Abstract

A procedure is described for determining the inhomogeneities of a sample of optical material in terms of small changes in index of refraction at discrete points over the measured surface. The method is used in the visible- and near-ir regions of the spectrum. An He–Ne gas laser source was used for the ir measurements. The variation in index was mapped with contour lines enclosing areas of similar index variations. The largest index change in 1-cm distance was noted and recorded as the maximum variation per centimeter. Over twenty-five different samples of six different materials were examined. The results are presented in a chart indicating the range of maximum refractive index variations of the samples observed. Data are shown to illustrate the possibility of determining index values to higher precision than the material justifies.

© 1966 Optical Society of America

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References

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  1. The Manufacture of Optical Glass and of Optical Systems, Ordnance Dept. Doc. No. 2037, (Government Printing Office, Washington1921), p. 29.
  2. F. Twyman, Prism and Lens Making (Hilger and Watts, London, 1952), D. C., p. 508.
  3. L. W. Tilton, F. W. Rosberry, F. T. Badger, J. Res. Natl. Bur. Std. 49, 21 (1952).
    [CrossRef]
  4. E. W. Deeg, Am. Ceram. Soc. Bull. 39, 674 (1960).

1960 (1)

E. W. Deeg, Am. Ceram. Soc. Bull. 39, 674 (1960).

1952 (1)

L. W. Tilton, F. W. Rosberry, F. T. Badger, J. Res. Natl. Bur. Std. 49, 21 (1952).
[CrossRef]

Badger, F. T.

L. W. Tilton, F. W. Rosberry, F. T. Badger, J. Res. Natl. Bur. Std. 49, 21 (1952).
[CrossRef]

Deeg, E. W.

E. W. Deeg, Am. Ceram. Soc. Bull. 39, 674 (1960).

Rosberry, F. W.

L. W. Tilton, F. W. Rosberry, F. T. Badger, J. Res. Natl. Bur. Std. 49, 21 (1952).
[CrossRef]

Tilton, L. W.

L. W. Tilton, F. W. Rosberry, F. T. Badger, J. Res. Natl. Bur. Std. 49, 21 (1952).
[CrossRef]

Twyman, F.

F. Twyman, Prism and Lens Making (Hilger and Watts, London, 1952), D. C., p. 508.

Am. Ceram. Soc. Bull. (1)

E. W. Deeg, Am. Ceram. Soc. Bull. 39, 674 (1960).

J. Res. Natl. Bur. Std. (1)

L. W. Tilton, F. W. Rosberry, F. T. Badger, J. Res. Natl. Bur. Std. 49, 21 (1952).
[CrossRef]

Other (2)

The Manufacture of Optical Glass and of Optical Systems, Ordnance Dept. Doc. No. 2037, (Government Printing Office, Washington1921), p. 29.

F. Twyman, Prism and Lens Making (Hilger and Watts, London, 1952), D. C., p. 508.

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Figures (9)

Fig. 1
Fig. 1

Shadowgraph of the standards for optical glass striae grade. Grade D is at left and Grade A at right. Grade B has a small surface scratch near center. Striae can be seen in Grade A on original photograph. These standards are 10.16 cm thick.

Fig. 2
Fig. 2

A pictorial display of a preliminary test procedure used to determine whether or not a sample is suitable for interferometric testing. Sample A does not pass the visual test. Sample B is strained. C contains striae. Sample D contains inclusions, but is otherwise a good sample.

Fig. 3
Fig. 3

A drawing showing the dimensional requirements for a sample to be used in an interferometric test. The small wedge between faces provides the necessary number of reflection fringes. The measurements are in millimeters. Both surfaces are polished and flat to within two fringes. If index refraction is below 1.5, Δt is 5 μ to 6 μ. If index of refraction is between 1.5 and 2.0, Δt is 4 μ to 5 μ. If index of refraction is between 2.0 and 2.5, Δt is 3.5 μ to 4.5 μ. If index of refraction is 3.4 (silicon), Δt is 3.0 μ.

Fig. 4
Fig. 4

A sample set of fringe pictures from a disk of fused silica. The picture at left shows the reflection fringes, center picture shows the transmission and field fringes, and picture at right shows only field fringes.

Fig. 5
Fig. 5

Contour maps of two samples showing index variation values at points corresponding to the intersections of the grid wires. The upper sample is fused silica; the lower is silicon. The contour lines are drawn along lines of equal index variation values. (a) Value for variation of index of refraction Δn × 10−6. Maximum variation per centimeter is 10.9 × 10−6; total variation is 16.4 × 10−6. (b) Silicon single crystal 1:1:1 P type Δn × 10−5. Maximum variation per centimeter is 12.4 × 10−5; total variation is 17.6 × 10−5.

Fig. 6
Fig. 6

Chart showing a method of intercomparing samples of different materials. Bottom four materials are represented by only one sample of each kind. The sample of zinc sulfide was the thinnest sample tested and therefore benefited from the less-smoothing effect that added thickness gives. The small circles indicate values of (maximum variation/centimeter)/n−1 from Table I.

Fig. 7
Fig. 7

A shadowgraph of a sample of polycrystalline calcium fluoride showing the high concentration of bubbles or voids in this sample.

Fig. 8
Fig. 8

A shadowgraph of silicon made with an image converter. Horizontal streak in upper half of sample indicates an area of striae.

Fig. 9
Fig. 9

Shadowgraph through a glass prism on the same path used by the spectrometer when a nominal value for index was determined.

Tables (1)

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Table I Average Maximum Values of Index of Refraction Variation/Centimeter Expressed Relative to n

Equations (4)

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1 2 m t λ = ( n - 1 ) Δ t + t Δ n .
m t = 2 / λ [ ( n 0 - 1 ) ( Δ t 1 - Δ t 2 ) + t 0 ( Δ n 1 - Δ n 2 ) ] ,
1 2 m r λ = n Δ t + t Δ n .
m r = 2 / λ [ t 0 ( Δ n 1 - Δ n 2 ) + n 0 ( Δ t 1 - Δ t 2 ) ] .

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