JPH0240177B2 - KOGAKUHIKYUMENNOKENSAHO - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical aspheric surface inspection method that is an improved Ronchi inspection method.

The Ronchi inspection method is a method in which a linear grating is placed at or near the focal point of the optical element to be tested, and the distorted lattice pattern (Ronchi fringes) observed from behind is analyzed to check for aberrations and the quality of the polished surface. It is applied to measurements of optical systems with relatively large quantities, and is characterized by the fact that the grating used at that time can be coarse.

Figure 1 is an explanatory diagram of such a conventional Ronchi inspection method. In the optical system as 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, 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. As a result, Figure 2 A, B, C,
A pattern like D is observed.

Here, Figure 2A shows the Ronchi fringes that are seen when the test lens has spherical aberration and the grating is placed on the focal plane of the test lens, and Figure 2B, C, and D show the Ronchi fringes for the test lens. These are the Ronchi fringes obtained when the position of the grating is advanced in the direction of the lens under test, and these are the shadows of the grating used.

In this way, the conventionally known Ronchi inspection method is geometrically analyzed on the assumption that the amount of aberration is large, the grating is coarse, and the interference phenomenon can be ignored, and the results will be shown below.

When the exit pupil plane 1 of the optical element to be tested has a wavefront aberration of W (ξ, η) according to the arrangement shown in FIG.
The ray exiting from the exit pupil plane (ξ, η) can be approximately expressed as ∂W/∂ξ=−x/R (1) ∂W/∂η=−y/R (2). Here, (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 expressed as f(x)=1+γ·cos2π/dx (3). Here, d is the grating pitch and γ is the fringe contrast. Therefore, assuming that the ray emitted from one point (ξ, η) of the exit pupil hits the grid line on the grid plane (x, y), from (1) and (3), f(x) = 1 + γ・cos2π/ d・R・αW/αξ(4) As shown above, it has been analyzed that the Ronchi fringe is a contour pattern of the first derivative of wavefront aberration (transverse aberration).

Therefore, the lateral aberration distribution can be obtained from the analysis result of the Ronchi fringe pattern, and by integrating this, the wavefront aberration can be obtained. Therefore, the accuracy of the Ronchi test depends on the accuracy of determining ∂W/∂ξ from the fourth equation.

Furthermore, the fringe peak position in the fourth equation is expressed as follows: 2π/d・R・∂W/∂ξ=2πm (m=0±1) Therefore, ∂W/∂ξ=d/Rm (6 ), we can find the differential ∂W/∂ξ of W(ξ, η).

As can be seen from such conventionally known geometrical analysis, the conventional Ronchi inspection method uses only information about fringe peaks. 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. Furthermore, since the conventional Ronchi inspection method uses a coarse grating to generate Ronchi fringes, the fringes of the Ronchi fringes that are generated are coarse, making it impossible to precisely determine the lateral aberration distribution by analyzing the Ronchi fringes. It was hot.

The purpose of the present invention is to modulate the Ronchi fringes by moving a linear grating pattern perpendicular to the optical axis, thereby improving the reading accuracy of the Ronchi fringes, and to quantitatively and automatically detect aberrations of aspherical optical elements with extremely high precision. The objective is to provide a method for measuring

This purpose 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, and to eliminate a plurality of interference fringes caused by the intermittent movement at at least one fixed point on the projection surface. The light intensity is detected, and the sign of the lateral aberration at the position of the aspherical surface corresponding to the fixed point is determined from the change in the light intensity with respect to the reference value. It represents the position of the optical grating before movement, and also detects the light intensity of multiple interference fringes caused by intermittent movement at at least two fixed points on the projection surface, and detects changes in the light intensity at one fixed point. As a reference value, measurement of the lateral aberration is achieved by determining the sign of the lateral aberration at the position of the aspherical lens corresponding to this other fixed point from the change in light intensity at the other fixed point with respect to this reference value. .

The principles and features of the present invention will be analyzed in detail.

In the Ronchi inspection method shown in Figure 1 (explanatory diagram of the conventional Ronchi inspection method), when the optical grating 2 placed at or near the focal point of the aspheric surface to be tested is moved perpendicular to the grating line in a direction perpendicular to the optical axis direction. , the lattice can be written as f(x ₁ δ)=1+γcos2π/d(x+δ) (7). However, δ is a position term caused by the movement of the grating. Therefore, the Ronchi fringe is f(x, δ)=1+γ・cos2π/d(R∂W/∂ξ+
δ) (8) can be written. The lattice spacing is divided into 1/N equal parts of d, and δ _o = d/Nn (n = 0, 1...N-1) (9) If the lattice is moved intermittently by δ _o , the Ronchi of Equation 8 is obtained. The phase of the fringe changes, resulting in various fringe patterns. In other words, f (x, δ _o ) = 1 + γ・cos (2π/d・R ∂W (ξ, η)/∂ξ+2πd/N・n) (10) Desired information ∂W is obtained from these many Ronchi fringes with different phases. Extract (ξ, η)/∂ξ.

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 shows an example of Ronchi fringes of an aspherical lens, and (a) to (f) show that the phase of the grating is changed. A cross section of this Ronchi stripe pattern (in Figure 3
When the intensity distribution of the AA' cross section is measured and the grating phase δ _o is expressed as a parameter, Figure 4 is obtained.
If we focus on two specific points on the AA' cross section and express the intensity changes at those points using the phase δ _o as a parameter, we obtain FIG. 5. As shown by Equation 10, the Ronchi fringe intensity changes sinusoidally with respect to the change in n. Therefore, the initial phase of the sine function can be found from the curve in this figure. If this initial phase is δ _p , then 2π/dR∂W (ξ, η)/∂W=δ _p (11) From ∂W (ξ, η)/∂ξ=δ _p /2π・d/R (12 ) can be used to find the lateral aberration.

By performing this operation for each point on the cross section of AA', a transverse aberration distribution can be obtained, and by integrating this, an aberration curve of the cross section AA' can be obtained.

An example of this procedure can be described mathematically as follows. First, by Fourier expanding equation 10 with respect to x, f(x, δ _o )=a ₀ /2+ _∞ 〓 ^r=1 a _r・cos2π/N ・r+ _∞ 〓 ^r=1 b _r sin2π/N・r (13 ) However, a _r = 2/N _N-1 〓 ^r=0 f(x, δ _o )・cos2π/N (14) b _r = 2/N _N-1 〓 ^r=0 f(x, δ _o )・sin2π/Nr (15) Therefore, a ₁ = γ・cos2π/d・R∂W (ξ, η)/∂ξ (16) b ₁ = γ・sin2π/d・R∂W (ξ, η)/∂ ξ (17) Therefore ∂W (ξ, η)/∂ξ=1/2π・d/Rtan ^-1 b ₁ /a ₀
(18) ∂W(ξ, η)/∂ξ is obtained.

What these mean is that, as shown in Equation 9, the grating is intermittently vibrated by an amount equal to 1/N of one period d of the grating, and the light intensity of the Ronchi fringe obtained in each case is detected. Then, the fringe intensity is given a weight amount cos2π/Nr or sin2π/Nr (r=0
, 1, 2...N-1), calculate the total sum, and then calculate the differential of the amount of wavefront aberration based on Equation 16.

In the method described above, if the phase peak of the Ronchi fringe is obtained from the amount of phase change of the grating using Equation 18, and expressed with respect to the location x, FIG. 6 is obtained. In Fig. 6, tan ^-1 is collapsed into the range from 0 to 2π for convenience of calculation, since the principal values are obtained from 0 to 2π.
Figure 7 is obtained by correcting the 2π phase jump of this folded curve. FIG. 7 shows the lateral aberration curve of the optical element to be measured. By integrating this curve, the aberration curve shown in FIG. 8 is obtained. The broken line in FIG. 8 is the integral of the curve in FIG. 7 and includes the Tilt component. When this is removed, the solid line in FIG. 8 is obtained.

The method described above was to find the lateral aberration from the change in the Ronchi fringe by knowing the amount of phase change in the grating, but by focusing on two points on the Ronchi fringe pattern and using one point as a reference point. By knowing the intensity change of the Ronchi fringe at one point with respect to this point, the amount of transverse aberration can be determined. FIG. 4 is an explanatory diagram of such a method, and when the Ronchi fringe degree at two points x=x ₁ and x=x ₂ is illustrated, FIG. 5 is obtained.

It can be seen from FIG. 5 that the fringe intensity 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 measure the action of this analog phase meter using a digital computer. Possible methods include a method of calculating the phase difference instead, or a method of approximating these two signals with a sine function using the method of least squares and using the phase of the sine function as the phase difference between the two signals.

FIG. 9 is a block diagram of an apparatus for implementing the present invention. Light from a point light source 3 is passed through a collimator 4 to form parallel light 5, and this parallel light is applied to an aspherical lens 1 to be tested, and a Ronchi grating 2 is placed at the paraxial focal position. This Ronchi grating 2 is moved by a stage 6 controlled by a pulse motor to change the phase of the Ronchi grating 2. The Ronchi pattern is detected by a TV camera or a two-dimensional image sensor 7, A/D converted by an image memory 8, and input to a computer 9.

By changing the phase of the Ronchi grating 2, recording N Ronchi patterns, and performing fringe analysis using the method described after Equation 13, the transverse aberration pattern (first-order differential of wavefront aberration) of the aspherical lens 1 to be tested can be determined. By integrating this, the wavefront aberration can be found.

As described above, the present invention has a feature that the accuracy of reading the amount of aberration is higher than that of the conventional Ronchi method, and that it is also possible to determine fringe orders other than the peak position of Ronchi fringes (interpolation of fringes is possible). Therefore, the measurement accuracy of the fracture surface aberration obtained by integrating these values is also high.

[Brief explanation of drawings]

Figure 1 is an explanatory diagram of the conventional Ronchi test method, Figure 2
Figures A, B, C, and D are Ronchi fringes obtained by the conventional method. Fig. 3 shows Ronchi fringes obtained by applying the present invention to an aspherical lens, Fig. 4 shows the intensity distribution of one cross-section of the Ronchi fringes measured and the grating phase δ _o is displayed as a parameter, and Fig. 5 The figure shows intensity changes at two specific positions in a cross section expressed using the phase as a parameter, Figure 6 shows the phase peak of the Ronchi fringe and displays it with respect to location x, and Figure 7 Transverse aberration curve of the device under test. 8th
The figure shows the aberration curve. FIG. 9 is a block diagram for realizing the present invention. (1 in the figure is the exit pupil plane of the optical element,
2 is a Ronchi grating, 3 is a point light source, 4 is a collimator, 5 is a flat light beam, 6 is a stage, 7 is a TV camera or two-dimensional image sensor, 8 is an image memory, and 9 is a computer. )

Claims (1)

[Claims] 1. An optical grating placed at or near the focal point of an optical aspherical surface to be tested is moved intermittently, and a plurality of interference fringes generated by the intermittent movement are generated at at least one fixed point on the projection surface. 1. A method for inspecting an optical aspherical surface, comprising detecting light intensity and determining irregularities at a position of the optical aspherical surface corresponding to the fixed point based on a change in the light intensity with respect to a reference value. 2. The method for inspecting an optical aspherical surface according to claim 1, wherein the reference value represents the position of the optical grating before intermittent movement with respect to the optical axis of the optical aspherical surface to be inspected. 3 Detect the light intensity of a plurality of interference fringes caused by intermittent movement at at least two fixed points on the projection surface, use the change in light intensity of one fixed point as a reference value, and calculate the change in the light intensity of the other fixed point with respect to this reference value. The method for inspecting an optical aspherical surface according to claim 1, wherein the unevenness at the position of the optical aspherical surface corresponding to the other fixed point is determined from the change in light intensity.