CN110793440A - Optical deflection transient measurement method - Google Patents

Abstract

The invention provides a transient measurement method based on optical deflection, and relates to the technical field of measurement. This method uses the frequency carrier fringes in orthogonal directions to separately modulate the four-step phase-shifting fringes in the x and y directions (a total of eight phase-shifting patterns), and couples the combined patterns of the four phase-shifting fringes in different directions to different Color channel, and finally get a color image, that is, the pattern projected by the projection screen. A camera is used to collect the deformed pattern after the deflection of the element to be tested, and color separation is used to separate the composite pattern in the x and y directions, and then eight phase-shift patterns are obtained through demodulation operation, that is, four amplitude shift patterns in each of the x and y directions. Phase fringe diagram, from which phase information is obtained through the phase demodulation correlation algorithm, the slope of the measured wave surface is obtained through calculation, and finally the reconstructed wave surface is obtained through the integration algorithm. The method solves the technical problem that in the traditional optical deflection detection, the phase-shifting fringes are continuously projected on the component to be tested, and the transient measurement cannot be performed.

Description

An Optical Deflection Transient Measurement Method

technical field

The invention relates to the technical field of measurement, in particular to a high-precision large dynamic instantaneous measurement method based on a frequency carrier method, a phase shift method and a color coding method.

Background technique

With the development of optical design and manufacturing, the application of optical components in testing is becoming more and more extensive. Components are evaluated using wavefront testing. Traditional wavefront detection methods mainly use interferometry, such as Twyman-Green interferometer, Fizeau interferometer, point diffraction interferometer, etc. However, the dynamic range of conventional interferometers limits its application to large-scale transmission wavefront measurements in practical situations.

In recent years, the deflection method has been widely used and successfully applied to the test of astronomical telescope reflectors, spherical, aspherical, specular free mirrors, and precision X-ray mirrors. Compared with the interferometric method, the deflection method has lower requirements on the system configuration, and only needs components such as a projection screen, a CCD camera, and the component to be tested. Based on the ray tracing method, the light from the projection screen to the camera after passing through the component to be tested is determined, and the detected wavefront is obtained through the light aberration. Use binary method, gray scale method, phase shift method, hybrid method, etc. to create maps and realize optical deflection measurement. The multi-step phase-shifting method is a commonly used detection method, and the projection screen continuously displays the projected phase-shifted sinusoidal fringes. However, this traditional method can only display and acquire one phase shift pattern at a time. In traditional optical deflection measurement, it is necessary to continuously project at least three sinusoidal phase-shift fringes on the component to be measured, so traditional optical deflection measurement cannot achieve transient measurement, which also makes this method suitable for practical applications. Limitations also limit the measurement speed.

SUMMARY OF THE INVENTION

Aiming at the deficiencies of the prior art, the purpose of the present invention is to solve the technical problem that transient measurement cannot be performed because multiple phase shift fringes are continuously projected on the measured object in the traditional method. The invention provides a high-precision large dynamic instantaneous measurement method based on a frequency carrier method, a phase shift method and a color coding method.

The present invention is achieved through the following technical solutions: a method for measuring the transient state of optical deflection, comprising the steps of:

Establish an optical detection system based on the optical deflection method, the optical detection system includes a projection screen, a component to be tested, a camera and a computer;

Design the projection coding pattern of the projection screen: use the frequency carrier fringes in orthogonal directions to separately modulate the eight phase-shifting patterns obtained by four-step phase-shifting, and couple the combined pattern of the four phase-shifting fringes in two different directions of x and y to the two. different color channels to get a color image;

Image demodulation: Project the color map to the component to be tested, and separate the color channels of the deformed color map collected by the camera after the component to be tested is deflected to obtain a composite pattern in the x and y directions. The composite pattern is demodulated to obtain four phase-shift fringe patterns in each of the x and y directions, with a total of eight phase-shift patterns;

The above-mentioned eight phase-shifting patterns are processed through the four-step phase-shifting algorithm and the phase unwrapping algorithm to obtain phase information, thereby obtaining the actual spot distribution, and comparing the actual spot distribution with the ideal obtained by ray tracing in the optical detection system The light spot distributions are compared, and the light aberration is obtained, and the light aberration is the result of the processing error of the element to be tested;

The slope distribution of the wavefront is obtained according to the relationship between the wave aberration and the light aberration, and the wavefront reconstruction is completed by integrating the slope of the wavefront.

Preferably, the designing the projection screen to project the coding pattern comprises the steps of:

Four phase-shifting patterns I _pxn and I _pyn with sinusoidal intensity are designed in the x and y directions respectively. The combined pattern of is coupled to the red R channel, and the combined pattern of the four phase-shifted fringes in the y direction is coupled to the blue B channel, and finally a color image is obtained, which can be represented by a mathematical formula:

Among them, A and B are the background intensity and modulation intensity coefficient of the sinusoidal stripes, respectively; I _x , I _y are the intensities in the x and y directions, respectively; I _R , I _B , and I _G are the intensities in the R, G, and B channels, respectively ; n is the number of phase shift steps, and (n=1,2,3,4); I _fx,n =cos(2πf _{fx, n} x),I _fy,n =cos(2πf _fy,n y) are respectively Different frequency carriers in the x and y directions; f _fx,n , f _fy,n are the design frequencies of each carrier in the x and y directions, respectively.

Preferably, the I _pxn and I _pyn are respectively:

I _pxn =1/2+cos(2πfx+n·π/2)

I _pyn =1/2+cos(2πfy+n·π/2)

where f is the frequency of the phase shift pattern, and x, y are the pixel coordinates on the projection screen, respectively.

Preferably, the image demodulation comprises the steps of:

Perform color channel separation on the deformed color image collected by the camera after passing through the component to be tested, and obtain composite phase-shift fringes in the x and y directions;

The composite patterns of different carrier frequencies and phase-shift fringes are separated by band-pass filters, and there are four patterns in each of the x and y directions;

According to the principle of Butterworth filter, the band-pass filter is designed with f _n as the center. The cutoff frequency of each frequency band is designed as f _n =(f _n-1 +f _n )/2, where n = 1, 2, 3, . . . , N and the baseband channel f ₀ =0, the The stripe information is:

是沿载波方向以f_n频率为中心的带通滤波器；I_CP(x,y)是复合相移条纹图的强度；I′_n(x,y)是相移条纹图的强度；in

is the band-pass filter centered at f _n frequency along the carrier direction; I _CP (x, y) is the intensity of the composite phase-shift fringe pattern; I′ _n (x, y) is the intensity of the phase-shift fringe pattern;

After processing by a low-pass filter, the carrier fringe information in the orthogonal direction is filtered out, so as to separate the carrier frequency and phase shift fringe pattern:

为最终恢复的相移条纹图的强度，h′_LP(x)是低通滤波器。in

is the intensity of the final recovered phase-shifted fringe pattern, h' _LP (x) is the low-pass filter.

Compared with the prior art, the present invention has the beneficial effects that: the method breaks the traditional measurement speed limitation, uses one color image to replace the multiple phase-shift images generated by multi-step phase-shifting, realizes the transient measurement, and reduces the cost. When designing the coding pattern, the existing method is three-step phase-shifting to generate three phase-shifting diagrams, which are respectively coupled to the three channels R, G, and B. The problem is that the channels will crosstalk each other, but this method , the fringe pattern coupled into the channel is already a modulated composite phase shift pattern, which also solves the problem of channel crosstalk. Since the wavelengths of R, G, and B are different, after the three phase shift diagrams are coupled into the R, G, and B channels, errors will be introduced in the subsequent phase shift calculation. However, in this method, the four phase shift diagrams in the same direction will This kind of error is avoided by modulating the shift image into a composite phase shift image into the same channel. The invention provides a high-precision large dynamic instantaneous measurement method based on a frequency carrier method, a phase shift method and a color coding method for optical deflection detection.

Description of drawings

Fig. 1 is the flow chart of the present invention;

2 is a schematic diagram of a detection optical path system according to a specific embodiment of the present invention;

3 is a schematic diagram of the design of the frequency carrier composite color map coding used for instantaneous deflection measurement projection screen projection according to the present invention;

Fig. 4 is the demodulation process diagram of the camera collecting the deformed color image after the component to be tested of the present invention;

FIG. 5 is a phase recovery diagram after demodulation of the present invention.

Detailed ways

The present invention will be described in detail below with reference to the specific embodiments shown in the accompanying drawings, but these embodiments do not limit the present invention, and those of ordinary skill in the art can make structural, method, or functional transformations according to these embodiments. All are included in the protection scope of the present invention.

Please refer to FIG. 1 , a method for measuring optical deflection transients proposed by the present invention includes the steps:

S1: Establish an optical detection system based on optical deflection method for transmission element detection. As shown in FIG. 2 , the optical detection system includes a projection screen 1 , a transmission element to be measured 2 , a camera 3 and a computer. The projection screen 1, the component to be tested 2, and the camera 3 are placed according to the reverse Hartmann structure. The front end of the camera 3 has a small filter hole to overcome the aperture imaging aberration. Projection screen 1 displays a color pattern finally formed by modulating the frequency carrier stripes in orthogonal directions and coupling the combined pattern of four phase-shifting stripes in different directions to different color channels. 3. The complete imaging of the component to be tested 2 can be obtained, and the deformed color map after imaging can be collected. The color channel separation is performed on the deformed color image to obtain a composite pattern in the x and y directions, and the composite pattern is demodulated. The camera 3 and the projection screen 1 are respectively connected with the computer.

S2: Design the projected coding pattern of the required projection screen. The method described below is used: the four-step phase-shifted fringe pattern in the x and y directions can be expressed as:

I _pxn =1/2+cos(2πfx+n·π/2) (1)

I _pyn =1/2+cos(2πfy+n·π/2) (2)

Where f is the frequency of the phase shift pattern, x and y are the pixel coordinates on the projection screen respectively, and n is the number of phase shift steps (n=1, 2, 3, 4).

After four-step phase shifting, four phase-shifting patterns are obtained in each of the x and y directions. The eight-amplitude phase-shifting patterns are individually modulated by the frequency carrier fringes in the orthogonal directions, and the combined patterns of the four-amplitude phase-shifting fringes in different directions are coupled. to a different color channel:

where I _x and I _y are the intensities in the x and y directions respectively; A and B are the background intensity and modulation intensity coefficient of the sinusoidal stripes respectively; I _fx,n =cos(2πf _fx,n x), I _fy,n = cos(2πf _fy,n y) are the different frequency carriers in the x and y directions respectively; f _fx,n , f _fy,n are the design frequencies of each carrier in the x and y directions, respectively.

The combined pattern of the four phase-shifted fringes in the x-direction is coupled to the red R channel, and the combined pattern of the four phase-shifted fringes in the y-direction is coupled to the blue B channel, and finally a color image is obtained. A color map can be represented by a mathematical formula:

Among them, A and B are the background intensity and modulation intensity coefficient of the sinusoidal stripes, respectively; I _x , I _y are the intensities in the x and y directions, respectively; I _R , I _B , and I _G are the R, G, and B channels, respectively. Intensity; n is the number of phase shift steps, and (n=1, 2, 3, 4); I _fx,n =cos(2πf _fx,n x), I _fy,n =cos(2πf _fy,n y) respectively are different frequency carriers in the x and y directions; f _fx,n , f _fy,n are the design frequencies of each carrier in the x and y directions, respectively.

S3: Image demodulation. The following method is adopted: project the color map to the component to be tested, and separate the color channels of the deformed color map collected by the camera after the component to be tested is deflected to obtain a composite of x and y directions. Stripes, using a Butterworth-based band-pass filter to separate out the composite phase-shift maps in different directions to achieve uniform filtering of the phase-shift maps. A low-pass filter is applied to the direction of the frequency carrier to obtain the deformed phase shift pattern, and finally eight phase shift patterns are obtained, that is, four phase shift patterns are collected in each of the x and y directions. (Take the x direction as an example, the same can be obtained in the y direction) According to the principle of Butterworth filter, a band-pass filter is designed with f _n as the center. The cutoff frequency for each band is designed to be:

where n=1, 2, 3, ..., N and baseband channel f ₀ =0. The fringe information after bandpass filtering is:

是沿正交方向以f_n频率为中心的带通滤波器；I_CP(x,y)是复合相移条纹图的强度；I′_n(x,y)是相移条纹图的强度。经过低通滤波器进行处理，滤除掉正交方向的载波条纹信息，从而分离出载波频率和相移条纹图：in

is the bandpass filter centered at the f _n frequency along the orthogonal direction; I _CP (x,y) is the intensity of the composite phase-shift fringe pattern; I′ _n (x,y) is the intensity of the phase-shift fringe pattern. After processing by a low-pass filter, the carrier fringe information in the orthogonal direction is filtered out, so as to separate the carrier frequency and the phase shift fringe pattern:

是最终恢复的相移条纹图的强度；h′_LP(x)是低通滤波器。in

is the intensity of the final recovered phase-shifted fringe pattern; h′ _LP (x) is the low-pass filter.

S4: Wavefront reconstruction: phase information is obtained through the four-step phase shifting algorithm and phase unwrapping algorithm to obtain the actual spot distribution, and the actual spot distribution is compared with the ideal spot distribution obtained by ray tracing in the optical deflection detection system , the light aberration is obtained, and the measured light aberration is the result of the processing error of the component to be tested. Therefore, the relationship between the wave aberration and the light aberration can be used to obtain the slope distribution of the wavefront, and the slope of the wavefront can be integrated to complete the wavefront reconstruction.

Example

Figure 2 is an optical path layout of an optical deflection transient measurement system for transmission element detection. The component to be tested 2 is a convex lens with a diameter of 25.4 mm and a refractive index of 1.51. The size of the measurement field is set to 1080×1080 pixels, and the carrier frequencies in the x and y directions are set to the frequencies corresponding to 40, 70, 100, and 130 cycles, and each phase shift fringe pattern has about 12 cycles. The number of pixels of the LCD screen used for fringe projection is 1920(H)×1080(V), the corresponding pixel size is 0.265mm(H)×0.265mm(V), a three-channel 8-bit color CCD camera is used, and the resolution is 1348 ( H) × 1280 (V), and the pixel size is 3.63 μm (H) × 3.63 μm (V).

Step 1, as shown in Figure 3, design the coding pattern required for projection on the projection screen: design four phase-shifting patterns I _pxn and I _pyn whose intensity varies sinusoidally in the x and y directions, and use the frequency carrier fringes in the orthogonal directions to pair The eight phase-shifting patterns are individually modulated, and the combined pattern of the four phase-shifting fringes in the x-direction is coupled to the red channel, and the combined pattern of the four-phase-shifting fringes in the y-direction is coupled to the blue channel, and finally a color image is obtained. A color map can be represented by a mathematical formula:

Among them, IR, _IB , IG are the light intensity in _R , _G , B channels respectively

Step 2: As shown in Figure 4, the color channel separation is performed on the deformed color image collected by the camera after passing through the component to be tested, to obtain composite stripes in the x and y directions, and use a Butterworth-based bandpass filter to separate out the different color channels. The composite phase shift map of the direction, which realizes uniform filtering of the phase shift map. The low-pass filter is applied to the direction of the frequency carrier to obtain the phase shift diagram, and finally eight phase shift diagrams are obtained, that is, four phase shift diagrams are collected in each of the x and y directions.

Step 3, as shown in Figure 5, after obtaining eight phase-shifting patterns, the four-step phase-shifting algorithm and the phase unwrapping algorithm are used to obtain phase information. The wrapped phase maps collected in the x and y directions are shown in (a) and (d) in Figure 5, and the corresponding unwrapped phases are shown in Figure 5 (b) and (e). (c) and (f) in Figure 5 are the comparison data for the ideal phase. From this, the actual light spot distribution can be obtained, and the actual light spot distribution can be compared with the ideal light spot distribution obtained by ray tracing in the optical deflection detection system, and the light aberration can be obtained. the result of. Therefore, the relationship between the wave aberration and the light aberration can be used to obtain the slope distribution of the wavefront, and the slope of the wavefront can be integrated to complete the wavefront reconstruction.

Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those of ordinary skill in the art will appreciate that various Improvements, additions and substitutions are possible.

Claims (4)

1. an optical deflection transient measurement method is characterized in that: Establish an optical detection system based on the optical deflection method, the optical detection system includes a projection screen, a component to be tested, a camera and a computer; Design the projection coding pattern of the projection screen: use the frequency carrier fringes in orthogonal directions to separately modulate the eight phase-shifting patterns obtained by four-step phase-shifting, and couple the combined pattern of the four phase-shifting fringes in two different directions of x and y to the two. different color channels to get a color image; Image demodulation: Project the color map to the component to be tested, and separate the color channels of the deformed color map collected by the camera after the component to be tested is deflected to obtain a composite pattern in the x and y directions. The composite pattern is demodulated to obtain four phase-shift fringe patterns in each of the x and y directions, with a total of eight phase-shift patterns; Wavefront reconstruction: The above-mentioned eight phase-shifting patterns are processed through the four-step phase-shifting algorithm and the phase unwrapping algorithm to obtain phase information, thereby obtaining the actual spot distribution, and comparing the actual spot distribution with the light passing through the optical detection system. The ideal spot distributions obtained by tracing are compared, and the light aberration is obtained, which is the result of the processing error of the element to be measured; the slope of the wavefront is obtained according to the relationship between the wave aberration and the light aberration distribution, and the slope of the wavefront is integrated to complete the wavefront reconstruction. 2. A kind of optical deflection transient measurement method according to claim 1, is characterized in that: described designing projection screen projection coding pattern comprises the steps: Four phase-shifting patterns I _pxn and I _pyn with sinusoidal intensity are designed in the x and y directions respectively. The combined pattern of is coupled to the red R channel, and the combined pattern of the four phase-shifted fringes in the y direction is coupled to the blue B channel, and finally a color image is obtained, which can be represented by a mathematical formula:

Among them, A and B are the background intensity and modulation intensity coefficient of the sinusoidal stripes, respectively; I _x , I _y are the intensities in the x and y directions, respectively; I _R , I _B , and I _G are the intensities in the R, G, and B channels, respectively ; n is the number of phase-shifting steps, and n=(1, 2, 3, 4); I _fx,n =cos(2πf _fx,n x), I _fy,n =cos(2πf _fy,n y) are respectively Different frequency carriers in the x and y directions; f _fx,n , f _fy,n are the design frequencies of each carrier in the x and y directions, respectively. 3. a kind of optical deflection transient measurement method according to claim 2, is characterized in that: described I _pxn and I _pyn are respectively: I _pxn =1/2+cos(2πfx+n·π/2) I _pyn =1/2+cos(2πfy+n·π/2) where f is the frequency of the phase shift pattern, and x, y are the pixel coordinates on the projection screen, respectively. 4. A kind of optical deflection transient measurement method according to claim 1, is characterized in that: described image demodulation comprises the step: Perform color channel separation on the deformed color image collected by the camera after passing through the component to be tested, and obtain composite phase-shift fringes in the x and y directions; The composite patterns of different carrier frequencies and phase-shift fringes are separated by band-pass filters, and there are four patterns in each of the x and y directions; According to the principle of Butterworth filter, the band-pass filter is designed with f _n as the center. The cutoff frequency of each frequency band is designed as f _n =(f _n-1 +f _n )/2, where n = 1, 2, 3, . . . , N and the baseband channel f ₀ =0, the The stripe information is:

in is the band-pass filter centered at f _n frequency along the carrier direction; I _CP (x, y) is the intensity of the composite phase-shift fringe pattern; I′ _n (x, y) is the intensity of the phase-shift fringe pattern; After processing by a low-pass filter, the carrier fringe information in the orthogonal direction is filtered out, so as to separate the carrier frequency and phase shift fringe pattern:

in

is the intensity of the final recovered phase-shifted fringe pattern, h' _LP (x) is the low-pass filter.

Images