CZ2022204A3 - Equipment for measuring spectral reflectance, especially concave spherical mirror surfaces - Google Patents

Abstract

The device for measuring the spectral reflectance, especially of concave spherical mirror surfaces, using a light source and reference measurement, contains a mutually adjustable illumination block (2) and a detection block (3), which is made up of a hollow integrating sphere (31) equipped on part of its surface with an input through a hole (311) and connected to a photodetector (33) connected to a control and evaluation computer (4), the essence of which is that the illumination block (2) and the detection block (3) are installed near the center (11) of the curvature of the measured mirror surface (1). The lighting block (2) consists of a light source (21) and a beam limiter (22) equipped with a through hole (221), and an impact surface (312) for measurement is defined against the entrance hole (311) on the opposite part of the inner surface of the integrating sphere (31) radiation characteristics of the light beam (211) initiated in the light source (21) and passing through the through hole (221) of the limiter (22) of the beam and the entrance hole (311) of the integrating sphere (31). The photodetector (33) is made up of a spectrophotometer and the light source (21) is wide-spectrum.

Description

Equipment for measuring spectral reflectance, especially concave spherical mirror surfaces

Field of technology

The invention falls into the field of measuring technology in optics, it is primarily intended for the production and control of optical mirror surfaces. The invention relates to a device for measuring spectral reflectance, in particular concave spherical mirror surfaces.

Current state of the art

In the chain of the production process of optical elements, the measurement of optical parameters has a pivotal role due to the guarantee of quality and the guarantee of the functionality of the resulting optical element. Furthermore, the measurement results are used as feedback for the production process. One of the most important parameters of an optical element, especially mirrors, is its reflectivity for given wavelengths of light. In principle, reflectance is measured by determining the spectral characteristics of the light incident on the measured element and the spectral characteristics of the light reflected from the measured element, and the reflectance of the surface is determined from these values. The methods of measuring reflectivity differ in the determination of these two quantities. Reflectance can be measured for one wavelength or for a certain spectral range.

Currently, a reference sample is used to quantify the spectral reflectance of optical surfaces, with which the measured surface is compared. This reference sample has two main drawbacks. With use, the sample wears out and thus changes its properties, and it is necessary to measure it regularly in a certified laboratory. An example of this measurement is for example described in the publication "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”.

Furthermore, methods are used with a fundamentally limited size of the light beam, where the reflectance is measured in a limited area, which has a diameter in the order of millimeters. To measure entire large areas, it is necessary to repeat this measurement several times, which is why the measurement is lengthy and demanding. As a rule, these devices allow measuring only planar samples or samples with small deviations from flatness and allow samples with limited dimensions to be inserted into the device. This method is described, for example, 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 “http://www.perkinelmer.com/product/lambda-850-uv-vis-spectrophotometer-1850”.

Measurement of reflectivity, spectral reflectivity and measurement of specular surfaces is also described in several patents, examples of which are files JP2002328103 (INSTRUMENT AND METHOD FOR MEASURING REFLECTIVITY FOR DETERMINING REFLECTIVITY IN SELECTED MEASURING PORTION OF SPECTRUM-DEPENDENT REFLECTIVE MEASURING OBJECT), where point measurement of spectral reflectivity of a small planar sample, or JPH10148572A (SPECTRAL REFLECTIVITY MEASUREMENT DEVICE), where a reference measurement is used and a reference sample is used in the reference branch, which introduces measurement uncertainty. In the file JPH10239154A (REFLECTIVITY METER) a reference sample with known spectral reflectance is used and it is compared with the spectral reflectance of the measured sample, while the measured area is very small and a narrow collimated beam of light is used for the measurement. In the file JPH10111240A (APPARATUS FOR MEASURING SPECTRAL REFLECTANCE), a special optical system is used to measure the spectral reflectance at several points at once, by photographing the reflected spectrum with a CCD camera. In file US2002071118A1 (DEVICE FOR MEASUREMENT OF THE SPECTRAL REFLECTANCE AND PROCESS FOR MEASUREMENT OF THE SPECTRAL REFLECTANCE) the device and the measurement method are described

- 1 CZ 2022 - 204 A3 of the spectral reflectance of a concave optical surface, but only locally at one point of the optical surface using a narrow beam of light, and in the document JPH1090063A (SPECTRAL REFLECTANCE MEASURING EQUIPMENT) a method is described that enables only point measurement of spectral reflectance and in addition reference sample is used. In the file CN105203048A (MEASURING SYSTEM AND METHOD FOR RADIUS OF CURVATURE) there is a method using also the center of curvature of a spherical mirror, but only for measuring the radius of curvature of these spherical concave mirrors. Furthermore, a device according to CN107037007A is known, containing an auxiliary device using a reference sample and a collimated beam of light, i.e. a beam with parallel light rays, so that the measured area must be comparable to the size of the collimating optics of the transmitting part and the size of the integrating sphere in the receiving part.

The collimated beam must also fall into the integrating sphere, which means that the reference sample must not significantly change the parameters of the beam, so it must be flat. Also known is the device described in JP2011154047A for measuring absolute reflectance without a reference sample. The device presented in this document also uses a collimated beam of light, so the measured area must be comparable to the size of the collimating optics of the transmitting part and the size of the area of the integrating sphere in the receiving part.

The task of the present invention is to present a new device for measuring the spectral reflectance, especially of large concave spherical mirror surfaces, which does not use a reference sample and measures the effective spectral reflectance of the entire element, i.e. the surface of the mirror, because for the practical use of optical elements, the effective reflectance of its entire optical surface is of particular interest .

The essence of the invention

The set goal is achieved by the invention, which is a device for measuring spectral reflectance, especially concave spherical mirror surfaces, using a light source and reference measurement and containing a mutually adjustable illumination block and a detection block, which is made up of a hollow integrating sphere equipped with an entrance hole on part of its surface and connected to a photodetector connected to a control and evaluation computer, the essence of which is that the illumination block and the detection block are installed near the center of curvature of the measured mirror surface, while the illumination block consists of a light source and a beam limiter equipped with a through hole, and is opposed to to the entrance opening on the opposite part of the inner surface of the integrating sphere, an incident surface is defined for measuring the radiation characteristics of the light beam initiated in the light source and passing through the through hole of the beam limiter and the entrance opening of the integrating sphere, while the photodetector consists of a spectrophoto meter and the light source is broad spectrum.

In a preferred embodiment, the integrating sphere is connected to the photodetector by an optical fiber.

The invention achieves a new and higher effect in that it is possible to measure the spectral course of the reflectivity of a concave mirror, and that of 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 another source of uncertainty into the measurement. In addition, the device does not use a collimated light beam, but uses a divergent light beam, which makes it possible to measure objects of almost any and unlimited dimensions with a single device, which is especially advantageous for large curved surfaces, namely spherical and convex. The method of measurement is not sensitive to the exact adjustment of the assembly due to the use of an integrating sphere. An important member of the assembly is the beam limiter in the lighting part, which ensures that the light beam illuminates only the mirror surface. The size and shape of the through hole of the limiter and the distance from the light source are variable, while changing these parameters changes the divergence of the light beam. The shape of the limiter opening determines the size and shape of the illuminated (measured) area on the mirror surface, i.e. if the mirror is rectangular (circular), the shape of the opening will also be rectangular (circular). It is therefore not necessary to measure a large number of points, but in one measurement the reflectivity of the entire mirror surface is characterized more than once, which significantly simplifies and shortens the measurement time.

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The method uses the center of curvature of a spherical mirror, which has the property that a beam of light emanating from this point will return to this point after being reflected from the spherical surface. This phenomenon can also be advantageously used in the off-axis variant. Another important member of the assembly is the integrating sphere, which records, i.e. detects, the entire beam of radiation entering the sphere through the entrance hole and sends part of this radiation to the detection electronics.

Clarification of drawings

A specific example of the embodiment of the invention is shown schematically in the attached drawings, where

- Fig. 1 is a diagram of the device with the arrangement of its individual members during reference measurement a

- Fig. 2 is a diagram of the device with the arrangement of its individual members during the actual measurement of the reflectance of the optical surface.

The drawings that illustrate the present invention and subsequently described examples of specific embodiments in no way limit the scope of protection stated in the definition, but only clarify the essence of the invention.

Examples of implementation of the invention

The device for measuring the spectral reflectance of concave spherical mirror surfaces 1 basically consists of an illumination block 2 and a detection block 3, which are installed near the center 11 of the curvature of the measured mirror surface L. The illumination block 2 consists of a light source 21 and a beam limiter 22. The detection block 3 is formed by a hollow integrating sphere 31, which is connected by an optical fiber 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 on part of its surface with an entrance hole 311, against which an impact surface 312 is defined on the opposite part of the inner surface of the integrating sphere 31 for measuring the radiation characteristics of the light beam 211, which is initiated in the light source 21 and cut by the through-hole 221 of the beam limiter 22.

The method of measurement on the device described above basically takes place 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 calculations of the spectral reflectance of the measured mirror surface L In the reference measurement shown in Fig. 1, the lighting block 2 is installed so that the restrictor 22 is placed in front of the entrance opening 311 of the integrating sphere 31 and only the incident surface 312 of the inner surface of the integrating sphere 31 is illuminated by the light beam 211 from the light source 21 through the through opening 221 of the restrictor 22.

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

Subsequently, the functional members of the illumination block 2 and the detection block 3 are adjusted according to Fig. 2 so that the light beam 211 illuminates only the mirror surface 1 through the through opening 221 of the limiter 22. The integrating sphere 31 is installed so that its entrance opening 311 is oriented against the mirror surface 1 so that the reflected light beam 211 after reflection from the mirror surface 1 is concentrated on the same impact surface 312 of the rear part of the inner surface of the integrating sphere 31. If this is fulfilled, the photodetector 33 measures the spectral response of the reflected light beam 211 in the detection block 3 and the result is recorded again in the evaluation program on the computer 4.

In the last step of the measurement, the computer 4 evaluates the data from the reference and own measurement and calculates the reflectivity of the mirror surface L This completes the entire measurement process.

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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 fiber 32 can be omitted and the photodetector 33 can be integrated directly on the integrating sphere 31.

In the described basic assembly, a broad-spectrum light source 21 must be used, which covers all the required 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 a photodetector 33 of the type e.g. photodiode or photomultiplier can be used in the detection block 3, 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 measuring even mirrors with a relatively large deviation from the ideal spherical shape. Other shapes can also be measured, which enable point-to-point display, i.e. the measured area from a real object at a finite distance creates a real image at a finite distance.

Industrial applicability

The spectral measuring assembly 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 elements. It can therefore be used in the post-production verification and certification of the quality of mirror surfaces, as well as in the routine inspection of the quality and wear of mirror surfaces.

Claims (1)

1. Equipment for measuring spectral reflectance, especially concave spherical mirror surfaces, using a light source and reference measurement and containing mutually adjustable 5, an illumination block and a detection block, which is formed by a hollow integrating sphere provided on part of its surface with an entrance hole and connected to a photodetector connected to a control and evaluation computer, characterized in that the illumination block (2) and the detection block (3) are installed near the center (11) of the curvature of the measured mirror surface (1), while on the one hand the lighting block (2) consists of a light source (21) and a beam limiter (22) equipped with a through hole (221) and on the other hand it is against the entrance hole (311) on the opposite part of the inner surface of the integrating sphere (31) defines an incident surface (312) for measuring the radiation characteristics of the light beam (211) initiated in the light source (21) and passing through the through hole (221) of the limiter (22) of the beam and the entrance hole (311) of the integrating sphere (31), while the photodetector (33) is formed by a spectrophotometer and the light source (21) is wide-spectrum. 15 2. Device according to claim 1, characterized in that the integrating sphere (31) is connected to the photodetector (33) by an optical fiber (32).