CN117268721A - Focal length measuring device and method - Google Patents
Focal length measuring device and method Download PDFInfo
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- CN117268721A CN117268721A CN202311263623.3A CN202311263623A CN117268721A CN 117268721 A CN117268721 A CN 117268721A CN 202311263623 A CN202311263623 A CN 202311263623A CN 117268721 A CN117268721 A CN 117268721A
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- 238000005259 measurement Methods 0.000 claims description 12
- 238000013461 design Methods 0.000 claims description 10
- 238000009792 diffusion process Methods 0.000 claims description 10
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
- G01M11/00—Testing of optical apparatus; Testing structures by optical methods not otherwise provided for
- G01M11/02—Testing optical properties
- G01M11/0207—Details of measuring devices
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
- G01M11/00—Testing of optical apparatus; Testing structures by optical methods not otherwise provided for
- G01M11/02—Testing optical properties
- G01M11/0242—Testing optical properties by measuring geometrical properties or aberrations
- G01M11/0257—Testing optical properties by measuring geometrical properties or aberrations by analyzing the image formed by the object to be tested
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
- G01M11/00—Testing of optical apparatus; Testing structures by optical methods not otherwise provided for
- G01M11/02—Testing optical properties
- G01M11/0292—Testing optical properties of objectives by measuring the optical modulation transfer function
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Abstract
The invention discloses a focal length measuring device, which sequentially comprises a collimator, a reflector, an objective table and an image acquisition module from the object side to the image side, wherein a dividing plate with a knife edge and a scribing pair is arranged on the focal plane of the collimator, the knife edge and the scribing pair are arranged in parallel, the image acquisition module comprises an object side telecentric lens and an area array CCD detector which are arranged on a linear displacement table, and parallel light emitted by the collimator is converged through a tested lens arranged on the objective table; the focal length measuring method based on the degradation model is also disclosed, the repeated focusing process near the normal focal position is omitted, the degradation model caused by defocusing is estimated through the edge characteristics at any defocusing position, the blurred line pair image is restored according to the degradation model, and the focal length measuring precision and efficiency are improved.
Description
Technical Field
The invention relates to the field of optical property testing, in particular to a focal length measuring device and a focal length measuring method based on degradation model estimation.
Background
An optical lens is an integral component in an optical imaging system. Along with the rapid development of optoelectronic technology, the application of the optical lens is also more and more extensive, and the optical lens plays an important role in the fields of machine vision, security monitoring, consumer electronics, military national defense, biomedicine and the like.
The focal length is a basic parameter of the lens, and is a key for determining the object-image relationship of the lens. However, machining and assembling errors are unavoidable in the steps of lens machining, lens assembling and the like. There is also a certain error between the actual focal length of the lens and the design value. The accuracy of the focal length directly affects the imaging effect of the overall optical system. It is therefore necessary to measure the effective focal length of the lens with high accuracy.
The current method for measuring the focal length of the lens is mainly an amplification method. The magnification method for measuring the focal length of the lens has the advantages of simple measuring process, convenient operation, larger measuring range, higher precision, better stability and lower difficulty in production, and is widely applied to various universities and optical product manufacturers. However, in the magnification method, the reticle image needs to be focused accurately, and then the measurement and calculation of the focal length of the lens are performed.
Therefore, whether to accurately focus and how fast focus limit the accuracy and efficiency of lens focal length measurement.
Disclosure of Invention
One of the objectives of the present invention is to design a focal length measuring device for solving the technical problems existing in the prior art.
The technical scheme adopted for solving the technical problems is as follows: the utility model provides a focal length measuring device, includes collimator, speculum, objective table and image acquisition module in proper order from the object side to the image side, be provided with the division board that has edge of a knife and line to the on the focal plane in collimator the place ahead, wherein edge of a knife and line to the parallel arrangement of pair, image acquisition module include object side telecentric lens and area array CCD detector that set up on the linear displacement platform, the collimator is with the parallel light outgoing, gathers through the measured lens of installing at the objective table, shoots the division board image that defocus through the driven image acquisition module of linear displacement platform.
The second purpose of the invention is to provide a focal length measurement method based on degradation model estimation, so as to solve the problems of poor focal length measurement precision and low efficiency caused by inaccurate focusing and slow focusing speed in the existing focal length measurement method.
The technical scheme adopted for solving the technical problems is as follows: a focal length measurement method based on a degradation model comprises the following steps:
s1, a lens to be tested is arranged on an objective table, an image acquisition module driven by a linear displacement table shoots out-of-focus reticle images, and a moving image acquisition module coarsely focuses the reticle images;
s2, acquiring a reticle image of any defocusing position through an image acquisition module;
s3, extracting knife edge features and reticle pair features in the reticle image, and calculating defocusing MTF;
s4, dividing the defocused MTF by an MTF design value at a positive focal position of an object side telecentric lens in the image acquisition module to obtain an image degradation model caused by defocusingWherein MTF (u) is a modulation transfer function at the defocus position, MTF' (u) is a modulation transfer function design value at the normal focus position, and u is a spatial frequency
S5, combining the image degradation model D (u), and restoring the defocused line pair characteristics through inverse filtering to obtain a clear line pair characteristic imageWherein g (x, y) is a defocused reticle pair characteristic image, and IDF is an inverse Fourier transform operation;
s6, obtaining the positions of the recovered reticle pairs and calculating the reticle pair spacing x of the image side;
s7, according to the actual distance x' between the score line and the object space and the focal length f of the collimator c Calculating focal length of the lens to be tested
Further, in the step S3, the calculating step of the defocus MTF specifically includes:
s31, performing four-time supersampling on the edge characteristics to obtain an edge diffusion function ESF;
s52, carrying out mean value filtering on the edge diffusion function ESF, and then differentiating to obtain a line diffusion functionWherein, is convolution operation;
s53, performing one-dimensional discrete fourier transform on the linear spread function LSF to obtain mtf=dft (LSF) at the defocus position, where DFT is a discrete fourier transform operation.
The beneficial effects of the invention are as follows:
the focal length measuring device adopts the object space telecentric lens 6, the lens has fixed magnification at different object distances, the requirement on focusing accuracy is lower in measurement, the modulation transfer function MTF of the image at the defocusing position is obtained through the knife edge characteristics and is compared with the MTF design value of the positive focal position, the degradation model D (u) of the image is obtained, the blurred line pair image at the defocusing position is restored into a clear line pair image through the degradation model D (u), then the line pair distance measurement is carried out, the focal length is calculated, the repeated focusing process at the positive focal position is omitted, and the focusing efficiency is improved.
Compared with the prior art, the method omits the repeated focusing process near the normal focus position, estimates the degradation model caused by defocusing through the knife edge characteristics at any defocusing position, restores the blurred line pair image according to the degradation model, and improves the precision and efficiency of focal length measurement.
Drawings
FIG. 1 is a schematic view of a focal length measuring device according to the present invention;
FIG. 2 is a schematic diagram of the structure of the reticle of the present invention.
The reference numerals are as follows: 1-reticle, 2-collimator, 3-reflector, 4-objective table, 5-linear displacement table, 6-object space telecentric lens and 7-area array CCD detector.
Detailed Description
The principles and features of the present invention are described below with reference to the drawings and examples, which are provided for illustration only and are not intended to limit the scope of the invention.
In order to solve the problems of low focusing speed, low focusing accuracy, low measuring efficiency, low focusing accuracy and the like in focal length measurement, the focal length measurement device designed by the invention sequentially comprises a collimator 2, a reflecting mirror 3, an objective table 4 and an image acquisition module from an object side to an image side, wherein a reticle 1 with a knife edge and a scribing pair is arranged on a focal plane in front of the collimator 2, the knife edge and the scribing pair are arranged in parallel, the knife edge is a diameter line positioned in the middle of the scribing in the drawing, as shown in fig. 2, the image acquisition module comprises an object side telecentric lens 6 and an area array CCD detector 7 which are arranged on a linear displacement table 5, the collimator 2 emits parallel light, the parallel light is converged through a measured lens arranged on the objective table 4, and an image acquisition module driven by the linear displacement table 5 shoots an out-of-focus reticle 1.
The focal length measurement method based on degradation model estimation comprises the following steps:
s1, a lens to be tested is arranged on an objective table 4, an image acquisition module driven by a linear displacement table 5 shoots an out-of-focus reticle 1 image, and a moving image acquisition module coarsely focuses the reticle 1 image.
S2, acquiring the image of the reticle 1 at any defocusing position through an image acquisition module.
S3, extracting knife edge features and reticle pair features in the images of the reticle 1, and calculating the defocus MTF.
And S31, performing four-time oversampling on the knife edge characteristics to obtain an edge diffusion function ESF.
S32, carrying out mean value filtering on the edge diffusion function ESF, and then differentiating to obtain a line diffusion function
Where is a convolution operation.
S33, performing one-dimensional discrete Fourier transform on the linear diffusion function LSF to obtain the MTF at the defocusing position
MTF＝DFT(LSF)，
Where DFT is a discrete fourier transform operation.
S4, dividing the defocus MTF by an MTF design value at the positive focal position of the object side telecentric lens 6 in the image acquisition module to obtain an image degradation model D (u) caused by defocus
Wherein MTF (u) is a modulation transfer function at the defocus position, MTF' (u) is a modulation transfer function design value at the normal focus position, and u is a spatial frequency.
S5, since the D (u) degradation model is obtained by measurement and has a certain error, the D (u) degradation model is an estimated degradation model. Combining the estimated image degradation model D (u), restoring the defocused line characteristics through inverse filtering to obtain a clear line-to-characteristic image
Where g (x, y) is the out-of-focus reticle pair feature image and IDF is the inverse fourier transform operation.
S6, obtaining the positions of the recovered reticle pairs and calculating the reticle pair spacing x of the image side.
S7, according to the actual distance x' between the score line and the object space and the focal length f of the collimator 2 c Calculating focal length of the lens to be tested
The focal length measuring device adopts the object space telecentric lens 6, the lens has fixed magnification at different object distances, the requirement on focusing accuracy in measurement is lower, the modulation transfer function (MTF, modulation Transfer Function) of the image at the defocusing position is acquired through the knife edge characteristics, the modulation transfer function is compared with the MTF design value at the positive focal position to obtain the degradation model D (u) of the image, the fuzzy line pair image at the defocusing position is restored into a clear line pair image through the degradation model D (u), and then the line pair distance measurement and the focal length calculation are carried out, so that the repeated focusing process at the positive focal position is avoided, and the focusing efficiency is improved.
It will be readily appreciated by those skilled in the art that the foregoing description is merely a preferred embodiment of the invention and is not intended to limit the invention, but any modifications, equivalents, improvements or alternatives falling within the spirit and principles of the invention are intended to be included within the scope of the invention.
Claims (3)
1. A focal length measuring device, characterized by: the image acquisition device comprises a collimator (2), a reflecting mirror (3), an objective table (4) and an image acquisition module in sequence from an object space to an image space, wherein a reticle (1) with a knife edge and a reticle pair is arranged on a focal plane of the collimator (2), the knife edge and the reticle pair are arranged in parallel, the image acquisition module comprises an object space telecentric lens (6) and an area array CCD detector (7) which are arranged on a linear displacement table (5), and parallel light emitted by the collimator (2) is converged through a tested lens arranged on the objective table (4).
2. A focus measurement method of a focus measurement device as claimed in claim 1, comprising the steps of:
s1, a lens to be tested is arranged on an objective table (4), an image acquisition module driven by a linear displacement table (5) shoots an out-of-focus reticle (1) image, and a moving image acquisition module coarsely focuses the reticle (1) image;
s2, acquiring an image of a reticle (1) at any defocusing position through an image acquisition module;
s3, extracting knife edge characteristics and reticle pair characteristics in the images of the reticle (1), and calculating defocusing MTF;
s4, dividing the defocus MTF by an MTF design value at the positive focal position of the object side telecentric lens (6) to obtain an image degradation model caused by defocusWherein MTF (u) is the modulation degree at the defocus positionA transfer function, MTF' (u) is a modulation transfer function design value at a positive focal position, and u is a spatial frequency;
s5, combining the image degradation model D (u), and restoring the defocused line pair characteristics through inverse filtering to obtain a clear line pair characteristic imageWherein g (x, y) is a defocused reticle pair characteristic image, and IDF is an inverse Fourier transform operation;
s6, obtaining the positions of the recovered reticle pairs and calculating the reticle pair spacing x of the image side;
s7, according to the actual distance x' between the score line and the object space and the focal length f of the collimator (2) c Calculating focal length of the lens to be tested
3. The focal length measuring device based on the degradation model according to claim 2, wherein the step of calculating the defocus MTF in step S3 specifically comprises:
s31, performing four-time supersampling on the edge characteristics to obtain an edge diffusion function ESF;
s32, carrying out mean value filtering on the edge diffusion function ESF, and then differentiating to obtain a line diffusion functionWherein, is convolution operation;
s33, performing one-dimensional discrete fourier transform on the linear spread function LSF to obtain mtf=dft (LSF) at the defocus position, where DFT is a discrete fourier transform operation.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
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| CN202311263623.3A CN117268721A (en) | 2023-09-27 | 2023-09-27 | Focal length measuring device and method |
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| Application Number | Priority Date | Filing Date | Title |
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| CN202311263623.3A CN117268721A (en) | 2023-09-27 | 2023-09-27 | Focal length measuring device and method |
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