An infrared focal plane array modulation transfer function test apparatus and method

Abstract

The application discloses an infrared focal plane array modulation transfer function testing device and method, which comprises an infrared light source, a knife-edge target, a manual rotating table, an imaging optical system, an infrared focal plane array detector, a three-dimensional displacement mechanism, a driving and data acquisition system and a control computer; the infrared light source comprises two light sources of a surface source black body and an integrating sphere, the surface source black body is used for testing a focal plane detector in a mid-infrared and far-infrared response band, and the integrating sphere light source is used for testing a short-wave infrared focal plane detector; the knife-edge target is installed on the manual rotating table, the imaging optical system is located between the knife-edge and the infrared focal plane array detector, the infrared focal plane array detector is arranged on the three-dimensional displacement mechanism, the driving and data acquisition system is connected with the infrared focal plane array detector and the control computer respectively, and the control computer is also connected with the three-dimensional displacement mechanism and the infrared light source respectively. The application has the advantages of wide application, convenient testing, comprehensive testing parameters, high testing accuracy and the like.

Description

Technical Field

[0001] This invention belongs to the field of testing technology, specifically relating to a device and method for testing the modulation transfer function of an infrared focal plane array. Background Technology

[0002] With the development of device technology and the increasing demands of aerospace applications, infrared focal plane arrays are being increasingly used in infrared imaging systems. The modulation transfer function (MTF) is a key parameter connecting device fabrication and overall system application. Essentially, it reflects the attenuation of signal frequency components within the cutoff frequency range after passing through the photoelectric imaging system, accurately reflecting the spatial resolution and imaging quality of the infrared detector. MTF has gradually become an important indicator for evaluating the imaging performance of infrared detectors.

[0003] The modulation transfer function (MTF) of an infrared focal plane array (IRFPA) is the ratio of the modulation depth of the output signal to the modulation depth of the input signal under sinusoidal spatial frequency modulation irradiation within the Nyquist frequency range. MTF is a function of spatial frequency.

[0004]

[0005] Where M o (f) represents the modulation index of the output signal, M i (f) represents the modulation degree of the input signal.

[0006] For infrared focal plane arrays (IRFAs), traditional MTF measurement methods include the contrast method and the scanning method. The contrast method measures the response of the IRFPA to a sinusoidal pattern (or bar-shaped target), obtaining the MTF from the ratio of the object's contrast to the image's contrast at different spatial frequencies; it is a direct measurement method. Generally speaking, the contrast method has limited measurement accuracy and poor applicability, and therefore is rarely used both domestically and internationally.

[0007] The scanning method involves moving the target and measuring the detector's response to the target signal, then performing a Fourier transform to obtain the MTF (Mean Transform Factor). It is an indirect measurement method. Depending on the target used, indirect measurement methods can be categorized into point source scanning, slit scanning, and knife-edge scanning methods, among others. A typical MTF testing system structure is as follows: Figure 1As shown, different optical signal targets are imaged onto the infrared focal plane photosensitive surface by an imaging optical system. A precision displacement mechanism drives the measured focal plane array to scan, followed by data acquisition and processing. Point source scanning and slit scanning methods require correction factors and have low signal-to-noise ratios, thus their applications are limited. The knife-edge scanning method involves differentiating the edge spread function (ESF) to convert the data into the line spread function (LSF) for processing, and finally performing a Fourier transform to obtain the MTF. The knife edge must be perfectly straight and defect-free, and its length only needs to be sufficient. During measurement, the knife edge should be perpendicular to the scanning direction. Due to the differential operation used, it is sensitive to noise; therefore, noise should be minimized to increase the target signal value and ultimately improve the signal-to-noise ratio. Furthermore, the scanning method mainly tests one or several pixels in the device, and the final MTF value is represented by the MTF value of a single pixel or several pixels. Due to limitations in device manufacturing processes, the final test results cannot fully characterize the MTF value of the entire focal plane array detector. Moreover, the scanning method itself also has disadvantages such as high operational difficulty and long testing time.

[0008] Traditional slit and knife-edge scanning methods require high-precision scanning mechanisms, have high measurement standards, and are difficult to operate. Therefore, the tilted knife-edge method was developed, which is an improvement on the knife-edge scanning method. The tilted knife-edge method images the detector with the knife edge slightly tilted, with the knife edge direction making a small angle with the row or column of the detector array. This results in a slight displacement of the knife edge position from one scan row to another on the array surface. During the positional registration of multiple rows of knife edge data and projection along the knife edge direction, more sampling points can be obtained, thus improving the sampling rate.

[0009] While the tilting knife-edge method offers significant convenience in testing, the slight angle between the knife edge and the detector array surface makes it susceptible to noise in the acquired knife-edge image, leading to deviations in the determined knife-edge direction and affecting the accuracy of the final measurement results. Furthermore, existing tilting knife-edge methods often only perform MTF testing in one direction, failing to conveniently achieve MTF testing in both vertical and horizontal directions, thus hindering a comprehensive evaluation of the MTF of focal plane array devices. Summary of the Invention

[0010] In view of the above-mentioned technical problems existing in the prior art, the present invention proposes an infrared focal plane array modulation transfer function testing device, which is reasonably designed, overcomes the shortcomings of the prior art, and has good effect.

[0011] To achieve the above objectives, the present invention adopts the following technical solution:

[0012] An infrared focal plane array modulation transfer function testing device includes an infrared light source, a knife-edge target, a manual rotary table, an imaging optical system, an infrared focal plane array detector, a three-dimensional displacement mechanism, a drive and data acquisition system, and a control computer. The infrared light source comprises two light sources: a blackbody and an integrating sphere. The blackbody is used for testing focal plane array detectors in the mid-to-far infrared response band, and the integrating sphere is used for testing short-wave infrared focal plane array detectors. The knife-edge target is mounted on the manual rotary table. The imaging optical system is located between the knife-edge target and the infrared focal plane array detector. The infrared focal plane array detector is placed on the three-dimensional displacement mechanism. The drive and data acquisition system is connected to both the infrared focal plane array detector and the control computer. The control computer is also connected to both the three-dimensional displacement mechanism and the infrared light source.

[0013] Furthermore, the surface source blackbody and the integrating sphere are placed on an adjustable lifting platform, and an adjustable rotating reflector is provided between them. The infrared radiation emitted by the surface source blackbody and the integrating sphere illuminates the knife-edge target after passing through the reflector, and then is imaged onto the photosensitive surface of the infrared focal plane array detector by the imaging optical system.

[0014] Furthermore, the surface blackbody radiation surface has a size of 100mm×100mm, a temperature range of 0℃~100℃, and an emissivity of 0.98±0.02; the integrating sphere light source has an inner diameter of 120mm~200mm, an output aperture of 30mm~60mm, and uses a halogen tungsten lamp as the built-in light source to provide light radiation in the range of 400nm~2500nm.

[0015] Furthermore, the blade target adopts a single blade design, with the side of the blade closest to the imaging optical system being blackened, and the side furthest from the imaging optical system being provided with a bandpass filter, the center wavelength of which is 1.55μm, 4μm, or 10μm.

[0016] Furthermore, the manual rotary table is equipped with coarse and fine adjustment buttons. The coarse adjustment range is 360°, the fine adjustment range is ±3°, and the fine adjustment resolution is 0.2°. The fine adjustment button is spring-reset. The diameter of the manual rotary table surface is between 55mm and 80mm, with a central opening of 25mm to 40mm in diameter for placing the blade target.

[0017] Furthermore, the imaging optical system adopts a double parabolic off-axis reflection structure, consisting of two parabolic mirrors with the same technical parameters. The mirrors have an aperture of 180mm to 250mm, a system working distance of 50mm to 100mm, an F-number between F / 3 and F4, and a magnification of 1.

[0018] Furthermore, the three-dimensional displacement mechanism includes three mutually orthogonal movement axes. One axis is parallel to the optical axis of the imaging optical system and is used for precise focusing of the knife-edge image through programmable adjustment, with a repeatability accuracy of no more than ±1μm. The other two axes are used for alignment between the knife-edge image and the photosensitive surface of the detector through programmable or manual adjustment, which facilitates finding the initial imaging position.

[0019] A method for testing the modulation transfer function of an infrared focal plane array, using an infrared focal plane array modulation transfer function testing device as described above, includes the following steps:

[0020] S1. Select a suitable infrared light source according to the type of detector under test, adjust the position of the selected light source, and directly align the light source with the infrared focal plane array detector under test. Perform two-point non-uniformity correction and blind pixel detection on it to obtain the gain correction parameter G(i,j) and bias correction parameter O(i,j) of each pixel. Calculate the data after non-uniformity correction and remove blind pixels.

[0021] S2. Return the light source to its original position to ensure that it fills the field of view of the imaging optical system. At the same time, adjust the three-dimensional displacement mechanism so that the blade can be clearly imaged on the photosensitive surface of the infrared focal plane array detector.

[0022] S3. Select the region of interest at the knife edge imaging position. In the driving and data acquisition system, use the Canny edge detection operator to detect the knife edge position and perform least squares fitting on the edge position to obtain the knife edge tilt angle.

[0023] S4. Project all pixels along the direction of the blade to obtain the original ESF curve;

[0024] S5. Smooth and fit the original ESF curve to obtain the final ESF curve. The fitting algorithm adopts a three-term Fermi function fitting model.

[0025] S6. Calculate the three Fermi functions after fitting according to an appropriate sampling interval to obtain the final ESF curve sampling points with equal intervals.

[0026] S7. Differentiate the final ESF curve to obtain the LSF curve, then perform a Fourier transform on the LSF curve and subtract the MTF value of the imaging optical system to obtain the final MTF result of the infrared focal plane array detector under test.

[0027] Furthermore, the fitting model for the three Fermi functions is as follows:

[0028]

[0029] Where x is the pixel position, a i bi c i d1 and d2 are the fitting coefficients of the three Fermi function models, respectively.

[0030] The beneficial technical effects of this invention are as follows:

[0031] (1) The test parameters are more comprehensive. The knife-edge target can rotate 360°, which can realize the modulation transfer function test in both vertical and horizontal directions.

[0032] (2) Wide range of applications. Employing a total internal reflection optical system and a dual-source scheme of a surface-source blackbody and a halogen tungsten lamp integrating sphere, it can meet the testing requirements of focal plane detectors across various wavelengths from near-infrared to mid- and far-infrared. It can even be extended to the testing of ultraviolet and visible light array detectors, thus broadening its application scope.

[0033] (3) High measurement accuracy. The optical system adopts a double off-axis parabolic structure design, so the light only undergoes two reflections, and the central ray is unobstructed, which keeps the system's MTF value at a high level and improves the imaging quality. This greatly reduces the measurement error caused by the system's MTF. Non-uniformity correction is performed before testing, which can effectively avoid noise introduced by non-uniformity of response rate, reduce the difficulty of subsequent data processing, and improve measurement accuracy.

[0034] (4) Easy to operate. An adjustable rotating mirror is placed in front of the blade. Depending on the response band of the device under test, the surface source blackbody or integrating sphere light source can be selected by simply rotating the mirror to different positions, thus meeting the requirements of focal plane detector testing for various bands. Attached Figure Description

[0035] Figure 1 This is the structure of a typical infrared focal plane array modulation transfer function test system;

[0036] Figure 2 The structure of the modulation transfer function testing device proposed in this invention;

[0037] Figure 3 This is a schematic diagram of the manual rotary table proposed in this invention;

[0038] Figure 4 This is a schematic diagram of the imaging optical system proposed in this invention;

[0039] Figure 5 This is a flowchart of the modulation transfer function testing method proposed in this invention;

[0040] Among them, 1-knife edge target; 2-coarse adjustment screw; 3-fine adjustment screw; 4-reflector; Detailed Implementation

[0041] An infrared focal plane array modulation transfer function testing device, such as Figure 2 As shown, it includes an infrared light source, a blade target, a manual rotary table, an imaging optical system, an infrared focal plane array detector, a three-dimensional displacement mechanism, a drive and data acquisition system, and a control computer.

[0042] The infrared light source primarily provides uniform infrared light radiation for the infrared focal plane array detector. To meet the testing requirements of different types and bands of infrared focal plane array detectors, the infrared light source is designed as a combination of a blackbody and an integrating sphere. The blackbody is used for testing focal plane detectors in the mid-to-far infrared response band, while the integrating sphere is used for testing short-wave infrared focal plane detectors. The two light sources are placed on an adjustable lifting platform, with an adjustable rotating reflector between them. During actual testing, they can be moved via programmable control. The infrared radiation emitted by the blackbody and the integrating sphere illuminates the blade target after passing through the reflector, and then is imaged onto the photosensitive surface of the infrared focal plane array detector by the imaging optics system. The surface blackbody radiation surface has a size of 100mm×100mm, a temperature range of 0℃~100℃, and an emissivity of 0.98±0.02. It can achieve a temperature uniformity of ±0.01℃ within an ambient temperature range of ±5℃. The inner diameter of the integrating sphere light source is 120~200mm, and in this embodiment, an inner diameter of 200mm is selected. The output aperture is 30mm~60mm, and in this embodiment, an output aperture of 50mm is selected. A halogen tungsten lamp is used as the built-in light source to provide light radiation in the range of 400nm~2500nm. The uniformity at the outlet of the integrating sphere is better than 99%, which can meet the usage requirements for testing infrared focal plane array detectors. If necessary, it can also meet the testing requirements for array detectors in the ultraviolet and visible light bands.

[0043] The blade is mounted on a manual rotary table and features a single blade design. The side of the blade closest to the imaging optical system is blackened to effectively eliminate the influence of target radiation on the measurement results. The side furthest from the imaging optical system is equipped with a bandpass filter, which can be flexibly selected according to the response band of the detector under test and the MTF band of interest. Three filters with center wavelengths of 1.55μm, 4μm, or 10μm are preferred. The addition of the filter can, on the one hand, filter out stray light interference and meet the MTF test requirements of infrared focal plane array detectors at specific wavelengths in different bands such as short wave, medium wave, and long wave. On the other hand, it can accurately subtract the corresponding MTF value of the optical system according to the center wavelength of the filter, thereby improving the measurement accuracy.

[0044] The tabletop of the manual rotary table can rotate 360°, such as... Figure 3As shown, there are coarse adjustment buttons and fine adjustment buttons, which are implemented by coarse adjustment screw 2 and fine adjustment screw 3. The coarse adjustment range is 360°, the fine adjustment range is ±3°, and the fine adjustment resolution is 0.2°. The fine adjustment button uses spring reset, which has high resolution and no backlash. The diameter of the manual rotary table is between 55mm and 80mm, with a center hole with a diameter of 25mm to 40mm for placing the knife edge target 1.

[0045] The imaging optical system is located between the knife edge and the infrared focal plane array detector. It adopts a double parabolic off-axis reflective structure, consisting of two parabolic mirrors 4 with identical technical parameters, such as... Figure 4 As shown, the aperture of mirror 4 is 180mm–250mm, the system working distance is 50mm–100mm, the F-number is between F / 3 and F4, and the magnification is 1. This system has the following advantages:

[0046] (1) The system is a total internal reflection system, which is suitable for the visible to far-infrared bands and has a wide spectral range. It can be used for testing focal plane detectors in different bands. (2) The system has only two mirrors with consistent performance parameters, which is not only easy to process and assemble, but also, compared with the Offner three-mirror system, the light only undergoes two reflections, which can achieve very good imaging quality. (3) Compared with other double mirror structures such as Cassegrain, the system has no obstruction in the center, which avoids the decrease in system MTF caused by light loss. When performing system MTF subtraction in the later stage, it can effectively reduce the measurement error introduced by system MTF, which is conducive to improving measurement accuracy.

[0047] The three-dimensional displacement mechanism comprises three mutually orthogonal movement axes. One axis is parallel to the optical axis of the imaging optics system and is used for precise focusing of the knife-edge image via programmable adjustment. Its travel is no less than 50 mm, and its repeatability is no greater than ±1 μm. The other two axes are primarily used for alignment between the knife-edge image and the detector's photosensitive surface, facilitating the finding of the initial imaging position. These axes can be adjusted either programmably or manually. Compared to the knife-edge scanning method, only one-dimensional movement and focusing are required after the position is adjusted. This simplifies the system structure and testing process and avoids the decrease in measurement accuracy caused by positioning errors of the displacement stage and environmental interference such as vibrations during the scanning process.

[0048] The drive and data acquisition system is connected to the infrared focal plane array detector and the control computer, respectively. The control computer is also connected to the three-dimensional displacement mechanism and the infrared light source.

[0049] A method for testing the modulation transfer function of an infrared focal plane array, employing an infrared focal plane array modulation transfer function testing device as described above, such as... Figure 5 As shown, it includes the following steps:

[0050] S1. Select a suitable infrared light source according to the type of detector under test, adjust the position of the selected light source, and directly align the light source with the infrared focal plane array detector under test. Perform two-point non-uniformity correction and blind element detection on it to obtain the gain correction parameter G(i,j) and offset correction parameter O(i,j) of each pixel. Calculate the data after non-uniformity correction and perform blind element removal to eliminate the measurement error introduced by detector response non-uniformity, fixed pattern noise and blind elements.

[0051] S2. Return the light source to its original position to ensure that it fills the field of view of the imaging optical system. At the same time, adjust the three-dimensional displacement mechanism so that the blade can be clearly imaged on the photosensitive surface of the infrared focal plane array detector.

[0052] S3. Select the region of interest at the knife edge imaging position. In the driving and data acquisition system, use the Canny edge detection operator to detect the knife edge position and perform least squares fitting on the edge position to obtain the knife edge tilt angle.

[0053] S4. Project all pixels along the direction of the blade to obtain the original ESF curve;

[0054] S5. Perform data processing operations such as smoothing and fitting on the original ESF curve to eliminate data noise and obtain the final ESF curve. The fitting algorithm adopts a three-term Fermi function fitting model, which is more consistent with the ESF curve shape.

[0055] The fitting model for the three Fermi functions is:

[0056]

[0057] Where x is the pixel position, a i b i c i d1 and d2 are the fitting coefficients of the three Fermi function models, respectively;

[0058] S6. Calculate the three Fermi functions after fitting according to an appropriate sampling interval to obtain the final ESF curve sampling points with equal intervals.

[0059] S7. Differentiate the final ESF curve to obtain the LSF curve, then perform a Fourier transform on the LSF curve and subtract the MTF value of the imaging optical system to obtain the final MTF result of the infrared focal plane array detector under test.

[0060] Of course, the above description is not intended to limit the present invention, and the present invention is not limited to the examples given above. Any changes, modifications, additions or substitutions made by those skilled in the art within the scope of the present invention should also fall within the protection scope of the present invention.

Claims

1. A device for testing the modulation transfer function of an infrared focal plane array, characterized in that, The system includes an infrared light source, a knife-edge target, a manually operated rotary table, an imaging optical system, an infrared focal plane array detector, a three-dimensional displacement mechanism, a drive and data acquisition system, and a control computer. The infrared light source comprises two light sources: a blackbody and an integrating sphere. The blackbody is used for testing the focal plane detector in the mid-to-far infrared response band, while the integrating sphere is used for testing the short-wave infrared focal plane detector. The knife-edge target is mounted on the manually operated rotary table. The imaging optical system is located between the knife-edge target and the infrared focal plane array detector. The infrared focal plane array detector is placed on the three-dimensional displacement mechanism. The drive and data acquisition system is connected to both the infrared focal plane array detector and the control computer. The control computer is also connected to both the three-dimensional displacement mechanism and the infrared light source. The surface source blackbody and the integrating sphere are placed on an adjustable lifting platform, and an adjustable rotating reflector is provided between them. The infrared radiation emitted by the surface source blackbody and the integrating sphere illuminates the knife edge target after passing through the reflector, and then is imaged onto the photosensitive surface of the infrared focal plane array detector by the imaging optical system. The blade target adopts a single blade design. The side of the blade closest to the imaging optical system is blackened, and the side away from the imaging optical system is provided with a bandpass filter. The center wavelength of the bandpass filter is 1.55μm, 4μm or 10μm. The manual rotary table is equipped with coarse and fine adjustment buttons. The coarse adjustment range is 360°, the fine adjustment range is ±3°, and the fine adjustment resolution is 0.2°. The fine adjustment button is spring-loaded. The diameter of the manual rotary table surface is between 55mm and 80mm, with a central hole of 25mm to 40mm in diameter for mounting the cutting edge target.

2. The infrared focal plane array modulation transfer function testing device according to claim 1, characterized in that, The surface blackbody radiation surface has a size of 100mm×100mm, a temperature range of 0℃~100℃, and an emissivity of 0.98±0.02; the integrating sphere light source has an inner diameter of 120mm~200mm, an output aperture of 30mm~60mm, and uses a halogen tungsten lamp as the built-in light source to provide light radiation in the range of 400nm~2500nm.

3. The infrared focal plane array modulation transfer function testing device according to claim 1, characterized in that, The imaging optical system adopts a double parabolic off-axis reflection structure, which consists of two parabolic mirrors with the same technical parameters. The diameter of the mirror is 180mm~250mm, the system working distance is 50mm~100mm, the F number is between F / 3 and F4, and the magnification is 1.

4. The infrared focal plane array modulation transfer function testing device according to claim 1, characterized in that, The three-dimensional displacement mechanism includes three mutually orthogonal movement axes. One axis is parallel to the optical axis of the imaging optical system and is used for precise focusing of the knife-edge image through programmable adjustment, with a repeatability accuracy of no more than ±1μm. The other two axes are used for alignment between the knife-edge image and the photosensitive surface of the detector through programmable or manual adjustment, which facilitates finding the initial imaging position.

5. A method for testing the modulation transfer function of an infrared focal plane array, characterized in that, The infrared focal plane array modulation transfer function testing device as described in any one of claims 1-4 includes the following steps: S1. Select a suitable infrared light source according to the type of detector under test, adjust the position of the selected light source, and directly align the light source with the infrared focal plane array detector under test. Perform two-point non-uniformity correction and blind pixel detection on it to obtain the gain correction parameters for each pixel. and bias correction parameters The non-uniformity corrected data is calculated and blind cells are removed. S2. Return the light source to its original position to ensure that it fills the field of view of the imaging optical system. At the same time, adjust the three-dimensional displacement mechanism so that the blade can be clearly imaged on the photosensitive surface of the infrared focal plane array detector. S3. Select the region of interest at the knife edge imaging position. In the driving and data acquisition system, use the Canny edge detection operator to detect the knife edge position and perform least squares fitting on the edge position to obtain the knife edge tilt angle. S4. Project all pixels along the direction of the blade to obtain the original ESF curve; S5. Smooth and fit the original ESF curve to obtain the final ESF curve. The fitting algorithm adopts a three-term Fermi function fitting model. S6. Calculate the three Fermi functions after fitting according to an appropriate sampling interval to obtain the final ESF curve sampling points with equal intervals. S7. Differentiate the final ESF curve to obtain the LSF curve, then perform a Fourier transform on the LSF curve and subtract the MTF value of the imaging optical system to obtain the final MTF result of the infrared focal plane array detector under test.

6. The method for testing the modulation transfer function of an infrared focal plane array according to claim 5, characterized in that, The fitting model for the three Fermi functions is as follows: ； in, For pixel position, , , and These are the fitting coefficients for the three Fermi function models.

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