CN118565774A - Improved knife-edge measurement infrared optical transfer function measurement system adjustment method - Google Patents

Abstract

The application discloses a regulating and measuring method of an improved knife-edge measuring infrared optical transfer function measuring system, which relates to the field of optical lenses, wherein a relay lens is arranged between a knife edge of the system and an infrared detector; the knife edge, the rotating motor, the relay lens and the infrared detector form an image analyzer, and the image analyzer is arranged on a three-dimensional translation table and is controlled to move in three dimensions. According to the scheme, the knife edge is used as a marker, the position of the image point of the relay lens is roughly adjusted and finely adjusted by combining ESF signals, so that accurate and repeatable optimal image point positioning is realized, the uncertainty of the pixel positions of the knife edge and the detector is provided, a rapid criterion based on an ESF signal curve is provided, the approximate relative relation between the knife edge shielding and the pixel unit can be determined according to the curve form, and a rapid and accurate basis is provided for optimizing adjustment.

Description

Improved knife-edge measurement infrared optical transfer function measurement system adjustment method

Technical Field

The embodiments of the present application relate to the field of optical lenses, and in particular to a method for adjusting and testing an improved knife-edge measurement infrared optical transfer function measurement system.

Background Art

At present, the commonly used methods for adjusting the modulation transfer function (MTF) of infrared optical lenses are divided into two methods: the unit detector knife-edge scanning method and the area array camera image analysis method. Compared with the disadvantages of infrared area array cameras in terms of minimum pixel size, uneven response rate of different pixels, blind pixels and cost, the unit detector has advantages in terms of higher signal-to-noise ratio, more flexible measurement method, more intuitive measurement process and data processing algorithm. In a typical unit detector knife-edge scanning device based on infinite conjugate optical path, a three-dimensional motion platform is used to drive the light-shielding knife edge, from completely deviating from the image point of the lens under test (at this time, the image point power can be completely received by the detector) to completely covering the image point (at this time, the image point power cannot be received by the detector at all), to achieve the knife-edge scanning of the image formed by the slit of the target generator through the infrared lens, so as to obtain the edge spread function (abbreviated as ESF), and then calculate the corresponding line spread function (LSF) and the final MTF of the lens under test through signal processing.

On the one hand, due to the limitation of the target size of the unit detector, the detector needs to be moved to the corresponding off-axis image point position for the measurement of the off-axis field of view of the lens under test. Due to the design, manufacturing and assembly process of infrared detectors, relay lenses, etc., position deviation may also be introduced, that is, the uncertainty of the optimal receiving position of the detector. Therefore, the system will add an additional set of three-dimensional translation control to move the spatial position of the infrared unit detector, which greatly increases the system cost and the complexity of the structural design and control program.

On the other hand, since the unit detector can only receive power signals and cannot form images in real time, and since the infrared image point is invisible to the human eye, the system based on the knife-edge scanning method has the reality that it is difficult to locate the best image point of the infrared lens (the best image point refers to the position where the converged light spot is the smallest after the infrared lens converges the parallel incident light beam. Once it deviates from the best image point, the light spot will diverge, which will cause inaccurate MTF measurement). Especially for the measured lens with a long focal depth and a large image point, its best image point has a certain size in the three directions of space, which further increases the difficulty of locating the best image point. Especially for occasions with high measurement requirements, how to match the receiving surface of the detector with the best image point position of the measured lens as much as possible has always been an important key step in the application of the system. Usually, only experienced operators who have undergone long-term practice and training can complete this step quickly and accurately, which also leads to inconvenience in application in large-scale rapid production processes.

Summary of the invention

The embodiment of the present application provides a method for adjusting and testing an improved knife-edge measurement infrared optical transfer function measurement system to solve the problem of inaccurate MTF measurement. The method is used for an improved knife-edge measurement infrared optical transfer function adjustment and testing system, the system includes an infrared light source that generates a wide infrared spectrum, a target generator that provides an imaging object, a collimator for light collimation, an infrared lens, a knife edge imaged by the infrared lens, and an infrared detector for infrared imaging detection; the focus of the collimator is located at the target position of the target generator, the infrared lens is mounted on a turntable, and transmission imaging is performed on the parallel light collimated by the collimator, and the turntable controls the rotation direction and angle of the infrared lens;

The knife edge is located between the infrared lens and the infrared detector, and the root of the knife edge is installed on the rotating motor to control the knife edge to block the lens imaging; a relay mirror is also arranged on the optical path between the knife edge and the infrared detector, and the conjugate object image points on both sides of the relay mirror are respectively located on the rotating surface of the knife edge and the imaging surface of the infrared detector; the knife edge, the rotating motor, the relay mirror and the infrared detector form an image analyzer, and the image analyzer is jointly arranged on a three-dimensional translation stage to control the image analyzer to move in three dimensions, and the three-dimensional translation stage is connected to the turntable through an extended structural member to match the conjugate object image points and perform off-axis field of view measurement of the infrared lens;

The method comprises: in a coarse adjustment stage, controlling the maximum luminous flux output by the incident light through the target generator, and then adjusting the luminous flux output by the incident light one by one, taking the first maximum energy Imax1 received by the infrared detector as the target, performing coarse adjustments on the image analyzer for multiple times, and controlling the conjugate object point of the relay mirror to be close to the image point position of the infrared lens; when the target generator sets the through hole as the initial imaging object, controlling the maximum luminous flux output by the incident light through the target generator; when the target generator is switched to a slit, the luminous flux is reduced, and the coarse adjustment process takes the maximum energy that the pixel unit on the infrared detector can receive under the conditions as the target, and performs coarse adjustments in the order of the slit width direction, the optical axis direction and the slit length direction respectively;

In the fine-tuning stage, the image analyzer is controlled to move along the width direction of the target through the three-dimensional translation stage, so that the knife edge blocks the secondary image point projected by the infrared lens in the pixel unit, and the energy detected by the infrared detector is controlled to be reduced to 80%Imax1; on the basis of the energy being reduced to 80%Imax1, the axial position of the image analyzer is adjusted along the optical axis until the secondary image with the minimum size projected by the infrared lens is obtained in the pixel unit;

Control the three-dimensional translation stage to move the image analyzer along the width direction of the target until the knife edge does not block the secondary image, and record the second maximum energy Imax2 of the infrared detector; move the image analyzer along the width direction of the target so that the knife edge blocks the measured secondary image point again, and control the energy detected by the infrared detector to be reduced to 20% Imax2;

The axial position of the image analyzer is adjusted along the optical axis direction to obtain the target image point position under the minimum energy state detected by the infrared detector (9); wherein the axial position of the electric signal curve obtained by the infrared detector at the minimum value point corresponds to the optimal image point position.

Specifically, the collimator (3) is a reflective off-axis parabolic mirror or a transmissive lens group; the target generator (2) comprises a programmable rotatable electrically controlled target wheel and a plurality of different targets; the infrared light emitted by the infrared light source (1) passes through the target of the target generator (2) and is incident on the collimator (3); in the coarse adjustment stage, the target generator (2) controls the rotation of the electrically controlled target wheel to change the target into a through hole, controls the maximum luminous flux output by the incident light, and then rotates to adjust to a slit, and performs fine adjustment.

Specifically, the rotary motor controls the blade to rotate continuously by 90°, switching between the horizontal and vertical directions, to measure the optical transfer function MTF of the infrared lens in the meridian direction and the sagittal direction, respectively.

Specifically, during the process of the rotary motor controlling the knife edge, the edge of the knife edge is always located at the center of the pixel unit.

Specifically, when measuring the meridian direction, the knife edge is adjusted to a horizontal state, and the direction of the knife edge remains unchanged during the measurement process; when measuring the sagittal direction, the knife edge is adjusted to a vertical state, and the direction of the knife edge remains unchanged during the measurement process.

Specifically, after determining the position of the target image point, the method further includes:

Move the knife edge away along the slit width direction until the secondary image is completely received by the pixel unit of the infrared detector, and the energy received by the infrared detector is restored to the second maximum energy Imax2;

The image analyzer is moved in the horizontal direction and the vertical direction respectively until the infrared detector obtains the third maximum energy Imax3 and the corresponding third target image point position;

The polling fine-tuning step is repeated until the set polling value is reached and the difference between the two measured axial positions is less than the error threshold, thus completing the position correction and measuring the optical transfer function of the infrared lens based on the position of the image analyzer after correction.

The beneficial effects brought by the technical solution provided by the embodiment of the present application include at least: on the basis of traditional knife-edge scanning, the knife-edge, relay mirror and infrared detector are integrated into an image analyzer, and a three-dimensional displacement stage is used to control the overall movement of the image analyzer, thereby saving the system hardware cost, control complexity, and spatial structure. By using the knife-edge as a marker, a method for searching and locating the best position of infrared image points that is fast, convenient and programmable and repeatable is designed. The method is divided into two parts: coarse adjustment and fine adjustment. The coarse adjustment part has low difficulty and precision requirements and can be manually implemented by the operator. The fine adjustment part can avoid the error of the operator's subjective judgment through program control, and realize accurate and repeatable best image point positioning. In view of the uncertainty of the knife-edge and detector pixel position in the present invention, through a fast criterion based on the ESF signal curve, the approximate relative relationship between the knife-edge occlusion and the pixel unit can be determined according to the curve shape, thereby providing a fast and accurate basis for optimizing the adjustment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic structural diagram of an improved knife-edge measurement infrared optical transfer function measurement system provided in an embodiment of the present application;

FIG2 is a schematic diagram of knife-edge scanning provided in an embodiment of the present application;

FIG3 is a schematic diagram of the electronic control system of the improved knife-edge measurement infrared optical transfer function measurement system;

4 is a flow chart of a method for adjusting and testing an improved knife-edge measurement infrared optical transfer function measurement system;

FIG5 is a schematic diagram of adjusting the infrared detector output 80% Imax1 during the fine-tuning stage;

Fig. 6 is a schematic diagram of controlling the position of the target image point in the state of minimum output energy of the infrared detector during the fine-tuning stage;

FIG7 is a flow chart of a complete debugging method provided in an embodiment of the present application;

FIG8 is a schematic diagram of a process for determining the maximum output value of an infrared detector using a fitting algorithm;

FIG9 is a schematic diagram showing the attenuation caused by the ESF curve exceeding the pixel unit frame of the infrared detector;

FIG. 10 is a schematic diagram showing an ESF curve representing a state where light is not completely received by a pixel unit but is blocked by a knife edge.

Figure numerals: 1. infrared light source; 2. target generator; 3. collimator; 4. turntable; 5. infrared lens; 6. knife edge; 7. rotating motor; 8. relay mirror; 9. infrared detector; 10. pixel unit; 11. three-dimensional translation stage.

DETAILED DESCRIPTION

In order to make the objectives, technical solutions and advantages of the present application more clear, the implementation methods of the present application will be further described in detail below with reference to the accompanying drawings.

The term "multiple" as used herein refers to two or more than two. "And/or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and/or B can mean: A exists alone, A and B exist at the same time, and B exists alone. The character "/" generally indicates that the related objects are in an "or" relationship.

Figure 1 is a structural schematic diagram of an improved knife-edge measurement infrared optical transfer function measurement system provided in an embodiment of the present application, the system comprising an infrared light source 1, a target generator 2, a collimator 3, a turntable 4, an infrared lens 5, a knife edge 6, a rotating motor 7, a relay mirror 8, an infrared detector 9 and a test three-dimensional translation stage 11.

The infrared light source 1 is mainly used to generate infrared wide-spectrum light that meets the wavelength band of the infrared lens 5. For example, it can be silicon nitride (medium wave, long wave), halogen tungsten lamp (medium wave, short wave) or high-temperature black body (full spectrum). The light source can be equipped with a filter of a specific wavelength band to reduce the interference introduced by the out-of-band spectrum.

The target generator 2 is close to the infrared light source 1, which provides the initial imaging object required for measuring MTF, usually including through holes, dot holes, slits, cross marks and other imaging objects. According to the definition of MTF, the imaging object should be an infinitely small point (or an impact signal source on the object plane). However, in practical applications, the luminous flux that can be provided by a dot hole that is too small is very small, resulting in a significant deterioration in the signal-to-noise ratio of the measurement. Therefore, this application chooses to use a slit with a certain width as the imaging object. Slits of different widths and directions can be selected according to different measurement requirements, and the edges of the slits should be as horizontal and vertical as possible. The target generator 2 can use an electrically controlled wheel to achieve the switching of different initial imaging objects.

The function of the collimator 3 is to collimate the light beam of the infrared light source 1 through the target generator 2 into a beam of parallel light, that is, the position of the initial image in the target generator 2 is the focal position of the collimator 3. The type of collimator can be a reflective off-axis parabolic mirror or a transmissive lens group. In contrast, the reflective off-axis parabolic mirror has no chromatic aberration and can work in different bands. The processing cost and adjustment difficulty of the transmissive lens group are relatively small, and the overall size can be more compact. Different types and focal lengths of collimators can be selected according to different applications. The specific type is not limited in this application.

The infrared lens 5 is the actual detection target, which is mounted on a freely rotatable turntable 4. When measuring on the axis, it should be perpendicular to the parallel light beam emitted by the collimator 3, and converge the parallel light beam into the image point to be measured. The turntable 4 rotates with the entrance pupil position of the infrared lens 5 as the rotation center, so that the MTF measurement of the infrared lens 5 in the off-axis field of view can be realized.

The knife edge 6 is a component disposed between the infrared lens 5 and the infrared detector 9, and is used to block the imaging of the infrared lens 5 so that its energy cannot be received by the pixel unit 10 on the infrared detector 9, thereby realizing knife edge scanning. The knife edge 6 in this embodiment can be a metal reflection type or a coating absorption type, and it only needs to ensure that the transmittance of the light beam in the measured band is extremely low. The root of the knife edge 6 is installed on the knife edge rotating motor 7, and the rotation of the knife edge 6 is controlled by the rotating motor 7 to change the direction of the light receiving area of the pixel unit 10 on the infrared detector 9.

The scanning principle of the knife edge is shown in FIG2. After the slit of the target generator 2 is imaged by the infrared lens 5, it will no longer be a uniform slit, but a linear diffusion image with the largest width in the middle and decreasing width on both sides and a trailing line. When the knife edge 6 is controlled to rotate and the three-dimensional translation stage 11 moves the image analyzer to completely not block the pixel unit 10 on the infrared detector 9, the overall energy of the slit image will be detected in theory. As the three-dimensional translation stage 11 moves the image analyzer to gradually block the slit image, the infrared detector 9 receives less and less slit image energy until the knife edge 6 completely blocks the slit image. The data obtained at different knife edge scanning positions constitute the ESF signal. As shown in FIG2, the LSF signal is the first-order differential of the ESF signal. Since the root of the knife edge 6 is installed on a rotatable small motor, the horizontal and vertical directions can be switched, thereby realizing the measurement of the MTF in the meridian and sagittal directions of the infrared lens.

In this structure, in order to achieve a higher signal-to-noise ratio measurement, the infrared detector 9 is usually cooled by liquid nitrogen to reduce thermal noise, so the pixel unit 10 of the infrared detector 9 will be encapsulated inside the entire component.

In some embodiments, when measuring some infrared lenses 5 with small back intercept (back intercept refers to the distance from the outermost side of the overall mechanical structure of the lens to the image point. If the back intercept is negative, it means that the image point is inside the mechanical structure of the lens image side), the shell packaging structure of the component will make it impossible to match the pixel unit 10 with the image point of the infrared lens 5 in space. Therefore, a relay lens 8 can be installed between the knife edge 6 and the infrared detector 9 to image the image point onto the pixel unit 10 of the infrared detector 9 again. That is, the image point of the infrared lens 5 becomes the object point of the relay lens 8, and the image point of the relay lens 8 corresponds to the pixel unit 10 of the infrared detector 9, that is, it is received by the infrared detector 9. In this scheme, the output signal amplitude of the infrared detector 9 is proportional to the image light power projected onto the pixel unit 10.

In the above structure, the knife edge 6 (and the knife edge rotation motor 7), the relay lens 8 and the infrared detector 9 components constitute the image analyzer as a whole. The edge position of the knife edge 6 is correspondingly set at the center of the pixel unit 10 on the infrared detector 9, or as close to the center of the pixel unit 10 as possible. For the pixel unit 10 with a circular aperture, when the knife edge 6 is at the center, the length of the line segment projected from the knife edge 6 to the pixel unit 10 is the longest. At this time, when the image analyzer scans the image point through the infrared lens 5, the image can be completely incident on the pixel unit 10 without causing principle errors due to the pixel frame limitation of the infrared detector 9.

The relay lens 8 should select a suitable wavelength band and magnification to ensure that the secondary imaging of the slit image in the slit length direction is not too large to exceed the size of the pixel unit 10 of the infrared detector 9 in this direction. The size of the slit width direction is usually much smaller than the width direction size of the pixel unit 10 on the infrared detector 9 in the image analyzer, because the slit of the target generator 2 is usually selected to be smaller and is reduced after imaging by the collimator 3 and the infrared lens 5. The position of the knife edge 6 and the position of the pixel unit 10 are two conjugate object image points of the relay lens 8 (that is, in theory, the axial position of the image of the knife edge 6 by the relay lens 8 should be exactly on the surface of the pixel unit 10 of the infrared detector 9), and the image distance of the relay lens 8 (that is, the distance from the relay lens 8 to the pixel unit 10) is fixed by the structural design and assembly process.

The image analyzer is installed as a whole on a three-dimensional translation stage 11, which is used to realize the movement of the image analyzer in three dimensions, so as to match the best image point. Specifically, the grating ruler is used as an encoder to realize independent precision displacement in three dimensions, so as to realize the knife-edge scanning function of the infrared lens 5. The grating ruler is a commonly used device for position or angle measurement, which can realize the closed-loop measurement and control function of displacement, rotation and other movements.

Optionally, the three-dimensional translation stage 11 can be installed on the turntable 4 through an extended structural member, and can rotate with the turntable 4, so that after the turntable 4 is rotated to the off-axis field of view of the infrared lens 5, the image analyzer can still move three-dimensionally in space to match the best image point. After the three-dimensional translation stage 11 is linked with the turntable 4, the turntable 4 can rotate the infrared lens 5 and the image analyzer at the same time, realizing the off-axis field of view measurement function of the infrared lens 5.

The device part of the above-mentioned improved knife-edge measurement infrared optical transfer function measurement system, the present application also provides the electronic control part for the above-mentioned device, as shown in Figure 3, the system includes a blackbody light source temperature controller corresponding to the blackbody light source, a target wheel rotation controller, a target generator, a target chopper, a preamplifier circuit and a phase-locked amplifier circuit, and the system is globally controlled by a computer host computer.

In a possible implementation, the infrared light source uses a point source divergent black body, and selects the required infrared band through a filter. The black body light source controller can adjust the temperature of the black body to adjust the output optical power. The target generator is composed of a programmable rotatable target wheel and various different targets. By rotating the target generator target wheel, a suitable target or marker is selected. The infrared light beam after passing through the target of the target generator is frequency modulated by a mechanical chopper. The chopping frequency of the chopper is controlled by the program and serves as the reference frequency of the back-end phase-locked amplifier. The chopped infrared light beam is collimated by a collimator. A reflective or transmissive collimator can be selected, and the focus of the collimator is located at the target position of the target generator. The infrared light beam collimated by the collimator is incident on the infrared lens and forms a convergent image point. A programmable electric three-dimensional translation stage drives the image analyzer to perform knife-edge scanning on the convergent image point of the infrared lens to achieve the measurement of the ESF signal. The knife edge of the image analyzer is continuously rotated 90° by a rotating motor, thereby supporting the measurement function of the meridian and sagittal directions of the infrared lens. The signal output by the image analyzer is amplified by the pre-amplifier circuit, and the phase-locked amplifier circuit is used to filter out the clutter noise to obtain an output with a high signal-to-noise ratio, and then the data is collected by program control. The infrared lens and the three-dimensional translation stage can be rotated as a whole relative to the incident collimated light beam through a turntable. The rotation center of the turntable is matched with the entrance pupil plane of the infrared lens through the installation structure of the infrared lens, thereby realizing the measurement function under the off-axis field of view of the infrared lens. Specifically, a laser indicator line can be set at the center of the turntable, and the laser indicator is turned on when the turntable rotates. By moving the installation structure of the infrared lens, the vertex of the front surface of the infrared lens is moved to the laser indicator line to achieve center matching. All the above-mentioned program-controlled components are controlled by the computer host software to achieve the final data analysis.

Since the human eye cannot see infrared light, and the system and device use unit detectors that cannot be imaged directly, the image point energy received by the image analyzer can only be used to indirectly determine whether the receiving optical path matches the optimal image point position formed by the infrared lens. In actual operation, in order to accurately measure the optical transfer function of the infrared lens, it is necessary to measure it at its optimal image point, that is, the axial position where the light spot is the smallest. When the axial position deviates from the optimal image point, the light spot diffuses and the image quality deteriorates, and the measured optical transfer function is inaccurate and degraded, so it is necessary to find the optimal image point. To this end, the present application, based on the above-mentioned control system, uses a knife edge integrated with an infrared detector as a marker to provide an improved knife-edge measurement infrared optical transfer function measurement system adjustment method, which is mainly to match the optimal image point position of the infrared lens and the corresponding optical path. The specific steps are divided into two stages: coarse adjustment and fine adjustment.

FIG4 is a flow chart of a method for adjusting and testing an improved knife-edge measurement infrared optical transfer function measurement system, which specifically includes the following steps:

S1, coarse adjustment stage, the maximum luminous flux output by the incident light is controlled by the target generator, the luminous flux output by the incident light is adjusted by the target generator one by one, the first maximum energy Imax1 received by the infrared detector is taken as the target, the image analyzer is coarsely adjusted multiple times, and the conjugate object point of the relay lens is controlled to be close to the image point position of the infrared lens;

In the initial stage, the initial imaging object (target) of the target generator is first switched to a through hole. At this time, the incident light beam has the maximum luminous flux, and the image point energy of the infrared lens is the largest (i.e., the first maximum energy Imax1). The operator can easily move the pixel unit in the image analyzer (i.e., the infrared detector, so that its pixel unit is moved to the vicinity of the matching image point, thereby receiving most of the image point power) to the vicinity of the convergent image point of the infrared lens (the conjugate object point of the relay lens) by moving the three-dimensional translation stage. Further, the operator switches the target generator to a suitable slit. At this time, although the luminous flux is greatly reduced compared to the previous through hole, the conjugate object point of the image analyzer is already near the convergent image point of the infrared lens, and the slit and through hole in this process are both located at the focal position of the collimator. In the process of switching between the through hole and the slit target, except for the change in the emitted luminous flux, the image point position does not change. The infrared detector will still receive enough signal energy to distinguish it from noise. At this time, the operator can continue to adjust the three-dimensional translation stage so that the energy signal received by the infrared detector reaches the maximum (the first maximum energy Imax1).

During the process of coarse position adjustment, the position in the slit width direction has the greatest influence, followed by the axial (i.e., optical axis) position, and the position in the slit length direction has the least influence. Therefore, different displacement axes can be appropriately selected to adjust the three-dimensional translation stage according to the trend of signal changes. For example, first adjust the slit width direction until the signal reaches a maximum of Imax1’, then adjust the axial direction until the signal reaches a maximum of Imax1”, and finally adjust the slit length direction until the signal reaches a maximum of Imax1. Repeat the above cycle 2-3 times to complete the coarse adjustment. The coarse adjustment process can be completed manually by the operator, because for coarse adjustment, it is not required to maximize the signal output of the infrared detector very accurately, but only to basically reach the maximum output.

It should be noted that during this period, the relative positions of the knife edge and the infrared detector in the image analyzer are fixed. In addition, when measuring the MTF in the meridian direction of the lens, the knife edge needs to be adjusted to a horizontal state, and the direction of the knife edge remains unchanged during the measurement process; when measuring the MTF in the sagittal direction of the lens, the knife edge needs to be adjusted to a vertical state, and the direction of the knife edge remains unchanged during the measurement process.

Although the coarse adjustment can adjust the signal output to the maximum, so that the pixel unit receiving optical path of the image analyzer basically matches the best image point of the infrared lens under test, further fine adjustment is still needed for accurate testing. For example, for a lens with a working center wavelength of 10um, a focal length of 100mm, and a light aperture of 50mm, the convergent spot diameter of the ideal Gaussian beam is about 25.5um, and its confocal length is about 102um, that is, the image point energy does not change much within the range of 25.5um diameter and 102um axial length, which makes it difficult to accurately find the best image point position during coarse adjustment. The reason is that the image point optical power changes very little within the three-dimensional space determined by the above-mentioned spot diameter and confocal length, and the positioning accuracy of the best image point in the axial direction needs to be in the order of um. Therefore, it is difficult to accurately locate the best image point by searching for the maximum value of the image point energy, especially the position along the axial direction. On this basis, further fine adjustment is still needed to improve the matching of the receiving optical path and the best image point, thereby improving the measurement accuracy of MTF.

S2, fine-tuning stage, the image analyzer is moved along the width direction of the target by a three-dimensional translation stage, so that the knife edge blocks the secondary image point projected by the infrared lens in the pixel unit, and the energy detected by the infrared detector is controlled to be reduced to the first target ratio of the first maximum energy Imax1; the object distance of the relay lens is adjusted along the optical axis direction until the secondary image of the minimum size projected by the infrared lens is obtained in the pixel unit;

This embodiment makes full use of the knife edge as a marker to achieve fine adjustment of the optimal image point. The principle of fine adjustment is shown in Figures 5 and 6. The shaded part is the shielding of the knife edge 6, the rectangular frame is the receiving target surface of the infrared detector pixel unit 10, the red shadow is the secondary image 12 formed on the pixel unit after the image point of the infrared lens passes through the relay lens, and the red area that does not overlap with the shadow represents the area of the secondary image that is not blocked by the knife edge, and the area is proportional to the image point energy signal received by the infrared detector.

In the fine adjustment stage, the image analyzer (the knife edge, relay lens and infrared detector) is first moved as a whole along the width direction of the slit image by the translation stage, so that the knife edge blocks a small portion of the image point energy of the infrared lens until it is reduced to the first target ratio of the maximum signal amplitude in the previous state. In the embodiment of the present application, the signal energy can be adjusted to 80%Imax1, which is shown on the left side of Figure 5, where the energy of a small portion of the secondary image 12 is blocked by the knife edge 6.

Then, based on the first target ratio, the object distance of the relay mirror is adjusted along the optical axis until the infrared lens projection is obtained in the pixel unit to obtain the secondary image of the minimum size. This process specifically adjusts the position of the three-dimensional translation stage as a whole along the axial direction. This adjustment process actually changes the object distance of the relay mirror (the distance from the relay mirror to the infrared lens imaging point). According to the lens imaging theory, the lateral magnification of the relay mirror will also change accordingly. When the object point of the relay mirror is close to the optimal conjugate position, the size of the secondary image formed along the slit width direction is the smallest, as shown on the right side of Figure 5. At this time, the energy loss of the secondary image blocked by the knife edge is the smallest, that is, the portion blocked by the knife edge is the smallest, and most of the image energy can be received by the image analyzer, so the energy loss is also the smallest. Correspondingly, the signal amplitude output by the infrared detector is the largest.

S3, control the three-dimensional translation stage to move the image analyzer along the width direction of the target until the knife edge does not block the secondary image, and record the second maximum energy Imax2 of the infrared detector; move the image analyzer along the width direction of the target to allow the knife edge to block the measured secondary image point again, and control the energy detected by the infrared detector to decrease to the second target ratio of the second maximum energy Imax2;

Continue fine-tuning in the previous state, and move the image analyzer by the three-dimensional translation stage along the width direction of the slit image until the knife edge recovers and no longer blocks the secondary image. The purpose is to determine the maximum energy value under this object distance state, that is, to record the second maximum energy Imax2 of the infrared detector.

The image analyzer is continuously moved along the width direction of the slit image by the translation stage, so that the knife edge blocks most of the image point energy of the infrared lens until it is reduced to the second target ratio of the maximum signal amplitude in the previous state. In the embodiment of the present application, the signal energy can be adjusted to 20%Imax2, that is, the state shown on the left side of FIG6 , in which most of the energy of the secondary image 12 is blocked by the knife edge 6.

S4, adjusting the object distance of the relay lens along the optical axis direction to obtain the target image point position under the minimum energy state detected by the infrared detector; wherein the axial position of the infrared detector obtaining the electrical signal curve at the minimum value point corresponds to the optimal image point position.

Further, the position of the three-dimensional displacement stage is adjusted along the optical axis. Based on the similar principle in the first step of fine-tuning, when the object point of the relay mirror is further close to the position of the optimal conjugate object point, the size of the secondary image is further converged. Most of the secondary images will be blocked by the knife edge and cannot be received by the pixel unit of the infrared detector, that is, the energy received by the infrared detector reaches the minimum, see the state shown on the right side of Figure 6. In this process, when deviating from the optimal image point, the detected image is in a divergent state, and a part of the blocked image will be received by the infrared detector due to the divergence, thereby increasing the received energy. At the optimal image point, the image is in the best convergence state. At this time, the image energy received by the infrared detector is the least because most of it is blocked by the knife edge. The infrared detector outputs an electrical signal proportional to the received light energy. When the image analyzer moves axially through the optimal image point, the amplitude of the electrical signal will drop to a minimum point and then increase. The axial position of this signal minimum point is the optimal image point position, that is, the optimal image point position in this step.

After completing the above steps, the purpose of accurately measuring MTF can basically be achieved. However, during the installation and adjustment process, the movement axis of the three-dimensional translation stage along the optical axis considered in the structural design may not completely coincide with the actual optical axis of the system, resulting in displacement of the image plane outside the optical axis when the translation stage moves along the axial direction. In this case, additional fine-tuning of the position of the infrared detector in the width and length directions of the slit image is required. On this basis, the adjustment method can also include:

S5, moving the knife edge along the slit width direction until the secondary image is completely received by the pixel unit of the infrared detector, and the energy received by the infrared detector is restored to the second maximum energy Imax2;

In step S4, the output of the infrared detector appears as a minimum value on the curve. The knife edge is first moved away along the width direction of the slit image so that all secondary images can be received by the infrared detector and restored to Imax2.

S6, moving the image analyzer along the meridian direction and the sagittal direction respectively until the infrared detector obtains the third maximum energy Imax3 and the corresponding third target image point position;

S7, polling fine-tuning step, until the set polling value is reached and the difference between the two measured axial positions is less than the error threshold, the position correction is completed, and the optical transfer function of the infrared lens is measured based on the position of the image analyzer after correction.

Based on the above steps, the flowchart of the complete debugging algorithm provided in this embodiment is shown in FIG7 , including:

Step 701, input initial parameters, axial position error threshold is δz, maximum initial value Imax1, image analyzer position;

Step 702, moving the image analyzer along the slit image width direction until the signal is adjusted to 80% Imax1;

Step 703, moving the image analyzer along the axial direction until the signal reaches a maximum;

Step 704, move the image analyzer along the width direction of the slit image until the knife edge no longer blocks the secondary image and the infrared detector output signal is the maximum Imax2;

Step 705, moving the image analyzer along the slit image width direction until the signal is 20%Imax2;

Step 706, moving the image analyzer along the axial direction until the signal reaches a minimum;

Step 707, moving the image analyzer along the width direction of the slit image until the knife edge no longer blocks the secondary image and the infrared detector output signal is maximum;

Step 708, move the image analyzer along the width and length directions of the slit image until the infrared detector outputs a maximum signal Imax3, and record the position of the image analyzer at this time;

The above process can usually be set to rotate three times, and the axial position error threshold δz and the image analyzer position or the image point position (x, y, z) of the pixel unit will be input in the output fine-tuning process, where z represents the axial coordinate value. After each rotation fine-tuning, the latest image analyzer position or the image point position (x’, y’, z’) of the pixel unit is obtained. After completing three rotation fine-tuning, the difference in the axial position between two adjacent coordinates is determined. When |z’-z| is much smaller than δz, it is determined that the final adjustment process is completed. When the above conditions are not met, continue to adjust with the latest coordinate values and parameters until the fine-tuning is completed.

In summary, the improved knife-edge measurement infrared optical transfer function measurement system and adjustment method provided by the present application integrates the knife-edge, relay mirror and infrared detector into an image analyzer on the basis of the traditional knife-edge scanning, and uses a three-dimensional displacement stage to control the overall movement of the image analyzer, thereby saving the system hardware cost, control complexity, and spatial structure. By using the knife-edge as a marker, a method for searching and locating the best position of infrared image points that is fast, convenient and programmable and repeatable is designed. The method is divided into two parts: coarse adjustment and fine adjustment. The coarse adjustment part has low difficulty and accuracy requirements and can be manually implemented by the operator. The fine adjustment part can avoid the error of the operator's subjective judgment through program control, and realize accurate and repeatable best image point positioning. In view of the uncertainty of the knife-edge and detector pixel position in the present invention, through a fast criterion based on the ESF signal curve, the approximate relative relationship between the knife-edge occlusion and the pixel unit can be determined according to the curve shape, thereby providing a fast and accurate basis for optimizing the adjustment.

In addition, with respect to the fine-tuning process, in step 703, the present application moves the image analyzer axially until the signal reaches a maximum, and its maximum value may not be equal to the initial maximum Imax1, because at this time a portion of the blade blocks the image in the previous step. In this step, when deviating from the optimal image point, since the image is in a divergent state, a portion of the image energy that should have been received by the infrared detector is blocked by the blade due to the divergence. Therefore, compared with the convergent state of the optimal image point, the received energy is smaller (i.e., more energy is blocked). By moving the image analyzer axially and observing the signal amplitude output by the infrared detector, the maximum value can be determined.

Correspondingly, in step 707, the image analyzer is moved along the slit image width direction until the blade no longer blocks the secondary image, and the infrared detector output signal is the maximum, wherein the maximum value may not be equal to Imax2, because compared with the above, the axial positions of the two may have changed, and their maximum values may not be equal. In the fine adjustment process, there is also a step of determining the maximum and minimum values of the infrared detector output signal. Unlike the coarse adjustment in which the maximum value of the positioning signal does not need to be very accurate, in the fine adjustment, the position of the maximum and minimum values is required to be as accurate as possible. Due to the influence of system vibration, infrared detector noise, temperature change, etc., if only the operator subjectively judges the extreme value position, a certain measurement error will be introduced, especially when judging the minimum value, because the secondary image energy received by the infrared detector at this time is very small, it is very easy to be interfered by noise and cause misjudgment. Considering the possibility that the infrared lens imaging point presents a Gaussian distribution in both the lateral (slit width) and axial directions, a fitting algorithm can be used, and the extreme value can be automatically determined by a computer program, which greatly increases the accuracy of the extreme value position. The programmatic determination method of the extreme value position is shown in Figure 8.

The specific steps include:

Step 801, input parameters: axial scanning position error threshold δz;

Step 802, move the image analyzer along the axial direction with a step length of δz, and record the infrared detector signal output I at each axial position z to obtain a set of (z, I) data;

Step 803: determine whether the (z, I) data presents an intermediate maximum value, that is, the signal values of the endpoints on both sides are greater than/less than the intermediate value of the array by more than 20%; if this step does not reach more than 20%, it is necessary to supplement another part of the (z', I') data in the missing direction and combine it with the previous (z, I) data to form a new (z, I) data;

Step 804: smoothing filter the (z, I) data to obtain filtered data (z, I'), which may be Savitzky-Golay filtering or moving window smoothing.

Step 805: Perform nonlinear Gaussian fitting on the filtered data (z, I') using the Levenberg-Marquardt algorithm;

Step 806: Find the position Z0 corresponding to the maximum/minimum value of I” for the fitted data (z, I”) according to the set requirements.

It should be noted that the parameter settings of the filtering algorithm and the nonlinear fitting algorithm do not need to be optimized too much, because in FIG8, only the axial position corresponding to the extreme point needs to be found, and the true value of the signal corresponding to each axial position does not need to be accurately found. Ideally, the edge of the knife should be aligned with the center of the pixel unit in the infrared detector, so as to maximize the use of the imaging area of the infrared detector. If it is far from the center, if a small circular detector pixel is encountered (the smaller the detector size, the smaller the corresponding inhomogeneity and the total amount of external noise) or a large secondary imaging (the focal length of the measuring lens is longer or the magnification of the relay lens is higher), there will be inaccurate knife-edge scanning ESF data, as shown in FIG9 and FIG10. In this design scheme, the coarse adjustment and fine adjustment during the test process are only for the object point of the relay lens, because the image analyzer is a whole, and the relative position of the relay lens and the infrared detector pixel unit is fixed, that is, the image point position remains unchanged, and there is no need to adjust the image point. Of course, when the components are assembled, the image point of the relay lens needs to be matched with the axial relative position of the pixel unit of the infrared detector.

During the assembly and adjustment process, the knife edge and the relay mirror can be designed as a circular aperture, and a structural mounting part can be used for good center alignment. However, due to the limitations of the packaging process, there is a certain uncertainty in the position of the pixel unit of the infrared detector in the component, so it is impossible to achieve alignment through a pre-designed method. It can only be determined by leaving room for adjustment during the design and judging according to the corresponding ESF curve shape. For example, the ESF curve in Figure 9 shows that when the secondary image 12 is not blocked by the knife edge 6, part of it is attenuated because it exceeds the pixel unit frame of the infrared detector, indicating that the knife edge only blocks a small part of the detector pixels. Correspondingly, the ESF curve in Figure 10 shows that the secondary image 12 has not been completely received by the pixel unit 10 of the infrared detector, but has been blocked by the knife edge 6, indicating that the knife edge blocks most of the detector pixels. The ESF curves in both cases deviate seriously from normal data, resulting in incorrect measurement results.

According to the two situations in Figures 9 and 10, the relative position relationship between the blade in the image analyzer and the center of the detector pixel can be quickly determined during the assembly process, so as to make corresponding position corrections and finally obtain an ideal ESF signal. This also indirectly confirms the feasibility and accuracy of the rapid judgment based on the ESF signal curve in the design scheme of the present application.

This specific embodiment is merely an explanation of the present invention and is not a limitation of the present invention. After reading this specification, those skilled in the art may make non-creative modifications to the present embodiment as needed. However, such modifications are protected by patent law as long as they are within the scope of the claims of the present invention.

Claims (6)

1. An adjusting and measuring method of an improved knife-edge measuring infrared optical transfer function measuring system is used for the improved knife-edge measuring infrared optical transfer function adjusting and measuring system and is characterized by comprising an infrared light source (1) for generating infrared wide spectrum, a target generator (2) for providing an imaging object, a collimator (3) for collimating light, an infrared lens (5), a knife edge (6) for shielding imaging of the infrared lens (5) and an infrared detector (9) for infrared imaging detection; the focus of the collimator (3) is positioned at a target position of the target generator (2), the infrared lens (5) is arranged on the turntable (4) to perform transmission imaging on the collimated parallel light through the collimator (3), and the turntable (4) controls the rotation direction and the rotation angle of the infrared lens (5);

The knife edge (6) is positioned between the infrared lens (5) and the infrared detector (9), the root of the knife edge (6) is arranged on the rotating motor (7), and the knife edge (6) is controlled to shade the lens for imaging; a relay lens (8) is further arranged on a light path between the knife edge (6) and the infrared detector (9), and conjugate image points at two sides of the relay lens (8) are respectively positioned on a rotating surface of the knife edge (6) and an imaging surface of the infrared detector (9); the knife edge (6), the rotating motor (7), the relay lens (8) and the infrared detector (9) form an image analyzer, the image analyzer is arranged on the three-dimensional translation table (11) together, the image analyzer is controlled to move in three dimensions, the three-dimensional translation table (11) is connected with the turntable (4) through an lengthening structural member, and conjugate image points are matched and the off-axis vision field measurement of the infrared lens (5) is carried out;

The method comprises the following steps: in the coarse adjustment stage, the target generator (2) is used for controlling the maximum luminous flux of the output of the incident light, then the luminous flux of the output of the incident light is adjusted successively, the first maximum energy Imax1 received by the infrared detector (9) is used as a target, the image analyzer is subjected to coarse adjustment for a plurality of times, and the position of a conjugate object point of the relay lens (8) close to the image point of the near infrared lens (5) is controlled; when the target generator (2) sets the through hole as an initial imaging object, the target generator (2) controls the incident light to output the maximum luminous flux; when the target generator (2) is switched into a slit, the luminous flux is reduced, and the coarse adjustment process takes the maximum energy which can be received by a pixel unit (10) on the infrared detector (9) under the condition that the luminous flux is positioned as a target, and coarse adjustment is respectively carried out according to the sequence of the width direction of the slit, the direction of the optical axis and the length direction of the slit;

in the fine tuning stage, the three-dimensional translation stage is used for controlling the image analyzer to move along the width direction of the target, so that a knife edge (6) shields a secondary image point projected by an infrared lens (5) on a pixel unit (10), and the energy detected by an infrared detector (9) is controlled to be reduced to 80% Imax1; on the basis of reducing the energy to 80% Imax1, adjusting the axial position of the image analyzer along the optical axis direction until the pixel unit (10) acquires a secondary image with the minimum size projected by the infrared lens (5);

Controlling the three-dimensional translation stage to move the image analyzer along the width direction of the target until the knife edge (6) does not shade the secondary image, and recording the second maximum energy Imax2 of the infrared detector (9); moving an image analyzer along the width direction of the target, enabling the knife edge (6) to shield the detected secondary image point again, and controlling the energy detected by the infrared detector (9) to be reduced to 20% Imax2;

The axial position of the image analyzer is adjusted along the optical axis direction, and the position of a target image point under the state of minimum energy detected by the infrared detector (9) is obtained; the infrared detector (9) acquires the axial position of the electric signal curve at the minimum point as the optimal image point position.

2. The method according to claim 1, characterized in that the collimator (3) is a reflective off-axis parabolic mirror or a transmissive lens group; the target generator (2) comprises an electric control target wheel capable of rotating in a program-controlled manner and a plurality of different targets, infrared light emitted by the infrared light source (1) enters the collimator (3) after passing through the targets of the target generator (2), the targets are controlled to rotate through the target generator (2) in a rough adjustment stage to change into through holes, the maximum luminous flux of the incident light is controlled to be output, then the incident light is rotationally adjusted into slits, and fine adjustment is carried out.

3. Method according to claim 2, characterized in that the rotating electric machine (7) controls the knife-edge (6) to rotate at 90 ° continuously, switching in horizontal and vertical directions, for measuring the optical transfer function MTF of the infrared lens (5) in meridian direction and in sagittal direction, respectively.

4. A method according to claim 3, characterized in that the edge position of the knife edge (6) is always in the centre of the pixel cell (10) during the control of the knife edge (6) by the rotating electrical machine (7).

5. Method according to claim 2, characterized in that the knife edge (6) is adjusted to a horizontal state when measuring the meridian direction, and the measuring procedure knife edge (6) direction is kept unchanged; when measuring the sagittal direction, the knife edge (6) is adjusted to be in a vertical state, and the direction of the knife edge (6) is kept unchanged in the measuring process.

6. The method of claim 5, wherein after determining the target image point location, the method further comprises:

the knife edge is moved away along the width direction of the slit until the secondary image is completely received by a pixel unit (10) of the infrared detector (9), and the energy received by the infrared detector (9) is restored to a second maximum energy Imax2;

Respectively moving the image analyzer along the horizontal direction and the vertical direction until the infrared detector (9) obtains third maximum energy Imax3 and the corresponding third target image point position;

And a polling fine tuning step, namely, finishing position correction until a set polling value is reached, and the difference value of the axial positions measured by the two times is smaller than an error threshold value, and measuring the optical transfer function of the infrared lens (5) based on the corrected position of the image analyzer.