CN109001141B - Infrared optical material impurity testing method - Google Patents
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Abstract
The invention relates to an infrared optical material impurity testing method, and relates to the technical field of optical material impurity testing. The method utilizes the field depth characteristic and the scene imaging principle of the infrared photographic system to perform one-time imaging of the field depth range of the infrared photographic system on the impurity condition existing in the infrared material which is not transparent to visible light to obtain all the impurity conditions in the infrared optical material, has simple requirement on equipment for testing the impurities in the infrared material which is not transparent to visible light, does not need a precise platform for bearing two-dimensional automatic movement of a sample and a precise one-dimensional automatic movement platform for bearing a microscope objective, and reduces the cost of testing equipment; the test method is one-time photographing real imaging, no overlapping imaging result is processed, and the reflected test result is real; the test method can be completed by taking a picture once, and is quick in test and short in time.
Description
Technical Field
The invention relates to the technical field of optical material impurity testing, in particular to an infrared optical material impurity testing method.
Background
Impurities in the optical material are heterogeneous material defects such as bubbles, stones and the like left after the optical material is manufactured, and the heterogeneous material defects can seriously affect the imaging quality of the optical system. Testing for impurities in optical materials is a necessary step in evaluating whether optical materials can be used satisfactorily. For the optical material with visible wave band, the impurities in the material are irradiated by a diffuse reflection light source, the quantity and the size of the impurities are recorded by direct observation of human eyes by a magnifying glass, and then the impurities are substituted into a formula to be calculated and then the grade of the impurities and the defects of the optical material are evaluated.
For infrared optical materials (wavelength of 0.8 μm to 14 μm) which cannot transmit visible light (wavelength of 0.4 μm to 0.78 μm), such as infrared chalcogenide optical glass, infrared crystal, infrared ceramic, etc., impurities in the materials cannot be directly observed by eyes as optical materials in the visible wavelength band, and therefore, an infrared test system needs to be established for testing.
A layered scanning test method of microscopic imaging for testing the uniformity of infrared optical material is disclosed. The main principle of the method is that infrared light source radiation is reflected to a microscope objective system forming a 90-degree angle with a semi-transparent reflector forming a 45-degree angle with an optical axis of the light source system, the microscope objective projects the light radiation to an optical material to be detected, the microscope objective performs object point imaging on the optical material irradiated by the microscope objective, and the image is imaged on an infrared detector. The impurities in the whole infrared optical material need to perform line-by-line transverse full-plane scanning and longitudinal layer-by-layer stepping on a test sample placed on a precise two-dimensional moving table to perform another plane scanning until all the layered scanning of the test sample is completed, and the principle and the method are shown in fig. 1 and fig. 2.
The layered scanning test method for the infrared optical material uniformity microscopic imaging has five problems: firstly, the test method adopts a microscopic principle, is limited by the short object distance of a microscope objective, can only test samples with very thin thickness, and usually does not exceed 5mm, so that the test method has no practical value (the maximum test thickness of the infrared optical part can reach 20 mm); secondly, the splicing relation between the test process and the test result is too complex, the point-by-point scanning imaging test is firstly carried out on the two-dimensional plane according to the millimeter scale range, then the stepping is carried out layer by layer according to the near micron level, the whole test is a huge scanning test workload process, and the splicing of the two-dimensional scanning image and the splicing of the longitudinal layer are also extremely complex, such as the situation shown in fig. 2; thirdly, the test result is difficult to accurately restore the actual result, because the tested microscopic images between circles and layers have the factors of deformation of image quality difference of different positions of the field of view of the microscopic objective, extensibility of depth-of-field imaging and the like, impurity images in transverse and longitudinal splicing overlapping regions can be repeatedly counted, repeated parts are difficult to be strictly distinguished, and deviation of the material impurity images from the actual condition can be caused; fourthly, the testing device is complex in composition, high in precision requirement and high in cost, and a T-shaped light path formed by a plurality of devices, a two-dimensional automatic moving platform of a high-precision test sample and a high-precision longitudinal (one-dimensional) automatic moving platform of a microscope objective are needed; fifthly, the testing time is long, and the testing process time is long due to the complexity of the testing.
Disclosure of Invention
Technical problem to be solved
The technical problem to be solved by the invention is as follows: how to design a test principle and a method of infrared optical material impurities to solve the problem of actual test of the optical material impurities, reduce the complexity of the composition of a test device and the test process, obtain the result according with the actual condition of the material impurities, reduce the test cost and shorten the test time.
(II) technical scheme
In order to solve the technical problem, the invention provides an infrared optical material impurity testing method, which comprises the following steps:
s1, establishing an infrared optical material depth of field photographic impurity testing system, which sequentially comprises an infrared diffusion light source system 1, a tested infrared optical material sample 2, an infrared photographic objective 3 and an infrared detector 4 in the light path direction; wherein, the tested infrared optical material sample 2 is placed on the sample placing table 6; the infrared diffusion light source system 1 consists of an infrared light source and a diffusion screen 101 for transmitting infrared spectrum, wherein the infrared light source is a full-spectrum infrared light source 102 or a monochromatic infrared light source 103;
s2, determining the maximum thickness of the infrared optical material sample 2 to be tested in the test system to be used as the design basis of the object depth of field of the infrared photographic objective lens 3;
s3, determining the object resolution of the infrared photographic objective 3 according to the minimum impurity size to be counted of the infrared optical material sample 2 to be detected;
s4, selecting the wave band response type of the infrared detector according to the transparent wave band of the predetermined infrared optical material sample 2 to be detected, namely determining the type of the infrared detector 4;
s5, establishing a matching relation between the maximum side length of the infrared detector 4 and the maximum testing range of the maximum caliber of the infrared optical material sample 2 to be tested;
s6, determining the image resolution interval sigma according to the object resolution of the infrared photographic objective 3 determined in the step S3;
s7, calculating the maximum F number allowed by the infrared photographic objective lens 3 according to the image space resolution distance sigma of the infrared photographic objective lens 3 determined in the step S6;
s8, selecting the size d of the detecting element 601 of the infrared detector 4, wherein d is less than or equal to sigma, so that the infrared detector 4 can meet the resolution of the image space resolution interval sigma of the infrared photographic objective 3;
s9, determining the size of the diffraction spot of the image point of the infrared photographic objective lens 3, namely the image point diffuse spot 602, by using the F number of the infrared photographic objective lens 3 obtained in the step S7;
s10, setting a series of alternative focal length values of the infrared photographic objective lens 3;
s11, calculating a series of object distances of the infrared photographic objective lens 3 according to the magnification of the infrared photographic objective lens 3 calculated in S5 and the series of focal length values set in step S10; calculating the series focal length of the infrared photographic objective lens 3 and the series object space front depth of field and the series object space rear depth of field corresponding to the object distance according to the allowable F number determined in the step S7, the series focal length value set in the step S10, the series object distance corresponding to the series focal length, and the allowable diameter of the out-of-focus diffusion circle; then according to the volume size allowed by the test system, according to the requirement that the infrared photographic objective lens 3 images the infrared optical material impurities within the thickness range of the sample to be tested on the infrared detector 4, selecting and calculating the object space depth of field Delta L, the imaged object distance L and the imaged image distance L 'of the infrared photographic objective lens 3 which reach a series of focal length values f' of a preset requirement, wherein the object space depth of field Delta L is the sum of the front depth of field of the object space and the back depth of field of the object space;
step S12, arranging the test system according to the following principle: the thickness middle surface 801 of the infrared optical material sample 2 to be detected is not overlapped with the object surface 802, but the thickness middle surface 801 of the infrared optical material sample 2 to be detected is arranged on the left side of the object surface 802 to form the object front depth of field 804 and the object rear depth of field 805 which effectively occupy the thickness of the sample, and the object surface 802 is a depth of field interface, namely an object plane corresponding to the image sensing plane 803 of the infrared detector 4;
and step S13, carrying out size measurement and quantity counting on the impurity images in the detected infrared optical material sample 2 received by the infrared detector 4 by using image processing and calculation software, and giving out a test result of impurity standard evaluation.
Preferably, in step S1, if the infrared light source is the monochromatic infrared light source 103 and is a laser infrared light source, a laser beam expanding optical system 104 is further disposed between the laser infrared light source and the diffuse reflection screen of the infrared transmission spectrum, and the wavelength of the laser infrared light source is within the transmission band of the infrared optical material and within the radiation response operating band of the infrared detector 4.
Preferably, in step S2, for the sample with the diameter of the testing aperture as large as 100mm, the maximum thickness of the tested infrared optical material sample 2 is determined to be not more than 25mm by using the near infrared or short wave infrared light source, and the maximum thickness tested by using the medium wave and long wave infrared light source can be thicker, which can be 2 times of the short wave infrared and more.
Preferably, in step S3, for the infrared optical material operating in the short-wave infrared band, the short-wave infrared light source is used to perform the impurity test on the infrared optical material, and the minimum size of the infrared optical material impurity to be resolved is 0.1 mm; for infrared optical materials working in a medium-wave infrared band or a long-wave infrared band, when a radiation source with a corresponding band is used for testing, the minimum size of impurities of the infrared optical materials needing to be resolved is increased along with the lengthening of the wavelength.
Preferably, in step S4, if the germanium crystal infrared optical material to be tested is not considered, a near-infrared or short-wave infrared detector is selected, and if impurities of various infrared optical materials to be tested are considered, a medium-wave infrared detector or a long-wave infrared detector is selected.
Preferably, in step S5, the matching relationship is obtained by dividing the maximum test dimension 502 of the infrared optical material sample 2 to be measured by the maximum effective dimension 501 of the infrared detector 4 as the magnification β of the infrared photographic objective 3.
Preferably, in step S6, the image-side resolution interval σ is the object-side resolution of the infrared photographic objective 3 divided by the magnification β of the infrared photographic objective 3.
Preferably, let the test wavelength be λ, and F ═ σ/(1.22 λ).
Preferably, in step S8, the image-side resolution interval σ of the infrared photographic objective 3 is the distance between two adjacent image elements.
Preferably, in step S9, if the F-number of the infrared camera objective 3 is halved to affect the desired depth of field length of the infrared camera objective 3, the diffraction spot diameter of the image point of the infrared camera objective 3 is determined to be 2 σ, and the diffraction spot diameter 603 of the image point is determined to be two detector sizes, i.e., 2 d; if the F-number of the infrared camera objective 3 is reduced by half without affecting the desired depth of field length of the infrared camera objective 3, the size of the diffraction spot diameter 603 of the image point of the infrared camera objective 3 is determined to be equal to the size of the detector element 601 of the infrared detector 4.
(III) advantageous effects
The method utilizes the field depth characteristic and the scene imaging principle of the infrared photographic system to perform one-time imaging of the field depth range of the infrared photographic system on the impurity condition existing in the infrared material which is not transparent to visible light to obtain all the impurity conditions in the infrared optical material, has simple requirement on equipment for testing the impurities in the infrared material which is not transparent to visible light, does not need a precise platform for bearing two-dimensional automatic movement of a sample and a precise one-dimensional automatic movement platform for bearing a microscope objective, and reduces the cost of testing equipment; the test method is one-time photographing real imaging, no overlapping imaging result is processed, and the reflected test result is real; the test method can be completed by taking a picture once, and is quick in test and short in time.
Drawings
FIG. 1 is a schematic diagram of a prior art micro-scanning test method;
FIG. 2 is a scan-spliced view of a test field of a prior art micro-scanning test;
FIG. 3 is a schematic diagram of a depth of field photographic contamination testing system for infrared optical materials in accordance with the present invention;
FIG. 4 is a diagram of a system configuration of the infrared diffuse light source of the present invention;
FIG. 5 is a schematic diagram showing the matching relationship between the size of an infrared optical material test sample and the size of an infrared detector according to the present invention;
FIG. 6 is a schematic diagram showing the matching relationship between the infrared detector and the image diffuse spot according to the present invention;
FIG. 7 is a schematic diagram of the relationship of infrared optical material impurities imaged on a detector according to the present invention;
FIG. 8 is a graph showing the matching of the depth of field of an object to the thickness of a sample in accordance with the present invention.
Detailed Description
In order to make the objects, contents and advantages of the present invention clearer, the following detailed description of the embodiments of the present invention will be made in conjunction with the accompanying drawings and examples.
The inventor of the invention finds that the impurities in the infrared optical material can be imaged once by applying the depth of field of the infrared photographic system, so that the comprehensive actual possession condition of the impurities in the optical material can be obtained, and the impurity test of the infrared optical material is simplified and realized. The invention is based on two basic points, namely, the characteristics and the performance relation of the imaging of the optical system are deeply known (the performance relation of the optical system is mainly the relation among resolution, depth of field, F number and the like), and the practical application condition of the infrared optical material, the allowable degree of impurities and the calculation method of the impurities are deeply known. The first basic point is to design an infrared photographic optical system for testing the impurities of the infrared optical material, the second basic point is to solve the requirement that the reasonably designed depth of field length of the photographic system can cover most of the infrared optical parts (the thickness of most of the infrared optical parts is not more than 20mm, the invention can design the depth of field of near infrared and short wave infrared when the magnification beta of an infrared photographic objective lens 3 is 10 and the refractive index of the material is about 2.5 to 25mm, the depth of field of medium wave infrared can be designed to be 2 times or more of the short wave, the depth of field of long wave infrared can be designed to be longer, when the magnification beta of the infrared photographic objective lens 3 is reduced and the refractive index of the material is increased, the longer depth of field increased according to the change proportion relation can be designed, the maximum size of a corresponding sample test when the magnification beta of the infrared photographic objective lens 3 is 10 is 100mm, the magnification beta of the infrared photographic objective lens 3 is opposite to the change relation of the depth of field delta L, depth of field decreases with increasing magnification and vice versa), the object-side resolution of a properly designed camera system can satisfy the resolution and imaging of the smallest size of impurities that need to be counted for infrared optical materials. Although there may be no limitation on the thickness of the test sample using tomography (but the limitation is more severe with microscopic scanning due to the short object distance of the microscope), the thickness of the sample actually tested will generally not or not necessarily exceed 25mm (near infrared or short wave infrared testing can almost meet the test requirements for most infrared optical part thicknesses), except in special cases. The reason why the infrared optical components are generally not made of a thick material is mainly that the infrared optical materials are expensive and the weight reduction of the optical system is considered, and therefore, the reflecting function in the infrared optical system is generally to use a mirror instead of a thick, expensive prism. The use of micro-layered scanning for the testing of ultra-thick infrared optical material impurities has few practical requirements in infrared optical testing.
Based on the above deep grasp of the actual conditions of the infrared optical system and the optical material impurities, the invention creates a test method of the infrared optical material impurities by applying the one-time imaging test principle of the depth of field of the infrared photographic system, and the test method comprises the following steps:
step S1, a depth-of-field photographic impurity testing system for infrared optical materials is established, which is composed of an infrared diffusion light source system 1, a to-be-tested infrared optical material sample 2, a sample placing table 6, an infrared photographic objective 3 (depth-of-field imaging system), an infrared detector 4, an infrared image collecting, processing, and displaying system 5, as shown in fig. 3.
As shown in fig. 4, the infrared diffusion light source system 1 is composed of an infrared light source and a diffusion screen 101 for transmitting infrared spectrum, and is a wide-spectrum infrared diffusion light source system shown in fig. 4a or a monochromatic infrared diffusion light source system shown in fig. 4 b. The infrared light source can be the full-spectrum infrared light source 102 in fig. 4a, i.e. the infrared light source can emit light radiation with the wavelength range of 0.78-14 μm; the monochromatic infrared light source 103 in fig. 4b, such as a laser infrared light source, may be used, and when a laser infrared light source is used, the laser beam expanding optical system 104 is also used. When the monochromatic infrared light source 103 is used, the wavelength of the monochromatic infrared light source should be within the transmission band of the infrared optical material and also within the radiation response operating band of the infrared detector 4. The diffused light emitted by the infrared diffusion light source system 1 should be as uniform as possible, so as to avoid the interference of the nonuniform light source to the test imaging.
Step S2, determining the maximum thickness of the infrared optical material sample 2 to be tested in the infrared optical material depth-of-field photographic impurity testing system, and this parameter will be used as the design basis for the depth of field of the object space of the infrared photographic objective lens 3. According to the design condition of the infrared optical part, the thickness of the infrared optical part is generally less than 20mm, therefore, for a sample with the diameter of 100mm, a near infrared or short wave infrared light source is used for testing, the maximum thickness of the tested infrared optical material sample 2 is determined to be not more than 25mm (if necessary, the depth of field of the infrared photographic objective lens 3 can be increased by reducing the testing caliber of the tested infrared optical material sample 2 and increasing the F number of the infrared photographic objective lens 3 according to the caliber reduction proportion), and the maximum thickness of the tested infrared optical material sample (2) is determined to be more than 2 times of the short wave in the medium wave and long wave infrared wave bands.
And step S3, determining the object space resolution of the infrared photographic objective lens 3 according to the minimum impurity size to be counted of the infrared optical material sample 2 to be detected. For infrared optical materials working in a short-wave infrared band (1-3 microns), a short-wave infrared light source is used for carrying out impurity test on the infrared optical materials, and the minimum size of the infrared optical material impurities needing to be resolved is 0.1 mm. For infrared optical materials working in a medium wave infrared band (3-5 μm) or a long wave infrared band (8-14 μm), when a radiation source with a corresponding wave band is used for testing, the minimum size of impurities of the infrared optical materials needing to be resolved increases along with the lengthening of the wave length. Except that the germanium crystal material is not transparent in infrared short wave band, almost all infrared optical materials are transparent from infrared short wave band to infrared long wave band, so that all infrared optical materials can be tested by selecting a medium wave band, and most infrared optical materials can be tested by selecting a short wave band.
And step S4, selecting the wave band response type of the infrared detector according to the transparent wave band of the predetermined infrared optical material sample 2 to be detected, namely determining the type of the infrared detector 4. If the infrared optical material of the tested germanium crystal is not considered, a near-infrared or short-wave infrared detector can be selected, and the advantages of large number of area array pixels, high image resolution and low price (the detection waveband covers 0.78-1 μm) of the detector are achieved. If impurities of various infrared optical materials are to be tested, a medium-wave infrared detector or a long-wave infrared detector can be selected.
Step S5, establishing a matching relationship between the maximum side length dimension (the maximum effective dimension 501 in fig. 5) of the infrared detector 4 and the maximum testing range of the maximum caliber (the maximum testing dimension 502 in fig. 5) of the tested infrared optical material sample 2. As shown in fig. 5, the maximum test size 502 of the infrared optical material sample 2 to be tested is divided by the maximum effective size 501 of the infrared detector 4 as the magnification β of the infrared photographic objective 3. The matching relation is used for calculating the image space resolution of the test, so that the maximum allowable size test of the infrared optical material sample 2 can also meet the resolution requirement of the test when the test system meets the required resolution for the small-caliber size test of the infrared optical material sample 2.
Step S6, determining the image resolution interval σ according to the object resolution of the infrared photographic objective 3 determined at S3. The image-side resolution interval σ is the object-side resolution of the ir-photographic objective 3 divided by the magnification β of the ir-photographic objective 3.
Step S7, calculating the maximum F number allowed for the infrared camera objective lens 3 based on the image-side resolution interval σ of the infrared camera objective lens 3 determined in step S6. Assuming that the test wavelength is λ, and F ═ σ/(1.22 λ), this F number allows the infrared photographic objective 3 to have a resolution to the minimum impurity size to be tested in the infrared optical material sample 2 to be tested.
Step S8, the size (side length) d of the detecting element 601 of the infrared detector 4 is selected, where d ≦ σ, so that the infrared detector 4 can satisfy the resolution of the image space resolution interval σ (minimum resolution interval) of the infrared photographic objective lens 3, as shown in fig. 6. The minimum resolution of the infrared detector 4 is the distance between two adjacent pixels, and the distance between two adjacent pixels of the infrared detector 4 with a large duty ratio can be approximately equal to the side length of the detection element.
Step S9, determining the size of the diffraction spot (i.e. the image point diffuse spot 602) of the image point of the infrared camera objective lens 3 by using the F number of the infrared camera objective lens 3 obtained in step S7. The method comprises the following steps: the diameter of the diffraction spot of the image point of the infrared photographic objective 3 is 2 sigma, the diameter 603 of the diffraction spot of the image point (i.e. the image point diffuse spot) is two detector sizes, i.e. 2d, and the matching relationship between the diameter 603 of the diffraction spot and four detector sizes is shown in fig. 6, but it is more preferable that: if the F number of the infrared photographic objective 3 is reduced by half without affecting the expected depth of field length of the infrared photographic objective 3, the size of the diffraction spot diameter 603 of the image point of the infrared photographic objective 3 is equal to or slightly larger than the size of the detection element 601 of the infrared detector 4, so as to ensure that enough radiation energy acts on the detection element 601 and meet the requirement that the detection element 601 has a good response signal to the image of the object point.
And step S10, setting alternative series of focal length values (such as focal length of 10mm, 20mm, 30mm, … …, 80mm and the like) of the infrared camera objective lens 3, so as to design various optical system total size schemes and optical system caliber schemes (calculated according to the F number and the focal length allowed by the infrared camera objective lens 3 determined in the step S7), and screening the optical system total size and the detector exposure for weighing the test system total size and the detector exposure.
Step S11 of calculating a series of object distances of the infrared photographic objective lens 3 based on the magnification of the infrared photographic objective lens 3 calculated in step S5 and the series of focal length values set in step S10; permission determined according to step S7Calculating a plurality of F numbers, the series focal length value set in step S10, the series object distance corresponding to the series focal length, and the allowable diameter of the out-of-focus diffusion circle, and calculating the series focal length of the infrared photographic objective lens 3, the series object space front depth of field corresponding to the object distance, and the series object space rear depth of field corresponding to the object distance; according to the volume size allowed by the testing equipment, according to the requirement that the infrared photographic objective lens 3 can image the infrared optical material impurities in the thickness range of the sample to be tested on the infrared detector 4, the above optical imaging parameters such as the object space depth of field delta L (the sum of the object space front depth of field and the object space back depth of field), the imaged object distance L and the imaged image distance L 'of the infrared photographic objective lens 3 with the satisfactory series of focal length values f' are selected in a balanced manner, as shown in FIG. 7, Xi、Xi+1Respectively is the ith impurity and the i +1 impurity X in the infrared optical material sample 2 to be detectedi’、Xi+1' are images of the ith and i +1 th impurities in the infrared optical material sample 2 to be measured, respectively.
Step S12, in order to ensure effective utilization of the depth of field length of the infrared optical system, in the arrangement of the optical material impurity testing system shown in fig. 3, the thickness middle plane 801 of the infrared optical material sample 2 to be tested does not coincide with the object plane 802 (depth of field interface, i.e. the object plane corresponding to the image sensing plane 803 of the infrared detector 4), but the thickness middle plane 801 of the infrared optical material sample 2 to be tested is near the left of the object plane 802, i.e. the object plane 802 is near the right of the thickness middle plane 801 of the infrared optical material sample 2 to be tested, so that the object front depth of field 804 and the object rear depth of field 805 effectively occupy the sample thickness (for the maximum thickness sample allowed), as shown in fig. 8. The position corresponding relation is calibrated by the track positioning arrangement or the track scale of the arrangement of the infrared optical material sample 2 to be detected.
And step S13, inputting the impurity images in the detected infrared optical material sample 2 received by the infrared detector 4 into the image acquisition, processing and display system 5, measuring the sizes of the impurities of the sample, counting the number of the impurities and calculating the impurity images through the image acquisition, processing and display system 5, and outputting a test result of impurity standard evaluation.
The method can be used for testing the optical material impurities in various infrared bands (near infrared, short wave infrared, medium wave infrared and long wave infrared) which are not transparent to visible light, and can also be used for testing the impurities of other radiation transparent materials, and only an infrared radiation source, an infrared optical imaging system and an infrared detector need to be replaced by a radiation source, an imaging system and a detector in corresponding bands, such as visible light bands, ultraviolet bands, terahertz bands and the like. Although the impurities of the optical material can be directly tested (or inspected) by human eyes in the visible light wave band, when the method is applied to testing the impurities of the visible light optical material, the testing (or inspection) can be automated, manual testing (or inspection) is not needed, the operating efficiency and the defect evaluation computing efficiency of the impurity testing (or inspection) of the visible light optical material are greatly improved, and the objectivity of the impurity testing (or inspection) of the visible light optical material is also greatly improved.
The above description is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, several modifications and variations can be made without departing from the technical principle of the present invention, and these modifications and variations should also be regarded as the protection scope of the present invention.
Claims (10)
1. The infrared optical material impurity testing method comprises the following steps: s1, establishing a testing system, and sequentially comprising an infrared diffusion light source system (1) and a tested infrared optical material sample (2) in the light path direction; wherein, the infrared optical material sample (2) to be measured is placed on the sample placing table (6); the infrared diffusion light source system (1) consists of an infrared light source and a diffusion screen (101) for transmitting infrared spectrum, wherein the infrared light source is a monochromatic infrared light source (103); it is characterized in that the preparation method is characterized in that,
in the step S1, the established test system is an infrared optical material depth of field photographic impurity test system, and the test system sequentially comprises an infrared photographic objective lens (3) and an infrared detector (4) behind the infrared optical material sample (2) to be tested in the light path direction; the infrared light source may also be a full spectrum infrared light source (102);
the following steps are also included after step S1:
s2, determining the maximum thickness of the infrared optical material sample (2) to be tested in the test system, and using the maximum thickness as the design basis of the object space depth of field of the infrared photographic objective lens (3);
s3, determining the object space resolution of the infrared photographic objective lens (3) according to the minimum impurity size to be counted of the infrared optical material sample (2) to be detected;
s4, selecting the wave band response type of the infrared detector according to the transparent wave band of the predetermined infrared optical material sample (2) to be detected, namely determining the type of the infrared detector (4);
s5, establishing a matching relation between the maximum side length of the infrared detector (4) and the maximum testing range of the maximum caliber of the infrared optical material sample (2) to be tested;
s6, determining the image resolution interval sigma according to the object resolution of the infrared photographic objective (3) determined in the step S3;
s7, calculating the maximum F number allowed by the infrared photographic objective lens (3) according to the image space resolution distance sigma of the infrared photographic objective lens (3) determined in the step S6;
s8, selecting the size d of a detection element (601) of the infrared detector (4), wherein d is less than or equal to sigma, so that the infrared detector (4) can meet the resolution of an image space resolution interval sigma of the infrared photographic objective lens (3);
s9, determining the diffraction spot of the image point of the infrared photographic objective lens (3), namely the size of the image point diffuse spot (602), by using the F number of the infrared photographic objective lens (3) obtained in the step S7;
s10, setting a series of alternative focal length values of the infrared photographic objective lens (3);
s11, calculating the series object distance of the infrared photographic objective lens (3) according to the magnification of the infrared photographic objective lens (3) calculated in S5 and the series focal length value set in the step S10; calculating the series focal length of the infrared photographic objective lens (3) and the series object space front depth of field and the object space back depth of field corresponding to the object distance according to the allowable F number determined in the step S7, the series focal length value set in the step S10, the series object distance corresponding to the series focal length and the allowable out-of-focus diffusion circle diameter; then according to the volume size allowed by the test system, according to the requirement that the infrared photographic objective lens (3) images the infrared optical material impurities in the thickness range of the sample to be tested on the infrared detector (4), selecting and calculating the object space depth of field delta L, the imaged object distance L and the imaged image distance L 'of the infrared photographic objective lens (3) which reach a series of focal length values f' meeting the preset requirement, wherein the object space depth of field delta L is the sum of the front depth of field of the object space and the back depth of field of the object space;
step S12, arranging the test system according to the following principle: the thickness middle surface (801) of the infrared optical material sample (2) to be detected is not overlapped with the object surface (802), the thickness middle surface (801) of the infrared optical material sample (2) to be detected is arranged on the left side of the object surface (802), the object front depth of field (804) and the object rear depth of field (805) are formed to effectively occupy the thickness of the sample, and the object surface (802) is a depth of field interface, namely an object plane corresponding to an image sensing plane (803) of the infrared detector (4);
and S13, carrying out size measurement and quantity counting on the impurity images in the detected infrared optical material sample (2) received by the infrared detector (4), and giving out a test result of impurity standard evaluation.
2. The method according to claim 1, wherein in step S1, if the infrared light source is a monochromatic infrared light source (103) and is a laser infrared light source, a laser beam expanding optical system (104) is further disposed between the laser infrared light source and the diffuse reflection screen of the infrared transmission spectrum, and the wavelength of the laser infrared light source is within the transmission band of the infrared optical material and within the radiation response operating band of the infrared detector (4).
3. The method of claim 1, wherein in step S2, for a sample having a test caliber up to 100mm in diameter, the maximum thickness of the infrared optical material sample (2) to be tested is determined to be not more than 25mm in the near infrared or short wave infrared band, and the maximum thickness of the infrared optical material sample (2) to be tested is determined to be more than 2 times as large as the short wave in the medium wave and long wave infrared bands.
4. The method of claim 1, wherein in step S3, for infrared optical material operating in the short-wave infrared band, the infrared optical material is tested for impurities using a short-wave infrared light source, and the minimum size of the infrared optical material impurities to be resolved is 0.1 mm; for infrared optical materials working in a medium-wave infrared band or a long-wave infrared band, when a radiation source with a corresponding band is used for testing, the minimum size of impurities of the infrared optical materials needing to be resolved is increased along with the lengthening of the wavelength.
5. The method of claim 1, wherein in step S4, if the germanium crystal infrared optical material to be tested is not considered, a near-infrared or short-wave infrared detector is selected, and if impurities of various types of infrared optical materials are considered to be tested, a medium-wave infrared detector or a long-wave infrared detector is selected.
6. The method according to claim 1, wherein in step S5, the matching relationship is obtained by dividing the maximum test dimension (502) of the infrared optical material sample (2) to be tested by the maximum effective dimension (501) of the infrared detector (4) as the magnification β of the infrared photographic objective (3).
7. The method according to claim 6, characterized in that in step S6, the image-side resolution interval σ is the object-side resolution of the IR camera objective (3) divided by the magnification β of the IR camera objective (3).
8. The method of claim 7, wherein let λ be the test wavelength, and F ═ σ/(1.22 λ).
9. The method according to claim 8, characterized in that in step S8, the image-side resolution of the infrared photographic objective (3) is separated by an interval σ of two adjacent image elements.
10. The method according to claim 9, characterized in that in step S9, if the F-number of the ir-photographic objective (3) minus half affects the desired depth of field length of the ir-photographic objective (3), the diffraction spot diameter of the image point of the ir-photographic objective (3) is determined to be 2 σ, the diffraction spot diameter of the image point (603) is determined to be two detector sizes, 2 d; determining the size of the diffraction spot diameter (603) of the image point of the IR camera objective (3) to be equal to the size of the detector element (601) of the IR detector (4) if a halving of the F-number of the IR camera objective (3) does not affect the desired depth of field length of the IR camera objective (3).
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