JPH0466832A - Measuring instrument of surface separation of lenses - Google Patents

Abstract

PURPOSE:To enable measurement of surface separation between component lenses in a state wherein a plurality of lenses are incorporated, by constructing an instrument so that a lens system or a convergent light can be transferred freely and by measuring a distance of transfer of the lens system or the convergent light whereat wavefront aberration is minimum. CONSTITUTION:A light emitted from a laser 1 is turned to be a parallel light by a collimator 6 and falls on a reference lens 7, and it is partly transmitted through the end surface of the lens 7 and partly reflected thereby. A convergent light transmitted is reflected at the vertex of a lens 8 to be inspected, falls on the lens 7 and interferes with a reflected light. An interference light is imaged on a CCD camera 10 and an interference image thereof is analyzed by a computer 15 through a frame memory 12. By making a motor driver 19 drive a stage 20 to transfer a lens system in the direction of the optical axis and by displacing a piezo-transducer 18 minitely, the computer 15 determines wavefront aberration, makes a length measurer 21 measure a distance of transfer of the lens system whereat the wavefront aberration becomes minimum, and measures surface separation between lenses on the basis of a measured value of the distance.

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention uses an interferometer that measures the surface spacing (air spacing) between each constituent lens with a plurality of lenses assembled in a lens frame. This invention relates to a surface distance measuring meter for lenses.

[Background of the Invention] When a single lens system is constructed by combining multiple lenses, in order to construct a high-precision lens system, the spacing (planar spacing) between the constituent lenses constituting the lens system must also be given preferential design benefits. It must be the same.

The following methods are known as methods for measuring this interplanar spacing.

■ Before incorporating multiple lenses, measure the radius of curvature, axial thickness, etc. of each lens using a known method.

■ Measure the shape of the lens frame (place 1, diameter of the abutting surface), diameter, length, etc. of the spacer, etc. Measure the surface spacing (air spacing) between the lenses (constituent lenses) by calculating from ■ and ■. .

For example, to find the surface distance t2 between the first lens 30 and the second lens 32 in a simple two-element lens as shown in FIG. 6, use the radius of curvature r2 of the surface of the second lens 30 on the second lens 32 side The inner diameter Ds of the spacer 34, the length ts of the spacer 34, the depth S3 of the surface of the second lens 32 on the second lens 32 side, and from these, first 52=r2+r2-(Ds,/21 2(
r2<OS2<O) is calculated, and then the surface spacing t2 t2=ts-33+52 (S3<0) is calculated.

However, the sign is the radius of curvature r of the first surface (1 is an integer greater than or equal to 2)
Regarding i, it is assumed that when the center of curvature is on the left side of each surface, it is negative, and the length in the other optical axis direction is measured with the vertex of each surface as the origin, and the right side is positive.

In addition, in FIG. 6, 36 is a main lens frame, and 38 is a holding ring.

[Problems to be Solved by the Invention] In this way, even for a simple lens frame, in order to measure the surface spacing of the lens system, the necessary dimensions of the constituent lenses and lens frame components are measured, and the final surface distance is measured. To find the interval, you have to do complicated calculations while paying attention to the sign.

Therefore, as the number of lens elements increases, the surface spacing calculation process becomes significantly complicated.

In addition, for lenses with variable lens spacing, such as zoom lenses, it is also necessary to measure the three-dimensional shape of the cam, the position of the bins, etc., and as a result, it is practically possible to determine the surface spacing of this type of lens system. is extremely difficult.

Therefore, this invention solves these conventional problems, and provides a lens that makes it possible to measure the interplanar spacing between constituent lenses in a state where multiple lenses are incorporated, without having to measure the lens frame or the like. This paper proposes a surface distance measuring meter.

[Means to solve the problem]

In order to solve the above-mentioned problems, the present invention includes a means for generating test light that is convergent light, and a test lens incorporating a plurality of component lenses or a convergence point of the convergent light, which is moved in the optical axis direction of the test lens. means for measuring the amount of movement thereof; means for detecting that the convergent light is reflected at the apex near the apex of each surface of the lens constituting the test lens; and the means for measuring the amount of movement and detection. Determine the convergence point or the apex reflection position on each surface of the lenses constituting the test lens from the means, and measure the surface spacing between these constituent lenses from this and the radius of curvature and refractive index of the constituent lenses of the test lens. The device is characterized in that it is comprised of a surface distance measuring means.

[Function] The radius of curvature, aspherical coefficient, and refractive index of each component lens surface are known values because they are measured by other means.

The distance at which vertex reflection occurs when the lens is moved is detected based on the amount of wavefront aberration from the interference fringes, and the distance S of lens movement obtained at this time and the initial surface spacing t of the lens are determined from the known radius of curvature and refractive index. is calculated.

Regarding the next lens, the surface spacing t of this lens is calculated from the movement amount S obtained for this lens, the radius of curvature, the refractive index of that lens, and the data of the surface spacing t of the previous lens.

This process is executed up to the last lens.

Since the position of the vertex reflection is detected by an interferometer, the interplanar spacing of the constituent lenses can be measured while the lens is still installed.

[Example 1] Next, an example of a lens surface distance measuring meter according to the present invention will be described in detail with reference to FIG. 1 and subsequent figures.

First, the principle of lens surface distance measurement related to this invention will be explained in the second section.
This will be explained with reference to the figures.

If we ignore aberrations for now, the paraxial theory of lenses holds true.

Therefore, for example, the paraxial theory of lenses described in "R Lens Design Method J" (Kyoritsu Shuppan Yoshiya Matsui, published November 5, 1972, Chapter 2) is used as the basis.

We formulate a paraxial theory for a lens system using multiple lenses in a similar manner to that described in this paper.

Now, it is assumed that the radius of curvature, axial thickness, and surface spacing (air spacing) of each lens from the first surface to the first surface (1 is an integer greater than or equal to 2) are known.

At this time, if the first surface to the first surface is considered as one lens system, the paraxial quantities such as the focal length, principal point position, and overall length of this lens system can be determined from the above-mentioned theoretical basis.

Let Pi, Pi', and Fi' be the object-side principal point, image-side principal point, and image-side focal point of the lens system from the first surface to the i-th surface, respectively. Also, fi composite focal length M of the lens system from the first surface to the first surface (image side focal point) pl from the first surface of the lens system from the first surface to the principal point on the convergent light incident side Distance (object-side principal point distance) 21' Distance from the first surface of the lens system from the first surface to the i-th surface to the principal point on the opposite side to the convergent light incidence (image-side principal point distance).

Let X be the point where the incident light intersects the optical axis, and let Si be the distance from the first surface of the lens to X.

At this time, the distance 11i between the object-side principal point Pi of the lens and xt
ai is expressed as ai=Si−pi.

If bi is the distance from the image-side principal point Pl' of the lens to the point (i+1) where the ray exiting the lens system intersects the optical axis, then from the image formation formula based on the principal point p1, ni is the i-th surface. As the refractive index of the medium between the
When ti, the ray returns along the optical path that it also traveled at the vertex of the i+1th surface.

Therefore, from the formula (2), ni/1l=1Zai+1/f1' (3) Therefore, the interplane distance ti between the first surface and the i+1th surface is as follows, and tl can be found.

Here, fi', pi, and pi' mentioned above are the radius of curvature rj (j=1 to 1) of each lens surface from the first surface to the first surface.
, the surface spacing tj between the third surface and the j+1th surface (j=1 to j
-1), refractive index nj (j=
It can be determined from 1 to i).

FIG. 3 is an explanatory diagram for actually measuring the interplanar distance ti.

This corresponds to a case where the lens is composed of two lenses and the incident light is fixed.

(A) is the apex reflection on the first surface, and from this position as a reference, the lens is moved to the left as shown in CB>, (C), and (D).

(B), (c), CD> are respectively the second side,
This is the vertex reflection on the third and fourth surfaces. At this time, the amount of lens movement is S1, S2. It is represented by S3.

22. As a practical matter, it is difficult to determine to what extent the lens should be moved among these measured quantities to achieve vertex reflection on the first, second, third, and fourth surfaces. There is. Therefore, it is assumed that the rough values of the movement amounts S'l, S2, and S3 are known in advance. (1) For the first surface, n2 (air) = 0, so pl = p1' = 0 (1) Equation Therefore, al=31-pi =31 From equation (4), (2) Regarding the second surface, focal length f2' from the first surface to the second surface, principal point position Ip2
．． p2' is rl, r2, nl,
n2=1. Determined from tl.

Therefore, using this, using formulas (1) and (3) from the measured value S2, a2 = 82 - p2 ・ (10) Sl is the measured value, and nl and rl are measured using different methods. , Therefore, tl can be found from equation (9).

Note that in this case, tl is the axial thickness of the first lens 30, and can be measured by another method to obtain t2.

(3) For the third surface, t3 is found in the same manner as described above.

Therefore, in general, in a lens system with a cranial lens surface,
r1~rk and nl~nk were measured by known methods, and 81~5k-I! was determined by the method of the present invention. When measured with , it is possible to sequentially calculate and determine the interplanar distances ti to tk-1 between the respective lenses.

In order to perform such calculation processing, it is necessary to detect the vertex reflection position at each movement value 1, but when using an interferometer for the test light, which is convergent light, as a detection means, it is necessary to detect changes in the interference fringes. The vertex reflection position can be detected based on .

That is, the amount of wavefront aberration is determined from the number, density, etc. of interference fringes using a fringe scanning interferometer. This calculation of the amount of wavefront aberration is performed each time the test lens is moved. In this case, the amount of wavefront aberration is measured in the vicinity where vertex reflection occurs.

As described above, since the major position of the vertex reflection position is known in advance, it may be moved manually to the vicinity of that position.

Then, the relationship between the amount of wavefront aberration and the amount of movement of the lens becomes as shown in FIG. The horizontal axis indicates the amount of lens movement, and the vertical axis indicates the amount of wavefront aberration Wrms at that time.

Distance S1 at which vertex reflection occurs. S2. Changes in the amount of wavefront aberration Wrw+s near S3 (FIG. 3) all draw a downwardly convex parabola, and reach a minimum point at the position where the apex reflection occurs.

Therefore, while observing the interference fringes, if the position (movement amount) S at which this minimum point is detected is detected by the computer built in the interferometer, the movement amount SL to the position at which the minimum point is reached, S2.33, etc. tl can be calculated from

Even when the lens is fixed and the convergent light and thus the interferometer are moved, tl can be calculated in the same way by simply reversing the moving direction.

In the measurement described above, lens aberrations are ignored, but it is clear that measurement accuracy is improved if aberrations are taken into consideration.

In addition to the paraxial amount calculated from the radius of curvature from the first surface to the first surface, the axial spacing of each surface, and the refractive index of each medium, the The amount of wavefront aberration generated in the lens is calculated by giving the numerical aperture NA.

Then, find the defocus amount △ that minimizes the spherical aberration, set Sj' = Si + Δ, and use this new Sl' instead of Sl to calculate the surface spacing t.
If i is recalculated, the surface spacing t1 takes into account spherical aberration, so the accuracy of determining the dorsal side is improved.

In addition to this, there is also a method in which, for example, the position where the value obtained by subtracting the above-mentioned calculated value of the amount of wavefront aberration from the measured wavefront aberration is minimized is set as St, and the surface spacing t1 is calculated using this position.

The surface accuracy of each surface should also be corrected when the axial spacing of each surface needs to be determined more precisely. This can be determined by analyzing interference fringes.

Needless to say, if the lens has an aspherical surface, a ray tracing formula that includes the aspherical surface can be used when calculating the wavefront aberration.

The most accurate means of detecting the position of the vertex reflection is to use an interferometer and check the changes in the interference fringes, so below we will explain a lens surface spacing meter that uses an interferometer. do.

Figure 1 shows an example of a lens spacing measurement meter using a fringe scanning interferometer.The light emitted from a laser 1, which functions as a means to generate a convergent test light, is reflected by a reflecting mirror 2. Bendable, condensing lens 3, pinhole 4
The light passes through the beam splitter 5 and becomes parallel light at the collimator 6.

This parallel light enters the reference lens 7, and part of it is transmitted through the final surface of the reference lens 7, which is a reference surface, and part of it is reflected.

The transmitted convergent light is reflected at the vertex of the incident lens surface of the test lens 8, and the vertex reflection travels the optical path in the opposite direction, enters the reference lens 7 again from the opposite direction, and interferes with the reflected light on the reference surface mentioned above. do.

The interference light passes through the collimator 6 and is sent to the beam splitter.
The interference image captured by the CCD camera 10 is reflected by the image pickup lens 9 and is reflected by the image pickup lens 9 onto the CCD camera 10.
The signal is A/D converted by a converter 11, passed through a frame memory 12, and analyzed by a computer 15. Buta, monitor 14
Then, the image passed through the frame memory 12 is D/A converted by the D/A converter 13 and displayed.

Now you can observe interference fringes.

The computer 15 calculates wavefront aberration from the interference fringes captured by the CCD camera 10.

In order to obtain wavefront aberration with higher accuracy, the computer 15
By slightly displacing the piezo transducer 18 through the D/A converter 17 and performing fringe scanning using a known method, the wavefront aberration can be determined with an accuracy of 1/200 to 1/1000 (λ is the wavelength). I can do it.

Further, the computer 15 via the motor driver 19,
By electrically driving the stage 20, the lens system is moved in the optical axis direction (to the left in FIG. 3).

The amount of movement is measured by a length measuring device 21 such as a laser interference length measuring device or Magnescale, and the measured value is converted into a digital amount by an A/D converter 22 and then sent to the computer 15.

This computer 15 calculates the aforementioned focal length and principal point position, stores them in a memory 16, and uses them as needed.

Note that the radius of curvature, aspheric coefficient, and refractive index of each surface are measured by other means and input into the computer 15 as known values.

FIG. 5 shows a typical flowchart for measuring the interplanar distance in consideration of spherical aberration. As a premise, Runs 30
Assume that is already in the 1st place in Figure 3A.

When the control program for surface distance measurement starts,
First, step 51''Ci = 1 is set, and 10 = 0.Next, when the lens system is moved and the position shown in Figure 3B is determined (the position is determined by interference as described above), In step 52, Sl is measured based on the output of the length measuring device 21.

When the measurement of Si is completed, rjnj and tj are input in step 53, and then fi, p are calculated based on the paraxial theory.
i and p1' are calculated.

After these values are calculated, in step 55, ai and the surface distance t between the first surface and the i+1th surface are calculated based on a calculation formula. Then, in the next step 56, in order to take into account the spherical aberration of the lens system, a perfect defocus amount △i that minimizes the spherical aberration is calculated from the amount of wavefront aberration, and based on this, in step 57, the S ″△
Then, a new ai is obtained from this S″△.

Once a new ai is calculated in consideration of spherical aberration, the surface spacing t1 is determined from this in step 58.

These processes are continued until the last lens is reached (step 59), and if there is a subsequent lens, the same process is repeated while incrementing i to calculate the interplanar distance ti with that lens. (Step 60.
52 to 58), when the calculation of the surface distance of the last lens is completed, this surface distance measurement processing program ends.

In the above-mentioned embodiments, a measurement system using a fringe scanning interferometer controlled by the computer 15 was exemplified, but for example, the rZYGo Ma
In addition to using a commercially available interferometer such as "rklV", this interferometer is equipped with a stage for moving the lens system and a length measuring device for measuring the amount of movement of the lens system, and while manually moving the stage, the minimum value of the wavefront aberration can be determined. It is possible to determine Si by detecting the wavefront aberration by calculating the wavefront aberration or by visual inspection.

Since calculation of the surface spacing is somewhat complicated, computer calculation is preferable in this case, and it is necessary to perform the calculation together with the amount of correction using wavefront aberration using a separate computer.

The position of the vertex reflection is not the interference fringe, but the pinhole 4 in the first section.
It is also possible to place the CCD camera 10 at a common position and detect the position where the spot image is minimum.

However, unlike an interferometer, it is not possible to calculate spherical aberration correction, so in that case the measurement accuracy will be slightly lower.

In addition to vertex reflection on each surface, interference also occurs when light rays are incident on each surface in the normal direction of the surface, so it is necessary to distinguish between surface reflection, but this point is explained above. ,
Since the approximate position 1 of Si is known in advance, there is no risk of erroneously calculating the wavefront aberration due to surface reflection.

In the method of the present invention, there is a possibility that errors may accumulate in the surface spacing of the lens at the back, counting from the incident convergent light.
It is also possible to measure the lens with the lens reversed.

The thickness of the lens can also be measured by this method, but if the lens is measured individually and this value is used, the error can be reduced. In this method, an interferometer is used and the lens system or convergent light is configured to be movable, the distance traveled by the lens system or convergent light is measured so that the wavefront aberration is minimized, and the surface spacing is calculated based on the measured value. This is how it was done.

According to this, it is possible to accurately and easily measure the interplanar distance between the constituent lenses in a state where the lenses are assembled without measuring the lens frame.

Therefore, according to the present invention, it becomes possible to measure the interplanar distance of a lens system including a zoom lens, which was impossible to measure with the prior art.

[Brief explanation of drawings]

1121 is a system diagram showing an example of a surface distance measuring meter for a lens according to the present invention, FIGS. 2 to 4 are diagrams for explaining the operation of surface distance measurement, and FIG. 5 is an example for surface distance measurement. The flowchart shown in FIG. 6 is a cross-sectional view of a lens system for explaining a conventional surface distance measurement method. Laser reference lens/lens system for emitting convergent light (test lens) Imaging lens/frame Memory monitor Computer piezo transducer stage Length measuring instrument Second lens/spacer Wr■s&l near the top A reflection of the primary mirror frame! Characteristics of Fig. 4 8: 'M experimental lens Fig. 6 Fig. 5

Claims (3)

[Claims] (1) A means for generating test light that is convergent light, a means for moving a test lens incorporating a plurality of component lenses or a convergence point of the convergent light in the optical axis direction of the test lens, and measuring the amount of movement thereof. means for detecting that the convergent light is reflected near the apex of each surface of the lens constituting the test lens; detecting the convergence point or the test lens from the movement amount measuring means and the detection means; It consists of a surface spacing measuring means that determines the apex reflection position on each surface of the constituent lenses, and measures the surface spacing between these constituent lenses from this and the radius of curvature and refractive index of the constituent lenses of the test lens. A lens surface distance measuring meter characterized by: (2) A lens surface spacing measuring instrument according to claim 1, characterized in that an interferometer is used as means for detecting the convergence point or vertex reflection on each surface of the lens constituting the test lens. (3) In claim 2, the point at which the wavefront aberration becomes minimum as the interferometer or the test lens is moved in the optical axis direction is defined as
A lens surface spacing meter characterized in that the apex reflection point of each surface of the constituent lenses is taken as the apex reflection point.