Device for the measurement of the spectral reflectance, in particular concave spherical mirror surfaces, and method of the measurement on this device

Abstract

A device for the measurement of the spectral reflectance, in particular concave spherical mirror surfaces, using a light source and a reference measurement, measurement, the essence of which consists of an interchangeable illumination block (2) and a detection block (3), which are installed near the centre (11) of curvature of the measured mirror surface (1), wherein both the illumination block (2) comprises a light source (21) and a beam reducer (22) provided with a aperture (221) and the detection block (3) comprises a hollow integrating sphere (31), to which a photodetector (33) connected to a control and evaluation computer (4) is connected, and secondly, the integrating sphere (31) is provided with an input port (311) on part of its surface, against which, on an opposite part of the inner surface of the integrating sphere (31), an affected area (312) is defined for measuring the radiation characteristics of a light beam (211) initiated in the light source (21) and passing through the aperture (221) of the beam reducer (22) and the input port (311) of the integrating sphere (31).

Description

[0001] Device for the measurement of the spectral reflectance, in particular concave spherical mirror surfaces, and method of the measurement on this device.

[0002] Field of Technology

[0003] The invention falls within the field of the measurement technique in optics, and is primarily intended for the production and inspection of optical mirror surfaces. The invention relates to a device for the measurement of spectral reflectance, in particular of concave spherical mirror surfaces, and the measurement method using this device.

[0004] State of the art to date

[0005] In the manufacturing process chain of optical elements, the measurement of optical parameters plays a crucial role in order to guarantee the quality and functionality of the resulting optical element. The measurement results are further used as feedback for the production process. One of the most important parameters of an optical element, especially mirrors, is its reflectance for given wavelengths of light. Reflectance is measured in principle by defining the spectral characteristic of the light incident on the element to be measured and the spectral characteristic of the light reflected from the element to be measured and determining the reflectance of the area from these quantities. The reflectance measurement methods differ in the determination of these two quantities. Reflectance can be measured for a single wavelength or for a specific spectral range.

[0006] Currently, the reference sample to which the measured area is compared is mainly used to quantify the spectral reflectance of optical surfaces. This reference sample has two main drawbacks. With use, the sample wears out and thus changes its qualities, so it must be periodically remeasured in a certified laboratory. An example of this measurement is described in "Reflectometers for Absolute and Relative Reflectance Measurements in the Mid-IR Region at Vacuum, Jinhwa Gene, Sensors 2021, 21(4), 1169' "available at "https: / / doi.org / 10.3390 / s21041169”. In addition, methods with a fundamentally limited light beam size are used, where reflectance is measured in a limited area with a diameter on the order of millimetres. To measure entire large areas, the measurement has to be repeated several times, which makes the measurement time-consuming and demanding. These instruments usually only allow the measurement of planar samples or samples with small deviations from planarity and allow samples with limited dimensions to be loaded into the device. This method is described e.g. in the publication "Measurements of angle-resolved spectral reflectance by Perkin Elmer Lambda 900 spectrophotometer Conference Paper in Proceedings of SPIE - The International Society for Optical Engineering, August 2002 ", available at "http: / / dx.doi.org / 10.1117 / 12.531032", or at

[0007] "https: / / www.perkinelmer.com / product / lambda-850-uv-vis-spectrophotometer-l850".

[0008] Measurement of reflectivity, spectral reflectance, and measurement of specular surfaces is also described in several patents, an example of which are the writings of JP2002328103 (INSTRUMENT AND METHOD FOR MEASURING REFLECTIVITY FOR DETERMINING REFLECTIVITY IN SELECTED MEASURING PORTION OF SPECTRUM-DEPENDENT REFLECTIVE MEASURING OBJECT), which describes a point measurement of the spectral reflectance of a small planar sample, or JPH10148572A (SPECTRAL REFLECTIVITY MEASUREMENT DEVICE), which uses a reference measurement and uses a reference sample in the reference branch to introduce measurement uncertainty. In JPH10239154A (REFLECTIVITY METER), a reference sample with a known spectral reflectance is used and this is compared with the spectral reflectance of the sample to be measured, where the area to be measured is very small and a narrow collimated beam of light is used for the measurement. In JPH10111240A (APPARATUS FOR MEASURING SPECTRAL REFLECTANCE), a special optical system is used to measure the spectral reflectance at several points simultaneously by photographing the reflected spectrum with a CCD camera. File US2002071118A1 (DEVICE FOR MEASUREMENT OF THE SPECTRAL REFLECTANCE AND PROCESS FOR MEASUREMENT OF THE SPECTRAL REFLECTANCE) describes a device and method for measuring the spectral reflectance of a concave optical surface, but only locally at a single point on the optical surface using a narrow beam of light, and in JPH1090063A (SPECTRAL REFLECTANCE MEASURING EQUIPMENT) a method is described which allows only spot measurements of spectral reflectance and in addition uses a reference sample. In file CN105203048A (MEASURING SYSTEM AND METHOD FOR RADIUS OF CURVATURE), a method using also the centre of curvature of a spherical mirror is presented, but only for measuring the radius of curvature of these spherical concave mirrors.

[0009] The purpose of the presented invention is to introduce a new device and method for measuring the spectral reflectance of large concave spherical mirror surfaces that do not use a reference sample and measure the effective spectral reflectance of the entire element, i.e., the mirror surface, because for the practical use of optical elements we are primarily interested in the effective reflectance of its entire optical surface.

[0010] Principle of the Invention

[0011] The stated object is achieved by the invention, which is a device for the measurement the spectral reflectance, in particular of concave spherical mirror surfaces, using a light source and a reference measurement, the essence of which consists of an interchangeable illumination block and a detection block, which are installed near the centre of curvature of the mirror surface to be measured, wherein both the illumination block comprises a light source and a beam reducer provided with a aperture and the detection block comprises a hollow integrating sphere, on which a photodetector is connected with a control and evaluation computer, and on the other hand, the integrating sphere is provided with an input port on a part of its surface, against which an affected area is defined on an opposite part of the inner surface of the integrating sphere for measuring the radiation characteristics of a light beam initiated in the light source and passing through the aperture of the beam reducer and the input port of the integrating sphere.

[0012] In a preferred embodiment, the integrating sphere is connected to the photodetector by an optical fibre, wherein the photodetector comprises a spectrophotometer and the light source is broad spectrum. Another essence of the invention is a method for measuring spectral reflectance, in particular of concave spherical mirror surfaces, on a device according to claim 1 or 2, implemented in such a way that firstly, in a first stage, the spectral response of the incident light beam on the affected area of the integrating sphere of the detection block is measured independently by means of a photodetector and the result is recorded in a computer evaluation program. Subsequently, in the second phase, the functional members of the illumination block and the detection block are set so that the light beam illuminates only the mirror surface through the aperture of the reducer and the reflected light beam after reflection from the mirror surface is concentrated on the same affected area of the rear part of the inner surface of the integrating sphere and the photodetector measures the response of the reflected light beam in the detection block and the result is again recorded in the computer evaluation program. Finally, in the third stage, the computer evaluates the data from the reference and actual measurements and calculates the reflectance of the mirror surface, thus completing the measurement process.

[0013] The invention achieves a new and higher effect in that it is possible to measure the reflectance of a concave mirror, and the entire surface at once. The principle of this method does not change even when measuring large mirror surfaces. The method used is absolute; there is no need to use a reference sample, which introduces an additional source of uncertainty into the measurement. Due to the use of an integrating sphere, the measurement method is not sensitive to the by the exact alignment of the assembly. An important part of the assembly is the beam reducer in the illuminating section, which ensures that the light beam illuminates only the mirror surface. The method uses the centre of curvature of a spherical mirror, which has the property that a ray of light coming from this point returns to this point after reflection from the spherical surface. This phenomenon can also be advantageously used in the off-axis variant. Another important part of the assembly is the integrating sphere, which detects, i.e. records, the entire beam of radiation entering the input through the input port of the sphere and sends part of this radiation to the detection electronics. Explanations of Drawings

[0014] A specific example of the invention design is shown in the accompanying drawings, where: fig .1 is a schematic of the device with the arrangement of its individual components in the reference measurement and fig.2 is a diagram of the device with the arrangement of its individual components during the actual measurement of the reflectivity of the optical surface.

[0015] Drawings illustrating the invention and the examples described thereafter specific embodiments in no way limit the scope of protection set out in the definition, but merely clarify the nature of the invention.

[0016] Examples of the invention designs

[0017] The device for the measurement of the spectral reflectance of concave spherical mirror surfaces 1 comprises, in a basic embodiment, an illumination block 2 and a detection block 3, which are installed near the centre H of curvature of the measured mirror surface 1. The illumination block 2 comprises a light source 21 and a beam reducer 22. The detection block 3 comprises a hollow integrating sphere 31, which is connected by an optical fibre 32 to a photodetector 33 realized in the form of a spectrophotometer. The photodetector 33 is connected to the control and evaluation computer 4. The integrating sphere 31 is provided with an input port 311 on a portion of its surface, against which an affected area 312 is defined on an opposite portion of the inner surface of the integrating sphere 31 for measuring the radiation characteristics of a light beam 2 H initiated in the light source 21 and clipped by the aperture 221 of the beam reducer 22.

[0018] The measurement method on the above described device is basically carried out in three steps and consists of a reference measurement, the actual measurement of the spectral characteristics of the light after reflection on the measured surface and the calculation of the spectral reflectance of the measured mirror surface 1. In the reference measurement illustrated in Figure 1 , the illumination block 2 is installed so that the reducer 22 is positioned in front of the input port 311. of the integrating sphere 31 and only the affected area 312 of the inner surface of the integrating sphere 31 is illuminated from the light source 21 through the aperture 221 of the reducer 22 by the light beam 211.

[0019] Using a photodetector 33 which is connected by an optical fibre 32 to an integrating sphere 31 the response of the incident light beam 211 in the detection section of the detection block 3 is measured, and the result is recorded in the evaluation program of the computer 4.

[0020] Subsequently, the functional members of the illumination block 2 and the detection block 3 are set according to FIG. 2 so that the light beam 211 illuminates only the mirror surface 1 through the aperture 221 of the reducer 22. The integrating sphere 31 is installed so that its input port 311 is oriented against the mirror surface 1, so that the reflected light beam 211_ is concentrated on the same affected area 312 after reflection from the mirror surface 1 the back of the inner surface of the integrating sphere 31 If this is satisfied, the photodetector 33 measures the spectral response of the reflected light beam 2L1 in the detection block 3 and the result is again recorded in the evaluation program in the computer 4.

[0021] In the last step of the measurement, the data from the reference and the actual measurement are evaluated in computer 4 and the reflectance of mirror surface 1 is calculated. This completes the measurement process.

[0022] The described embodiment of the measuring device assembly is not the only possible solution according to the invention. In the detection block 3, the optical fibre 32 may be omitted and the photodetector 33 may be integrated directly on the integrating sphere

[0023] 31

[0024] In the basic assembly described, a broad-spectrum light source 21 must be used that covers all of the desired wavelengths of light to be measured, then a photodetector 33 in the form of a spectrophotometer must be used. It is also possible to consider a version where only one wavelength of light is measured, then in the detection block 3 a photodetector 33 of the type e.g. photodiode or photomultiplier can be used, when this photodetector 33 must be sensitive for the given wavelength of the measured light. The measured mirror surface 1 does not have to be exactly spherical, the method allows to measure even mirrors with relatively large deviation from the ideal spherical shape. Different forms can be measured that allow point-to-point viewing, i.e., the measured area from a real object at a finite distance produces a real image at a finite distance.

[0025] Industrial applicability

[0026] The spectral measurement setup for measuring concave spherical mirror surfaces according to the invention is intended in particular for measuring the absolute spectral reflectance of optical surfaces, which is the most important optical parameter of mirror optical components. It can therefore be used in post-production verification and certification of mirror surface quality as well as in routine quality and wear control of mirror surfaces.

Claims

AMENDED CLAIMS received by the International Bureau on 16 May 2022 (16.05.2022)PATE NT CLAI MS1. A device for the measurement of the spectral reflectance, in particular concave spherical mirror surfaces, using a light source and a reference measurement and comprising a mutually reconfigurable illumination block and a detection block comprising a hollow integrating sphere provided with an entrance hole on part of its surface and connected to a photodetector connected to a control and evaluation computer, characterized in that illumination block (2) and a detection block (3), are installed near the centre (11) of curvature of the measured mirror surface (1), wherein both the illumination block (2) comprises a light source (21) and a beam reducer (22) provided with a aperture (221) an input port (311) on an opposite part of the inner surface of the integrating sphere (31), an affected area (312) is defined for measuring the radiation characteristics of a light beam (211) initiated in the light source (21) and passing through the aperture (221) of the beam reducer (22) and the input port (311) of the integrating sphere (31), wherein the photodetector (33) comprises a spectrophotometer and the light source (21) is broad spectrum.

2. Device according to claim 1, characterized in that the integrating sphere (31) is connected to the photodetector (33) by an optical fibre (32).

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