JPH03218432A - Method and device for measuring refraction factor distribution - Google Patents

Abstract

PURPOSE:To easily enable measurement by the naked eyes and to enable the fine control of matching liquid refraction factor difference by providing an interference gauge to observe reference light flux and sample transmitted measuring light flux, liquid immensing means to hold the sample by matching liquid, and temperature control means. CONSTITUTION:The emitted beam of a coherent light source 1 is splitted to a measuring beam and a reflected beam by a half mirror 4 and the former beam is reflected on a movable mirror 5 and transmitted through a transparent container 6 where matching liquid 8 is put in. In the container 6, a lens 10 to be detected and a glass sample 9, which refraction factor and shape are already known, are arranged. The latter beam is passed through a mirror 11 and a half mirror 12 and an interference stripe is generated while superimposing the latter beam with the former beam. By operating the temperature of a normal temperature tank 51 and the tilt of the mirror 5, the maximum refraction factor distribution of the lens 10 to be detected is measured from the change state of the interference stripe.

Description

[Detailed description of the invention] [Industrial use! 'F] The present invention relates to a refractive index distribution measuring method for measuring the refractive index distribution of a sample by matching the refractive indices of the sample and a matching liquid. [Conventional technology]

In order to maintain high precision of optical elements such as lenses and prisms, the refractive index distribution within the element must be uniform. Since non-uniformity in the refractive index distribution cannot be corrected, for example, if there is a large difference in the refractive index distribution of lenses used in an imaging system, this will directly affect the deterioration of image quality. In particular, plastic lenses, which have been widely used in recent years, tend to have uneven refractive index distribution compared to glass lenses, and the refractive index changes greatly within a single lens, so the refractive index distribution is measured one by one after molding. There is a need to. The refractive index distribution of an optical element (sample) has conventionally been measured by the following method. The apparatus used for the measurement is constructed by placing an immersion device in which the sample is immersed in a matching liquid having a slightly different refractive index from that of the sample, in the measurement optical path of a Mach-Zehnder deaf interferometer. The reason why the sample is immersed in the matching solution is to suppress the occurrence of crosshairs during observation due to the difference in refractive index between the sample and the surroundings.Also, when measuring a lens as a sample, the power of the lens is This is to make it disappear. Matching liquid is made by mixing multiple types of liquids with different refractive indexes. In measurement, one unit is the operation of measuring the number of interference fringes that appear between one measurement point and another measurement point and the difference in sample thickness at each point, and this operation is performed at many locations. This is done by doing it repeatedly. Interference fringes appear due to changes in the optical path length determined by the refractive index and thickness of the sample, so by determining the number of interference fringes and the difference in thickness, it is possible to measure the refractive index of the area that includes both measurement points. can.

[Problem to be solved by the invention]

However, the conventional method for measuring refractive index distribution of a sample described above requires a larger number of measurement points in order to increase the measurement accuracy, so it has been difficult to easily measure the refractive index distribution. In addition, since the matching liquid is made by mixing liquids with different specific gravities, it takes time for the air bubbles that enter during stirring to disappear and the matching liquid becomes stable, and the refractive index difference between the sample and the matching liquid is small. The problem was that it was difficult to make adjustments.

【the purpose】

This invention was made in view of the above problems,
It is an object of the present invention to provide a measuring device and method that can easily measure the refractive index of a sample and finely adjust the refractive index difference between the sample and a matching liquid.

[Means to solve the problem]

In order to achieve the above object, the sample refractive index distribution measuring device of the present invention includes an interferometer that performs observation by interfering a measurement light beam that has passed through the sample with a reference light beam that does not pass through the sample, and a measurement optical path of the interferometer. A liquid immersion means for holding the sample immersed in the matching liquid, and a temperature adjustment means for adjusting the temperature of the matching liquid. In addition, the method for measuring refractive index distribution of a sample according to the present invention involves placing the sample between transparent substrates facing each other and spaced apart in parallel, filling the space between the transparent substrate and the sample with a matching liquid, and matching the sample with the sample. It is characterized in that by changing the temperature of the liquid, the refractive index of both is equalized, and the measurement light beam that has passed through the sample and the matching liquid is made to interfere with the reference light beam that does not pass through them for observation.

[Example] First, the principle of the measurement method of this invention will be explained. As shown in FIGS. 7 and 8, the refractive index n.sub.l is approximately equal to the refractive index nl of the test lens (sample). Let us consider the interference pattern WII (x, y) obtained by the transmitted wave and the reference wave when a plane wave of wavelength λ is transmitted through the test lens immersed in the matching liquid. Taking the Z-axis in the optical axis direction and the x-Y coordinates in a plane perpendicular to this, the shapes of the first and second surfaces H of the tested lens are as follows: Z+ = S+ (x, y) ■ Surface: Represented by Z2 = 32 (x, y). If the center thickness of the lens is d, then Sa on both sides
The total amount of g Sag(x.y) is Sag(x,y) =S+(x.y)+32(x,y
) = {1). On the other hand, the refractive index nt of the test lens
(x, y) is separated into the average refractive index nts and the variation in refractive index at each position on the lens (difference from the average refractive index: refractive index distribution) Δold (x, y), Ws (x, y) = Sag(x.yl[ntll-
nJ+ [d − Sag(x.y)koΔnt
(x, y) − (2) holds true. From equation (2), the refractive index n of the matching liquid. If is a constant, the interference pattern WII(x, y) becomes a function only of the average refractive index nta and the refractive index distribution Δnt(x, y). Next, the average refractive index n+lI and the refractive index distribution Δnt (x,
Let us consider a method for separately determining the two parameters y) and y). When the above equation (2) is expanded using Zernike's polynomial, which is common in the expansion equation of liquid level aberration, Ws(x.
y) = Sag(x.yl[nts-n++]+[a
- Sag (x. y)] Δnt (x, y): C+
+C2pcosψ0C3psinψ+Ca(2ρ2-1
) + C6ρ”cos2ψ+ .Here, C2ρcosψ, Csρsinψ are tilt terms indicating the inclination of the wavefronts of the reference light and measurement light, and C4(2ρ2-1) is the focal point on the observation surface where interference fringes are observed. The focal shift term (W2) that indicates the shift C5ρ2cos2ψ+... is an aberration term (enemy) that indicates the aberration caused by optical non-uniformity (refractive index distribution, etc.) of the test lens.Then, it is matched with the test lens. Sag(x
, y) ・[ntiI- na] can be approximated by a quadratic function and is almost equal to the defocus term. In this case, [ntO −
nm] is obtained, and from the aberration term W, △nt (x, y)
is obtained, and it becomes possible to separate the two parameters as shown in the formula below. If the defocus term is set to 0, the observed fringe component is the aberration term,
In other words, only the refractory distribution is shown. The defocus term becomes 0 when ntll-nn·0, that is, when the average refractive index nts of the test lens and the refractive index n1 of the matching liquid are equal. Therefore, in the embodiment, by changing the degree of similarity between the test lens and the matching liquid, a state where ntl-ns=0 is created, and the average refractive index of the test lens, ntll, and the refractive index of the matching liquid, n. The fringe components caused by any difference are eliminated, and the refractive index distribution of the lens to be tested is measured from the state of the interference fringes at that time. In Figure 9, one surface is spherical (radius of curvature r), the other surface is flat (radius of curvature co), the diameter is D and there is no refractive index distribution, and three lenses (w@-3λ) are used. Or one (
When observing the fringe of Ha・λ), the aberration term (ΔW) and R
It shows the relationship with the number (R:r/D). If the reproducibility of fringe reading is λ/25 in Rms, R > 0.7 even in the case of 3 fringes, so most R-number lenses are below the reproducibility of fringe reading, and the observed interference fringes are The component represents only the defocus term. [First Embodiment] Next, a first embodiment of the present invention will be described based on FIGS. 1 to 6. FIG. 1 shows a measuring device used in the first embodiment. This device is basically a Mach-Zehnder type interferometer. 1 is a coherent light source that emits coherent light (wavelength λ), and for example, a He-Ne laser light source is used. The beam emitted by the coherent light source 1 is expanded in diameter by a beam expander constituted by a lens 2.3, and divided into a measurement beam and a reference beam by a first half mirror 4. The measurement light reflected by the first half mirror 4 is further reflected by the movable mirror 5 and transmitted through the transparent container 6. The angle of the movable mirror 5 can be moved (tilted) as desired. The transparent container 6 is made of undistorted glass. 7
is a lid that can be opened and closed. A matching liquid 8 is contained in the transparent container 6. In the transparent container 6, as shown in FIG. 2, a test lens lO and a glass sample 9 with a known refractive index ng and shape (θ, L) are placed at positions through which the measurement light passes independently. It is located in The lens 10 to be tested is generally a plastic lens such as polymethyl methacrylate (acrylic), but it may also be a glass lens. Although the glass sample 9 is not necessary for measuring the refractive index distribution, it is arranged for the refractive index measurement described later. The reference target transmitted through the first half mirror 4 is reflected by the first fixed mirror 11, and then reflected by the second half mirror 12.
The light waves are superimposed on the light waves that have passed through the lens 10 to be tested. Interference fringes are then generated by the overlap of the two light waves. 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 l2, a first imaging lens 22, and the like. It is composed of an imaging device 23 provided at an imaging position and a television monitor 24 that displays the imaged interference fringes so that they can be observed. Reference numeral 51 denotes a constant temperature bath, which is placed around the transparent container 6, and has a transparent window 52 fitted in the portion that corresponds to the optical path of the measurement light. Furthermore, a heater 53 for heating the inside of the constant temperature chamber 51, a temperature sensor 54 for detecting the temperature inside the constant temperature chamber 51, and a fan 55 for stirring the air in the constant temperature chamber 51 are arranged inside the constant temperature chamber 51. has been done. Further, S6 is a power supply for heating the heater 53, 57 is a control unit for controlling the power supply voltage, and the temperature sensor 5
The detection signal from 4 is input to the control unit 57 to control the heat generation state of the heater 53 and maintain the temperature in the thermostatic chamber 5l at an arbitrary constant temperature. In order to measure the refractive index distribution of a lens by the measuring method of the second invention, the temperature in the constant temperature bath 5l is controlled to change the temperature of the lens to be tested 10 and the matching liquid 8. Then, the number of interference fringes observed on the monitor television 24 changes. When the refractive index of the test lens 10 and the refractive index of the matching liquid 8 become equal, the interference fringes disappear and only partial brightness shading remains. Therefore, when the movable mirror 5 is tilted (or tilted), curved stripes are observed at the center of the stripes, as shown in FIG. In this state, the fringe component caused by the difference between the average refractive index nt@ of the test lens 10 and the refractive index n@ of the matching liquid 8 has disappeared, and nt@ ” n
This indicates that the state is s. In other words, the defocus term disappears from the interference fringes, and only the refractive index distribution component, which is the aberration term, is observed. In order to tilt the wavefront, the fixed mirror 11 may be made rotatable instead of the movable mirror 5. Measure the difference between the maximum and minimum refractive index values (maximum refractive index distribution) of the test lens 10 from the size S of the peripheral shift of the fringes in FIG. 3 observed in this state, for example. Can be done. In FIGS. 10 and 11, a plastic lens made of polymethyl methacrylate (acrylic) is used as the test lens lO, and a liquid mixture of dimethyl silicone oil and phenylmethyl silicone oil is used as the matching liquid 8.
Figure 10, which shows the results of measurements made while changing the temperature of the thermostatic chamber 51 between 19 and 25°C, shows the maximum refractive index distribution of the test lens 10; was found to be almost unaffected by temperature changes. On the other hand, FIG. 11 shows the temperature dependence relationship between the average refractive index of the test lens 10 and the refractive index of the matching liquid.
It can be seen that the rate of refractive index change due to temperature change is greater in the matching liquid 8 than in the test lens 10. Therefore, the average refractive index of the test lens 10 and the refractive index of the matching liquid 8 can be made to match by changing the temperature of the constant temperature board 51 as a whole and searching for the intersection of the straight lines of refraction. Next, the relationship between temperature changes and observed interference fringes will be explained. When the temperature of the constant temperature bath 51 is changed, w1(
λ) interference fringes are observed, and at temperature T2, H2(λ) interference fringes are observed, and the refractive index of the test lens and the matching liquid at temperature and T2 are expressed as nx and nt2, respectively
and nml+ ns2, W+-(nt+ -
nm+)・Sag/λW2; (nt2- nm2)・
Sag/λW2 − Wi : Sag((nt2
- nt+) (n*2 - 11mI冫)/λ And the refractor of the test lens and matching liquid is the first
As shown in Figure 1, it changes linearly with temperature change, so Nt=Δnt/ΔT: (nt2-nt+)/(T2-
T+)Nm =Δas/ΔT:Cn*2-na+)
/(T2-T+), then ΔW ='ph - W+ :Sag(Nt - Nm
It becomes lΔT/λ. So, for example, Nt − Nm = (−1.1 + 4
．． 31) When XIO-4 [1/'C Sag = 1 mm λ = 0.8328μ■, the change in interference fringe ΔW per ΔT = 1'C! Yo, ΔW
#0.5λ howls. Next, a method for measuring the refractive index of the matching liquid and the lens to be tested will be explained. As shown in FIG. 1, the measuring device of this embodiment includes:
A second imaging lens l3 and an image sensor 14 are arranged facing the glass sample 9. The second image sensor 14 is formed by, for example, sox so independent photoelectric elements. Further, an AD converter 15, a digital arithmetic circuit 16, and a display device 17 are sequentially connected to the output terminal thereof. Furthermore, a memory 18 for storing data necessary for calculation and an input circuit 19 for inputting known data are sequentially connected to the calculation circuit 16. As shown in FIG. 4, interference fringes 30 are generated due to the overlap between the light wave transmitted through the glass sample 9 and the reference light wave.
The image is formed on the image sensor 14. Then, based on the output from the image sensor 14 at that time, the spatial frequency F of that fringe is calculated in the calculation circuit 16. This can be done by a well-known discrete Fourier transform (DFT), but it is better to use a faster Fourier transform (FFT). As shown in FIG. 5, interference fringes 30 on the image sensor 14
From the output on the line segment 3l perpendicular to , a brightness intensity distribution having a substantially sine curve shape as shown in FIG. 6 is calculated.
Then, from the intensity distribution, the spatial shoulder wave number F of the interference fringes is calculated by FFT, and the refraction angle am of the matching liquid 8 can be determined from that value. That is, since F=k/L (na - no)L-tanθ=kλ, n. It is determined by = flo+λ・F/tanθ. On the other hand, since the temperature inside the thermostatic chamber 51, that is, the temperature of the test lens 10 and the matching liquid 8, can be known, the refractive index of the test lens 10 can be determined from the temperature and the refractive index n1 of the matching liquid 8. can be found. (Second Embodiment) Next, a second embodiment of the present invention will be described based on FIGS. 12 to 17. The second embodiment is aimed at measuring a sample smaller than the first embodiment. For example, it is applied to measurements on a Brevarato microscope. In Fig. 12, lot, 102 is a transparent substrate,
It is used to hold the sample 103. substrate 101,
For 102, a material with no striae, bubbles, or dust attached inside is used. In this example, the sample 103 is a parallel plane plate whose end surfaces 104 and 105 (see FIG. 14) that contact the substrates 101 and 102 are parallel to each other, and whose side surface 106 is parallel to both end surfaces 104 and 105 (see FIG. 14).
105, but the end faces 104, 105
There is a high possibility that a crack 107 has occurred near the edge between the side surface 106 and the side surface 106. In this embodiment, the substrate 101
A transparent thin film 109 is formed as a temperature regulating member. This transparent thin film 109 is a conductive film such as ITO or NESA used in liquid crystal display elements, and is provided on the surface 101' in contact with the sample 103. A pair of linear electrodes 110 and 111 are provided on the transparent thin film 109. 110' and 111' are lead wires connected to the linear electrodes 110 and 111. The sample 103 is sandwiched between the substrates 102, and the sample 103 is immersed in the matching liquid 108, and the sample holder 1
12 (see Figure 15). Then, either AC or DC voltage is applied to the linear electrode 110. When applied between 111 and 111, the matching liquid 108 is heated by the transparent thin film 109.
, the temperature of sample 103 increases. When the sample 103 is an optical glass such as BK7 or F2, or an electronic material glass such as soda lime glass or borosilicate glass, the temperature coefficient of refraction is approximately -3 to 8X 10-'/deg. On the other hand, as the matching liquid 108, immersion liquid for mineral research such as cedar oil and clove oil, Si (silicon) oil, etc. are used, but in the case of silicone oil, the refractive index temperature coefficient is -5X 10-'/ Before and after dog (in the case of water,
-8x 10-'/deg), which is about 102 times that of the sample. Therefore, the refractive index of the matching liquid can be changed without almost changing the refractive index of the sample (see Figure 10), and the refractive index of both can be matched within a very small temperature adjustment range of several degrees to 1 oaeg. can be done. Furthermore, if the temperature control member is a heating element, it is necessary to pay attention to the following points. As the temperature increases, the refractive index of 10 g of matching liquid such as water or silicone oil decreases, so when producing the matching liquid 108 by mixing,
It is desirable to manufacture the sample with a refractive index slightly higher than that of sample 103. The sample holder 112 has a measurement optical system 113 shown in FIG.
installed in the optical path of the As the measurement optical system 113, for example, a Leitz interference microscope (a type of Matsuhatsu-Ender interferometer) is used, and this is a measurement optical system that measures the refractive index distribution of the sample as the curvature of interference fringes. 1613th! ff Nioiite, 114 Ha light source, 115
HaA-7 Mi5116 and 117 are total reflection mirrors
119 and 120 are microscope objective lenses, 121 is a half mirror, and 122 is a microscope eyepiece. FIG. 17 shows an example in which interference fringes were observed by heating the sample holder 112, in which a sample with a refractive index distribution and a sample without a refractive index distribution were observed. It is assumed that the refractive index distribution occurs in the thickness direction inside the sample 103. Here, in the case of any sample, the left side of the drawing with the boundary portion 123 in between is an interference fringe 124 due to light passing through the sample, and the right side is an interference fringe 125 due to light passing through the matching liquid. If the refractive indices of the sample and matching liquid match within a predetermined accuracy, those without a refractive index distribution will be observed as uniform linear interference fringes, and those with a refractive index distribution will be observed as interference fringes 12.
4 shows a bend according to the refractive index distribution of the sample, and at the same time,
It is observed that the interference fringes 124 and the interference fringes 125 are smoothly connected at the boundary portion 123. On the other hand, when the refractive indices do not match, the interference fringe 1 appears at the boundary portion 123 regardless of the presence or absence of the refractive index distribution.
An abnormality occurs in which the interference fringes 125 and 24 are unnaturally bent. Therefore, by adjusting the temperature while observing the abnormality that occurs at this boundary portion 123, the refractor can be finely adjusted. In FIG. 17, 8T indicates the temperature difference between the sample 103 and the matching liquid 108 in each state, and is set to be 0 at the temperature when the refractive indexes match. According to the above method, the refractive index of the sample and 10-2 to 10-
A matching liquid having a refractive index matching on the order of 3 is prepared, a sample is immersed in the matching liquid and sandwiched therebetween, and the temperature of the matching liquid is changed by temperature control based on a temperature control member. As a result, the refractive index of the matching liquid changes, and the refractive index of the sample and the refractive index of the matching liquid can be made to match on the order of 10-3 to 10-5. In addition, by matching the refractive index of the sample and the matching liquid, it is possible to prevent vibrations occurring at the edge of the sample.

[It is also possible to reduce the disturbance of interference fringes caused by the chipping 107. Specific measurements can be performed in the same manner as in the first embodiment described above. Note that in the second embodiment, temperature control is performed using the transparent thin film 109, but temperature control may also be performed using a Beltier effect type element. [Effects of the Invention] According to the sample refractive index distribution measuring device and measuring method of the present invention, by applying a temperature change to the sample and the matching liquid, the refractive index distribution measuring device and the measuring method of the present invention can be used without the need for a special optical system or calculation.
The refractive index distribution of a sample can be easily measured with the naked eye.
In particular, quality control of plastic lenses, etc. can be performed simply and at a high level, and the difference between the refractive index of the sample and the refractive index of the matching liquid can be easily fine-tuned.

[Brief explanation of drawings]

1 to 6 show a first embodiment of the present invention, FIG. 1 is an explanatory diagram showing the entire measuring device, FIG. 2 is a partially enlarged view of FIG. 1, and FIG. FIG. 3 is an explanatory diagram showing the interference fringes observed with the apparatus shown in FIG. 1, and FIGS. 4 to 6 are explanatory diagrams showing the procedure for determining the refractive index of the matching liquid. Figures 7 to 9 are diagrams for explaining the principle of the invention.
Figures 0 and 11 are graphs showing changes in refractive index distribution and average refractive index with respect to temperature changes. Figure 12 ~ 1st
FIG. 7 shows the second embodiment of the present invention, and the first embodiment shows the second embodiment of the present invention.
2 is a perspective view of a sample holder, FIG. 13 is a perspective view of a substrate equipped with a transparent thin film and a wire electric wire, FIG. 14 is a partially enlarged view of the sample shown in FIG. 12, and FIG. Figure 12 is a view of the sample holder as viewed from the direction of arrow Q, Figure 16 is an explanatory diagram of the measurement optical system, and Figure 17 is an illustration of interference fringes observed using the measurement optical system shown in Figure 16. be. Figure 2 q Figure 3 Refer to Figure 7 Temporary Figure 8 Figure 9 Figure 10 Figure 11 Procedural amendment form (self-drawn)

Claims (4)

[Claims] (1) An interferometer that observes by interfering the measurement light beam that has passed through the sample with the reference light beam that does not pass through the sample, and an immersion liquid that is installed in the measurement optical path of the interferometer and holds the sample immersed in a matching liquid. 1. An apparatus for measuring refractive index distribution of a sample, comprising: a temperature adjusting means for adjusting the temperature of the matching liquid. (2) The refractive index distribution measuring device for a sample according to claim 1, wherein the temperature adjustment means is provided on at least one of a pair of substrates that sandwich the sample. (3) Placing a sample between transparent substrates facing each other in a spaced-apart manner, filling a space between the transparent substrate and the sample with a matching liquid, and changing the temperature of the sample and the matching liquid. A method for measuring refractive index distribution of a sample, characterized in that the refractive index of the sample and the matching liquid are made equal, and the measurement light beam that has passed through the sample and the matching liquid is caused to interfere with the reference light beam that does not pass through them for observation. (4) Claim 3, wherein the reference light beam and the measurement light beam are coherent lights emitted from the same light source.
A method for measuring refractive index distribution of a sample described in .