JP3495861B2 - Aspherical shape measuring method and device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an aspherical surface shape measuring method and apparatus for measuring the shape and distribution of refractive index of an optical component using an interferometer.

[0002]

2. Description of the Related Art Conventionally, an interferometer, which is an apparatus for detecting the wavefront shape of light as an interference fringe pattern by utilizing the phenomenon of light interference, has been used to precisely measure the shape of optical parts such as lenses and mirrors, It is widely used for industrial purposes to accurately measure the refractive index distribution of, and recently, by digitizing the interference fringe pattern as phase information for each pixel,
Very precise measurement is possible. Then, as a method of digitizing this interference fringe pattern as phase information,
Proceedings of the 1992 JSPE Spring Conference p. 371-3
72 discloses a phase shift electronic moire method.

FIG. 6 is a block diagram of a phase shift electronic moire analysis apparatus. The output of a CCD camera 1 for inputting an interference fringe image is connected to an image input means 2 for extracting a video signal for one screen, and an image is displayed. The output of the input means 2 and the outputs of the three image memories 3a, 3b, 3c for storing information for one screen are respectively connected to three multipliers 4a, 4b, 4c for multiplying between two images. In addition, the three low-pass filters 5a, 5b, 5c having the characteristic of removing high spatial frequencies in the image and passing only low spatial frequencies
Respectively connected to. Low-pass filters 5a, 5
The output from b is a subtractor 6a that performs the subtraction between the two images.
Further, the outputs of the low-pass filters 5b and 5c are respectively connected to the subtractor 6b, and the outputs of the two subtractors 6a and 6b are 2
It is connected to a divider 7 that divides between two images, and is further connected to a non-linear calculator 8 that calculates an arc tangent.

Interference fringes generated from the interferometer are intentionally generated by inclining the reference plane to generate a large number of fringes, which are then picked up by the CCD camera 1 using an appropriate lens, and the image input means 2 displays one screen. Minute information is extracted. On the other hand, information corresponding to a large number of striped images is written in advance in each of the image memories 3a, 3b, 3c, and the initial phase of the striped lines is π for each of the image memories 3a, 3b, 3c.
It is shifted by / 4, 3π / 4, 5π / 4. The measured image signal captured by the CCD camera 1 is stored in the image memories 3a, 3
The signals of b and 3c are respectively multiplied by the images in the multipliers 4a, 4b and 4c, and a kind of moire fringe having a low spatial frequency is generated by this calculation, and the moire fringes show the degree of bending of the image fringe to be measured. It represents.

Therefore, the low-pass filters 5a, 5b, 5
By cutting the signals of a large number of fringes that are carriers in c, an image equivalent to a state in which rough interference fringes are produced without tilting the reference surface in the interferometer can be obtained. At this time, since the initial phases of the stripes in the images of the reference image memories 3a, 3b, 3c are shifted by π / 4, 3π / 4, 5π / 4, they pass through the low-pass filters 5a, 5b, 5c. The initial phase of the image corresponding to the state in which the rough interference fringes are obtained is also shifted by π / 4, 3π / 4, and 5π / 4.

When the phase distribution of the interference fringes to be obtained is φ (x, y), each image can be expressed by the following equation.

[0007]     J1 (x, y) = sin {φ (x, y) + π / 4}             = {Sinφ (x, y) + cosφ (x, y)} / √2… (1)     J2 (x, y) = sin {φ (x, y) + 3π / 4}             = {-Sin (x, y) + cos φ (x, y)} / √2 (2)     J3 (x, y) = sin {φ (x, y) + 5π / 4}             ＝ (− sin (x, y) − cos φ (x, y)} / √2… (3)

Then, the phase distribution φ (x, y) can be calculated by performing the following calculation.

(J1-J2) / (J2-J3) = sinφ (x, y) / cosφ (x, y) = tanφ (x, y) (4) φ (x, y) = tan ^-1 { (J1-J2) / (J2-J3)}… (5)

Therefore, the calculation of the equation (4) is performed by the subtractors 6a and 6b.
And the divider 7 and the calculation of the equation (5) is performed by the non-linear calculator 8, the phase distribution φ (x, y) is obtained as an output. Thus, by obtaining the phase distribution φ (x, y) from one original image of the interference fringes, it is possible to avoid the influence of mechanical vibration during measurement, fluctuation of air, and the like.

A null lens is known as a method for measuring an aspherical shape, and a normal interferometer is used for an object to be measured having a large amount of aspherical surface that makes analysis impossible due to too many interference fringes. , Null test of the measured aspherical shape is performed by using a special lens that generates a light wavefront that matches the aspherical shape by design.

[0012]

However, although the electronic moire method in the above-mentioned conventional example is excellent as the phase detecting means, in the object to be measured having a large amount of aspherical surface, the spatial frequency band of the interference fringe becomes wide. There is a problem that an undetectable area is formed, a resolution of an image pickup device is limited, and fine interference fringes cannot be resolved, and a load of image calculation is large and a cost of a processing device is high. Further, the design of the null lens is very difficult, and the null lens cannot be evaluated by the interferometer, so that there is a problem that it is extremely difficult to manufacture a perfect null lens.

It is an object of the present invention to provide a simple aspherical surface shape measuring method and apparatus for quickly and intuitively determining the quality of a work surface at a processing site where aspherical lenses and the like are stably mass-produced.

[0014]

The aspherical surface shape measuring method according to the present invention for achieving the above object is an aspherical surface shape measuring method for measuring a shape of an optical element to be measured having a large amount of aspherical surface. An interference fringe image to be measured in which an optical wavefront conversion element generates an optical wavefront that substantially matches the aspherical shape of the measured optical element, and a spatial carrier wave is generated by a large number of interference fringes, and a frequency substantially equal to the spatial frequency of the carrier wave. And a plurality of reference images having different initial phases are calculated to measure the phase distribution of the interference fringe image to be measured by an electronic moire method, and the optical wavefront generated by the optical wavefront conversion element, and By writing a minute shape error pattern corresponding to the difference between the aspherical shape of the measurement optical element, as a reference image of the interference fringe phase analysis by the electronic moire method,
It is characterized in that a null test of the aspherical shape of the optical element to be measured is performed.

Further, the aspherical surface shape measuring apparatus according to the present invention has an optical wavefront converting element for generating an optical wavefront substantially matching the aspherical surface shape of the optical element to be measured having a large amount of aspherical surface, and has a large number of interferences. An interference fringe image to be measured which produces a spatial carrier by a fringe and a plurality of reference images having substantially the same frequency as the carrier and different initial phases are calculated by electronic moire interference fringe phase analysis to obtain the measured object. In the aspherical surface shape measuring device for measuring the phase distribution of the interference fringe image, as the reference image data in the electronic moire method interference fringes phase analysis, the optical wavefront generated by the optical wavefront conversion element and the aspherical surface shape of the measured optical element. The null test of the aspherical shape of the optical element to be measured is performed by a writing unit that writes a minute shape error pattern corresponding to the difference between

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail with reference to the embodiments shown in FIGS. FIG. 1 shows a block diagram of the first embodiment. In front of a coherent optical oscillator 10 which is a light source with high coherence, a polarization conversion means 11 for arbitrarily converting the polarization state of light, and a beam diameter are enlarged. A beam expander 12 and a bending mirror 13 that changes the traveling direction of light are sequentially arranged. In the reflection direction of the folding mirror 13,
A polarization beam splitter 14 that splits light according to the polarization state of light, a quarter-wave plate 15 that reversibly converts the polarization state between linearly polarized light and circularly polarized light, and a wavefront form of light is converted into a predetermined shape. The null lens 16, the aspherical workpiece T to be measured, and the aspherical workpiece T to be measured, which are designed and manufactured as described above, are sequentially arranged with the work holders 17 that can be freely adjusted in six directions with respect to the optical axis. ing. It should be noted that T'denotes the shape of the wavefront representing the wavefront form of the light converted by the null lens 16.

In the left reflection direction of the polarization beam splitter 14, a quarter-wave plate 18, a reference mirror 19 which is highly accurately flat-polished, and the reference mirror 19 are held and the tilt can be adjusted minutely with respect to the optical axis. A reference mirror holder 20 having such a fine movement mechanism is arranged, and in the right reflection direction of the polarization beam splitter 14, a polarizer 21 for combining and interfering light of orthogonal polarization components and an image of the work T to be measured are appropriately arranged. An imaging lens 22 for forming an image of a size and a two-dimensional image detector 23 such as a CCD camera are sequentially arranged.

The output of the two-dimensional image detector 23 is image input means 24 for extracting a video signal for one screen, temporal averaging, spatial averaging, background noise removal, camera sensitivity unevenness correction, and unnecessary area masking. The output of the pre-processing means 25 is connected to the pre-processing means 25 for performing pre-processing such as , Is connected to a multiplier 27 that performs multiplication between images at high speed, and the output of the multiplier 27 is connected to a television monitor 29 that displays a processed image through a low-pass filter 28 that passes only the lines of low frequency components in the image. Has been done.

The light emitted from the coherent light source 10 is
After being adjusted by the polarization conversion device 11 so that the polarization state becomes circularly polarized light or linearly polarized light having a polarization azimuth in the vicinity of 45 degrees, the beam expander 12 expands the light beam diameter,
The bending mirror 13 changes the traveling direction and makes it enter the polarization beam splitter 14. Polarizing beam splitter 1
Reference numeral 4 separates the incident light into two linearly polarized lights which are orthogonal to each other. One of the lights is reflected by the polarization beam splitter 14 toward the reference mirror 19 as reference light, and the other light is transmitted through the polarization beam splitter 14. The light to be measured goes to the workpiece T to be measured.

After the polarization state of the reference light is converted into circularly polarized light by the quarter-wave plate 18, it is reflected by the reference mirror 19 and again converted into linearly polarized light by the action of the quarter-wave plate 18. Returning to the polarization beam splitter 14, however, since the azimuth of linearly polarized light is rotated by 90 degrees at this time, the polarization beam splitter 14 goes straight and the two-dimensional image detection device 2
Head to 3. On the other hand, the test light has its polarization state converted into circularly polarized light by the quarter-wave plate 15 and is then converted into an aspherical wavefront T ′ by the null lens 16 and reflected by the surface of the workpiece T to be measured and returned to the null lens 16. Return, 1/4 wave plate 1
By the action of 5, the polarization state is converted into circularly polarized light and returns to the polarization beam splitter 14, but at this time, since the azimuth of the linearly polarized light is rotated by 90 degrees, the polarization beam splitter 14
Reflected toward the two-dimensional image detecting device 23.

Since the reference light and the test light which are directed to the two-dimensional image detecting device 23 are lights having polarization directions orthogonal to each other immediately after they are emitted from the polarization beam splitter 14, they do not cause interference, but approximately 45 degrees from the two polarization directions. Polarizer with tilted main axis 2
Passing 1 causes the polarization directions to coincide with each other, causing interference. As a result, the two light beams that have interfered with each other strengthen each other when the phases of the light wavefronts thereof coincide with each other, and weaken each other when the phase difference is 180 degrees, so that an interference fringe pattern corresponding to the shape of the wavefront is formed, and an appropriate pattern is formed. The image is formed on the imaging surface of the two-dimensional image detecting device 23 in a state where the magnification and the image forming relationship are satisfied by the image forming lens 22.

This interference fringe image is a two-dimensional image detecting device 23.
Is detected by the image input means 24 and converted into digital data for one screen by the image input means 24, and then the preprocessing means 25, if necessary, temporal average, spatial average, background noise removal, camera sensitivity unevenness correction, unnecessary area masking. And the like are preprocessed. Then, the preprocessed image is multiplied in the multiplier 26 for each pixel corresponding to the data in the reference image memory 27 at the video rate.

CC used in a two-dimensional image detecting device now
When the horizontal direction of the D camera is x and the vertical direction is y,
If a reference image having a high spatial frequency V in the x direction is prepared, the luminance signals Ir and Is of the reference image and the input image can be expressed by the following equations.

[0024]     Ir = Asin (Vx + P) + R (6)     Is = Bsin (Wx + Q) + S (7)

Here, A and B are the reference image and the luminance amplitude of the input image, Vx and Wx are the reference image and the spatial frequency of the input image in the x direction, P and Q are the reference image and the initial phase of the input image, respectively. , R, S are reference images,
It is the bias component of the input image.

Here, when the equations (6) and (7) are multiplied by the multiplier 27, the following calculation is performed.

[0027]     I = Ir ・ Is = A ・ Bsin (Vx + P) ・ sin (Wx + Q) + A ・ Ssin (Vx + P) ) + BRsin (Wx + Q) + RS       = (A · B / 2) [cos {(V−X) x + P−Q} −cos {(V + W) x + P + Q}] + A ・ Ssin (Vx + P) + B ・ Rsin (Wx + Q) + R ・ S (8)

This expression (8) is an image containing four spatial frequencies of V-W, V + W, V, and W. If it is left as it is, it is difficult to intuitively understand because it contains a complicated fringe pattern, so that it is low-pass. Filtering is performed in the filter 28.

That is, the cut-off frequency of the low-pass filter 28 is set to, for example, n stripes / screen so that the spatial frequencies Vx and Wx in the x direction of the reference image and the input image are n stripes / screen or more. If adjusted, the action of the low-pass filter 28 cuts the frequency components of V, W, and V + W, and only the V-W signal of the difference frequency passes through the low-pass filter 28, which is observed on the television monitor 29. The image looks like this:

[0030]     Im = (A ・ B / 2) cos {(V−W) x + P−Q} + R · S (9)

In the above adjustment, the spatial frequency V in the x direction of the reference image is actually set to n stripes / screen or more according to the cutoff frequency of the low-pass filter 28, and the inspection operator sets it on the television monitor 29. This can be achieved by adjusting the fine movement mechanism of the work holder 17 or the reference mirror holder 20 of the interferometer so that the fringes appear most coarsely so that the spatial frequency W of the input image matches the spatial frequency V of the reference image. This operation, when watching the image on the TV monitor 29,
This is almost the same as the alignment work performed by an ordinary interferometer so that the interference fringes are roughly produced, and the operator can operate it without feeling discomfort.

FIG. 2 shows a block circuit configuration diagram of a low-pass filter 28 which performs a filtering process. The input video signal VI is a luminance / sync separation circuit 28a and a filter circuit 28.
b, the video signal VO is obtained via the luminance / synchronization synthesis circuit 28c, and the luminance / synchronization separation circuit 28a
The sync signal SS from is output to the luminance / sync synthesizing circuit 28c.

FIG. 3 shows a time chart of an electric signal in the filtering process. For example, in the case of an NTSC standard video signal, the horizontal scanning frequency is Fh = 15.73.
Since the frequency is 4 kHz, the low-pass filter 28 having a cut-off frequency of Fh · n may be used to filter the vertical stripes of n lines / screen. Now, 50 / n
On the screen, the cutoff frequency of the low-pass filter 28 is Fc = Fh · n = 15.734 × 50 = 786.7 kHz.
Since z may be set, the filtering process can be easily performed by configuring an appropriate electric processing circuit.

However, in the actual video signal VI, the synchronization signal
Since SS is compounded, the brightness / sync separation circuit 28
After the luminance signal BS and the synchronizing signal SS are separated in a, the luminance circuit BS filters only the luminance signal BS to obtain the luminance signal BF, and the luminance / synchronization synthesizing circuit 28c synthesizes the luminance signal BS with the synchronizing signal SS again to synthesize the video signal VO. Need to be sent to the television monitor 30.

If the object to be measured has a simple shape such as a spherical surface or a flat surface, the shape can be inspected by using a simple linear stripe pattern as a reference image. In order for the null lens 16 to generate an aspherical wave that substantially matches the aspherical shape of the workpiece T to be measured by design, it is necessary to design and manufacture the null lens 16 within the wavelength order, which is extremely small. Difficult and costly. On the other hand, if the null lens 16 is not perfect, the inspection operator will judge the quality of the work T to be measured by looking at the distorted fringe pattern, and it is impossible to make a highly accurate judgment.

Therefore, the error of the null lens 16 is evaluated in advance, and a pattern corresponding to the error is converted into reference image data and written in the reference image memory 27, so that the result is displayed as the result of the image calculation. The stripe pattern on the TV monitor 29 that is used is a pure work T to be measured.
Will represent the shape error. In this way, the inspection operator can determine the interference fringes that are measured with the ideal straight fringe pattern having no distortion, so that the inspection operator can perform the same work as the inspection of a normal spherical surface or a flat surface, and perform high-precision operation. It is possible to determine the amount of aspherical surface.

As means for preliminarily evaluating the error of the null lens 19, for example, an aspherical master work whose accuracy is assured by another measuring device is mounted on this apparatus, and at this time, it is observed through the null lens 16. It is conceivable to save the image before multiplication in the reference image memory 27 and use it in normal measurement.

The two-dimensional image detecting means 23 is not limited to a CCD camera, but a photoelectric pickup tube such as a vidicon, a one-dimensional C
It may be a CD scanning device or the like, the image input means 25 may include an interface function for matching signals with various image pickup means, and the reference image memory 28 stores a read speed slower than the video rate. Means may be used.

Further, the multiplier 26 may be replaced with an adder or subtractor when only observing Moire fringes, and the low-pass filter 28 is an analog filter including a capacitor, a resistor, an operational amplifier, and a high speed. It is also possible to use a computer-based digital filter or the like, and there is no problem even if the cutoff frequency is fixed or variable. The null lens 16 may be a special optical element such as a computer generated hologram element (CGH) that applies an optical diffraction phenomenon to generate an arbitrary light wavefront.

FIG. 4 is a block diagram of the second embodiment, and FIG.
The same symbol as represents the same member. In this embodiment, a plurality of null lenses 16a, 16b, ..., 16n for converting the wavefront form of light into respective predetermined shapes are designed and manufactured, and the null lenses 16a, 16b, ..., 16n can be stocked in the stocker 30 and supplied into the optical path as needed. It is said that. Further, the output of the preprocessing means 25 is divided into two systems by the video branching means 31 having a mode changeover switch for changing over the mode at the time of measurement, and one output is multiplied by the same processing as that of the first embodiment. 27, a low pass filter 28, and a television monitor 29 are sequentially connected. On the other hand, the other output is connected to an input image memory 32 for temporarily storing an image subjected to measurement and preprocessing, and a general-purpose computer 33 for serially performing phase analysis processing and graphic processing. Further, a data storage means 34 for storing a plurality of reference image data in advance is provided,
The output of the data storage means 34 is connected to the reference image memory 26 and the computer 33, respectively.

The generated interference fringe image is detected by the two-dimensional image detection device 23, converted into digital data for one screen by the image input means 24, and then preprocessed by the preprocessing means 25 if necessary. The pre-processed image signal is branched into an alignment mode and a detection mode in the video branching unit 31, and the reference surface and the measured surface are aligned on the a side of the video branching unit 31. It should be noted that, while the alignment by a normal interferometer is intended to roughly produce interference fringes, the electronic moire method is intended to produce a large number of fringes corresponding to a predetermined spatial frequency V. Since it is necessary to perform special processing, the alignment mode is performed on the side a of the video branching unit 31.

The image for which the preprocessing is completed is multiplied by the data in the reference image memory 26 loaded according to the type of the null lens 16 selected in advance, in the multiplier 27 for each pixel corresponding to the video rate. Done.

Similar to the first embodiment, the luminance signals Ir and Is of the reference image and the input image are expressed by the equations (6) and (7), respectively.
It is calculated by the multiplier 27 and becomes the formula (8). Here, if the cutoff frequency of the low-pass filter 28 is set to a value that is slightly lower than the spatial frequency V of the reference image, the action of the low-pass filter 28 allows only the V-W signal of the difference frequency to pass. The image observed on the television monitor 29 is given by the equation (9) as in the first embodiment. Therefore, if the interferometer is adjusted so that the stripes appear most coarsely on the television monitor 29, the spatial frequency W of the input image matches the spatial frequency V of the reference image, and the purpose of alignment in the electronic moire method is achieved. To be done.

In this equation (9), when V-W is close to 0, the phase difference P-Q between the reference image and the input image appears as a stripe, so that the wavefront error and the refractive index distribution are approximated from the stripe pattern. Can be visually recognized.

When the alignment is completed, the process moves to the detection mode on the b side of the video branching means 31. In the detection mode, the preprocessed image is stored in the input image memory 13 as 1
Only the screen is saved. The shutter speed for one screen is about 1/30 to 1/1000 seconds in a normal CCD camera, and is 1 / 1,000,000 seconds or less if a special camera or pulse emission laser light source is used. It is possible to freeze and detect interference fringes over time. In this way, if the image to be measured is captured instantaneously, the subsequent arithmetic processing may be gradually performed in the arithmetic mode.

FIG. 5 shows a flow chart of the calculation processing. The calculation mode is a general-purpose calculation processing of the data of the input image memory 32 and the plurality of reference image data stored in the data storage means 34 prepared in advance. This is a mode for processing by the computer 33.

First, in step S1, the input image data is loaded from the input image memory 32, and then the first reference image data D1 corresponding to the null lens 16a selected in step S2.
Is loaded from a part of the data storage device means 34. At this time, a striped pattern having the same spatial frequency V as the reference image data used in the alignment mode is recorded in the reference image data D1.

The luminance signals Ir1 and Is of the reference image data D1 and the data of the input image memory 32 are expressed by the following equations similarly to the equations (6) and (7).

[0049]     Ir1 = Asin (Vx + P1) + R (10)     Is = Bsin (Wx + Q) + S (11)

If the equations (10) and (11) are multiplied by the images and the low-pass filtering is performed in steps S3 and S4, the equations (8) and (9) are obtained.
Since the same calculation is established as, the brightness distribution image Im1 is calculated as follows.

[0051]     Im1 = (A · B / 2) cos {(V−W) x + P1−Q} + R · S (12)

Since the spatial frequency (V-W) in the x direction is adjusted to 0 or a sufficiently small state by the alignment mode, the brightness distribution image Im1 becomes as follows.

[0053]     Im1 = (A ・ B / 2) cos (P1−Q) + R ・ S (13)

Then, in step S5, the calculation result is temporarily stored. A plurality of reference image data are prepared in advance, and each data has a stripe pattern with the same spatial frequency distribution, but the initial phase is slightly shifted, and the following series of data is recorded. Has been done.

[0055]     Ir1 = Asin (Vx + P1) + R     Ir2 = Asin (Vx + P2) + R                     ・                     ・     Irn = Asin (Vx + Pn) + R (14)

Therefore, if the reference plane image data and the input image data are processed by the loop of multiplication and low-pass filter processing in steps S6 and S7, the same result as the expression (13) is obtained, and the following brightness is obtained. The distribution images are Im1 to Imn.

[0057]     Im1 = (AB / 2) cos (P1-Q) + RS     Im2 = (A / B / 2) cos (P2-Q) + R / S                     ・                     ・     Imn = (A ・ B / 2) cos (Pn−Q) + R ・ S (15)

The bucket method is widely known as a method for calculating the initial phase distribution of interference fringes. This is a method for calculating the initial phase of a signal that changes in a sinusoidal shape from data of three or more representative points. By appropriately selecting the phase difference of the point data, the initial phase can be obtained by simple arithmetic operations. Therefore, in step S8, the phase calculation based on the bucket method is performed.

Now, the data of each representative point is expressed by the following equation.

[0060]     I1 = Kcos (θ + φ1) + L     I2 = Kcos (θ + φ2) + L                     ・                     ・     In = Kcos (θ + φn) + L (16)

Here, K is the amplitude of the signal, L is the offset of the signal, φn is the phase difference, and θ is the initial phase to be obtained.

For example, n = 3, φ1 = π / 4, φ2 = 3
If π / 4 and φ3 = 5π / 4 are selected, the following equation is obtained.

[0063]     I1 = K cos (θ + π / 4) + L = (K / √2) (cos θ−sin θ) + L     I2 = Kcos (θ + 3π / 4) + L = (K / √2) (− cos θ− sin θ) + L     I3 = Kcos (θ + 5π / 4) + L = (K / √2) (− cosθ + sinθ) + L                                                                 … (17)

Therefore, cos θ and sin θ can be expressed by the following equations.

[0065]      cos θ = (I1-I2) / (K√2)      sin θ = (I3−I4) / (K√2) (18)

The initial phase θ is given by the following equation.

Θ = tan ⁻¹ (sin θ / cos θ) = tan ⁻¹ {(I3−I2) / (I1−I2)} (19)

In this way, the influence of the amplitude K and the offset L can be eliminated, and the initial phase θ can be obtained.

An appropriate phase difference φ corresponding to n = 4, 5, ...
It is known that by giving n, the initial phase θ can be calculated by simple four arithmetic operations and tan ⁻¹ operation. Generally, the larger n is, the higher the measurement accuracy becomes. Therefore, the phase distribution can be calculated by the phase calculation based on the similar bucket method from the intermediate data of the present embodiment represented by the equation (15).

Further, the phase distribution calculated by the calculation of tan ⁻¹ can express only the range of ± π / 2. Therefore, if there is a phase distribution that exceeds this range, the originally supposedly smooth data looks discontinuous, so in step S9, discontinuity elimination processing called phase splicing processing is performed. Further steps
In S10 and S11, if necessary, visualization processing such as stereoscopic drawing, contour drawing, arbitrary section display, etc., and data reduction processing such as orthogonal polynomial fitting, etc., are displayed as information that can be understood by the measurer.

When measuring an aspherical workpiece T to be measured having a different design shape, the null lens 16 is replaced with an appropriate one, and the reference image data corresponding thereto is stored in the data storage means 34.
From the computer 33 to the reference image memory 26, and the measurement may be performed in the same procedure.

Also in this embodiment, the two-dimensional image detecting means 23 may be a photoelectric image pickup tube such as a vidicon or a one-dimensional CCD scanning device, and in the detection mode after alignment is completed, these photoelectric conversion means are used. It is also possible to use a video device that records the captured image, or data that is captured by a silver salt camera and read by a scanner device or the like and converted into an electrical signal. Further, the video branching means 31 may be a switch for switching the input image between the alignment side and the detection side instead of the distributor, and the data storage means 34 for storing the reference image data may be a semiconductor memory device, a hard disk device, a magneto-optical storage device, A magnetic tape recorder or the like can be used.

[0073]

As described above, the aspherical surface measuring method and apparatus according to the present invention can measure the spatial frequency of the interference fringes to a level at which an object having a large amount of aspherical surface can be observed using the optical wavefront conversion element. By applying the electronic moire method of displaying the error of the optical wavefront conversion element used as the reference image after reducing the distribution range, the quality of the shape can be intuitively determined, and the optical wavefront conversion element used Since it is not necessary to manufacture with high precision, an inexpensive system can be obtained. Also, by connecting an appropriate data analysis computer and performing appropriate calculations, it is possible to display the shape error of the aspherical workpiece as numerical data, and it is necessary to measure more aspherical shapes. Even at a certain time, it is possible to form a system that can be measured simply by replacing the optical wavefront conversion element according to each shape and calling the corresponding reference image data in which the error of each optical wavefront conversion element is stored in advance. be able to.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a first embodiment.

FIG. 2 is a block circuit configuration diagram of a low-pass filter.

FIG. 3 is a time chart diagram of a filtering processed signal.

FIG. 4 is a configuration diagram of a second embodiment.

FIG. 5 is a flowchart of a binarized data process.

FIG. 6 is a block circuit configuration diagram of a conventional example.

[Explanation of symbols]

10 Coherent optical oscillator 11 Polarization conversion means 14 Polarizing beam splitter 15, 18 1/4 wave plate 16, 16a to 16n Optical wavefront conversion element 19 Reference mirror 21 Polarizer 23 Two-dimensional image detection means 24 Image input means 25 Pretreatment means 26 Reference image memory 27 multiplier 28 Low-pass filter 29 TV monitor 31 video signal branching means 32 Input image memory 33 computers 34 Data storage means

Claims (6)

(57) [Claims] 1. An aspherical surface shape measuring method for measuring a shape of an optical element to be measured having a large amount of aspherical surface, wherein an optical wavefront converting element generates an optical wavefront that substantially matches the aspherical surface shape of the optical element to be measured. , An interference fringe image to be measured which produces a spatial carrier wave by a large number of interference fringes, and a plurality of reference images having a frequency substantially equal to the spatial frequency of the carrier wave and having different initial phases, The phase distribution of the interference fringe image is measured by an electronic moire method, and a minute shape error pattern corresponding to the difference between the optical wavefront generated by the optical wavefront conversion element and the aspherical shape of the optical element under measurement is measured by the electronic An aspherical surface shape measuring method characterized by performing a null test of an aspherical surface shape of an optical element to be measured by writing it as a reference image for interference fringe phase analysis by the Moire method. 2. The null lens having a structure in which one or more spherical lenses are combined as the optical wavefront conversion element.
The aspherical surface shape measuring method described in. 3. The aspherical surface shape measuring method according to claim 1, wherein the optical wavefront conversion element is a computer generated hologram element. 4. An interference to be measured which has an optical wavefront conversion element for generating an optical wavefront substantially matching the aspherical shape of the optical element to be measured having a large amount of aspherical surface, and which produces a spatial carrier wave by a large number of interference fringes. An aspherical surface for measuring the phase distribution of the interference fringe image to be measured by calculating a fringe image and a plurality of reference images having substantially the same frequency as the carrier wave and different initial phases by electronic moire method interference fringe phase analysis. In the shape measuring apparatus, as reference image data in the electronic moiré method interference fringe phase analysis, a minute shape error pattern corresponding to the difference between the optical wavefront generated by the optical wavefront conversion element and the aspherical shape of the measured optical element. An aspherical surface shape measuring apparatus, characterized in that a nulling test of the aspherical surface shape of the optical element to be measured is performed by a writing means for writing. 5. The null lens having a configuration in which one or more spherical lenses are combined as the optical wavefront conversion element.
The aspherical surface shape measuring apparatus according to. 6. The aspherical surface shape measuring apparatus according to claim 4, wherein the optical wavefront conversion element is a computer generated hologram element.