Method for testing modulation transfer function of optical system

Abstract

The invention relates to a method for testing a modulation transfer function of an optical system. The method sequentially comprises the following steps: S1, focusing a camera lens until a test surface is clearly imaged; s2, focusing the projection lens until the test surface is clearly imaged; s3, projecting a picture by a projector, and synchronously shooting a stored graph by a camera; and S4, performing graph post-processing, and calculating a contrast ratio. According to the invention, the MTF measurement of the system can be carried out without moving a fringe pattern card; the method can be used for carrying out MTF measurement on a single lens, and is especially suitable for carrying out system MTF measurement on an optical system containing a projector and a camera at the same time, such as a 3D structured light measurement system.

Description

Technical Field

[0001] The present invention relates to the technical field related to optical systems, and in particular to a method for testing a modulation transfer function of an optical system. Background Art

[0002] The resolution of an optical lens is a key indicator for evaluating imaging quality. In optics, modulation transfer function (MTF) is generally used to characterize the imaging quality of an optical lens. MTF represents the imaging capability of an optical system for periodic structures. The measurement of MTF generally involves the use of a test card. By imaging the image on the test card, the MTF of the lens or system is calculated. Commonly used test charts include the ISO12233 resolution test chart, the USAF1951 resolution board, etc. In addition to standard test methods, there are also some special methods that can be used to quantify resolution.

[0003] After massive searches, it was found that the Chinese patent announcement number is CN101452199A, which discloses a method for measuring the modulation transfer function value, which includes the following steps: providing a test board and an image sensor opposite to the test board, the test board having a plurality of test patterns of alternating light and dark, the image sensor being used to sense the brightness value of the image after each test pattern is imaged by the lens to be tested, and placing the lens to be tested between the test board and the image sensor; adjusting the imaging brightness of the image sensor to the average brightness of all test patterns after being imaged by the lens to be tested, driving the image sensor to move along the imaging optical axis of the lens to be tested, until the image sensor achieves the best imaging effect for all test patterns; driving the image sensor to move parallel to the arrangement direction of the test patterns, continuously recording the brightness values ​​of the test patterns sensed by the image sensor through the lens to be tested, obtaining the maximum brightness value and the minimum brightness value, and calculating the modulation transfer function value of the lens to be tested based on the maximum brightness value and the minimum brightness value.

[0004] In summary, a method for testing the clarity of a lens by moving a test plate using a fringe chart is now introduced. In this testing method, it is necessary to move the camera and the lens to complete the shooting of multiple sets of pictures and calculate the resolution.

[0005] In view of the above-mentioned defects, the designers have actively carried out research and innovation in order to create a testing method for the modulation transfer function of an optical system to make it more valuable for industrial use. Summary of the invention

[0006] In order to solve any of the above technical problems, an object of the present invention is to provide a method for testing the modulation transfer function of an optical system.

[0007] To achieve the above object, the present invention adopts the following technical solution:

[0008] A method for testing the modulation transfer function of an optical system comprises the following steps in sequence:

[0009] Step S1, focusing the camera lens until a clear image of the test surface is obtained;

[0010] Step S2, focusing the projection lens until a clear image of the test surface is formed;

[0011] Step S3, the projector projects the picture, and the camera synchronously captures and stores the graphic;

[0012] Step S4: Graphics post-processing, contrast calculation.

[0013] As a further improvement of the present invention, in step S3, the camera synchronously captures and stores the graphics to a control storage device.

[0014] As a further improvement of the present invention, the lens to be tested is a projection lens or a camera lens.

[0015] As a further improvement of the present invention, step S4 includes the following steps in sequence:

[0016] Step S41, projecting a sinusoidal pattern through a projector, and synchronously triggering the camera 3 to shoot;

[0017] Step S42, selecting a target area to extract stripe brightness;

[0018] Step S43, after obtaining the one-dimensional change of the sinusoidal fringes, extract the maximum and minimum values ​​and their position distribution from the sinusoidal curve, and then obtain the continuous change curve of the maximum value (Imax) and the minimum value (Imin);

[0019] Step S44: The contrast distribution curve is calculated from the distribution curves of the maximum and minimum values, that is:

[0020]

[0021] As a further improvement of the present invention, a rotation correction step is further included between step S41 and step S42, that is, the photographed stripes are rotated to a vertical state.

[0022] As a further improvement of the present invention, in step S42, pixels within a preset width are selected along the stripe direction for averaging to obtain an intensity distribution along the stripe direction.

[0023] As a further improvement of the present invention, in step S41, by changing the period of the projected sinusoidal fringes, a contrast distribution curve at different spatial frequencies is calculated.

[0024] As a further improvement of the present invention, step S4 includes the following steps in sequence:

[0025] Step S41, projecting a plurality of sinusoidal fringe patterns of the same frequency through a projector;

[0026] Step S42: The intensity distribution on the above-mentioned sinusoidal fringe group is:

[0027] I n =I0(1+sin(2π(x / x0+n / N))); n∈[0,N-1];

[0028] Step S43, taking N phase shift images by continuously triggering the projector and the camera, where N is a natural number greater than or equal to 2;

[0029] Step S44, calculating the whole field contrast through the phase shift diagram, that is:

[0030]

[0031] Among them, x and y represent the spatial coordinate position of the image, and max(In(x,y)) and min(In(x,y)) are the maximum and minimum brightness of the imaging pattern captured by the sinusoidal fringe group at a certain position in space.

[0032] As a further improvement of the present invention, in step S41, the lens to be tested is a projection lens, and the optical axis of the projector is parallel to the optical axis of the projection lens.

[0033] As a further improvement of the present invention, in step S41, the lens to be tested is a projection lens, and through the Sham imaging principle, the projection surface is conjugate with the test surface, and a clear image is projected on the test surface.

[0034] By means of the above scheme, the present invention has at least the following advantages:

[0035] The present invention does not need to move the fringe chart to measure the system MTF;

[0036] The present invention can be used to perform MTF measurement on a single lens, and is also particularly suitable for measuring the system MTF of an optical system including a projector and a camera, such as a 3D structured light measurement system.

[0037] The above description is only an overview of the technical solution of the present invention. In order to more clearly understand the technical means of the present invention and implement it according to the contents of the specification, the following is a detailed description of the preferred embodiments of the present invention with accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without creative work.

[0039] Figure 1 It is a schematic diagram of the principle of a method for testing the modulation transfer function of an optical system of the present invention;

[0040] Figure 2 It is a schematic diagram of the process of the present invention;

[0041] Figure 3 is a schematic diagram of a flow chart of a first embodiment of the present invention;

[0042] Figure 4 is an example of a fringe pattern captured by a camera based on the first embodiment of the present invention;

[0043] Figure 5 is a schematic diagram of a contrast distribution curve based on the first embodiment of the present invention;

[0044] Figure 6 is a schematic diagram of a flow chart of a second embodiment of the present invention;

[0045] Figure 7 is a schematic diagram of the imaging quality test results based on the second embodiment of the present invention in which the optical axis of the projector is parallel to the optical axis of the projection lens;

[0046] Figure 8 It is a schematic diagram of the imaging quality test results when the optical axis of the projector and the optical axis of the projection lens are tilted based on the use of the Sham structure in the second embodiment of the present invention.

[0047] The meanings of the various reference numerals in the figures are as follows.

[0048] Projector 1, projection lens 2, camera 3, camera lens 4, test surface 5, control storage device 6. DETAILED DESCRIPTION

[0049] The specific implementation of the present invention is further described in detail below in conjunction with the accompanying drawings and examples. The following examples are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0050] In order to enable those skilled in the art to better understand the scheme of the present invention, the technical scheme in the embodiment of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiment of the present invention. Obviously, the described embodiment is only a part of the embodiment of the present invention, rather than all the embodiments. The components of the embodiment of the present invention generally described and shown in the drawings here can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiment of the present invention provided in the drawings is not intended to limit the scope of the invention claimed for protection, but merely represents the selected embodiment of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without making creative work belong to the scope of protection of the present invention.

[0051] Example

[0052] like Figures 1 to 8 As shown,

[0053] A method for testing the modulation transfer function of an optical system comprises the following steps in sequence:

[0054] Step S1, the camera lens 4 is focused to form a clear image of the test surface 5;

[0055] Step S2, focusing the projection lens 2 until a clear image of the test surface 5 is formed;

[0056] Step S3, the projector 1 projects the image, and the camera 3 synchronously captures and stores the image;

[0057] Step S4: Graphics post-processing, contrast calculation.

[0058] Preferably, in step S3 , the camera 3 synchronously captures and stores the graphics to the control storage device 6 .

[0059] Preferably, the lens to be tested is a projection lens 2 or a camera lens 4 .

[0060] Preferably, step S4 includes the following steps in sequence:

[0061] Step S41, projecting a sinusoidal pattern through the projector 1, and synchronously triggering the camera 3 to shoot;

[0062] Step S42, selecting a target area to extract stripe brightness;

[0063] Step S43, after obtaining the one-dimensional change of the sinusoidal fringes, extract the maximum and minimum values ​​and their position distribution from the sinusoidal curve, and then obtain the continuous change curve of the maximum value (Imax) and the minimum value (Imin);

[0064] Step S44: The contrast distribution curve is calculated from the distribution curves of the maximum and minimum values, that is:

[0065]

[0066] Preferably, a rotation correction step is further included between step S41 and step S42, that is, the photographed stripes are rotated to a vertical state.

[0067] Preferably, in step S42, pixels within a preset width are selected along the stripe direction for averaging to obtain an intensity distribution along the stripe direction.

[0068] Preferably, in step S41, by changing the period of the projected sinusoidal fringes, a contrast distribution curve varying at different spatial frequencies is calculated.

[0069] Preferably, step S4 includes the following steps in sequence:

[0070] Step S41, projecting a plurality of sinusoidal fringe patterns of the same frequency through the projector 1;

[0071] Step S42: The intensity distribution on the above-mentioned sinusoidal fringe group is:

[0072] I n =I0(1+sin(2π(x / x0+n / N))); n∈[0,N-1];

[0073] Step S43, taking N phase shift images by continuously triggering the projector 1 and the camera 3, where N is a natural number greater than or equal to 2;

[0074] Step S44, calculating the whole field contrast through the phase shift diagram, that is:

[0075]

[0076] Among them, x and y represent the spatial coordinate position of the image, and max(In(x,y)) and min(In(x,y)) are the maximum and minimum brightness of the imaging pattern captured by the sinusoidal fringe group at a certain position in space.

[0077] Preferably, in step S41 , the lens to be tested is the projection lens 2 , the optical axis of the projector 1 is parallel to the optical axis of the projection lens 2 , and the test surface 5 is not perpendicular to the optical axis of the projection lens 2 .

[0078] Preferably, in step S41, the lens to be tested is a projection lens 2, and through the Sham imaging principle, the projection plane is conjugate with the test plane 5, and the projection is clearly imaged on the test plane 5, the test plane 5 is not perpendicular to the optical axis of the projection lens 2, and the projector 1 is not parallel to the projector optical axis.

[0079] The present invention discloses a method for testing the modulation transfer function of an optical system, and introduces a method for measuring the MTF of a system without moving a fringe chart. This method can be used to measure the MTF of a single lens, and is also particularly suitable for measuring the MTF of an optical system including a projector and a camera, such as a 3D structured light measurement system.

[0080] The measurement system designed by the present invention is as follows Figure 1 As shown, the system structure consists of a projector 1, a projection lens 2, a camera 3, a camera lens 4, and a control storage device 6.

[0081] The system performs measurement as follows: Figure 2 As shown in the flow chart. The projector focuses through the projection lens and projects a stripe pattern on the projection surface (i.e., the test surface 5). The camera end focuses on the projection surface through the camera lens and records the projection pattern. The imaging pattern is recorded by the camera and transmitted to the computing device (i.e., the control storage device 6). The computing device calculates the imaging MTF of the lens through the recorded pattern. The lens to be tested can be installed at the projection end, i.e., the projection lens 2; it can also be installed at the camera end, i.e., the camera lens 4.

[0082] There are many ways to calculate MTF. This invention focuses on disclosing two usable methods and the results of their application examples.

[0083] First embodiment:

[0084] The first method is the single-image mode, which calculates the contrast using one image. The process of this processing method is as follows Figure 3 As shown. First, a sinusoidal pattern is projected through the projector, and the camera is triggered to shoot synchronously. Figure 4 The pattern in is an example of an actual fringe pattern captured.

[0085] In actual applications, the camera and projector are prone to relative rotation during installation, so the first step of the algorithm processing is to perform rotation correction to rotate the captured stripes to a vertical state. If the system has undergone rigorous calibration inspection, this step can also be omitted.

[0086] The second step of the algorithm processing is to select the target area in the corrected image to extract the stripe brightness. In order to reduce the influence of noise in a single image, pixels within a certain width can be averaged along the stripe direction to obtain the intensity distribution along the stripe direction. Figure 5The solid line stripes in the middle figure are the intensity distribution obtained by taking values ​​along the middle part of the stripe pattern. After obtaining the one-dimensional change of the sinusoidal stripes, the next step of data processing is to extract the maximum and minimum values ​​and their position distribution from the sinusoidal curve, and then use the interpolation method to obtain the continuous change curve of the maximum value (Imax) and the minimum value (Imin), that is, the upper and lower envelope curves of the sinusoidal curve. Figure 5 The intensity curve is marked with a dotted line.

[0087] Finally, the contrast distribution curve can be calculated from the distribution curves of the maximum and minimum values, which can be calculated according to the following formula:

[0088]

[0089] By using this method to calculate contrast, the contrast along one-dimensional direction can be quickly calculated. The result of the contrast calculation is the MTF value of the system composed of the projector and the camera at the spatial frequency corresponding to the projected fringes.

[0090] In addition, if you need to calculate the MTF curve of the system at different spatial frequencies, you only need to change the period of the projected sinusoidal fringes and then recalculate using the same method. This method has the advantage of fast data acquisition speed.

[0091] Second embodiment:

[0092] Another method that can be used to calculate MTF is the multi-graph mode, the process is as follows Figure 6 As shown. In multi-image mode, the projector projects multiple images, which are all sinusoidal fringes of the same frequency, but there is a phase offset between the fringes. The intensity distribution on this set of fringes can be described by the following formula:

[0093] I n =I0(1+sin(2π(x / x0+n / N))); n∈[0,N-1]

[0094] Through the continuous firing of the projector and the camera, N phase shift images are taken. The value of N is at least two, and more phase shift images can make the results smoother and more accurate. The whole field contrast can be directly calculated through the phase shift image, and the calculation method is as follows:

[0095]

[0096] Among them, x and y represent the spatial coordinate position of the image;

[0097] max(In(x,y)) and min(In(x,y)) are the maximum and minimum brightness of the imaging pattern captured by the phase-shifted fringe group at a certain position in space.

[0098] When using the multi-image mode for contrast testing, the calculation method is simpler and the two-dimensional distribution of MTF can be obtained at one time.

[0099] Figure 6 and Figure 7 The following are the results of the imaging quality test of two lenses using the multi-image mode. Both lenses are installed in the position shown in the projection lens 2.

[0100] like Figure 6 The optical axis of the first lens projector is parallel to the optical axis of the projection lens, and the clear imaging surface is perpendicular to the optical axis of the lens. Therefore, when the image is imaged to the projection plane, the middle area is clear and the contrast of the upper and lower edges decreases.

[0101] like Figure 7 The second lens uses the Sham imaging principle, which tilts the object plane (i.e. the projector) so that a clear image can be formed on the entire projection plane. Figure 8 The test results also show that the contrast uniformity is relatively Figure 7 significant improvement.

[0102] In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside" and the like indicate positions or positional relationships based on the positions or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention. In addition, the terms "first", "second", etc. are only used for descriptive purposes, and cannot be understood as indicating or implying relative importance or implying the number of technical features indicated. Thus, features defined as "first", "second", etc. may explicitly or implicitly include one or more of the features. In the description of the present invention, unless otherwise specified, "multiple" means two or more.

[0103] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection, it can be a mechanical connection, it can be an electrical connection, it can be a direct connection, it can be indirectly connected through an intermediate medium, and it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0104] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. It should be pointed out that a person skilled in the art can make several improvements and modifications without departing from the technical principles of the present invention, and these improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A method for testing the modulation transfer function of an optical system, characterized in that: The following steps are included in sequence: Step S1, focusing the camera lens (4) to obtain a clear image of the test surface (5); Step S2, focusing the projection lens (2) until a clear image of the test surface (5) is formed; Step S3, the projector (1) projects the image, and the camera (3) synchronously captures and stores the image; Step S4: Graphics post-processing, contrast calculation.

2. A method for testing the modulation transfer function of an optical system as claimed in claim 1, characterized in that: In step S3, the camera (3) synchronously captures and stores the graphics to the control storage device (6).

3. The method for testing the modulation transfer function of an optical system according to claim 1, characterized in that: The lens to be tested is a projection lens (2) or a camera lens (4).

4. A method for testing the modulation transfer function of an optical system as claimed in claim 1, characterized in that: The step S4 comprises the following steps in sequence: Step S41, projecting a sinusoidal pattern through the projector (1), and synchronously triggering the camera (3) to shoot; Step S42, selecting a target area to extract stripe brightness; Step S43, after obtaining the one-dimensional change of the sinusoidal fringes, extract the maximum and minimum values ​​and their position distribution from the sinusoidal curve, and then obtain the continuous change curve of the maximum value (Imax) and the minimum value (Imin); Step S44: The contrast distribution curve is calculated from the distribution curves of the maximum and minimum values, that is:

5. A method for testing the modulation transfer function of an optical system as claimed in claim 4, characterized in that: A rotation correction step is also included between step S41 and step S42, that is, the photographed stripes are rotated to a vertical state.

6. A method for testing the modulation transfer function of an optical system as claimed in claim 4, characterized in that: In step S42, pixels within a preset width are selected along the stripe direction for averaging to obtain the intensity distribution along the stripe direction.

7. A method for testing the modulation transfer function of an optical system as claimed in claim 4, characterized in that: In step S41, by changing the period of the projected sinusoidal fringes, the contrast distribution curve at different spatial frequencies is calculated.

8. The method for testing the modulation transfer function of an optical system according to claim 1, characterized in that: The step S4 comprises the following steps in sequence: Step S41, projecting a plurality of sinusoidal fringe patterns of the same frequency through the projector (1); Step S42: The intensity distribution on the above-mentioned sinusoidal fringe group is: I n =I0(1+sin(2π(x / x0+n / N)));n∈[0,N-1]; Step S43, taking N phase shift images by continuously triggering the projector (1) and the camera (3), wherein N is a natural number greater than or equal to 2; Step S44, calculating the whole field contrast through the phase shift diagram, that is: Among them, x and y represent the spatial coordinate position of the image, and max(In(x,y)) and min(In(x,y)) are the maximum and minimum brightness of the imaging pattern captured by the sinusoidal fringe group at a certain position in space.

9. A method for testing the modulation transfer function of an optical system as claimed in claim 8, characterized in that: In step S41, the lens to be tested is a projection lens (2), and the optical axis of the projector (1) is parallel to the optical axis of the projection lens (2).

10. The method for testing the modulation transfer function of an optical system according to claim 8, characterized in that: In step S41, the lens to be tested is a projection lens (2), and through the Sham imaging principle, the projection surface is conjugate with the test surface (5), and a clear image is projected on the test surface (5).

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