JPS59116522A - Method for checking optical aspherical surface - Google Patents

Abstract

PURPOSE:To improve reading accuracy of Ronchi fringes and to measure the abberation of a nonspherical surface optical element quantitatively, automatically, and highly accurately, by intermittently moving an optical grating, which is arranged at or near the focal point of the nonspherical surface to be checked, detecting the optical intensity of a plurality of interference fringes, which are yielded at least at one fixed point on a projected surface, and judging the unevenness at the position of nonspherical surface based on the change in optical intensity with respect to a reference value. CONSTITUTION:Light from a point light source 3 is made to be parallel light 5 through a collimator 4. The parallel light is projected on a nonspherical surface lens 1 to be checked. A Ronchi grating 2 is placed at the position of paraxial focus. The Ronchi grating 2 is moved by a stage 6, which is controlled by a pulse motor, and the phase of the Ronchi grating 2 is changed. The Ronchi pattern is detected by a TV camera or a two dimensional image sensor 7, and undergoes A/D conversion by an image memory 8. The result is inputted to a computer 9. The phase of the Ronchi grating is changed and N sheets of the Ronchi patterns are recorded. Fringes are analyzed by a Fringe Scanning method. Then a traverse abberation pattern (linear differential of wavefront abberation) of the nonspherical surface lens 1 to be checked is obtained. The wavefront abberation is obtained by the integration of the patterns.

Description

[Detailed description of the invention] This relates to surface inspection methods.

The Ronchi inspection method places a curved grating at or near the focal point of the optical element under test, and analyzes the distorted lattice pattern (Ronchi fringes) observed from a distance to determine the presence of aberrations and the quality of the polished surface. This method is applied to the measurement of optical systems with relatively large aberrations of +0, and the grating used at that time is characterized by being coarse and wavy.

Figure 1 is an explanatory diagram of this conventional Ronchi inspection method. In the optical system shown in the figure, the (ξ, η) plane is the exit pupil plane 1 of the optical element (lens) to be tested, and the A (xy) plane is provided, and a linear grating 2 is placed there, and the exit pupil plane 1 of the optical element to be tested is viewed from behind through the linear grating. The first λ
Patterns such as those shown in Figures (A), (B), (C), and (D) are observed.

Here, Fig. 2 (Al is a Ronchi fringe that is seen when the test lens has spherical aberration and the grating is placed on the pointless surface of the test lens; Fig. 2 (B), (C), ( D) is the Ronchi density obtained when the position of the grating is moved forward in the direction of the lens to be tested, and this is the result of the grating used.

In this way, the conventionally known Ronchi inspection method has a large aberration, a coarse lattice, and the results of a somewhat complex analysis are shown below, assuming that the dryness phenomenon can be prevented.

Exit pupil plane 1 of the optical element to be tested according to the arrangement shown in Figure 1.
has a wavefront aberration of W(ξ, η), the exit pupil plane (
ξ, η) can be approximately expressed as αW x αξ R (11), where x, y) is the intersection of the lattice plane and the light beam, and R is the radius of curvature of the wavefront. Further, assuming that the grid lines are parallel to the y direction and the transmittance distribution of the grid is sinusoidal, the grid is displayed as follows. Here, d is the grating pitch and γ is the fringe contrast. Therefore, if we assume that the ray emitted from one point (ξ, η) of the exit pupil is like a lattice HK on the lattice plane (X%V), we obtain from (I) and (3), (The fringes were analyzed as an equal-height itiI pattern of the first derivative (lateral aberration) of wavefront aberration.) Therefore, the lateral aberration distribution can be obtained from the analysis results of the Ronchi vertical pattern, and by integrating this, , wavefront aberration can be obtained. Therefore, the viscosity of Ronchi horizontal αW is dependent on the accuracy of calculating from equation 7.

Further, the precision of the fringe peak position in Equation 9 is expressed as follows, and therefore αWd (6) αξ R In the conventional Ronchi inspection method, only information on the fringe peak was used. For this reason, there was a drawback that the sign (unevenness) of the lateral aberration distribution could not be analyzed from the Ronchi pattern of the fringe peaks. In the Ronchi inspection method for T9-following buds, Ronchi fringes were generated using a large grid, so the line spacing of the generated Ronchi fringes was coarse. It was impossible to determine the distribution.

The purpose of the present invention is to modulate the Ronchi fringe by moving a straight-static one-curve grating pattern in the vertical direction along the optical axis, thereby improving the reading accuracy of the Ronchi fringe and eliminating aberrations of aspherical optical elements with extremely high precision. This is a method that provides a quantitative and automatic measurement method.

The purpose of this is to intermittently move an optical grating placed at or near the focal point of the aspherical surface to be tested according to the present invention, so that at least one fixed point on the projection surface is touched by t. The light intensity of the generated number of interference fringes is detected, and the base 71'5
From the change in light intensity with respect to 11C1, determine the irregularities in the aspherical I☆ scraps corresponding to the above-mentioned 1-1 fixed point, and
The reference value of Vanguard 1 represents the position of the optical grating in front of the intermittent shifter with respect to the optical axis of the aspheric surface to be tested, and the intermittent shift%! at least one fixed point on the projection surface! Detect the light intensity of the interference fringes of the Neri touch caused by This is achieved by determining the unevenness at the position of the aspherical lens corresponding to the other toy fixed point.

Book 4 Analyze the principles and characteristics of Western Mei in detail.

In the Ronchi inspection method shown in Figure 1 (explanatory diagram of the conventional Ronchi tS! inspection method), an optical grating 2 placed at or near the focal point of the aspheric surface to be inspected is placed in the optical axis direction and vertical direction ((and direction)). When the vassal is transferred to T, the lattice becomes /(X1δ)-1+r CO9
It can be written as “” (x 10 δ) (7). It is assumed that the difference δ is caused by the movement of the lattice. Increment δ into equal parts of the grid spacing d. = −n (n = 011'−N −1)
(9!δ.If you move the grid intermittently by v,
The phase of the g-th Ronchi fringe changes, and various patterns can be obtained. In other words, as mentioned above, in the conventional method, fringes are analyzed by focusing only on the peak position of the Ronchi fringes. In the present invention, the Ronchi fringe is analyzed with extremely high accuracy by analyzing the change in the Ronchi fringe due to the phase change of the grating.

Figure 3 is an example of Ronchi fringes of an aspherical lens, (a)
From (n is the phase of the grating).The intensity distribution of one cross section of this Ronchi fringe pattern (A A' cross section in Fig. 3) is set as +'61', and the grating phase δ is calculated. If you display it as a meter, you will get a rising and falling diagram.If you focus on a specific position on the AA' cross section and display the intensity change at that point as a phase δ and/or a parameter, you will get g3[n1.The Ronchi 11 intensity is As shown by Equation 70, it changes sinusoidally with respect to the change in n.Therefore, from the curve in this figure, the first cry 1 phase of the sine number Ni can be found.

Let this initial phase be δ. Then, Than The four-yellow aberration can be found as

If this operation is performed for each point on the cross section of AA', t[1
There is an aberration distribution, and by integrating this, the aberration circumference of the AA' section can be obtained.

An example of this procedure Wj' can be expressed mathematically as follows. First, by Fourier expanding the first θ equation with respect to Detected, and its fringe intensity corresponds to the lattice shift mode)...N
-1), calculate the total sum, and based on that, the differential of the amount of wavefront aberration is found according to the first equation.

The method described above calculates the amount of phase change of the grating using the first g equation,
If the phase quality of the Ronchi fringe is determined and expressed with respect to location x, Figure 4 is obtained. Figure 4 is “tan” due to scheduling reasons.
” is obtained from 0 to 2π, so from 0 to 2π
It has been folded into divisions. If the phase jump of 2π of this folded curve is corrected by phase 1, FIG. 7 is obtained. FIG. 7 shows the loquat aberration curve of the 1j constant optical element. By summing this curve, the g-th 1st aberration report can be obtained. In Figure 3, eKa4 is the share of the curve in Figure 7, which is Ti
Contains 1t of components. If you remove this, the third
Get the figure's downfall.

The method described above (well, the method was to find the sum aberration from the change in the Ronchi fringe by knowing the phase digitization of the grating, but by focusing on two points in the Ronchi fringe pattern, one point By using this point as a reference point and knowing the strength ossification of Ronchi at one point with respect to this point, the small aberration ratio can be determined. Figure 1 is an explanation of such a method.
1 and X''X2, the 51st yen is obtained.

From FIG. 3, it can be seen that the cracking strength at any point changes periodically. To find the phase difference between these two curves, for example, you can D/A convert these two signal data and input it to an analog phase meter to find the phase difference, or you can calculate the action of this analog phase meter by deinotal calculation. A method for determining the phase difference using the book 11. : There is a method of approximating the sine function y using the submultiple method and using the phase of the sine 1η1 þ'J as the phase difference between the two.

Construction) (S is a block diagram of a device for implementing the present invention. Light from a point light source 3 is sent to a collimator 4; i+i L
, No. F line light 5, this parallel light is taken as an IIF aspherical lens 1, and a Ronchi grating 2 is placed at the paraxial position jf-t. This Ronchi grating 2 is turned +hh by a stainless steel rotor controlled by a pulse motor to change the phase of the Ronchi grating 20. The Ronchi pattern is detected by a TV camera or a two-dimensional image sensor 7, A/D converted by a 1LIII image memory 8, and input to a computer 9.

By changing the phase of the Ronchi grating 20, recording an Nh Ronchi pattern, and inverting it using the Fringe, 5can+ning method, the tffj aberration pattern (first-order differential of wavefront convergence) of the aspherical lens 1 to be tested can be found. By integrating this, the wave seizure difference can be found.

In addition, compared to the conventional Ronchi method, the present invention has higher accuracy in removing aberrations, and can also determine orders of 6, . ) is a clear long, and the wavefront aberration introduced by integrating these is also crystal clear.

[Brief explanation of the drawing]

Figure 17 is an explanatory diagram of the conventional Ronchi testing method.
(A) (s) (C) (ol is the Ronchi fringe obtained by the conventional method. Figure 3 is the Ronchi fringe obtained by applying the present invention to an aspherical lens. Figure 2 is one of the Ronchi fringes. The intensity distribution of the cross section is set at 6111, and the lattice position ona (δ is expressed as a parantha).The P S diagram is a diagram that focuses on a specific position ff'? In the figure shown in Figure 6, the phase term of the Ronchi fringe is calculated and displayed by raying it at the location X.
Phase aberration curve of 11+, l constant element. Fig. 3 shows the aberration curve e-〇 Fig. 9 is a block diagram for carrying out the present invention. (1 in the figure is the exit row 1 of the optical element) and 2 is the Ronchi grating,
3 is an imaginary light source, 4 is a collimator, 5 is a plane light, 6 is a stage, 7 is a TV camera or a two-dimensional image sensor 18
is an image memory, and 9 is a computer. ) 7) Figure 1 Engraving of the purchased drawing ζ No change in content) Figure 2 (△) (8) Engraving of the drawing (L with no change in content) (C) Figure 5 Figure 6 Ii1 Patent Office located Mr. Commissioner■, Indication of the case 1962 Patent application No. 22?7
．． 2♂ No. 2, Title of invention Inspection method for optical aspheric surfaces 3
, Relationship with the person making the amendment Applicant name (679) RIKEN 4, Agent 5, Date of amendment order Voluntary

Claims (1)

[Claims] /) An optical grating placed at or near the focal point of the optical aspherical surface to be tested is moved intermittently (IA1), and at least seven fixed points on the projection surface are It is characterized by detecting the light intensity of the interference fringes of the number of rice grains produced, and cutting out the unevenness at position # of the optical aspherical surface corresponding to the above-mentioned P.1 fixed point by 1'41 from the change in light intensity with respect to the quasi-value. 2) The above-mentioned reference value represents the position of the optical grating of the intermittent transition 'Mh rIiJ with respect to the optical peak of the flannel-like optical aspheric surface. The method for inspecting an optical aspheric surface according to item 1. 3) Calculate the light intensity of multiple interference fringes caused by intermittent movement at two identified points on the projection surface, use the change in light intensity at one fixed point as a reference value, and fix the other with respect to this reference value. The optical aspherical surface inspection method according to claim 1, wherein irregularities in the activation of the optical aspherical surface corresponding to the other fixed point are determined from changes between the wide light valves.