preferably hermetically sealed. Within the scope of the invention, hermetic encapsulation of the FTIR spectrometer means that, in particular, there is no exchange of gases between the internal structure of the FTIR spectrometer comprising the features recited in claim 1 and the space surrounding the FTIR spectrometer. Thus, the amount of water, particularly in the form of water vapor, remains constant inside the FTIR spectrometer. Water or water vapor exhibits characteristic vibration modes in the wavelength range typically of interest in the analysis of infrared spectra. Hermetic encapsulation has the advantage that the vibration modes remain constant during operation of the FTIR spectrometer and can be subtracted from the actual signal as background using a reference measurement. This improves the SNR.

[0081] The mode of operation of the FTIR spectrometer according to the invention is described below by way of example: The infrared radiation source is operated, e.g., with the aid of an electric current, and emits light at least in the infrared range. The light from the infrared radiation source is collimated, directed to the interferometer, and strikes the beam splitter in the interferometer. The beam splitter splits the light into two individual beams. A first individual beam is reflected in the first arm by a first mirror back to the beam splitter. A second individual beam is reflected by a second mirror back to the beam splitter. At least one of the two arms, or even both arms, are variable in length. In the case of a mirror that is movable along a linear axis, the control system periodically moves the mirror between a first and a second inflection point using an actuator, thus changing the length of the arm.In the case of a rotating rocker, the control system regulates the rocker's drive in such a way that the rocker performs a pendulum motion relative to the stationary beam splitter between two endpoints, with one arm shortening and the other arm lengthening relative to the beam splitter. After reflection from the mirrors of the two arms, the two individual beams are recombined in the beam splitter, interfere, and exit the interferometer.

[0082] To record a reference spectrum of infrared light, i.e. a spectrum of infrared light without interaction of the light with the sample, the infrared light is reflected towards the measuring cell by part of the mirror arrangement after leaving the interferometer. In the measuring cell, the light beam couples into the sample interface, e.g. an ATR crystal. However, the sample interface is not in contact with the sample or sample material. The infrared light that leaves the sample interface again carries the information characteristic of the sample interface, e.g. the absorption of the ATR crystal. The light is directed onto the infrared detector by means of another part of the mirror arrangement and measured there. This reference spectrum is later used to calculate the infrared spectra.

[0083] To record a sample spectrum, i.e., the recording of a spectrum of infrared light following interaction of the infrared light with the sample or sample material, the infrared light is reflected by part of the mirror arrangement towards the measuring cell after leaving the interferometer. In the measuring cell, the light beam couples into the sample interface, for example an ATR-Krista II. The light which leaves the sample interface, e.g., the ATR-Krista II, carries characteristic information for the sample or sample material and for the sample interface, e.g., the ATR crystal. The light is directed onto the infrared detector by reflection using another part of the mirror arrangement, and is detected by the infrared detector.

[0084] In addition to the infrared light leaving the sample, the infrared detector or a separate detector, such as a separate photodiode, preferably detects the reference laser beam, which is also passed through the interferometer and interferes there. The reference laser beam and the light beam from the infrared radiation source interact only negligibly or not at all.

[0085] The infrared light leaving the sample, captured by the infrared detector (i.e., the sample signal), and the reference laser beam signal are recorded and processed, for example, by the control system or a separate measurement computer. The sample signal is preferably Fourier transformed and corrected for the reference spectrum. Corresponding methods are known in the art. The signal of the reference laser beam is assigned a path difference of the arms in the interferometer. The desired infrared spectra are calculated from the processed sample signal and the path difference using methods known in the art.

[0086] The FTIR spectrometer has the advantage of eliminating the disadvantages of the prior art. In particular, the optical system of the FTIR spectrometer according to the invention can be manufactured using simple technical means. Furthermore, the optical system can be manufactured cost-effectively and quickly from materials that are widely available commercially and are easy to process. This significantly reduces both the manufacturing effort and the costs of the FTIR spectrometer. Furthermore, the simplified manufacturing effort makes the FTIR spectrometer according to the invention more sustainable than comparable known FTIR spectrometers.

[0087] In other words, the FTIR spectrometer according to the invention enables the coupling and decoupling of the largest possible amount of light from an extended, broadband light source into and out of a sample in contact with a sample interface, which can preferably be an ATR crystal, in order to maximize the SNR in the FTIR spectrometer. The manufacturing effort and the costs of the optical components in the form of mirrors, a key price factor of the FTIR spectrometer, are kept as low as possible without having to compromise on signal quality. This is achieved through an achromatic optical design, particularly the mirror arrangement, which consists partly or even exclusively of similar reflective mirrors and avoids absorption and dispersion in transmissive optics.

[0088] A further and surprising advantage of the FTIR spectrometer according to the invention is that all mirrors of the mirror array are arranged outside the interferometer, thus eliminating the need for high demands on wavefront errors and thus on the quality of the optical surface of the mirrors of the mirror array. Wavefront errors of the mirror array do not then have an effect in the form of an interferometric contrast, but rather only on the achievable transmission through the optical structure. The special arrangement of the mirrors of the mirror array outside the interferometer enables the use of the materials described in this invention for the base body.

[0089] In a preferred embodiment of the FTIR spectrometer, at least one mirror of the mirror arrangement outside the interferometer has a mirror shape or a combination of mirror shapes from the following list: an off-axis parabolic mirror, a parabolic mirror, a compound parabolic concentrator, a spherical concave mirror, a mirror that has the shape of at least one parabolic segment or a circular segment in at least one axis. Within the meaning of the invention, the mirror shape of a mirror of the mirror arrangement or a combination of mirror shapes of a mirror of the mirror arrangement describes either solely the geometric design of the reflective surface of the mirror of the mirror arrangement or the entire or partial geometric design of the mirror of the mirror arrangement.

[0090] For the purposes of the invention, a parabolic mirror is a concave mirror in the form of an axis-symmetric section of a paraboloid of revolution, with the focal point located on the axis of symmetry of the section of the paraboloid of revolution. A paraboloid of revolution is a concave surface described by the rotation of a parabola around an axis. For the purposes of the invention, an off-axis parabolic mirror is an asymmetric section of a paraboloid of revolution, with the section having an offset from the axis of symmetry of the paraboloid of revolution and from the focal point. For the purposes of the invention, a compound parabolic concentrator is a non-imaging mirror that focuses all incident light onto one surface within the largest possible acceptance angle. For the purposes of the invention, a spherical concave mirror is a concave mirror whose shape can be represented by a section of a hollow sphere.

[0091] Such mirrors have the advantage that they either effectively redirect and simultaneously focus incoming light, particularly infrared radiation (parallel incoming light rays) or redirect and simultaneously collimate it (divergent incoming light rays). Furthermore, such mirrors are easy to manufacture. A further advantage of the design using the mirrors described here is the reduction of absorption and dispersion of infrared light in optical elements in the FTIR spectrometer according to the invention outside the interferometer. This significantly reduces, in particular, wavelength-dependent transmission of infrared light.

[0092] In a preferred embodiment of the FTIR spectrometer, each mirror of the mirror arrangement outside the interferometer has a mirror shape or a combination of mirror shapes from the following list: an off-axis parabolic mirror, a parabolic mirror, a compound parabolic concentrator, a spherical concave mirror, a mirror which has the shape of at least one parabolic segment or a circular segment at least in one axis.

[0093] The advantage of this design with purely reflective optical elements outside the interferometer is the complete avoidance of absorption and dispersion of infrared light in the optical elements of the FTIR spectrometer according to the invention. This significantly reduces or even completely eliminates wavelength-dependent transmission of infrared light. Furthermore, the design of the FTIR spectrometer according to the invention is further simplified. The manufacturing effort and complexity of the optical components of the FTIR spectrometer are also significantly reduced, as the mirror designs are technically simple to manufacture by using the manufacturing processes and the material of the base bodies. This also has the significant advantage of reduced manufacturing costs for the mirrors and the FTIR spectrometer.

[0094] In a preferred embodiment of the FTIR spectrometer, at least one of the mirrors of the mirror arrangement, whose base body is made of a plastic material or comprises a plastic material, is manufactured by an injection molding process or a 3D printing process and the reflective surface is at least partially formed by a metal coating.

[0095] In the sense of the invention, the manufacture of a mirror of the mirror arrangement is understood to mean in particular the manufacture of the base body of the mirror as well as the manufacture of the reflective surface of the mirror.

[0096] For the purposes of this invention, an injection molding process describes a primary molding process known in the prior art, in which a plastic material is liquefied (plasticized) using an injection molding machine and injected under pressure into a mold, the injection mold. After the plastic material has cooled or the plastic material has crosslinked in the injection mold, the plastic material transforms into a solid state and can be removed.

[0097] For the purposes of this invention, a 3D printing process describes a manufacturing process known in the prior art from the field of additive manufacturing. Typical examples include the following technologies: fused deposition modeling (FDM), fused filament fabrication (FFF), direct ink writing (DIW), composite filament fabrication (CFF), stereolithography (SLA), digital light processing (DLP), and / or continuous liquid interface production (CLIP).

[0098] The metal coating can, for example, comprise one or more of the following materials or consist of one or more of the following materials or a combination of the materials: aluminum, gold, silver, rhodium, nickel, chromium, platinum, copper. The metal coating can, for example, be applied by vapor deposition of the surface of the base body to be coated, for example using the process of physical vapor deposition (PVD) or chemical vapor deposition (CVD). Alternatively or additionally, the metal coating can be applied by immersing at least the surface of the base body to be coated in a metal bath or by spraying the surface of the base body.

[0099] As an alternative to the aforementioned methods, it is also conceivable for at least one or all of the mirrors or their base bodies to be manufactured using milling or cutting methods known in the art. Preferably, at least one or all of the mirrors manufactured using one or more of the aforementioned methods can be post-processed in a step following the manufacturing process. Examples of preferred post-processing techniques include milling, cutting, grinding, and polishing.

[0100] Preferably, one or more mirrors of the mirror arrangement, the base body of which is made of a plastic material or comprises a plastic material and is produced by an injection molding process or a 3D printing process, or the reflecting surface of which is only partially illuminated. Preferably, the reflecting surface of the mirror of the mirror arrangement of the FTIR spectrometer according to the invention is only illuminated to a maximum of 98%, particularly preferably to 95%, even more preferably to 93% of the entire reflecting surface of the mirror(s). Most preferably, the reflecting surface is only illuminated in an area that contributes to the successful focusing or collimation of the infrared light.Successful focusing or collimation occurs when less than 5%, preferably less than 3%, of the reflected light fails to reach the nearest optical element in the beam path or the infrared detector. According to the invention, the illuminated area is preferably symmetrically designed and / or arranged symmetrically with respect to a center point of the reflecting surface. This has the advantage that the edge does not contribute to the reflection of the mirror. As a result, the reflection of the mirror is significantly more controlled, and the light reflected by the mirror is significantly more homogeneous and symmetrical.

[0101] The mirrors of the mirror assembly described within the scope of the invention are preferably held by a mirror holder. Preferably, part or all of the mirror holder can be manufactured using an injection molding process or a 3D printing process. The same materials as those already described within the scope of the invention in connection with the mirrors described above can be used. Alternatively, fiber-reinforced polyamide, for example, can also be used as the material for part or all of the mirror holder.

[0102] Mirrors manufactured in this way for reflecting infrared light have the advantage that, compared to mirrors manufactured using methods known in the prior art, they can be manufactured using simple and familiar means and with minimal technical effort. Furthermore, the described manufacturing processes allow for the production of large quantities in a short time. The aforementioned advantages also result in significantly lower manufacturing costs per mirror compared to known manufacturing processes for mirrors for reflecting infrared light.

[0103] Finally, the mirrors manufactured using the described processes are suitable for use in FTIR spectrometers. The mirrors described here for reflecting infrared light meet, in particular, the high quality requirements for optical components used in FTIR spectrometers. This overcomes a long-standing prejudice in the state of the art.In a preferred embodiment of the FTIR spectrometer, the plastic material is at least one material from the following list or comprises at least one material from the following list: polymethyl methacrylate (PMMA), polycarbonate (PC), cycloolefin polymer, cycloolefin copolymer, styrene acrylonitrile, styrene acrylonitrile, polycarbonate high temperature, polysulfone (PS), polyamide (PA), polycarbonate high refractive, polyester high refractive, polyethylene terephthalate (PET), polyethylene terephthalate with glycol (PETG), acrylonitrile-butadiene-styrene copolymer (ABS), nylon, polylactic acid (PLA), polyurethane (PU), a light-curing plastic (photopolymer), for example acrylic, epoxy and / or vinyl ester resin.

[0104] Preferably, the plastic material can also be a combination of two materials from the above list.

[0105] Particularly preferably, the plastic material comprises or is polycarbonate (PC). Mirrors according to the invention with a polycarbonate base body have the advantage of having a low wavefront error when reflecting infrared light. Furthermore, they are inexpensive to manufacture and easy to process and manufacture. Furthermore, it has been shown that polycarbonate mirrors can achieve an excellent surface roughness of <10 nm.

[0106] PLA and PETG are particularly easy to process in 3D printing. ABS has a higher melting point, is very stiff and scratch-resistant, as well as moisture-repellent, and, despite its high mechanical robustness, is easy to machine. Particularly smooth surfaces are possible when using PMMA and PC. Very smooth surfaces can also be produced using a light-curing plastic (photopolymer), such as acrylic, epoxy, and / or vinyl ester resin, for example, using stereolithography processes (SLA or DLP). These smooth surfaces are particularly suitable for use as a surface for applying a reflective surface to a mirror. All of the materials mentioned above have the advantage of being easy to process. Furthermore, metal coatings adhere particularly well to the materials mentioned here.The aforementioned materials are also suitable for the aforementioned manufacturing processes, particularly for use in injection molding and / or 3D printing. Furthermore, the aforementioned materials exhibit advantageous temperature properties for use in an FTIR spectrometer. The aforementioned materials are also easy to process and post-process, and cost-effective.

[0107] Preferably, the plastic material can comprise a fiber material in addition to the aforementioned materials, thus forming a composite material at least in sections. The fiber material can be, for example, carbon fibers or glass fibers. The addition of fibers generally improves the mechanical and, in particular, the temperature-dependent properties of the plastic material. In a preferred embodiment of the FTIR spectrometer, the reflective surface of at least one mirror of the mirror arrangement has, at least in some regions, a freeform optic.

[0108] For the purposes of the invention, a freeform optic is a reflective surface that differs from spherical and parabolic geometries. For example, a freeform optic can be a reflective surface that differs at least in some areas from the mirror shapes mentioned in claims 2 and 3 or combinations thereof.

[0109] By using freeform optics, beam-shaping and beam-steering properties of the mirrors can be controlled and achieved, which are not possible with simpler designs. This can further improve the efficiency of the FTIR spectrometer.

[0110] In a preferred embodiment of the FTIR spectrometer, the freeform optics has at least in some regions a shape deviation from one of the following mirror shapes: an off-axis parabolic mirror, a parabolic mirror, a compound parabolic concentrator, a spherical concave mirror, a mirror which has the shape of at least one parabolic segment or a circular segment at least in one axis.

[0111] In a particularly preferred embodiment of the FTIR spectrometer, the freeform optics has at least partially or completely a shape deviation from the following mirror shapes: an off-axis parabolic mirror, a parabolic mirror, a compound parabolic concentrator, a spherical concave mirror, a mirror which has the shape of at least one parabolic segment or a circular segment at least in one axis.

[0112] Simulations have shown that shape deviations can have a positive effect on beam shaping. For example, combining a spherical portion into a parabolic shape can achieve a more targeted, advantageous reflection and focusing of a light beam. The provision of mirrors with freeform optics can be implemented easily and cost-effectively within the scope of the manufacturing methods mentioned in this invention, particularly with regard to the methods known in the prior art.

[0113] In a preferred embodiment of the FTIR spectrometer, the freeform optics has at least a partial shape deviation in an edge region.

[0114] In a particularly preferred embodiment of the FTIR spectrometer, the freeform optics exhibit a shape deviation in an edge region. Preferably, the freeform optics of the FTIR spectrometer exhibit a shape deviation from the following mirror shapes in an edge region: an off-axis parabolic mirror, a parabolic mirror, a compound parabolic concentrator, a spherical concave mirror, or a mirror that has the shape of at least one parabolic segment or a circular segment along at least one axis.

[0115] In the sense of this invention, the edge region preferably describes the transition between the reflecting surface and the base body of a mirror.

[0116] Particularly preferably, the edge region has a minimum extension or minimum radius of 1 mm, preferably 2 mm, more preferably 3 mm. It has been shown that this region is particularly advantageous for beam shaping and guidance.

[0117] In a preferred embodiment of the FTIR spectrometer, the shape deviation is a convex regular or irregular fillet or chamfer or a combination of a convex regular or irregular fillet and / or a chamfer.

[0118] The provision of such a rounding or chamfer has the advantage of preventing unwanted stray light from reflecting infrared light. Alternatively or additionally, by appropriately designing the rounding and / or chamfer sections, stray light can be reflected into areas within the FTIR spectrometer according to the invention where it does not interfere with or negatively influence the measurement signal.

[0119] In a preferred embodiment of the FTIR spectrometer, at least one mirror of the mirror arrangement or each mirror of the mirror arrangement is designed and arranged such that when infrared light is reflected at the respective mirror of the mirror arrangement, the infrared light has a maximum wavefront error per mirror of 50 times the wavelength, preferably 25 times the wavelength of the infrared light.

[0120] In the sense of the invention, a wavefront error describes a spatial phase shift between light waves that together form a light beam.

[0121] The wavefront error is significantly determined by the macroscopic shape of the mirrors of the mirror arrangement outside the interferometer described within the scope of the invention. Alternatively, the wavefront error can be determined by the surface quality of the mirrors of the mirror arrangement outside the interferometer described within the scope of the invention. Alternatively, the wavefront error can be determined by a combination of the macroscopic shape and the surface quality of the mirrors outside the interferometer described within the scope of the invention. The mirrors of the mirror arrangement outside the interferometer described within the scope of this invention comprise a base body and a reflective surface. Within the scope of the invention, the macroscopic shape of a mirror according to the invention of the mirror arrangement outside the interferometer describes the external geometric design of the mirror orthe external geometric design of the reflective surface. A non-exhaustive list of examples of macroscopic design elements that can be combined with one another may include the following: bulges, indentations, notches, edges, planes, recesses, or other known regular or irregular surface designs.

[0122] Within the scope of the invention, the surface quality of a mirror according to the invention of the mirror arrangement outside the interferometer describes the microscopic design of one or more interfaces of the mirror according to the invention or of the reflective surface. The interface can be the reflective surface of the mirror or can encompass or support it. In particular, the interface of a mirror described within the scope of the invention can be the region beneath the reflective coating. Alternatively or additionally, the above-described interface of the mirror according to the invention can be the reflective surface of the mirror on the base body within the scope of the invention. An example of a measure of the surface quality is the roughness of a surface or of the interface.

[0123] The wavefront error can be determined significantly by the macroscopic shape of at least one mirror according to the invention of the mirror arrangement outside the interferometer. Preferably, the wavefront error can be determined significantly by the macroscopic shape of each mirror of the mirror arrangement outside the interferometer.

[0124] The wavefront error can be determined significantly by the surface quality of at least one mirror according to the invention of the mirror arrangement outside the interferometer. Further preferably, the wavefront error can be determined significantly by the surface quality of each mirror of the mirror arrangement outside the interferometer.

[0125] Particularly preferably, the wavefront error can be determined significantly by the macroscopic shape and surface quality of at least one mirror of the mirror arrangement outside the interferometer. Further particularly preferably, the wavefront error can be determined significantly by the macroscopic shape and surface quality of each mirror of the mirror arrangement outside the interferometer.

[0126] Preferred examples of the low optical quality optics described above are optics with a maximum wavefront error of 50 times the wavelength of the reflected light, preferably 25 times the wavelength, more preferably 12.5 times the wavelength, most preferably 10 times the wavelength.

[0127] Particularly preferably, the macroscopic shape of at least one mirror of the mirror arrangement outside the interferometer or of each mirror of the mirror arrangement outside the interferometer can be designed and configured such that when infrared light is reflected at the respective mirror of the mirror arrangement, the infrared light has a maximum wavefront error per mirror of 50 times the wavelength of the infrared light, preferably 25 times the wavelength of the infrared light, more preferably 12.5 times the wavelength, particularly preferably 10 times the wavelength.

[0128] Particularly preferably, the surface quality of at least one mirror of the mirror arrangement outside the interferometer or of each mirror of the mirror arrangement outside the interferometer can be designed and arranged such that when infrared light is reflected at the respective mirror of the mirror arrangement, the infrared light has a maximum wavefront error per mirror of 50 times the wavelength of the infrared light, preferably 25 times the wavelength of the infrared light, more preferably 12.5 times the wavelength, particularly preferably 10 times the wavelength.

[0129] Particularly preferably, the macroscopic shape and the surface quality of at least one mirror of the mirror arrangement outside the interferometer or of each mirror of the mirror arrangement outside the interferometer can be designed and arranged such that when infrared light is reflected at the respective mirror of the mirror arrangement, the infrared light has a maximum wavefront error per mirror of 50 times the wavelength of the infrared light, preferably 25 times the wavelength of the infrared light, more preferably 12.5 times the wavelength, particularly preferably 10 times the wavelength.

[0130] The mirror shapes described within the scope of this invention, with the structure of a mirror of the mirror arrangement comprising the base body and the reflective surface described within the scope of this invention, enable such a low maximum wavefront error, in particular due to the macroscopic shape of the mirror and / or the surface finish of the mirror. Furthermore, a grinding or polishing step can be performed before applying the metal coating to the base body. In this context, a metal coating is an example of an advantageous coating that provides a reflective surface. Grinding and polishing steps are examples of means known in the prior art for processing the macroscopic shape and surface finish.In addition to or as an alternative to the grinding and polishing steps, other means known in the prior art for processing the macroscopic shape and surface texture are also conceivable. For example, surface roughness can be minimized through grinding and / or polishing steps. This also minimizes the wavefront error. In particular, the wavefront error can be minimized by processing the surface texture of the base body before and / or after applying the metal coating or providing the reflective surface.

[0131] A deterioration of the wavefront outside the interferometer only results in a loss of efficiency, which, however, is not relevant for the recording and processing of the infrared spectra up to the above-mentioned maximum per mirror for the wavefront error.

[0132] Due to this surprising property, the efficiency of the mirror arrangement used in the invention, which has the above-mentioned maximum wavefront error, remains comparable to the efficiency of mirrors used in FTIR spectrometers known in the prior art, but at significantly reduced costs and lower manufacturing effort.

[0133] In a preferred embodiment of the FTIR spectrometer, the mirrors of the mirror array and the interferometer are designed and configured such that, upon reflection of infrared light from all mirrors of the mirror array, the infrared light exhibits a total maximum wavefront error of 300 times the wavelength of the infrared light. Preferably, the mirrors of the mirror array outside the interferometer are designed and configured such that, upon reflection of infrared light from all mirrors of the mirror array, the infrared light exhibits a total maximum wavefront error of 300 times the wavelength of the infrared light, preferably 200 times the wavelength of the infrared light.

[0134] Particularly preferably, the macroscopic shape and / or surface quality of the mirrors of the mirror arrangement outside the interferometer is designed and arranged such that when infrared light is reflected at all mirrors of the mirror arrangement outside the interferometer, the infrared light has a total maximum wavefront error of 300 times the wavelength of the infrared light, preferably 200 times the wavelength of the infrared light.

[0135] The wavefront of the infrared light is no longer modified within the interferometer. This means that there is no difference between the interference of two light beams with identical wavefronts with a high wavefront error compared to the interference of light beams with perfectly flat wavefronts. The wavefront error already present before entering the interferometer is thus retained after entering the interferometer, split in the beam splitter, and then recombined. Further deterioration due to errors in the optics, particularly the mirrors of the mirror arrangement, only results in a loss of efficiency, which, however, is not relevant for the recording and processing of the infrared spectra in the FTIR spectrometer according to the invention, up to the above-mentioned maximum for the total wavefront error caused by the mirror arrangement.Due to this surprising property, the efficiency of the mirror arrangement used in the invention remains comparable to the efficiency of known FTIR spectrometers in the state of the art at the above-mentioned maximum wavefront error, but at significantly reduced costs and lower manufacturing effort.

[0136] In a preferred embodiment of the FTIR spectrometer, the mirror arrangement comprises at least two off-axis parabolic mirrors with a first focal length and at least two parabolic mirrors with a second focal length.

[0137] Preferably, at least four mirrors are arranged in the following order along the beam path: off-axis parabolic mirror (with first focal length f1) - parabolic mirror (with second focal length f2) - sample interface (e.g., ATR crystal) - parabolic mirror (with second focal length f2) - off-axis parabolic mirror (with first focal length f1). The first and second focal lengths f1 and f2 preferably do not have the same values. The second focal length f2 can, for example, be in the range from 1 mm to 2.5 mm, preferably 1.7 mm. The ATR crystal preferably has a maximum area for contact with the sample that is less than 2.5 mm by 2.5 mm.

[0138] The use of the previously described mirror arrangement with the four mirrors allows for the provision of an intermediate focus for adjusting the resolution. Furthermore, parallel beams through an off-axis parabolic mirror before and after the sample interface, preferably an ATR crystal, allow a variable distance to the remaining optics in the FTIR spectrometer without changing the imaging properties. Furthermore, parallel beams before and after the sample interface in the measuring cell allow for easy replacement of the sample interface. For example, a sample interface in the form of an ATR crystal can be replaced with another single- or multiple-reflection ATR, transmission, and / or DRIFTS setup.

[0139] Alternatively, the parabolic mirror can be constructed as a single, integral parabolic mirror before and after the sample interface. An example of this is a parabolic mirror or CPC as used in conventional flashlights. In this case, the sample interface, such as the ATR crystal, can be arranged in an opening at the focal point of the single-piece parabolic mirror. Positioning the sample interface or ATR crystal at the focal point of the parabolic reflector allows for a very compact and cost-effective design for coupling light into and out of the sample. Furthermore, such a design is robust and avoids or reduces problems caused by misalignment of the mirror arrangement.

[0140] In a preferred embodiment of the FTIR spectrometer, the sample interface is an ATR crystal housed in a holder, wherein the holder is manufactured from metal using a 3D printing process. For the purposes of this invention, the 3D printing process for metal encompasses any suitable 3D printing process for printing metal known in the art.

[0141] The 3D printing process for metal preferably has a maximum print resolution of 230 pm per layer. This ensures the necessary fit of the ATR crystal in the holder.

[0142] Preferably, the holder has at least one web or receptacle designed and configured to act as an abutment for the diamond, transmitting or absorbing compressive forces on the diamond to the holder. Compressive forces can arise, for example, when samples are pressed against the ATR crystal. Such a design of the holder ensures long-lasting and safe use of the holder and the ATR crystal accommodated therein.

[0143] The web can divide an opening extending through the holder from a top side to a bottom side into two sections or two openings on a bottom side. When the ATR crystal is accommodated in the holder, the sections are configured to allow the infrared light entering and exiting the ATR crystal to pass through. The opening can be configured on the top side such that the ATR crystal can be inserted therein with a precise or almost precise fit and can be flush with a surface of the top side. In this case, the top side can be an area in which the ATR crystal can be brought into contact with a sample or sample material.

[0144] The holder is preferably printed from stainless steel or titanium. Stainless steel and titanium can withstand high tensile and compressive forces and are chemically inert.

[0145] Preferably, the ATR crystal is glued into the holder with an adhesive or soldered with a solder. Further preferably, the ATR crystal is glued into the holder in such a way that the opening in the top of the holder is sealed fluid-tight by the ATR crystal and the adhesive or solder. Both gluing and soldering are joining methods that can be performed with minimal technical effort, high precision, and low cost.

[0146] The solder used to solder the ATR crystal into the holder preferably comprises or is silver solder with or without a titanium content. Both of these solder types exhibit advantageous wetting and bonding properties with both the ATR crystal, preferably diamond, and the stainless steel or titanium holder. This creates a strong and durable bond between the ATR crystal and the holder.

[0147] Soldering is preferably performed in a vacuum furnace. This ensures that the ATR crystal, preferably a diamond, is not damaged during soldering. As already mentioned, the bonding or soldering of the ATR crystal in the holder preferably forms a ventilating and watertight seal around the opening in the top of the holder. This has the advantage that the hermetic encapsulation of the FTIR spectrometer is maintained when the holder with the ATR crystal is mounted in the FTIR spectrometer, preventing any additional water from entering the FTIR spectrometer.

[0148] The holder can be manufactured, for example, according to the following manufacturing process, taking into account the above-mentioned properties:

[0149] - printing the holder from metal, preferably from stainless steel or titanium, more preferably with a print resolution per layer of maximum 230 pm, and

[0150] - Soldering an ATR crystal, preferably in a vacuum furnace, more preferably with a silver solder with or without titanium content.

[0151] Preferably, the ATR crystal can be soldered into a stainless steel holder. This is possible due to the manufacture of the holder from 3D-printed metal in combination with the soldering of the ATR crystal into the holder, despite the different thermal expansion coefficients of stainless steel and, for example, diamond as the material for the ATR crystal. State-of-the-art molybdenum holders are significantly more complex and expensive to manufacture than the aforementioned design. Due to the lower cost of stainless steel, the entire holder can be 3D-printed in one piece, thus eliminating the need for precise and tight fitting of a molybdenum diamond holder into a larger stainless steel holder, as is common in the prior art.

[0152] The general advantage of manufacturing the holder using 3D printing from metal, and in particular according to the method described above, is that it is significantly simpler and more cost-effective than conventional manufacturing processes such as milling or spark erosion from solid material. Furthermore, the 3D printing process allows for the creation of geometries that are difficult or impossible to achieve using conventional manufacturing methods. Such a holder can be manufactured in small dimensions and can absorb the high pressures generated during contact between the ATR crystal and the sample or sample material without destroying or damaging the holder.

[0153] In a preferred embodiment of the FTIR spectrometer, the holder is configured to hold the ATR crystal stationary at a sample pressure of up to 130 bar against the ATR crystal.

[0154] Such pressures are necessary to ensure the necessary coupling and decoupling of light into and out of the sample through the ATR crystal. The holder described in the invention can withstand such pressures, particularly due to the provided web and the way the ATR crystal is mounted in the holder. In a preferred embodiment of the FTIR spectrometer, the ATR crystal has a maximum sample support area of ​​3 mm x 3 mm.

[0155] In the context of the invention, the sample support surface is the maximum surface of the ATR crystal that can come into contact with a sample or sample material. When using the holder described above, the sample support surface is located on top of the holder and is defined by the surface of the ATR crystal in the holder that is flush with the surface of the holder.

[0156] Preferably, the maximum sample support area is 2.8 mm x 2.8 mm, more preferably 2.5 mm x 2.5 mm, and even more preferably 2.0 mm x 2.0 mm. Small sample support areas are also reflected in the overall dimensions of the ATR crystal, which is why small ATR crystals can be used with small sample support areas. This requires less ATR crystal material, simplifying production and reducing costs.

[0157] In a preferred embodiment, the FTIR spectrometer according to the invention is used according to one of the above-mentioned embodiments for measuring a sample containing pharmaceutical substances. The FTIR spectrometer according to the invention is particularly preferably used according to one of the above-mentioned embodiments for determining the quality of a sample containing pharmaceutical substances. Such quality determinations can be carried out, for example, in pharmacies or by pharmacists. The quality determination can in particular comprise one or more steps: determining the identity of a substance, preferably a pharmaceutical one, determining a concentration of one or more pharmaceutical substances in the sample, determining the purity of one or more pharmaceutical substances in the sample, determining a concentration of impurities in the sample, and qualitatively determining impurities, in particular their type, in the sample.

[0158] It should be clarified that one or more of the preferred embodiments described above, as long as they are consistent, can be combined with one another and also represent preferred embodiments.

[0159] Preferred embodiments of the invention are explained and described in more detail below with reference to the accompanying drawings.

[0160] Fig. 1 is a schematic representation of the structure of an FTIR spectrometer,

[0161] Fig. 2a, b an exemplary schematic beam path of the FTIR spectrometer according to the invention from Figure 1 with two different embodiments of a spectrometer structure, Fig. 3a, b two views of an exemplary schematic structure of a mirror of the mirror arrangement of the FTIR spectrometer according to the invention,

[0162] Fig. 4 shows a second, alternative schematic beam path of a part of the FTIR spectrometer according to the invention,

[0163] Fig. 5 shows an exemplary beam path within a Compound Parabolic Concentrator mirror,

[0164] Fig. 6a-c different views of a structure of a holder for an ATR crystal 11,

[0165] Fig. 7a, b two views of an exemplary mirror holder in the interferometer of the FTIR spectrometer according to the invention,

[0166] Fig. 8a-h Measurement results for measured parameters of mirrors of the mirror arrangement of the FTIR spectrometer according to the invention in comparison with commercially available precision metal mirrors,

[0167] Fig. 9a, b Results of a simulation to determine the transmission or luminous intensity of the mirror arrangement as a function of the maximum wavefront error through the mirrors of the mirror arrangement in the FTIR spectrometer according to the invention,

[0168] Fig. 10a, b show exemplary FTIR spectra recorded with an FTIR spectrometer according to the invention based on the mirror arrangement according to the invention with injection-molded mirrors and a mirror arrangement with commercially available precision metal mirrors for comparison of the results, and

[0169] Fig. lla-h various spatially resolved measurements of wavefront errors of metal precision mirrors and various embodiments of mirrors according to the invention.

[0170] Figure 1 shows a schematic representation of the structure of an embodiment of an FTIR spectrometer 1 according to the invention. Figure 2a schematically shows an exemplary beam path 13 of the FTIR spectrometer 1 with a first embodiment of an interferometer. Figure 2b schematically shows an alternative structure of an interferometer. The FTIR spectrometer 1 is described below:

[0171] The FTIR spectrometer 1 comprises an infrared radiation source 3, an interferometer 5a, 5b, a measuring cell 7, an infrared detector 9, and a control system 11. The interferometer 5a typically has a first and a second arm 12a, 12b, at least one arm being a variable-length arm 14. For example, the control system 11 can regulate a mirror offset of a mirror of the first arm 12a of the interferometer 5a, which mirror is movable along a linear axis, by appropriately controlling a corresponding actuator or control element. This changes the distance of the mirror of the variable-length arm 14 from the beam splitter 10, i.e., the length of the first arm 12a, and thus the path length traveled by the light L in the first arm 12a.

[0172] Alternatively, the interferometer 5b can comprise a rocker 16 rotatable in a plane, as shown in Fig. 2b. The rocker 16 is designed such that it comprises, in particular, the mirrors of the interferometer 5b necessary for reflecting both individual beams coming from the beam splitter 10. The rocker 16 thus forms or comprises both arms of the interferometer 5b. The control system 11 regulates a rotational movement, for example with the aid of a drive of the rocker 16, such that the rocker 16 executes a pendulum movement about an axis 18 between two endpoints relative to the stationary beam splitter. The rocker 16 can be driven, for example, with the aid of a voice coil.

[0173] The FTIR spectrometer 1 also has a reference laser. Using the reference laser, the position and an inclination angle of at least one mirror of one or both arms of the interferometer 5a, 5b can be determined, or a relative path length difference between the mirrors of the first and second arms 12a, 12b of the interferometer 5a, 5b can be determined.

[0174] The measuring cell 7 has a sample interface and can preferably include an ATR crystal 15 therein or thereon, which can be brought into contact with a sample 17. The control system 11 is configured to change the length of at least one arm of the interferometer.

[0175] The infrared detector 9 is configured to measure the intensity of the infrared light directed onto the infrared detector 9 after the interaction in the ATR crystal 15 or the sample 17. The infrared detector 9 can, for example, be or comprise a pyroelectric sensor. Alternatively or additionally, the infrared detector can be or comprise a photodiode.

[0176] In addition, the FTIR spectrometer 1 comprises a mirror arrangement 13 outside the interferometer 5a, 5b with at least two mirrors, for example, four mirrors 19a, 19b, 19c, 19d as shown in Fig. 2a, b. Each mirror 19a-d comprises a reflective surface 21 and a base body 23 that comprises the reflective surface 21 (see Fig. 3a, b). The base body 23 of at least one mirror 19a-d or all mirrors 19a-d of the mirror arrangement 13 is / are made of a plastic material and / or 3D-printed metal. Alternatively, the base body 23 of at least one mirror 19a-d or all mirrors 19a-d can comprise plastic material and / or 3D-printed metal.

[0177] 34

[0178] REVISED SHEET (RULE 91) ISA / EP The mirror arrangement 13 is at least designed to direct a light beam, ie light L, from the infrared radiation source 3 through the interferometer 5a, 5b onto the sample interface of the measuring cell 7 and to direct the light beam from the sample interface of the measuring cell 7 onto the infrared detector 9.

[0179] The operation of the FTIR spectrometer 1 according to the invention is described below by way of example. The infrared radiation source 3 is operated and emits light L at least in the infrared range. The light L from the infrared radiation source 3 is collimated by the mirror 19a into a light beam L and impinges on a beam splitter 10 in the interferometer 5a, 5b. The beam splitter 10 splits the light beam into two individual beams. A first individual beam is reflected in the first arm 12a by a first mirror back to the beam splitter 10. A second individual beam is reflected by a second mirror in the second arm 12b back to the beam splitter 10. At least one of the two arms, or even both arms, are variable in length.In the case of a mirror that is movable along a linear axis, the control system 11 periodically moves the mirror of the first arm 12a between a first and a second inflection point relative to the stationary beam splitter 10, thus changing the path length of the light in the first arm 12a, whereby the arm itself is an arm 14 with variable length. In the case of a rotatable rocker 16, the control system 11 regulates the drive of the rocker 16 such that the rocker 16 performs a pendulum movement relative to the stationary beam splitter 10 between two endpoints, thereby shortening one arm 12a or 12b and lengthening the other arm 12b or 12a. After reflection from the mirrors, the two individual beams are recombined in the beam splitter 10, interfere there, and exit the interferometer 5a, 5b as light beam L.

[0180] The recording of a reference spectrum of the infrared light L has already been described above in connection with the invention.

[0181] A sample spectrum, i.e. a spectrum of the light that has left the ATR crystal 15 after interacting with the sample 17, is now recorded analogously to the description above as follows: after leaving the interferometer 5a, 5b, the infrared light L is directed and focused towards the measuring cell 7 by part of the mirror arrangement 13, in the case of Fig. 2a, b by mirror 19b. In the measuring cell 7, the incident light 25 enters the ATR crystal 15 at an angle 0. At the interface between the ATR crystal 15 and the sample 17, an evanescent wave 27 is created, which interacts with the sample material. The light L leaves the ATR crystal 15 at the same angle 0 as outgoing light 29 and now carries information characteristic of the sample 17 or the sample material. The light L is reflected by means of another part of the mirror arrangement 13, ie in the case of Fig.2a, b are directed and focused by mirrors 19c and 19d by reflection onto the infrared detector 9 and detected by the infrared detector 9.

[0182] 35

[0183] REVISED SHEET (RULE 91) ISA / EP In addition to the infrared light leaving the sample 17, the infrared detector 9 or a separate detector, e.g., in the form of a separate photodiode, preferably detects the reference laser beam, which has also been directed through the interferometer 5a, 5b and interferes there. The reference laser beam and the light beam from the infrared radiation source 3 interact only negligibly or not at all.

[0184] The infrared light L detected by the infrared detector 9, which leaves the sample 17 (i.e., the sample signal), and the signal of the reference laser beam are recorded and processed, for example, by the control system 11, which comprises, for example, a microcontroller or microprocessor or, alternatively or additionally, a separate measuring computer. The sample signal is preferably Fourier transformed, for example, using a known Fast Fourier Transformation (FFT), and corrected for the reference spectrum. Corresponding methods are known in the art. The signal of the reference laser beam is assigned a path difference of the arms in the interferometer 5a, 5b. The desired infrared spectra are calculated from the processed sample signal and the path difference using methods known in the art.

[0185] Figure 3a, b shows, by way of example, a schematic structure of a mirror 19 of the mirror arrangement 13 of the FTIR spectrometer 1 according to the invention. The mirror 19 can, for example, be one, several or all of the mirrors 19a, 19b, 19c and / or 19d from Fig. 2a, b.

[0186] The mirror 19 comprises a reflective surface 21 and a base body 23 that includes the reflective surface 21. The base body 23 is made of a plastic material and / or 3D-printed metal. Alternatively, the base body 23 may comprise a plastic material and / or 3D-printed metal.

[0187] The reflective surface 21 of the mirror 19 is preferably concave in sections and / or designed as a concave mirror. The base body 23 of the mirror can encompass the reflective surface or hold it directly or indirectly, e.g., via intermediate layers. The reflective surface 21 can be connectable to other parts of the FTIR spectrometer 1 according to the invention via the base body 23. If the reflective surface is, for example, a metal coating, the reflective surface can, for example, be applied directly to a section of the surface of the base body. Alternatively, the metal coating can be applied to intermediate layers. The intermediate layers (or at least one of them) can, in turn, be applied directly to the section of the surface of the base body.

[0188] The base body 23 can be designed as a single piece together with the reflective surface 21. In this case, the reflective surface 21 is applied directly to a surface portion of the base body 23. Alternatively, it is also conceivable that the reflective surface 21 is applied indirectly, e.g., on an intermediate layer on the surface portion of the base body 23. The base body 23 can

[0189] 36

[0190] REVISED SHEET (RULE 91 ) ISA / EP for example, be connectable to a part, e.g. a part of the housing or a base plate of the FTIR interferometer 1 according to the invention.

[0191] As an alternative to the one-piece design, the base body 23 can be designed in multiple pieces (not shown) with at least a first and a second part (and optionally further parts such as spacers or the like). In this context, it is conceivable, for example, that the first part of the base body 23, as in the one-piece case, comprises the reflective surface 21 directly on a surface section or comprises the reflective surface 21 indirectly via an intermediate layer. The first part of the base body 23 can then be connectable to the second part of the base body 23 (and optionally further parts of the base body 23) with a part of the housing or a base plate of the FTIR interferometer 1 according to the invention.

[0192] The mirror 19 or the reflecting surface 21 of the mirror 19 may have a mirror shape or a combination of mirror shapes from the following list: an off-axis parabolic mirror, a parabolic mirror, a compound parabolic concentrator, a spherical concave mirror, a mirror which has the shape of at least one parabolic segment or a circular segment at least in one axis.

[0193] In addition, the reflective surface 21 of the mirror 19 of the mirror arrangement 13 can have a freeform optic at least in some areas. The freeform optic can, for example, have a shape deviation from one of the following mirror shapes at least in some areas: an off-axis parabolic mirror, a parabolic mirror, a compound parabolic concentrator, a spherical concave mirror, a mirror that has the shape of at least one parabolic segment or a circular segment at least in one axis.

[0194] It is also conceivable that the shape deviation is a convex regular or irregular fillet or chamfer or a combination thereof.

[0195] Preferably, the mirror 19 of the mirror arrangement 13 is designed and configured such that when infrared light L is reflected by the mirror 19 of the mirror arrangement 13, the infrared light L has a maximum wavefront error of 50 times the wavelength, preferably 25 times the wavelength of the infrared light L.

[0196] Figure 4 shows a second, alternative schematic beam path of a portion of the FTIR spectrometer 1 according to the invention. The configuration in Figure 4 comprises a mirror arrangement 13' with at least two off-axis parabolic mirrors 31 with a first focal length f1 and at least two parabolic mirrors 33 with a second focal length f2. The first and second focal lengths f1 and f2 preferably do not have the same values. Figure 5 shows an example of a compound parabolic concentrator mirror (CPC) 32. The spatially extended infrared radiation source 3, the infrared detector 9, or the ATR crystal 15 can be accommodated at a focal point 34 of the CPC 32.

[0197] In the case that the spatially extended infrared radiation source 3 is arranged in the focal point 34 of the CPC 32, the CPC is designed to collimate light L emitted by the infrared radiation source 3.

[0198] In the case that the infrared detector 9 or the ATR crystal 15 is arranged at the focal point 34 of the CPC 32, the CPC is designed to focus the preferably collimated light L incident in the direction of the infrared detector 9 or the ATR crystal 15 at an angle of up to 0 onto the infrared detector 9 or the ATR crystal 15.

[0199] Figures 6a to 6c show various views of a holder 35 structure for an ATR crystal 15 that can be used within the scope of the invention. Figure 6a shows an oblique plan view of a top side 37 of the holder 35, i.e., the side of the holder 35 facing a sample 17. The top side 37 of the holder 35 has an opening 39 that is designed to receive the ATR crystal 15 flush with the surface and to seal it off using suitable means, e.g., solder.

[0200] Figure 6b shows a bottom side 41, i.e., a surface of the holder 35 that is opposite the top side 37. The bottom side 41 has two openings 43 and 45 that are connected to the opening 39 of the top side. The two openings 43 and 45 are separated from each other by a web 47.

[0201] Figure 6c shows a sectional view along section line AA from Fig. 6a. Fig. 6c also includes an ATR crystal 15. The ATR crystal 15 is designed such that it is flush with the surface 37 of the holder 35 and rests on the web 47. The ATR crystal 15 can be fastened in the holder 35, for example, with the aid of solder or adhesive. By resting the ATR crystal 15 on the web 47, the web 47 can absorb any compressive forces when a sample 17 is pressed onto the ATR crystal 15. The web thus acts as an abutment with respect to compressive forces from the direction of the upper side 37 of the holder 35 on the ATR crystal. This can prevent the ATR crystal 15 from breaking out of the holder 35.

[0202] Preferably, the holder 35 is manufactured from metal using a 3D printing process. The 3D printing process preferably has a print resolution of a maximum of 230 pm per layer. This ensures the necessary fit of the ATR crystal in the holder.

[0203] Figures 7a, b show two views of an exemplary mirror holder 49 in the interferometer 5a, 5b of the FTIR spectrometer 1 according to the invention. Figure 7a shows a side view of the mirror holder 49, and Figure 7b shows a top view of the mirror holder 49.

[0204] 38

[0205] CORRECTED SHEET (RULE 91) ISA / EP The mirror holder 49 can have a base body 51 which can be connected to a section of the interferometer or the FTIR spectrometer, e.g. a housing section. A first part 53 is connected to the base body 51. The first part 53 has a first spring steel sheet 55. The base body 51 is connected to the first part 53 by means of the first spring steel sheet 55. In this case, the first part 53 can be plate-shaped. A first screw 57 which is rotatably mounted in the base body 51 spaces the first part 53 from the base body 51. The first screw 57 has only a force-fitting connection to the first part 53. The first spring steel sheet 55 exerts a spring force such that the first part 53 is prestressed in the direction of the base body 51 and the first screw 57, or an end of the first screw 57 facing the first part 53, forms an abutment for the spring force of the first part 53.

[0206] The frictional connection of the screw end of the first screw 57 can, for example, be made directly with the first part 53 or with a separate material. The separate material is preferably abrasion-resistant and can withstand the forces exerted by the screw end of the first screw 57 on the first part through the frictional connection, particularly during frequent rotational movements. This extends the service life of the mirror holder 49.

[0207] The mirror holder also has a second part 59. The second part 59 is connected to the first part 53. The second part 59 has a second spring steel sheet 61. The second part 59 is connected to the first part 53 by means of the second spring steel sheet 61. A second screw 63, which is rotatably mounted in the first part 53, spaces the second part 59 from the first part 53 and / or the base body 51. The second screw 63 has only a force-fitting connection with the second part 59. The second spring steel sheet 61 exerts a spring force such that the second part 59 is prestressed in the direction of the first part 53 and the second screw 63, or an end of the second screw 63 facing the second part 59, forms an abutment for the spring force of the first part 53. A through hole 64 in the base body 51 allows access to the second screw 63.

[0208] The second part 59 additionally comprises a mirror 65. The mirror 65 can be attached to the second part 59, encompassed by the second part 59, or formed by the second part 59. In the absence of a second part 59, the mirror 65 can also be attached to the first part 53, encompassed by the first part 53, or formed by the first part 53.

[0209] The first and second parts 53, 59 have a cuboid shape. Furthermore, the second spring steel sheet 61 is arranged on one of the side surfaces 67 of the second part 59, which is perpendicular to the surface 69 that receives or provides the mirror. The surface normal of the first spring steel sheet 55 is arranged perpendicular to the normal of surface 69 and perpendicular to the normal of surface 61 and is received on a side surface 71 of the first part 53. Figures 8a-h show measurement results for measured parameters of a mirror 19' of the mirror arrangement 13 of the FTIR spectrometer 1 according to the invention in comparison with commercially available mirrors.

[0210] Figure 8a shows a metal precision mirror, such as is used in a mirror arrangement outside the interferometer in commercial FTIR spectrometers. Figure 8b shows a mirror 19' of the mirror arrangement 13 of the FTIR spectrometer 1 according to the invention. The measured mirror 19' has a base body 23 made of PMMA plastic and was manufactured by injection molding. Subsequently, the reflective surface was applied as a metallic gold coating.

[0211] Figures 8c and 8d show measurement data of the reflecting surfaces 21 of the mirrors shown in Figures 8a and 8b in the form of the measured height along the path indicated by arrow A. The measurement data shown were taken using a profilometer known in the art. The measurement data clearly show a parabolic profile.

[0212] Figures 8e and 8f each show two measurement results of the microscopic surface roughness of the metal precision mirror from Fig. 8a. Figures 8g and 8h each show two measurement results of the surface roughness of the mirror 19' of the mirror arrangement 13 of the FTIR spectrometer 1 according to the invention from Fig. 8b. The mean roughness in the case of Fig. 8e and Fig. 8f is 17.8 nm RMS (root mean square) and 14.5 nm RMS, respectively; the mean roughness in the case of Fig. 8g and Fig. 8h is 39.1 nm RMS and 17.3 nm RMS, respectively. Surprisingly, the microscopic surface roughness of the mirror in Fig. 8b is of the same order of magnitude as the roughness of the metal precision mirror in Fig. 8a and is only approximately a factor of two larger, thus significantly below the shortest wavelength of approximately 1 pm used in FTIR spectrometers.

[0213] Figure 9a shows a setup 73 used in a simulation of an exemplary beam path in the FTIR spectrometer 1 according to the invention. The setup 73 shown essentially corresponds to the beam path from Fig. 2a, b up to the ATR crystal 15. The setup 73 comprises a circular infrared radiation source 3' with a diameter of 2 mm assumed for the simulation. In addition, a parabola 75 with a defined phase error is provided, in which the light from the infrared radiation source 3' is collimated. The simulated setup 73 takes into account the aperture 77 of a beam splitter in the interferometer. The setup 73 also comprises a second, focusing parabola 79 as well as the apertures of the ATR crystal and the ATR crystal holder.

[0214] In the simulation, the parabola 75 collimates the light from the infrared radiation source 3'. A phase error of the parabola 75 is introduced into the simulation using Zernike polynomials known in the art. By introducing phase errors on the first parabola

[0215] 40

[0216] REVISED SHEET (RULE 91) ISA / EP 75 using Zernike polynomials, the transmission can be influenced by the setup. The power transmitted in the simulated setup 73 is 10% of the power emitted by the infrared radiation source 3'.

[0217] Figure 9b shows the result of the simulation in the form of several curves representing the transmission through the optical system as a function of the deviation from an ideal parabolic shape. Each dashed curve corresponds to a different Zernike polynomial. In addition, the mean of all curves shown is shown as a solid line. The three insets in Fig. 9b are 2D interferograms of the beam propagated through the system with an unmodified reference beam. They demonstrate the influence of the introduced phase error. Depending on the type of introduced phase error (linear left / right or spherical), the transmission through the simulated system can be increased or decreased. For a wavelength of X = 2 pm and a randomly oriented phase error (average of all curves in Fig. 9b), a wavefront error of approximately 300 X, i.e., 300 times the wavelength, is tolerable without reducing the efficiency of the optical system.Therefore, surprisingly, the use of mirrors whose base bodies are made of plastic or consist of plastic, and in particular the use of inexpensive injection-molded plastic optics, which can introduce wavefront errors up to the aforementioned level, is generally unproblematic.

[0218] Figure 10a shows two exemplary single-shot FTIR spectra, each recorded with an inventive FTIR spectrometer 1 based on the inventive mirror arrangement with injection-molded mirrors and a spectrometer based on a similar mirror arrangement with precision metal mirrors. Single spectrum I was recorded with the inventive FTIR spectrometer 1. Single spectrum C was recorded with the same spectrometer using commercial precision metal mirrors.

[0219] The spectra shown were not averaged. The FTIR spectrometer 1 used according to the invention comprised exclusively mirrors 19 in the mirror assembly 13, whose base bodies 23 comprised a plastic material and were manufactured by an injection molding process.

[0220] Figure 10b shows the calculated difference spectrum D of the spectra C and I shown in Fig. 10a. Figure 10b shows only minimal difference values ​​between the spectra at different wavenumbers. Deviations are mainly due to slightly different adjustments of the two FTIR spectrometers used.

[0221] Figure 11 shows various spatially resolved measurements of wavefront errors in the central part of metal precision mirrors and various embodiments of mirrors according to the invention. The measurements were performed using a Shack-Hartmann wavefront sensor with a collimated laser beam at a wavelength of 556 nm. The mirrors used for the measurements in Figure 11 each had a parabolic shape.

[0222] The subfigures of Figure 11 show the spatial position of the mirror's reflecting surface (x and y positions) and the measured wavefront error (encoded as grayscale). The peak-to-valley (PV) value above each subfigure describes the maximum measured wavefront error (difference between the highest and lowest points in the wavefront profile) on the depicted surface of the mirror used. The root mean square (RMS) value above each subfigure describes the root mean square of the wavefront error on the depicted surface of the mirror used.

[0223] Figures 11a and 11b show measurements of the spatially resolved wavefront error of two different metal precision mirrors (Metallic-1 and Metallic-2). Figures 11c to 11h show measurements of the spatially resolved wavefront error of embodiments of a mirror of the mirror arrangement outside the interferometer of the FTIR spectrometer according to the invention. For the measurements in Figures 11c and 11d, two different mirrors made of polymethyl methacrylate (PMMA-1 and PMMA-2) were used. For the measurements in Figures 11e and 11f, two different mirrors made of polyurethane (PU-1 and PU-2) were used. For the measurements in Figures 11g and 11h, two different mirrors made of polycarbonate (PC-1 and PC-2) were used.

[0224] The measurements in Figures 11c to 11h show an absolute wavefront error of up to 5 pm (PV), i.e., a multiple of the reference wavelength of 2 pm. As already described above, these mirrors would not be suitable for use in an interferometer of an FTIR spectrometer. However, surprisingly, all the mirrors shown were usable in the FTIR spectrometer according to the invention for measuring samples.

Claims

Patent claims 1. FTIR spectrometer (1) with - an infrared radiation source (3), - an interferometer (5a, 5b) with at least one arm (14) variable in length, - a reference laser, - a measuring cell (7) with a sample interface, preferably an ATR crystal (15), which can be brought into contact with a sample (17), - an infrared detector (9), - a control system (11) arranged to change the length of at least one arm of the interferometer (5), and - a mirror arrangement (13) outside the interferometer (5) with at least two mirrors (19a, 19b, 19c, 19d), each with a reflecting surface (21) and a base body (23) which comprises the reflecting surface (21), wherein the mirror arrangement (13) is at least configured to direct a light beam (L) from the interferometer (5a, 5b) to the sample interface and to direct the light beam (L) from the sample interface to the infrared detector (9), - wherein the base body (23) of at least one mirror (19a, 19b, 19c, 19d) or of all mirrors (19a, 19b, 19c, 19d) of the mirror arrangement (13) is / are made of a plastic material and / or of 3D printed metal or the base body (23) of at least one mirror (19a, 19b, 19c, 19d) or of all mirrors (19a, 19b, 19c, 19d) comprises / comprising plastic material and / or 3D printed metal.

2. FTIR spectrometer (1) according to claim 1, wherein at least one mirror (19a, 19b, 19c, 19d) of the mirror arrangement (13) outside the interferometer (5a, 5b) has a mirror shape or a combination of mirror shapes from the following list: an off-axis parabolic mirror, a parabolic mirror, a compound parabolic concentrator, a spherical concave mirror, a mirror which has the shape of at least one parabolic segment or a circular segment at least in one axis.

3. FTIR spectrometer (1) according to claim 1 or 2, wherein each mirror (19a, 19b, 19c, 19d) of the mirror arrangement (13) outside the interferometer (5a, 5b) has a mirror shape or a combination of mirror shapes from the following list: an off-axis parabolic mirror, a parabolic mirror, a compound parabolic concentrator, a spherical concave mirror, a mirror which has the shape of at least one parabolic segment or a circular segment at least in one axis.

4. FTIR spectrometer (1) according to one of claims 1 to 3, wherein at least one of the mirrors (19a, 19b, 19c, 19d) of the mirror arrangement (13), the base body (23) of which is made of a plastic material or comprises a plastic material, is produced by an injection molding process or a 3D printing process and the reflective surface (21) is at least partially formed by a metal coating. FTIR spectrometer (1) according to one of claims 1 to 4, wherein the plastic material is at least one material from the following list or comprises at least one material from the following list: polymethyl methacrylate (PMMA), polycarbonate (PC), cycloolefin polymer, cycloolefin copolymer, styrene acrylonitrile, styrene acrylonitrile, high-temperature polycarbonate, polysulfone (PS), polyamide (PA), high-refractive polycarbonate, high-refractive polyester, polyethylene terephthalate (PET), polyethylene terephthalate with glycol (PETG), acrylonitrile-butadiene-styrene copolymer (ABS), nylon, polylactic acid (PLA), polyurethane (PU), a light-curing plastic (photopolymer), for example acrylic, epoxy and / or vinyl ester resin. FTIR spectrometer (1) according to one of claims 1 to 5, wherein the reflecting surface (21) of at least one mirror (19a, 19b, 19c, 19d) of the mirror arrangement (13) has, at least in some regions, a free-form optic.The FTIR spectrometer (1) according to claim 6, wherein the freeform optics exhibit, at least in some regions, a shape deviation from one of the following mirror shapes: an off-axis parabolic mirror, a parabolic mirror, a compound parabolic concentrator, a spherical concave mirror, or a mirror that exhibits the shape of at least one parabolic segment or a circular segment at least in one axis. The FTIR spectrometer (1) according to claim 7, wherein the freeform optics exhibit, at least in some regions, a shape deviation in an edge region. The FTIR spectrometer (1) according to claim 8, wherein the shape deviation is a convex regular or irregular rounding or chamfer, or a combination of a convex regular or irregular rounding and / or a chamfer.FTIR spectrometer (1) according to one of claims 1 to 9, wherein at least one mirror (19a, 19b, 19c, 19d) of the mirror arrangement (13) or each mirror (19a, 19b, 19c, 19d) of the mirror arrangement (13) is designed and configured such that when infrared light is reflected at the respective mirror (19a, 19b, 19c, 19d) of the mirror arrangement (13), the infrared light has a maximum wavefront error per mirror of 50 times the wavelength, preferably 25 times the wavelength of the infrared light. FTIR spectrometer (1) according to one of claims 1 to 10, wherein the mirrors (19a, 19b, 19c, 19d) of the mirror arrangement (13) and of the interferometer (5a, 5b) are designed and arranged such that when infrared light is reflected by the mirrors (19a, 19b, 19c, 19d) of the mirror arrangement (13), the infrared light has a total maximum wavefront error from the infrared radiation source (3) to the infrared detector (9) of 300 times the wavelength of the infrared light. The FTIR spectrometer (1) according to one of claims 1 to 11, wherein the mirror arrangement (13) has at least two off-axis parabolic mirrors (19a, 19b, 19c, 19d) with a first focal length and at least two parabolic mirrors (19a, 19b, 19c, 19d) with a second focal length. The FTIR spectrometer (1) according to one of claims 1 to 12, wherein the sample interface is an ATR crystal (15) received in a holder (35), the holder (35) being manufactured from metal using a 3D printing process. The FTIR spectrometer (1) according to one of claims 1 to 13, wherein the holder (35) is configured to hold the ATR crystal (15) stationary at a contact pressure of the sample against the ATR crystal (15) of up to 130 bar. FTIR spectrometer (1) according to one of claims 1 to 14, wherein the ATR crystal (15) has a maximum sample support area of ​​3 mm by 3 mm.

Images