Dual-Surface Defect Imaging Method and Device for Transparent Components Based on Short-Coherence Light

Abstract

The present invention discloses a method and device for imaging double-surface defects of a transparent component based on short-coherent light. The output beam generated by a short-coherent light source is divided into a detection beam and a reference beam. The reference beam is split into a front-surface reference beam and a rear-surface reference beam. The front-surface reference beam and the rear-surface reference beam are respectively reflected by a reference mirror and returned along the original path. The detection beam is emitted to the transparent component to be measured, and the front-surface reflected beam and the rear-surface reflected beam of the transparent component to be measured are received. The front-surface reflected beam and the rear-surface reflected beam respectively interfere with the front-surface reference beam and the rear-surface reference beam. The interference images are recorded by a CCD, and the front and rear surface defect images are respectively obtained through filtering processing. The present invention can record the front and rear surface defect imaging of the transparent component to be measured through one exposure, and can synchronously separate and read the defect information, having the advantages of convenient and efficient use.

Description

Technical Field

[0001] The present invention relates to the technical field of optical imaging, and particularly to a method and device for imaging double-surface defects of a transparent member based on short-coherence light. Background Art

[0002] Currently, there is a great demand for quality detection technologies for surface defects of transparent media such as glass. For example, the cover glass of a mobile phone is the outermost glass of the mobile phone touch screen. Its production process includes processes such as substrate cutting, surface grinding and polishing, and screen printing. Each process may produce surface defects such as ink residue, dirt, and dust impurities. Therefore, effective defect detection of the front and back surfaces of the cover glass of a mobile phone is crucial for controlling the yield rate of mobile phones and liquid crystal product screens.

[0003] The existing mainstream defect detection uses machine vision to detect defects on the glass surface. This method can quickly identify defects, but it cannot determine whether the defect is specifically on the front surface or the back surface, which has become a technical shortcoming in the defect detection of transparent media. Currently, there is a method using swept-source optical tomography, which can accurately locate the defects on the glass surface and inside, but it requires a swept-source short-coherence light source, the detection thickness is limited, and it is mainly designed for internal defects of conductive glass. For surface defect detection, the efficiency is not high. There is also a method of applying thermal stress to the transparent material, and its refractive index will change. By studying the refractive index distribution information of the measured material, the specific position of the defect is determined, but heating the sample may damage the sample. There is also a method that utilizes the propagation law of linearly polarized light in a transparent medium and realizes single-sided imaging of glass surface defects based on polarization extinction. However, this method can only record single-surface imaging information in one imaging, and rotating or replacing the analyzer is required to record the other surface. Summary of the Invention

[0004] The purpose of the present invention is to provide a method and device for imaging double-surface defects of a transparent member based on short-coherence light. The present invention can record the imaging of the front and back surface defects of the transparent member to be measured through one exposure, and can synchronously separate and read the defect information, with the advantages of convenient and efficient use.

[0005] The technical solution of the present invention: A method for imaging double-surface defects of a transparent member based on short-coherence light, the output beam generated by the short-coherence light source is divided into a detection beam and a reference beam, the reference beam is split into a front-surface reference beam and a back-surface reference beam, and the front-surface reference beam and the back-surface reference beam are respectively reflected by the reference mirror and return along the original path; the detection beam is emitted to the transparent member to be measured and the front-surface reflected beam and the back-surface reflected beam of the transparent member to be measured are received, the front-surface reflected beam and the back-surface reflected beam respectively interfere with the front-surface reference beam and the back-surface reference beam, and the interference images are recorded by a CCD, and the front and back surface defect images are respectively obtained through filtering processing.

[0006] In the above-mentioned method for imaging double-surface defects of a transparent part based on short-coherence light, in the polarization optical path, the output beam generated by the short-coherence light source is polarized by a polarizer, and then divided into a detection beam and a reference beam by a first polarization beam splitter after passing through a half-wave plate; the reference beam is divided into a front-surface reference beam and a back-surface reference beam by a second polarization beam splitter after passing through a first quarter-wave plate. The front-surface reference beam is reflected by a front-surface reference mirror after passing through a third quarter-wave plate and then returns along the original path. The back-surface reference beam is reflected by a back-surface reference mirror after passing through a fourth quarter-wave plate and then returns along the original path. The returned front-surface reference beam and back-surface reference beam are combined by the second polarization beam splitter; the detection beam irradiates the transparent part to be measured after passing through a second quarter-wave plate and a glass brick. The front-surface reflection beam and the back-surface reflection beam reflected by the transparent part to be measured return along the original path and are combined by the first polarization beam splitter. By adjusting the optical path difference of each interference optical arm, the front-surface reference light and the back-surface reference light respectively interfere with the front-surface and back-surface reflection lights of the glass to be measured, and an interference image is formed on the CCD through an imaging lens, and then the defect images of the double surfaces are respectively extracted through filtering.

[0007] In the above-mentioned method for imaging double-surface defects of a transparent part based on short-coherence light, in the non-polarization optical path, the output beam generated by the short-coherence light source is divided into a detection beam and a reference beam by a first beam splitter. Among them, the reference beam is divided into a front-surface reference beam and a back-surface reference beam by a second beam splitter. The front-surface reference beam is reflected by a front-surface reference mirror after passing through a first attenuation sheet and then returns along the original path. The back-surface reference light is reflected by a back-surface reference mirror after passing through a second attenuation sheet and then returns along the original path. The returned front-surface reference beam and back-surface reference beam are combined by the second beam splitter; the detection beam irradiates the transparent part to be measured after passing through a second quarter-wave plate and a glass brick. The front-surface reflection beam and the back-surface reflection beam reflected by the transparent part to be measured return along the original path and are combined by the first polarization beam splitter. By adjusting the optical path difference of each interference optical arm, the front-surface reference light and the back-surface reference light respectively interfere with the front-surface and back-surface reflection lights of the glass to be measured, and an interference image is formed on the CCD through an imaging lens, and then the defect images of the double surfaces are respectively extracted through filtering.

[0008] In the above-mentioned method for imaging double-surface defects of a transparent part based on short-coherence light, the interference length of the short-coherence light source is less than the optical path difference generated by the light traveling back and forth once on the double surfaces of the transparent part to be measured; by adjusting the optical path differences of the interference optical arms of the front-surface reference mirror and the back-surface reference mirror, the optical paths of the interference optical arms of the front-surface reference mirror and the back-surface reference mirror are respectively made consistent with the optical paths of the front-surface reflection beam and the back-surface reflection beam of the transparent part to be measured, and two sets of interference fringes are received on the CCD. The interference equation is expressed as:

[0009]

[0010] where: A and B are the reflected light fields of the front surface and the rear surface; R A and R B are the reference beams corresponding to the front surface and the rear surface; |γ(l)| represents the coherence degree when the optical path difference is l, which is related to the light source selection, and the subscript of l corresponds to the optical field; Re{} represents taking the real part.

[0011] For the aforementioned method for imaging double-surface defects of a transparent part based on short-coherent light, after obtaining the interference fringe pattern, perform a Fourier transform on it to obtain a frequency spectrum diagram, so that the two sets of fringes are separated, and then extract the signals of the two-surface imaging through band-pass filtering respectively. Perform an inverse Fourier transform on each and take the square of the modulus, and then divide by their respective reference light fields to obtain the front and rear surface defect images.

[0012] The device for the aforementioned method for imaging double-surface defects of a transparent part based on short-coherent light includes a short-coherent light source. The short-coherent light source is connected to a half-wave plate through a polarizer, and the half-wave plate is connected to a first polarization beam splitter prism; the first polarization beam splitter prism is connected to a first quarter-wave plate, a second quarter-wave plate and an analyzer; the first quarter-wave plate is connected to a third quarter-wave plate and a fourth quarter-wave plate through a second polarization beam splitter prism; the third quarter-wave plate is connected to a front surface reference mirror; the fourth quarter-wave plate is connected to a rear surface reference mirror; the second quarter-wave plate is connected to a glass brick; the analyzer is connected to a CCD through an imaging lens.

[0013] The device for the aforementioned method for imaging double-surface defects of a transparent part based on short-coherent light includes a short-coherent light source. The short-coherent light source is connected to a second beam splitter prism, a glass brick and an imaging lens through a first beam splitter prism; the second beam splitter prism is connected to a first attenuation sheet and a second attenuation sheet; the first attenuation sheet is connected to a front surface reference mirror, and the second attenuation sheet is connected to a rear surface reference mirror; the imaging lens is connected to a CCD through a third attenuation sheet.

[0014] Compared with the prior art, the present invention utilizes the limited coherence length of the short-coherent light source. The output beam generated by the short-coherent light source is divided into a detection beam and a reference beam. The reference beam is split into a front surface reference beam and a rear surface reference beam. Two reference beams are introduced to interfere with the double surfaces of the transparent part to be measured simultaneously. The imaging information of the double-surface defects is recorded simultaneously through one exposure and synchronously separated and extracted, so as to obtain the front and rear surface defect images, which has the advantages of convenient and efficient use. The present invention can perform pipeline detection after determining the surface shape, refractive index and thickness of the transparent part to be measured, selecting the corresponding light source and modulating the optical path. Compared with the existing methods, the present invention has the advantages of high efficiency, convenient use and the ability to obtain three-dimensional defect information, and has a good application prospect. Description of the Drawings

[0015] Figure 1It is a schematic structural diagram of the device in Embodiment 1 of the present invention;

[0016] Figure 2 It is a schematic structural diagram of the device in Embodiment 2 of the present invention;

[0017] Figure 3 It is the scratch image before separation in Embodiment 2 of the present invention;

[0018] Figure 4 It is the scratch image of the front surface obtained by separation;

[0019] Figure 5 It is the scratch image of the back surface obtained by separation. Specific implementation manner

[0020] The present invention will be further described below in conjunction with the accompanying drawings and embodiments, but it is not used as a basis for limiting the present invention.

[0021] Embodiment 1: A method for imaging double-surface defects of a transparent part based on short-coherent light. The output beam generated by a short-coherent light source is divided into a detection beam and a reference beam. The reference beam is split into a front-surface reference beam and a back-surface reference beam. The front-surface reference beam and the back-surface reference beam are respectively reflected by a reference mirror and return along the original path. The detection beam is emitted to the transparent part to be measured, and the front-surface reflection beam and the back-surface reflection beam of the transparent part to be measured are received. The front-surface reflection beam and the back-surface reflection beam respectively interfere with the front-surface reference beam and the back-surface reference beam. The interference images are recorded by a CCD, and the front and back surface defect images are respectively obtained through filtering processing.

[0022] In this embodiment, the device for implementing the above method is as Figure 1As shown in the figure, it includes a short coherence light source 1. The short coherence light source 1 is connected to a half-wave plate 3 through a polarizer 2. The half-wave plate 3 is connected to a first polarization beam splitter prism 4. The first polarization beam splitter prism 4 is connected to a first quarter-wave plate 5, a second quarter-wave plate 11, and an analyzer 14. The first quarter-wave plate 5 is connected to a third quarter-wave plate 7 and a fourth quarter-wave plate 9 through a second polarization beam splitter prism 5. The third quarter-wave plate 7 is connected to a front surface reference mirror 8. The fourth quarter-wave plate 9 is connected to a back surface reference mirror 10. The second quarter-wave plate 11 is connected to a glass brick 12, and in front of the glass brick 12 is a transparent part to be measured 13. The analyzer 14 is connected to a CCD 16 through an imaging lens 15. The polarizer and the half-wave plate are used to adjust the light intensity of the detection beam and the reference beam. The first quarter-wave plate is used to adjust the polarization state of the reference beam so that it can be split by the second beam splitter prism and can return along the original path and interfere with the detection beam through the first beam splitter prism. The second quarter-wave plate is used to adjust the polarization state of the detection beam so that the detection beam reflected by the transparent part to be measured can pass through the first beam splitter prism and interfere with the reference light, and can adjust the measurement light intensity. The third quarter-wave plate and the fourth quarter-wave plate are respectively used to adjust the light intensity of the front surface and back surface reference lights. By rotating the polarizer and each quarter-wave plate, the brightness of the two sets of interference fringes can be adjusted to make their brightness approximately equal. The glass brick is used to compensate for the optical path difference generated by the reference light passing through the second polarization beam splitter prism. The analyzer is used to adjust the light intensity ratio of the interference between the reference light and the measurement light.

[0023] In the polarization optical path formed by the above device, the output beam generated by the short coherence light source is polarized by the polarizer and then divided into a detection beam and a reference beam by the first polarization beam splitter prism after passing through the half-wave plate. The reference beam is divided into a front surface reference beam and a back surface reference beam by the second polarization beam splitter prism after passing through the first quarter-wave plate. The front surface reference beam is reflected by the front surface reference mirror and returns along the original path after passing through the third quarter-wave plate. The back surface reference beam is reflected by the back surface reference mirror and returns along the original path after passing through the fourth quarter-wave plate. The returned front surface reference beam and back surface reference beam are combined by the second polarization beam splitter prism. The detection beam irradiates the transparent part to be measured after passing through the second quarter-wave plate and the glass brick. The front surface reflected beam and the back surface reflected beam reflected by the transparent part to be measured return along the original path and are combined by the first polarization beam splitter prism. By adjusting the optical path difference of each interference optical arm, the front surface reference light and the back surface reference light respectively interfere with the front surface and back surface reflected lights of the glass to be measured, and the interference image is formed on the CCD through the imaging lens, and then the defect images of the double surfaces are respectively extracted through filtering.

[0024] The interference length of the short coherence light source is less than the optical path difference generated by the light traveling back and forth between the two surfaces of the transparent part to be measured; by adjusting the optical path difference of the interference light arms of the front surface reference mirror and the back surface reference mirror, the optical paths of the interference light arms of the front surface reference mirror and the back surface reference mirror are respectively made consistent with the optical paths of the reflected light beams from the front surface and the back surface of the transparent part to be measured, and two sets of interference fringes are received on the CCD. The interference equation is expressed as:

[0025]

[0026] In the formula: A and B are the reflected light fields of the front surface and the back surface; R A 、R B are the reference light beams corresponding to the front surface and the back surface; |γ(l)| represents the coherence degree when the optical path difference is l, which is related to the light source selection, and the subscript of l corresponds to the light field; Re{} represents taking the real part.

[0027] For different transparent parts to be measured, by matching a short coherence light source with a coherence length less than the optical path difference between the upper and lower surfaces, the interference term 2Re{AB *}|γ(l AB )| can be made much smaller than the target term And |R A | 2 、|R B | 2 、|A| 2 、|B| 2 are DC terms.

[0028] Use a computer to perform a two-dimensional Fourier transform on the obtained interference image, extract the spectra of the target term respectively, and perform an inverse Fourier transform. Take the square of the modulus respectively to get (R A A) 2 、(R B B) 2 , and then divide by the front and back surface reference light images respectively to obtain the front and back surface defect images A 2 and B 2 .

[0029] Embodiment 2: A method for imaging double - surface defects of a transparent part based on short - coherence light. The output beam generated by a short - coherence light source is divided into a detection beam and a reference beam. The reference beam is split into a front - surface reference beam and a rear - surface reference beam. The front - surface reference beam and the rear - surface reference beam are reflected by reference mirrors respectively and then return along the original path. The detection beam is emitted to the transparent part to be measured, and the front - surface reflected beam and the rear - surface reflected beam of the transparent part to be measured are received. The front - surface reflected beam and the rear - surface reflected beam interfere with the front - surface reference beam and the rear - surface reference beam respectively. The interference images are recorded by a CCD, and the front - surface and rear - surface defect images are obtained through filtering processing.

[0030] In this embodiment, the device for implementing the above method is as Figure 1 shown, and it includes a short - coherence light source 1. The short - coherence light source is connected to a second beam - splitting prism 3 through a first beam - splitting prism 2, a glass brick 8, and an imaging lens 10. In front of the glass brick 8 is the transparent part 9 to be measured; the second beam - splitting prism 3 is connected to a first attenuation sheet 4 and a second attenuation sheet 6; the first attenuation sheet 4 is connected to a front - surface reference mirror 5, and the second attenuation sheet 6 is connected to a rear - surface reference mirror 7; the imaging lens 10 is connected to a CCD 12 through a third attenuation sheet 11. The first attenuation sheet and the second attenuation sheet are respectively used to adjust the light intensities of the front - surface and rear - surface reference beams; the third attenuation sheet is used to adjust the light intensity of the interference - pattern fringes; by rotating each attenuation sheet, the brightness of the two sets of interference fringes can be adjusted to make their brightnesses approximately equal; the glass brick is used to compensate for the optical path difference generated by the reference light passing through the second beam - splitting prism.

[0031] In the non - polarized optical path formed by the above device, the output beam generated by the short - coherence light source is divided into a detection beam and a reference beam by the first beam - splitting prism. Among them, the reference beam is divided into a front - surface reference beam and a rear - surface reference beam by the second beam - splitting prism. The front - surface reference beam returns along the original path after being reflected by the front - surface reference mirror after passing through the first attenuation sheet, and the rear - surface reference beam returns along the original path after being reflected by the rear - surface reference mirror after passing through the second attenuation sheet. The returned front - surface reference beam and rear - surface reference beam are combined by the second beam - splitting prism; the detection beam passes through a second quarter - wave plate and the glass brick and then irradiates the transparent part to be measured. The front - surface reflected beam and the rear - surface reflected beam reflected by the transparent part to be measured return along the original path and are combined by the first polarization beam - splitting prism. By adjusting the optical path differences of each interference optical arm, the front - surface reference light and the rear - surface reference light interfere with the front - surface and rear - surface reflected lights of the transparent glass respectively, and the interference images are formed by the imaging lens on the CCD, and then the defect images of the double surfaces are extracted through filtering respectively.

[0032] The interference length of the short - coherence light source is less than the optical path difference generated by the light traveling back and forth once between the double surfaces of the transparent part to be measured; as Figure 3As shown, by adjusting the optical path difference of the interference light arms of the front surface reference mirror and the rear surface reference mirror, the optical paths of the interference light arms of the front surface reference mirror and the rear surface reference mirror are made consistent with the optical paths of the reflected beams from the front surface and the rear surface of the transparent part to be measured respectively, and two sets of interference fringes are received on the CCD. The interference equation is expressed as:

[0033]

[0034] In the formula: A and B are the reflected light fields of the front surface and the rear surface; R A , R B are the reference light beams corresponding to the front surface and the rear surface; |γ(l)| represents the coherence degree when the optical path difference is l, which is related to the light source selection, and the subscript of l corresponds to the light field; Re{} represents taking the real part.

[0035] For different transparent parts to be measured, by matching a short coherence light source with a coherence length less than the optical path difference between the upper and lower surfaces, the interference term 2Re{AB *}|γ(l AB )| can be made far less than the target term while |R A | 2 , |R B | 2 , |A| 2 , |B| 2 are DC terms.

[0036] Use a computer to perform a two-dimensional Fourier transform on the obtained interference image, extract the spectra of the target terms respectively, and perform an inverse Fourier transform. Take the square of the modulus respectively to get (R A A) 2 , (R B B) 2 , and then divide by the front and rear surface reference light images respectively to obtain the front and rear surface defect images A 2 and B 2 of the sample to be measured.

[0037] In this embodiment, the separation effect is as shown in the appendix Figures 3 to 5 . Among them, the thickness of the sample to be measured is 0.55 mm. Figure 3 is the scratch image before separation. Figure 4 is the scratch image of the front surface obtained by separation. Figure 5 is the scratch image of the rear surface obtained by separation. It can be seen from Figures 3 - 5 that the present invention can image and separate the defects on the front and rear surfaces of the transparent part at one time. Figure 3 and Figure 4The defects are clearly visible in the imaging diagram, which shows the advantages of the present invention in terms of high efficiency, convenient use, and the ability to obtain three-dimensional defect information, and has good application prospects.

Claims

1. A method for imaging double - surface defects of a transparent part based on short - coherence light, characterized in that: The output beam generated by the short coherence light source is divided into a detection beam and a reference beam. The reference beam is split into a front surface reference beam and a back surface reference beam. The front surface reference beam and the back surface reference beam are reflected by the reference mirrors respectively and return along the original paths. The detection beam is emitted to the transparent part to be measured and the front surface reflection beam and the back surface reflection beam of the transparent part to be measured are received. The front surface reflection beam and the back surface reflection beam interfere with the front surface reference beam and the back surface reference beam respectively. The interference images are recorded by the CCD, and the front and back surface defect images are obtained respectively through filtering processing. In the polarization optical path, the output beam generated by the short coherence light source is polarized by the polarizer, and then divided into a detection beam and a reference beam by the first polarization beam splitter after passing through the half-wave plate. The reference beam is divided into a front surface reference beam and a back surface reference beam by the second polarization beam splitter after passing through the first quarter-wave plate. The front surface reference beam is reflected by the front surface reference mirror after passing through the third quarter-wave plate and returns along the original path. The back surface reference beam is reflected by the back surface reference mirror after passing through the fourth quarter-wave plate and returns along the original path. The returned front surface reference beam and back surface reference beam are combined by the second polarization beam splitter. The detection beam is irradiated on the transparent part to be measured after passing through the second quarter-wave plate and the glass brick. The front surface reflection beam and the back surface reflection beam reflected by the transparent part to be measured return along the original paths and are combined by the first polarization beam splitter. By adjusting the optical path differences of each interference optical arm, the front surface reference light and the back surface reference light interfere with the front surface and back surface reflection lights of the glass to be measured respectively. The interference images are obtained by imaging on the CCD through the imaging lens, and the defect images of the double surfaces are extracted respectively through filtering. In the non-polarization optical path, the output beam generated by the short coherence light source is divided into a detection beam and a reference beam by the first beam splitter. Among them, the reference beam is divided into a front surface reference beam and a back surface reference beam by the second beam splitter. The front surface reference beam is reflected by the front surface reference mirror after passing through the first attenuation sheet and returns along the original path. The back surface reference light is reflected by the back surface reference mirror after passing through the second attenuation sheet and returns along the original path. The returned front surface reference beam and back surface reference beam are combined by the second beam splitter. The detection beam is irradiated on the transparent part to be measured after passing through the second quarter-wave plate and the glass brick. The front surface reflection beam and the back surface reflection beam reflected by the transparent part to be measured return along the original paths and are combined by the first beam splitter. By adjusting the optical path differences of each interference optical arm, the front surface reference light and the back surface reference light interfere with the front surface and back surface reflection lights of the glass to be measured respectively. The interference images are obtained by imaging on the CCD through the imaging lens, and the defect images of the double surfaces are extracted respectively through filtering.

2. The method for imaging double-surface defects of a transparent part based on short-coherence light according to claim 1, wherein: The interference length of the short coherence light source is less than the optical path difference generated by the light traveling back and forth between the double surfaces of the transparent part to be measured. By adjusting the optical path differences of the interference optical arms of the front surface reference mirror and the back surface reference mirror, the optical paths of the interference optical arms of the front surface reference mirror and the back surface reference mirror are made consistent with the optical paths of the front surface reflection beam and the back surface reflection beam of the transparent part to be measured respectively, and two groups of interference fringes are received on the CCD. The interference equation is expressed as: Where: A and B are the reflected light fields of the front surface and the rear surface; R A , R B are the reference beams corresponding to the front surface and the rear surface; |γ(l)| represents that the coherence degree when the optical path difference is l is related to the light source selection, and the subscript of l is the corresponding light field; Re{} represents taking the real part.

3. The method for imaging double-surface defects of a transparent component based on short-coherence light according to claim 2, wherein: After obtaining the interference fringe pattern, perform Fourier transform on it to obtain the frequency spectrum diagram, so that the two sets of fringes are separated. Then, extract the signals of the two surface images through band-pass filtering respectively, perform inverse Fourier transform on them respectively, take the square of the modulus, and divide by their respective reference light fields to obtain the front and rear surface defect images.

4. The apparatus for the method of imaging double-surface defects of a transparent member based on short-coherence light according to claim 1, characterized in that: It includes a short-coherence light source. The short-coherence light source is connected to a half-wave plate through a polarizer, and the half-wave plate is connected to a first polarization beam splitter prism; the first polarization beam splitter prism is connected to a first quarter-wave plate, a second quarter-wave plate and an analyzer; the first quarter-wave plate is connected to a third quarter-wave plate and a fourth quarter-wave plate through a second polarization beam splitter prism; the third quarter-wave plate is connected to a front surface reference mirror; the fourth quarter-wave plate is connected to a rear surface reference mirror; the second quarter-wave plate is connected to a glass brick; the analyzer is connected to a CCD through an imaging lens.

5. The device for the method of imaging double-surface defects of a transparent part based on short-coherence light according to claim 1, characterized in that: It includes a short-coherence light source. The short-coherence light source is connected to a second beam splitter prism, a glass brick and an imaging lens through a first beam splitter prism; the second beam splitter prism is connected to a first attenuation sheet and a second attenuation sheet; the first attenuation sheet is connected to a front surface reference mirror, and the second attenuation sheet is connected to a rear surface reference mirror; the imaging lens is connected to a CCD through a third attenuation sheet.

Images