JPH028726A - Measurement of refractive index of lens - Google Patents

Abstract

PURPOSE:To enable measurement of a refractive index and a refractive index distribution without destroying a lens by arranging a glass sample and the lens to be inspected with a refractive index and shape known to generate an interference fringe using two matching liquids with the refractive index thereof slightly different therebetween and a computation is performed from an output of an intensity distribution thereof. CONSTITUTION:A glass sample 9 with a refractive index and shape known and a lens 10 to be inspected with a refractive index and shape unknown are immersed into a first matching liquid 8 and a coherent light from a light source 1 is transmitted therethrough. An imaging lens 13 and an image sensing element 14 are made movable vertically together and moved to positions facing the lens 10 to be inspected or the glass sample 9. An interference fringe forms an image on the image sensing element 14 as generated with the overlapping of a light wave transmitted through the lens 10 or the glass sample 9 with a reference light wave. Then, a second matching liquid obtained by changing a mixing ratio of the matching liquid 8 is used to generate an interference fringe likewise. Thus, an average refractive index and a refractive index distribution of the lens 10 to be inspected can be determined quantitatively from an output of the intensity distribution of the interference fringes.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for measuring the refractive index of a lens, and in particular, to a method for measuring the refractive index and refractive index distribution of a plastic lens whose shape and refractive index are unknown. The present invention relates to a method for measuring the refractive index of a lens, which is suitable for

[Prior Art] Plastic lenses have come into widespread use in recent years due to the need for lightweight lenses, cost reductions, and aspheric lenses.

However, from the viewpoint of stability of physical properties, plastic lenses have the disadvantage that, compared to glass lenses, the refractive index and its distribution are unstable and fluctuate widely due to manufacturing reasons. Therefore, it is necessary to measure the refractive index and its distribution of each plastic lens after molding. In this case, since the lens cannot be destroyed, its measurement is not easy, especially if it is an aspherical lens.

Conventionally, the following methods have been used to measure the refractive index of such plastic lenses.

The device used as a whole is a Matsu/Zehnder interferometer, in which the light beam is used as a plane wave reference beam, and the test lens is placed in the light beam, and the refractive index is the same as that of the test lens. Dip and set in approximately equal amount of matting liquid. Furthermore, for refractive index reference, a glass sample with a known refractive index that is close to that of the test lens is also immersed and set at the same time. In the observation of interference fringes produced by placing such an immersion device in the center of a Matsuha-Zehnder interferometer, the difference in the thickness of the sample is measured while N interference fringes are observed.
The refractive index of the lens to be tested was determined from the measured values.

[Problems to be Solved by the Invention] However, the refractive index of the matching liquid changes with changes in temperature and the like. Therefore, in the above measurement method,
If the temperature changes, the range where N-tree interference fringes occur will also change,
The thickness of the sample at that part must be measured each time, and if the refractive index distribution of the lens is not uniform, irregularly shaped interference fringes will occur, so there are only N fringes. A situation arises in which it is difficult to know how to interpret this. Therefore, in refractive index measurement in such cases, quantitative interpretation is difficult. Additionally, it has been difficult to automate measurements.

This invention eliminates such conventional drawbacks.

A method for measuring the refractive index of a lens that can measure the refractive index and refractive index distribution of a lens whose shape and refractive index are unknown without destroying the lens, and which also allows for easy quantitative interpretation and automated measurement. The purpose is to provide

[Means for Solving the Problems] In order to achieve the above object, the method for measuring the refractive index of a lens according to the present invention uses a glass sample whose refractive index and shape are known, and a test lens whose refractive index and shape are unknown. are immersed in the first matching liquid, coherent light is transmitted through them, and the transmitted light wave is superimposed on the reference light wave to generate interference fringes, and the intensity distribution of the interference fringes caused by the light waves transmitted through the glass sample is determined. The refractive index of the first matching liquid is determined from the output, and the intensity distribution of the interference fringes caused by the light waves transmitted through the test lens is output, and from the output, the refractive index of the first matching liquid is determined. Find the polynomial of 1 and obtain the first order,
The glass sample and the lens to be tested are immersed in a second matching liquid whose refractive index is slightly different from that of the first matching liquid, the coherent light is transmitted through them, and the transmitted light wave is superimposed on the reference light wave. Together, they generate interference fringes,
The intensity distribution of interference fringes generated by the light waves transmitted through the glass sample is output, and the refractive index of the second matching liquid is determined from the output, and the intensity distribution of the interference fringes generated by the light waves transmitted through the test lens is determined. A second polynomial representing the interference fringes is determined from the output, and the shape of the lens to be tested is determined from the refractive indices of the first and second matching liquids and the first and second polynomials. Then, the defocus term and the aberration term of the first or second polynomial are separated, and the average of the test lens is determined from the defocus term and the refractive index of the first or second matching liquid. The present invention is characterized in that the refractive index is determined, and the refractive index distribution of the lens to be tested is determined from the aberration term.

[Operation] FIGS. 9 and 10 show the measurement principle of the method for measuring the refractive index of a lens according to the present invention. m X + y, Z is the coordinate.

First, when a plane wave containing a wavelength is transmitted through a test lens immersed in a matching liquid whose refractive index nII is approximately equal to the refractive index n of the test lens, the interference pattern Wo obtained by the transmitted wave and the reference wave is Consider (x, y).

The shape of each surface of the lens to be tested.

Surface: Z+ =S+ (X, y) 11th surface' Z2
=82 (X * y) and the center wall thickness is d, the total amount of Sag on both sides Sag (x, y) is Sag (x, y) = S + (x, y) + S
2 (x, y) · (1).

On the other hand, if the refractive index n t (x + y) is separated into the average refractive index nto and the refractive index distribution Δnt (x, y), then Wo (x, y) = Siag (x, y) @ [nto-
11+a I+[d-9ag(!,ri]Δnt(x,
y) ++ (2) If the refractive index n11 of the matching liquid is measured from equation (2), w, )(x, y) are n ta and Δnt(x.

y), but generally these two parameters cannot be determined separately.

Now, if we expand equation (2) above using Zernike's polynomial, which is common in wavefront aberration expansion equations, we get Wo(x, y)-9ag(x, y) * Into-+
+a]++d-9ag(i, month Δnt (x, y)
・CI +C2fcosLf+C3fsjnf'<4
It can be written as (2j2-1)+C5/"cos2 de 4-*-.

C2a cascJ), C3a5inf is the tilt term,
ca (2/)2-1) is the defocus term (W2)C
sJ''cos2p+... is an aberration term (W).

And now [n (6nB)] #0.

Sag(X, V) [nton nm] is 2
It can be approximated by the following function, and is almost equal to the defocus term.

Figure 11 shows various R numbers (R=r/D) where one surface is spherical (radius of curvature r), the other surface is flat (radius of curvature oo), the diameter is D, and there is no refractive index distribution. Aberrations when considering a lens and assuming that the matching liquid is prepared for that lens so that three stripes (WQ = 3 people) and 1 stripe (Wo = 1 person) can be observed. term (Δ
W) is shown. If the reproducibility of stripe reading is set to /25 in RIIls, R>0.7 even in the case of three manuscripts.

Therefore, most R-number lenses have less than the fringe reading reproducibility, indicating that the observed interference fringe component is represented only by the defocus term.

Therefore, [nto-nml is obtained from the defocus term of the read fringe WO (x, y), and Δnt (x, y) is obtained from the aberration term W.

It becomes possible to separate the two parameters. In other words, the average refractive index of Wazu is determined.

Therefore, in the present invention, during measurement, first and second matching liquids with slightly different refractive indexes are used, and the Sag amount of the lens to be tested is first determined from interference fringe analysis in each case. .

That is, the refractive index of the first and second matching liquids is n Ial
+n112, and the interference fringes that occur when using the first and second matching liquids are Wo, ,Wwa, then Wo+
(x,y)-9ag(x,7)*[nto-nm+
] + [d-Sag (x, y) lΔnt (x, y) °
−(3) Wdx, y)=Sag(x, y)* [nto
−nm2]+[d−3ag(x, July Δnt(x, r...
(I) Therefore, from (3) and (4), (Se1. Se2 is the maximum Sag amount on both sides of the lens, as shown in FIG. 9.) Therefore, the Sag amount, that is, the shape of the tested lens is different. For example, if the refractive index distribution of the lens is found from the aberration term of the polynomial and the refractive index of the matching liquid is known, then the lens to be tested is nm2-n1ll from the defocus term of the polynomial, and the refractive index of each matching liquid n1ll + n112 can be found. For example, the Sag amount of the lens to be tested can be found from equation (5).

Note that the refractive index of the matching liquid may be determined from the known refractive index and shape of the glass sample.

[Example] FIG. 1 shows a measuring device used in the present invention,
This device is basically a Matsuha Red Zender interferometer. 1 is a coherent light source that emits coherent light (wavelength λ); for example, a) Ie-Ne laser light source is used. A beam of light emitted from a coherent light source 1 is expanded by a beam expander lens 2 and turned into a parallel beam of light by a collimator lens 3. 4 is a first half mirror, and the light beam reflected by this half mirror 4 is further reflected by a movable mirror 5 and sent to a transparent container 6.
Transparent. The movable mirror 5 can adjust its angle slightly (
tilt).

The transparent container 6 is made of undistorted glass. 7
is a lid that can be opened and closed. Inside the transparent container 6, the material of the test lens lO is polymethyl methacrylate (acrylic).
In this case, a matching liquid 8 containing, for example, a mixture of dimethyl silicone oil and phenylmethyl silicone oil is contained.

As shown in FIG. 2, the matching liquid 8 contains a glass sample 9 whose refractive index ng and shape (θ, L) are known.
Refractive index nt and shape (d, Sag(x, y)
) is in parallel with the unknown test lens 10.

It is arranged perpendicular to the transmitted light beam. Test lens l
O generally targets lenses made of plastic such as polymethyl methacrylate (acrylic), but may also target lenses made of glass or other materials.

The light beam transmitted through the first half mirror 4 is reflected by the first fixed mirror 11, and then further reflected by the second half mirror 1.
2 and is superimposed on the light wave that has passed through the test lens lO or the glass sample 9. Then, the interference fringes caused by the overlap of the two light waves are transmitted to the first imaging lens 1.
3, an image is formed on the surface of the image sensor 14.

The first imaging lens 13 and the image sensor 14 are provided so as to be able to move in the vertical direction in the figure as a unit. Therefore, although the first imaging lens 13 faces the test lens 10 in FIG. 1, the first imaging lens 13 may also face the glass sample 9 as shown in FIG. can.

Returning to FIG. 1, the image sensor 14 is formed of, for example, 50×50 independent photoelectric elements. An AD converter 15 and a digital arithmetic circuit 16 are connected to the output terminal.
and a display device 17 are sequentially connected. The arithmetic circuit 16 also includes a memory 18 that stores data necessary for the arithmetic operation.
and an input circuit 19 for inputting known data are sequentially connected. Note that as the arithmetic circuit 16, a microcomputer or other arithmetic device can be used.

Reference numeral 20 denotes an observation device for observing the state inside the transparent container 6, which includes a second fixed mirror 21 provided opposite to the second half mirror 12, a second imaging lens 22, and the like. Imaging device 23 and television monitor 2 provided at the imaging position
4.

In order to measure the refractive index of the lens 10 to be tested using the measurement method of the present invention, first, known data (n g *θ, L) regarding the glass sample 9 is input to the memory 18 from the input circuit 19 and stored. put.

Next, the refractive index of the matching liquid 8 is adjusted. This adjustment is performed on the TV monitor 24 so that the number of interference fringes caused by the light waves transmitted through the test lens 10 is, for example, 3 or less.
It is possible that the number of interference fringes becomes one or less and disappears. Specifically, the lid 7 of the transparent container 6 is removed, and one of the two types of silicone oil constituting the matching liquid 8 is dripped into the container 6 using a dropper or the like.
The matching liquid 8 may be stirred and mixed. The liquid whose refractive index nfold is thus determined is the first matching liquid in the present invention.

Next, as shown in FIG. 3, the first imaging lens 13
to face the glass sample 9. Then, as shown in FIG. 4, interference fringes 30 generated by the overlap of the light wave transmitted through the glass sample 9 and the reference light wave are imaged on the image sensor 14. Then, based on the output from the image sensor 14 at that time, the spatial frequency a F + of the stripe is calculated by the calculation circuit 16.
Calculate at. This can be done by a well-known discrete Fourier transform (DFT), but preferably by a faster first Fourier transform (FFT).

In this case, from the output on a line segment 31 as shown in FIG. 5, which is perpendicular to the interference fringes 30 on the image sensor 14,
A substantially sinusoidal brightness intensity distribution as shown in FIG. 6 is calculated. Then, from the intensity distribution, FFT
The spatial frequency F1 of the interference fringes is calculated, and the refractive index nat of the first matching liquid 8 is determined from the calculated value.

That is, F,=kl/L (nllll-ng) L fist tan θ
= λk.

Therefore, it is determined by nmt=ng+in・F+/lanθ. And the value of tfllll is memory 1
Remember it in 8.

Next, as shown in FIG. 1, the first imaging lens 13
is made to face the lens 10 to be tested. Then, test lens 1
As shown in FIG.
4. The output from the image sensor 14 at that time is
The arithmetic circuit 16 first performs high-precision fringe analysis. This analysis is performed by rotating the movable mirror 5 by a small angle to give a tilt to the fringe, for example, by a well-known spatial fringe scanning method, determining the phase, and then calculating the first polynomial W in the arithmetic circuit 16. 0 expanded into l(x, y) In this embodiment, for example, it is expanded into a Zernike polynomial and stored in the memory 18.

Next, the mixing ratio of the matching liquid 8 is slightly changed. In this case as well, one of the two types of silicone oil constituting the matching liquid 8 is dripped into the container 6 using a dropper or the like, and mixed by stirring. The liquid whose refractive index n theory 2 has been determined in this way is the second matching liquid in the present invention. In this case, the refractive index nl1 of the matching liquid Ml is
If the refractive index of O is slightly larger than nl, then the refractive index n mt of the second matching liquid is adjusted to be slightly smaller than nl, and conversely, if n Ill is slightly smaller than nt, then nm! It is best to adjust the value to be slightly larger than nl.

Next, in exactly the same way as when using the first matching liquid described above, the refractive index nu of the second matching liquid is determined from the interference fringes generated by the overlap of the light wave transmitted through the glass sample 9 and the reference light wave. The value is stored in the memory 18.

Next, in exactly the same manner as when using the first matching liquid described above, interference fringes are generated by the overlap of the light wave transmitted through the test lens IO and the reference light wave. FIG. 8 illustrates an example of interference fringes 50 that are imaged on the image sensor 14 at that time. Then, based on the output of the image sensor 14 at that time, the arithmetic circuit 16 calculates interference fringes using a second polynomial W(x
, y). Then, the defocus term and the aberration term of this polynomial are stored in the memory 18.

Next, first, the first and second polynomials Wo1. The difference between Wo2 and the refractive index of Pi4i and the second matching liquid n1m1゜n
From the difference of 1lt, the amount of Sag (i.e. shape) of the lens to be tested
is determined as sag(x,y)=WX-WXnllffi-nt.

Then, find the maximum Sag amount (Sel+Se2), read out the refractive index n1Lt of the second matching liquid from the memory 18, and find the average refractive index nto of the test lens 10 from the defocus term of the second polynomial as, The results are output and displayed on the display device 17.

Further, the wall thickness [d − Sag (x, y)] of each part of the test lens lO is read from the memory 18, and the refractive index distribution Δn of the test lens 10 is calculated from the aberration term (W) of the second polynomial. (x, y).

The result is output and displayed on the display device 17.

In the above embodiment, the defocus term and the aberration term of the second polynomial were stored in the memory 18 and used for measurement analysis, but the first polynomial was used instead of the second polynomial. In that case, the average refractive index nto of the lens lO to be tested is determined as follows. The n set is the refractive index of the first matching liquid.

[Effects of the Invention] According to the method for measuring the refractive index of a lens according to the present invention, since the lens to be tested is simply immersed in a matching liquid, there is no need to destroy the lens during measurement.Moreover, from the output of the intensity distribution of interference fringes, By extracting the defocus term of the polynomial that represents interference fringes, the average refractive index of the test lens whose refractive index and shape are unknown can be determined, and the refractive index distribution can be determined from the aberration term of the polynomial, so it can be determined quantitatively. Yes.

Therefore, it has excellent effects such as being able to easily perform automatic layer 1 analysis.

[Brief explanation of the drawing]

1 to 3 are schematic diagrams showing an example of a measuring device for carrying out measurements according to the present invention, FIGS. 4 to 6 are schematic diagrams illustrating the procedure for determining the refractive index of a matching liquid, and FIGS. 8th
The figure is a schematic diagram showing interference fringes generated on the surface of the image sensor by light waves transmitted through the test lens, Figures 9 and 10 are schematic diagrams explaining the measurement principle of the present invention, and Figure 11 is a diagram illustrating the measurement principle of the present invention. FIG. 3 is a diagram illustrating measurement accuracy based on the measurement principle. Agent Patent Attorney Kazuhiko Mitsui See Figure 5, Figure 4, Figure 6, Figure 7, jJ8, Figure 9, Figure 10.

Claims (1)

[Claims] Immersing a glass sample whose refractive index and shape are known and a test lens whose refractive index and shape are unknown in a first matching liquid,
Coherent light is transmitted through these, and the transmitted light wave is superimposed on a reference light wave to generate interference fringes, and the intensity distribution of the interference fringes generated by the light waves that have passed through the glass sample is output, and from that output, the first In addition to determining the refractive index of the matching liquid, the intensity distribution of interference fringes caused by the light waves transmitted through the test lens is output, and from the output, a first polynomial representing the interference fringes is determined. The sample and the lens to be tested are immersed in a second matching liquid whose refractive index is slightly different from that of the first matching liquid, the above-mentioned healed light is transmitted through them, and the transmitted light wave is superimposed on the reference light wave. Generating interference fringes, outputting the intensity distribution of the interference fringes caused by the light waves transmitted through the glass sample, and determining the refractive index of the second matching liquid from the output; The intensity distribution of the interference fringes is output, a second polynomial representing the interference fringes is determined from the output, and the above is calculated from the refractive indices of the first and second matching liquids and the first and second polynomials. The shape of the lens to be tested is determined, and the defocus term and the aberration term of the first or second polynomial are separated, and the defocus term and the refractive index of the first or second matching liquid are separated. A method for measuring the refractive index of a lens, characterized in that the average refractive index of the lens to be tested is determined, and the refractive index distribution of the lens to be tested is determined from the aberration term.