A method for testing the spectral performance of a dispersive spectral imaging system

Abstract

The present invention relates to a method for testing the spectral performance of a dispersive spectral imaging system. The method employs a one-dimensional pinhole array as a test target, which is imaged to an entrance slit via a dual-telecentric relay system. A laser frequency comb is used as a light source, and a rotating integrating sphere is used to uniformly illuminate the test target. A planar array detector mounted on the image plane of the tested spectral imaging system is used for imaging, obtaining a diffuse spot image of the target for spectral performance testing. The centroid position of each light spot is measured to obtain the spectral line bending and chromatic distortion of the tested spectral imaging system. The peak half-width and distribution function of each light spot are measured to obtain the spectral resolution and response function of the tested spectral imaging system, thereby testing the spectral performance of the dispersive spectral imaging system. The test method provided by the present invention does not require mechanical scanning, has a high test speed, and has multiple test functions. It can simultaneously test multiple spectral performance parameters and is suitable for performance measurement of different models of spectral imaging instruments, thus having a wide range of applications.

Description

Technical Field

[0001] The present invention relates to a method for testing spectral performance, and in particular to a method for testing spectral performance of a dispersion type spectral imaging system. Background Art

[0002] Spectral performance is a key performance metric for spectral imaging systems, including spectral resolution, spectral line bending, chromatic aberration, and response function. Currently, testing spectral performance can be divided into two steps. First, a narrow-linewidth light source, such as a spectral lamp or tunable laser, is used to illuminate the slit of the spectral imaging system to test spectral resolution and spectral line bending. Second, a broadband light source and a test optical system are used to generate a broadband point source at the slit of the spectral imaging system to measure the system's chromatic aberration. Mouroulis et al. from JPL in the United States describe the specific testing method and process in their paper, "Pushbroom Imaging Spectrometer with High Spectroscopic Data Fidelity: Experimental Demonstration" (Optical Engineering, Vol. 39, No. 3, p. 808, 2000). However, this test requires switching the light source and test optical system, which is inefficient and cannot simultaneously measure spectral line bending and chromatic aberration. In their paper "Efficient method to measure the spectral distortions using periodically distributed slit in hyperspectral imager" (Optics Express, Vol. 25, No. 17, p. 20340, 2017), Hong Jinsuk et al. from South Korea proposed a method for simultaneously measuring spectral bending and chromatic distortion. The test device in this method employs a periodic slit array, placed closely adjacent to the slit. Testing is performed using a krypton lamp to illuminate the slits, improving test efficiency. However, this method has the following drawbacks: the slit array must be placed closely adjacent to the input slit of the spectral imaging system, which can easily scratch or damage the slit surface; the slit array orientation is difficult to ensure perpendicularity to the input slit, which can easily lead to measurement errors; and the use of an elemental lamp as the light source limits the number of spectral lines output. Summary of the Invention

[0003] In view of the shortcomings of existing spectral performance measurement methods, the present invention proposes a method for quickly and accurately testing the spectral performance of a dispersion-type spectral imaging system.

[0004] The technical solution for achieving the purpose of the present invention is to provide a method for testing the spectral performance of a dispersive spectral imaging system, comprising the following steps:

[0005] (1) The slit plane of the measured spectral imaging system is overlapped with the image plane of the dual telecentric relay system, a one-dimensional pinhole array target is mounted on the object plane of the dual telecentric relay system, a laser frequency comb light source is incident on a rotating integrating sphere, and the outlet of the rotating integrating sphere illuminates the one-dimensional pinhole array target; the target substrate of the one-dimensional pinhole array target is non-transparent, and the target pattern is a plurality of light-transmitting pinholes of the same diameter arranged at equal intervals on a straight line;

[0006] (2) Installing the array detector on the image plane of the spectral imaging system to be measured to receive the diffuse spot image of the spectral imaging system to be measured;

[0007] (3) Measure the diffuse spot image. By measuring the centroid position of each light spot, the spectral line bending and color distortion of the measured spectral imaging system can be obtained. By measuring the peak half-height width and distribution function of each light spot, the spectral resolution and response function of the measured spectral imaging system can be obtained.

[0008] In the technical solution of the present invention, measuring the diffuse speckle image includes the following steps:

[0009] (1) The acquired diffuse speckle image is segmented using a threshold segmentation algorithm to obtain multiple sub-regions, each containing a diffuse speckle. The number of sub-regions is equal to the product of the number of pinholes within the slit length of the measured spectral imaging system and the number of wavelengths output by the laser frequency comb.

[0010] (2) Calculate the centroid of the diffuse speckle in each sub-region;

[0011] (3) Based on the centroid coordinates of the dispersion direction of the diffuse spot corresponding to the same wavelength, the slit image curve of the wavelength is obtained by quadratic curve fitting; the maximum deviation of the dispersion direction coordinates at both ends of the fitting curve and the center of the curve is taken and multiplied by the inverse linear dispersion rate of the measured spectral imaging system to obtain the spectral line curvature value of the wavelength;

[0012] (4) Perform straight line fitting based on the coordinates of the centroid of the diffuse spot in the same field of view. The chromatic distortion value of the measured spectral imaging system is calculated by multiplying the slope difference of the fitting line between the edge field of view and the center field of view by the image plane width in the dispersion direction and the inverse linear dispersion rate.

[0013] (5) Calculate the peak half-maximum width of the diffuse spot in each sub-region in the dispersion direction and multiply it by the inverse linear dispersion rate to obtain the spectral resolution of the measured spectral imaging system at different wavelengths and different fields of view.

[0014] (6) Based on the intensity distribution of the diffuse spots in each sub-region in the dispersion direction and the spatial direction, the response function of the measured spectral imaging system at different wavelengths and different fields of view is obtained.

[0015] The inverse linear dispersion, as used in the technical solution of this invention, is the inverse of the linear dispersion, representing the dispersion spectrum per unit length on the image plane, typically expressed in nm / mm. Because a laser frequency comb can output multiple wavelengths, a single image can simultaneously capture spectral curvature values at multiple wavelengths.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] 1. The test method provided by the present invention uses a one-dimensional pinhole array as a test target and uses a dual telecentric relay system to image the target to the incident slit. No mechanical scanning is required during the test process, and the test speed is fast.

[0018] 2. The present invention uses a laser frequency comb that can output multiple wavelengths simultaneously as a light source, and evenly illuminates the test target through a rotating integrating sphere. It has multiple testing functions and can simultaneously complete the testing of multiple spectral performance parameters.

[0019] 3. Use the area array detector installed on the image plane of the spectral imaging system to obtain images, analyze and obtain spectral performance test results, which can be used to measure the performance of different models of spectral imagers and has a wide range of applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a working principle diagram of the spectral performance test provided by an embodiment of the present invention.

[0021] Figure 2 Schematic diagram of a one-dimensional pinhole array target for the spectral performance testing method provided in an embodiment of the present invention.

[0022] Figure 3 This is an image obtained on a detector by the spectral performance testing method provided in an embodiment of the present invention.

[0023] Figure 4 This is a data processing flow chart of the spectral performance test process according to an embodiment of the present invention.

[0024] In the figure, 1. One-dimensional pinhole array target; 2. Dual telecentric relay system; 3. Laser frequency comb source; 4. Rotating integrating sphere; 5. Planar array detector; 6. Entrance slit of the measured spectral imaging system; 7. The measured spectral imaging system. DETAILED DESCRIPTION

[0025] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0026] Example 1:

[0027] See attached Figure 1, is a test working principle diagram of the spectrum performance test method provided in this embodiment. The tested spectrum imaging system is a dispersion spectrum imaging system, which uses a grating or a prism as a spectroscopic element to obtain a target spectrum image. Figure 1 As can be seen, the testing device includes a one-dimensional pinhole array target 1, a dual-telecentric relay system 2, a laser frequency comb 3, a rotating integrating sphere 4, and a planar array detector 5. The test device structure is as follows: the slit plane of the spectral imaging system 7 under test coincides with the image plane of the dual-telecentric relay system 2, and the entrance slit 6 of the spectral imaging system 7 under test is used to limit the field of view of the spectral imaging system; the one-dimensional pinhole array target 1 is mounted on the object plane of the dual-telecentric relay system 2; the laser frequency comb light source 3 is incident on the rotating integrating sphere 4, and the exit of the integrating sphere 4 illuminates the pinhole array target 1; and the planar array detector 5 is mounted on the image plane of the spectral imaging system 7 under test. In this embodiment, the dual-telecentric relay system 2 can adopt a reflective or transmissive optical system, capable of high-quality imaging of the linear field of view. The dual-telecentric feature makes the test device insensitive to slight defocus, thereby improving the system's test uncertainty. The laser frequency comb 3 can provide a rich laser longitudinal mode output, providing a narrow linewidth laser output with multiple different wavelengths for spectral performance testing. The rotating integrating sphere 4 is used to generate uniform illumination light to illuminate the one-dimensional pinhole array target 1. The area array detector 5 is used to receive images from the measured spectral imaging system.

[0028] The optical path of the dual-telecentric relay system in this embodiment utilizes an Offner concentric structure, offering advantages such as simplicity, lack of chromatic aberration, and excellent imaging quality. A dual-telecentric relay system can also be implemented using a transmissive optical system, such as a dual-Gauss system. Compared to reflective systems, the greatest difficulty with transmissive systems lies in chromatic aberration correction, especially when operating over a wide wavelength band. Chromatic aberration correction can be achieved by combining various glass types.

[0029] Figure 2 Schematic diagram of the structure of a one-dimensional pinhole array target for the test method provided in this embodiment. The target substrate is opaque, while the pinhole area is translucent. The entire target pattern consists of a series of evenly spaced pinholes, and the total length of the pinhole array is greater than the total length of the slit of the spectral imaging system being tested. To prevent the pinhole size from affecting the spectral performance test results, the pinhole radius should be as small as possible, typically less than the Airy disk radius of the spectral imaging system being tested.

[0030] The specific steps of the spectral performance test method are:

[0031] (1) The slit plane of the measured spectral imaging system is overlapped with the image plane of the dual telecentric relay system, a one-dimensional pinhole array target is mounted on the object plane of the dual telecentric relay system, a laser frequency comb light source is incident on a rotating integrating sphere, and the outlet of the rotating integrating sphere illuminates the one-dimensional pinhole array target; the target substrate of the one-dimensional pinhole array target is non-transparent, and the target pattern is a plurality of light-transmitting pinholes of the same diameter arranged at equal intervals on a straight line;

[0032] (2) Installing the array detector on the image plane of the spectral imaging system to be measured to receive the diffuse spot image of the spectral imaging system to be measured;

[0033] (3) Measure the diffuse spot image. By measuring the centroid position of each light spot, the spectral line bending and color distortion of the measured spectral imaging system can be obtained. By measuring the peak half-height width and distribution function of each light spot, the spectral resolution and response function of the measured spectral imaging system can be obtained.

[0034] See attached Figure 3 , is an image acquired by the area array detector when the spectral performance of the imaging spectrometer is tested using the testing method provided in this embodiment. Figure 3 The vertical direction is the spatial direction, corresponding to the slit length of the spectral imaging system under test; the horizontal direction is the dispersion direction, corresponding to the slit width of the spectral imaging system under test. The white circles in the figure represent the diffuse light spots formed by the pinhole in the target substrate on the image plane of the spectral imaging system under test at different fields of view and wavelengths. The size and center of mass of the light spot are affected by diffraction and aberrations of the spectral imaging system under test.

[0035] Image data processing is performed based on the image acquired by the area array detector of this embodiment. By measuring the position of each light spot on the image, the spectral line bending and color distortion of the spectral imaging system under test can be obtained; by measuring the peak half-maximum width and distribution function of each light spot, the spectral resolution and response function of the spectral imaging system under test can be obtained.

[0036] See attached Figure 4 , is a flowchart of image data processing during the test process, and the specific method includes the following steps:

[0037] (1) The acquired diffuse spot image is segmented using a threshold segmentation algorithm to obtain multiple sub-regions, each containing a diffuse spot. The number of sub-regions is equal to the product of the number of pinholes within the slit length range of the measured spectral imaging system and the number of wavelengths output by the laser frequency comb.

[0038] (2) Calculate the centroid of the diffuse spot in each sub-region.

[0039] (3) Based on the centroid coordinates of the dispersion direction of the diffuse spot corresponding to the same wavelength, the slit image curve at that wavelength is obtained by quadratic curve fitting. The maximum deviation between the two ends of the fitting curve and the dispersion direction coordinates at the center of the curve is taken and multiplied by the inverse linear dispersion rate of the measured spectral imaging system to obtain the spectral line curvature value at that wavelength. The inverse linear dispersion rate is the inverse of the linear dispersion rate, which represents the dispersion spectrum range corresponding to unit length on the image plane, usually in nm / mm. Since the laser frequency comb can output multiple wavelengths, the spectral line curvature values at multiple wavelengths can be obtained simultaneously by taking an image.

[0040] (4) Perform a straight line fitting based on the coordinates of the centroid of the diffuse spot in the same field of view. The chromatic distortion value of the measured spectral imaging system is calculated by multiplying the slope difference of the fitted straight line between the edge field of view and the center field of view by the image plane width in the dispersion direction and the inverse linear dispersion rate.

[0041] (5) Calculate the peak half-maximum width of the diffuse spot in each sub-region in the dispersion direction and multiply it by the inverse linear dispersion rate to obtain the spectral resolution of the measured spectral imaging system at different wavelengths and different fields of view.

[0042] (6) According to the intensity distribution of the diffuse spot in each sub-region in the dispersion direction and the spatial direction, the response function of the measured spectral imaging system at different wavelengths and different fields of view can be obtained.

Claims

1. A method for testing the spectral performance of a dispersive spectral imaging system, characterized in that The steps include: (1) The slit plane of the measured spectral imaging system (7) is overlapped with the image plane of the dual telecentric relay system (2), a one-dimensional pinhole array target (1) is installed on the object plane of the dual telecentric relay system, a laser frequency comb light source (3) is incident on a rotating integrating sphere (4), and the outlet of the rotating integrating sphere illuminates the one-dimensional pinhole array target; the target substrate of the one-dimensional pinhole array target is non-transparent, and the target pattern is a plurality of light-transmitting pinholes of the same diameter arranged at equal intervals on a straight line; (2) installing the array detector (5) on the image plane of the spectral imaging system to be measured, and receiving the diffuse spot image of the spectral imaging system to be measured; (3) Measure the diffuse spot image. By measuring the centroid position of each light spot, the spectral line bending and color distortion of the measured spectral imaging system can be obtained. By measuring the peak half-height width and distribution function of each light spot, the spectral resolution and response function of the measured spectral imaging system can be obtained.

2. The method for testing the spectral performance of a dispersion-type spectral imaging system according to claim 1, characterized in that Measuring the diffuse speckle image includes the following steps: (1) The acquired diffuse speckle image is segmented using a threshold segmentation algorithm to obtain multiple sub-regions, each containing a diffuse speckle. The number of sub-regions is equal to the product of the number of pinholes within the slit length of the measured spectral imaging system and the number of wavelengths output by the laser frequency comb. (2) Calculate the centroid of the diffuse speckle in each sub-region; (3) Based on the centroid coordinates of the dispersion direction of the diffuse spot corresponding to the same wavelength, the slit image curve of the wavelength is obtained by quadratic curve fitting; the maximum deviation of the dispersion direction coordinates at both ends of the fitting curve and the center of the curve is taken and multiplied by the inverse linear dispersion rate of the measured spectral imaging system to obtain the spectral line curvature value of the wavelength; (4) Perform straight line fitting based on the coordinates of the centroid of the diffuse spot in the same field of view. The chromatic distortion value of the measured spectral imaging system is calculated by multiplying the slope difference of the fitting line between the edge field of view and the center field of view by the image plane width in the dispersion direction and the inverse linear dispersion rate. (5) Calculate the peak half-maximum width of the diffuse spot in each sub-region in the dispersion direction, multiply it by the inverse linear dispersion rate, and obtain the spectral resolution of the measured spectral imaging system at different wavelengths and different fields of view; (6) Based on the intensity distribution of the diffuse spots in each sub-region in the dispersion direction and the spatial direction, the response function of the measured spectral imaging system at different wavelengths and different fields of view is obtained.

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