CN102506759A - Lonky detection method of aspheric surface with heavy calibre - Google Patents

Abstract

A large-aperture aspheric Ronchi detection method, using ray tracing and phase information of sinusoidal fringes to design a compensated sinusoidal grating, and display it on a transmissive liquid crystal display, which is also a phase shifting device, which can make the display The compensating sinusoidal grating on it produces phase shift, and the light emitted by an off-axis point light source is reflected by the mirror and passes through the compensating grating, and the camera records the phase shift fringe pattern carrying the deviation information of the mirror relative to its ideal surface shape. By analyzing the phase shift fringe pattern, the actual lateral aberration of the measured mirror surface and the corresponding ideal lateral aberration are determined, and the deviation gradient of the measured mirror surface is obtained based on the geometric principle of Ronchi detection, and the deviation of the measured mirror surface is obtained by integrating it, and then Reconstruct the three-dimensional surface shape of the measured mirror. The invention does not need to specially manufacture compensation gratings for each type of aspheric mirror, has the advantages of flexibility, convenience, and high detection accuracy, and has important application prospects in the detection of large-diameter aspheric surfaces.

Description

A large aperture aspheric Ronchi detection method

technical field

The invention relates to an optical detection technology, in particular to the detection of a rotationally symmetrical large-diameter aspheric mirror in the stages of fine grinding and initial polishing, and belongs to the field of advanced optical manufacturing and detection technology.

Background technique

Aspherical mirrors have the advantages of increasing the freedom of optical design, simplifying system structure, reducing weight, and improving the image quality of optical systems. With the development of advanced optical manufacturing technology and the improvement of imaging quality requirements, aspheric mirrors are widely used in space communication, celestial observation, military and civilian industries. However, compared with spherical mirrors, the manufacture of aspherical mirrors is much more difficult, which puts forward higher requirements for its processing and inspection technology.

At present, the detection techniques of aspheric mirrors mainly include surface contour method, zero compensation method and Ronchi test method. The three-coordinate measuring instrument is a detection instrument in the surface profile method. It uses the measuring head to directly act on the surface of the aspheric mirror to directly measure the three-dimensional coordinates of each point on the surface of the aspheric mirror. This measurement method has high precision, but the measurement The head is easy to damage the surface of the mirror under test, the measurement efficiency is low, and the measurement aperture is also limited.

The zero compensation method needs to place a compensator in the detection system to compensate for the spherical aberration produced by the measured aspheric surface. The principle is that each light emitted by the interferometer is incident on the aspheric surface along the normal line of the aspheric surface after passing through the compensation system, and is reflected by the mirror surface and returns along the original path to interfere with the reference wave. If the compensated wavefront completely matches the reference wavefront, the interference fringes are straight fringes, otherwise, the interference fringes are curved fringes, and the degree of curvature of the fringes reflects the deviation of the measured aspheric surface from its ideal surface shape. Although the precision of the zero position detection method is very high, the corresponding compensator must be specially designed and produced for each type of aspheric mirror, which lacks versatility, and the production, detection and assembly accuracy of the compensator will directly affect the detection results.

The Ronchi method can both qualitatively and quantitatively detect the surface shape of aspheric mirrors. It mainly calculates the deviation of the measured mirror surface relative to its ideal surface shape based on the deformation of the actual fringe relative to its ideal fringe. According to whether the stripes on the grating are curved or not, the Ronchi detection method can be divided into the standard Ronchi detection method and the zero Ronchi detection method. The ideal fringes are curved when using the standard Ronchi method to detect specular surfaces, while the ideal fringes are straight when using the zero Ronchi method to detect specular surfaces. Therefore, the Zero Ronchi test method makes it easier for technicians to qualitatively judge the deviation of the measured mirror surface according to the degree of curvature of the stripes. In addition, the zero Ronchi detection method can also avoid the fringe diffusion caused by the diffraction effect, thereby improving the detection accuracy. However, the traditional zero-Ronchi detection method has some defects. 1) It cannot provide enough measurement data, mainly because the information of the area between the fringes cannot be obtained; 2) the moiré fringe pattern formed by the coaxial detection will be 3) The edge of each band on the traditional compensation grating has a jagged shape; 4) When detecting different mirror surfaces, the corresponding compensation grating must be specially made, which brings inconvenience to the actual detection.

Contents of the invention

The object of the present invention is to propose a large-aperture aspheric Ronchi detection method using an off-axis point light source to address the shortcomings in the prior art. This method can realize non-contact full-field detection of large-aperture aspheric surfaces, it can avoid the error caused by the moiré pattern in the coaxial Ronchi detection method to the measurement results, and eliminate the zigzag shape of each fringe band edge on the compensation grating , to obtain enough information on the points to be measured on the mirror surface, there is no need to specially make corresponding compensation gratings for each type of aspheric mirror.

The object of the present invention is to adopt following technical scheme to realize:

The detection device is mainly composed of the measured mirror, optical fiber, optical fiber lighting source, CCD camera, transmission liquid crystal display and computer. Among them, the transmission liquid crystal display is used to display the compensated sinusoidal grating in both vertical and horizontal directions, and as a phase shifting device, it is placed perpendicular to the optical axis of the measured mirror. The curved fringes on the compensated sinusoidal grating are designed according to the phase information and ray tracing of the given straight sinusoidal fringes on the ideal surface. The sinusoidal fringes are selected because every point on them has a phase, so every point on the designed compensation grating There is no phase at all. For the compensation grating at different positions, the shape of the curved stripes is different, so a corresponding compensation grating must be designed for a given position. Since the compensation of asphericity depends heavily on the location of the compensation grating design, the compensation grating must be placed exactly where it was designed. To this end, a circular mark is displayed on the screen displaying the grating, so that the mark coincides with the projection of the edge of the mirror surface to be tested on the screen to ensure that the compensating sinusoidal grating is placed in the designed position.

It is necessary to calibrate the internal and external parameters of the camera before detection. First, the internal parameters of the camera are calibrated by phase target and Fourier fringe analysis technology, and then a checkerboard calibration target perpendicular to the optical axis is placed in the center of the mirror to calibrate the external parameters of the camera. During the detection process, the light emitted by the fiber optic lighting source is transmitted to the side of the center of curvature of the vertex of the measured mirror through a single-mode fiber to form a point light source. In order to ensure full-aperture measurement, the relative diameter of the optical fiber must not be smaller than the relative diameter of the measured mirror. , the brightness is controlled by a DC voltage-regulated fiber optic lighting source. This point light source illuminates the entire measured mirror surface, and the camera focused on the mirror surface records the phase shift fringe pattern carrying the mirror surface deviation information through the compensation grating. The continuous phase distribution φ of the acquired fringe pattern can be calculated by the phase shifting technique and the corresponding phase unwrapping technique, and this phase is called the relative phase. During detection, a mark is encoded on the compensating grating, and this mark is used as a reference to convert relative phase to absolute phase. In order to reconstruct the measured surface shape, the actual lateral aberration of the measured mirror surface and the actual reflected light passing through each point on the grating are determined by first finding the same name point of each pixel point of the camera on the grating and the checkerboard calibration target, and then The ideal lateral aberration corresponding to the intersection point of the actual reflected light and the ideal surface shape is obtained by ray tracing, and then based on the geometric principle of Ronchi detection, the deviation gradient of the measured mirror surface relative to its ideal surface shape is obtained, and the measured mirror surface is obtained by integrating it deviation, and then reconstruct the three-dimensional surface shape of the measured mirror.

Compared with the prior art, the present invention has the following advantages:

1. The present invention is a non-contact full-field detection. Compared with a three-coordinate measuring instrument and an interferometer, the detection structure is simple, the operation is convenient and fast, the required devices are easy to manufacture, and the price is relatively low.

2. The present invention will use a transmissive liquid crystal display to generate a compensated sinusoidal grating, which can easily change the number and direction of the compensation grating’s stripes, precisely control the phase shift of the grating, and avoid the trouble of repeatedly describing the compensation grating when measuring different types of mirrors .

3. Compared with the traditional zero Ronchi detection method, the present invention can eliminate the sawtooth shape of each fringe band edge on the compensation grating, and can obtain enough information on the points to be measured on the mirror surface.

4. The present invention uses an off-axis point light source to avoid the detection error caused by the moiré fringe pattern in the coaxial Ronchi detection method.

Description of drawings

Fig. 1 is a schematic diagram of the device of the detection method of the present invention.

Fig. 2 is a schematic diagram of the design of the compensated sinusoidal grating in the present invention.

Fig. 3 is a schematic diagram of the detection method of the present invention.

Fig. 4 is the compensated sinusoidal grating designed by the present invention in the actual detection process, wherein 4(a) is the compensated sinusoidal grating in the vertical direction, and 4(b) is the compensated sinusoidal grating in the horizontal direction.

Fig. 5 is the fringe pattern collected by the CCD camera in the present invention, wherein 5(a) is the fringe pattern in the vertical direction, and 5(b) is the fringe pattern in the horizontal direction.

Fig. 6 The present invention reconstructs the mirror surface shape actually detected.

Detailed ways

The present invention will be further described in detail below in conjunction with the accompanying drawings, working principles and embodiments.

Figure 1 is a schematic diagram of the detection device, wherein 1 is the mirror to be tested, 2 is the optical fiber, 3 is the optical fiber lighting source, 4 is the CCD camera, 5 is the transmission liquid crystal display screen, and 6 is the computer. During detection, a compensated sinusoidal grating needs to be displayed on the transmission liquid crystal display 5 .

The design of the compensated sinusoidal grating is shown in Figure 2. When designing the compensated sinusoidal grating, it is assumed that there are given straight sinusoidal fringes on the ideal surface, and the curved fringes on the compensated grating are calculated using ray tracing and phase information of the fringes. In Fig. 2, a Cartesian coordinate system is established with the center o of the ideal surface shape as the coordinate origin, and the z- axis coincides with the optical axis of the ideal surface shape. N is a point on the ideal surface, S is an off-axis point light source, A is the intersection point of the reflected light passing through point N and the display screen, L is the intersection point of the reflected light passing through the mirror origin o and the display screen, AL is the lateral aberration . According to the law of reflection, the unit vector of the reflected light passing through any point N on the mirror surface can be obtained.

Given the coordinates of point N on the ideal surface and the unit vector of the reflected ray passing through point N , the reflected ray passing through point N and the intersection point A of this reflected ray with the display screen can be determined. If point L is the center of the display screen, then according to the coordinates of point L and the pixel size of the display screen, the coordinates of each pixel on the display screen can be determined. By using the equi-grid points ( N _x , N _y ) on the given straight sinusoidal fringe pattern and the points with the same name ( A _x , A _y ) on the display screen, it is possible to find each pixel on the display screen at a given The phase of each pixel on the display screen is the phase of the point of the same name on the given straight sinusoidal fringe pattern. Therefore, the intensity distribution of the curved fringe pattern on the display screen, i.e. compensated sinusoidal grating, can be expressed as:

                                                 (1)

(1)

a ₁ and a ₂ are constants, and φ ( x , y ) is the phase. When the vertical stripes are given on the ideal surface, φ =2π x / p ₁ ; when the horizontal stripes are given on the ideal surface, φ =2π y / p ₂ . p1 and p2 are _the periods of the vertical and horizontal sinusoidal stripes _, respectively.

The designed compensated sinusoidal grating is used to detect the aspheric mirror. Before the detection, the internal parameters of the camera are first calibrated by using the phase target and Fourier fringe analysis technology, and then a checkerboard calibration target perpendicular to the optical axis is placed in the center of the mirror surface. The extrinsic parameters of the camera. The world coordinate system selected during calibration is the same as the Cartesian coordinate system established in Figure 2. The plane with z = 0 is the checkerboard calibration target surface, and the x- axis and y- axis are respectively parallel to the horizontal and vertical directions of the feature points on the target surface.

Figure 3 is the geometric principle of detection, as shown in Figure 3, A 'is the projection point of point A on the plane Moz , L ' is the projection point of the center point L of the display screen on the plane Moz . The display screen displays the compensated sinusoidal gratings in the vertical and horizontal directions, which respectively correspond to the given vertical and horizontal sinusoidal stripes on the ideal surface shape. In the detection, the light emitted from the point light source is reflected by the specular surface and passes through the compensation grating, thereby generating a fringe pattern carrying specular deviation information and recorded by the camera. The continuous phase distribution φ of the acquired fringe pattern can be calculated by the phase shift technique and the corresponding phase unwrapping technique. This phase is called the relative phase, which is relative to the starting point of the phase unwrapping. In the present invention, a mark is added in each grating with different modulation degrees, and this mark can convert the relative phase into the absolute phase.

The essence of the Ronchi test method is to measure the lateral aberration. In order to obtain the actual lateral aberration of the measured mirror surface, it is necessary to find the point with the same name of each pixel on the camera on the grating. The process of finding the same-named point can be divided into two steps. First, use the information of the same phase to find the same-named point N of each pixel point on the camera on the given straight stripe image through interpolation, and then use ray tracing to find the point N The homonymous point A on the grating thus determines the actual lateral aberration AL of the mirror under test. From the calibration of the camera, it can be seen that the camera coordinate system has been calibrated in the world coordinate system, so the point B with the same name of each pixel on the camera can be found on the checkerboard calibration target surface. Knowing points A and B , the actual reflected ray BA can be determined.

，点A′在径向的分量记作

，那么两径向分量之差，这个差值与被测镜面相对其理想面形的偏差有关。 Using the expression of the actual reflected light BA and the ideal surface shape, their intersection point M can be calculated. Using ray tracing, the ideal reflected ray passing through the point M and the intersection point C of the ideal reflected ray with the display screen can be calculated, thus obtaining the ideal lateral aberration CL . The projection point of point C on the plane Moz is C ′, if the component of point C ′ in the radial direction is recorded as

, the component of point A ′ in the radial direction is denoted as

, then the difference between the two radial components , this difference is related to the deviation of the measured mirror surface from its ideal surface shape.

Assuming that the deviation of the measured mirror surface is small, the deviation of the mirror surface can be measured from the normal direction of the mirror surface based on the geometric principle of Ronchi detection. Suppose the surface shape of an ideal aspheric surface is f ( r ), then the gradient of the ideal surface shape is tan β = df ( r )/ dr . If the measured mirror deviation along the normal direction is represented by g ( r ), then in the case of a point light source off-axis, the gradient of the mirror deviation along the normal direction can be deduced as

                                        (2)

(2)

D is the distance between the transmissive liquid crystal display and the center of the mirror, r = ( x ² + y ² ) ^1/2 . From formula (2), the gradients in the x and y directions of the mirror surface can be deduced, and the gradients in these two directions can be integrated to recover the mirror surface deviation g ( r ) along the normal direction, and then reconstruct the measured mirror surface shape

                                                         (3)

(3)

Figure 4 is a compensated sinusoidal grating designed according to phase information of sinusoidal fringes and ray tracing in actual detection, where Figure 4(a) is the compensation grating in the vertical direction, and Figure 4(b) is the compensation grating in the horizontal direction. Compensation gratings in two directions will be displayed on the transmissive LCD screen respectively.

Figure 5 is the process of using the designed compensation grating to detect a concave mirror, two phase-shift fringe images collected by the camera and carrying the mirror deviation information, in which Figure 5(a) is the vertical fringe figure, and Figure 5(b ) is a fringe pattern in the horizontal direction.

Fig. 6 is the actual detected mirror surface shape reconstructed by the present invention.

Claims (5)

1. a large aperture aspheric Ronchi detection method, is characterized in that, comprises the following steps: a) Compensate the curved fringes on the sinusoidal grating by ray tracing and the phase information design of the given straight sinusoidal fringes on the ideal surface, and display the curved fringes on the transmissive liquid crystal display; b) Use the light emitted by an off-axis point light source to illuminate the entire mirror surface under test, and the reflected light passes through the compensating sinusoidal grating; c) Record the fringe pattern carrying the deviation information of the mirror surface relative to its ideal surface shape through the camera; d) Analyze and collect the fringe pattern to obtain the deviation gradient of the measured mirror surface, integrate the deviation gradient, restore the deviation of the measured mirror surface, and then reconstruct the three-dimensional surface shape of the measured mirror surface. 2. The method according to claim 1, characterized in that, the transmissive liquid crystal display screen displaying the curved fringe pattern is used as a compensating sinusoidal grating, and as a phase shifting device, causing the compensating sinusoidal grating to generate a phase shift. 3. The method according to claim 1, wherein the compensating sinusoidal grating comprises a vertical compensating grating and a horizontal compensating grating, which respectively correspond to given vertical sinusoidal stripes and horizontal sinusoidal stripes on the ideal surface shape. stripe. 4. The method according to claim 1, wherein said one off-axis point light source makes the detection system more convenient, and there is no error caused by the moiré pattern in the coaxial Ronchi detection to the measurement result. 5. The method according to claim 1, wherein the analysis and collection of the fringe pattern is to determine the actual lateral aberration by looking for each pixel point of the camera on the grating with the same name, and by ray tracing calculation The ideal lateral aberration corresponding to the intersection point of the actual reflected light of the grating and the ideal surface shape, and then use the geometric principle of Ronchi detection to obtain the mirror surface deviation gradient along the normal direction of the aspheric surface when the point light source is off-axis.

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