Abstract

A study of the reflection characteristics of over a hundred mirrors of thirty-seven kinds has been made to get data that will permit both a comparison of mirrors and of the various methods and instruments for testing them. Tests were made of specular reflectance at angles from 15 degrees to 82.5 degrees and a new gonioreflectometer was built particularly for this work. The angular distribution data were used to compute average reflectances, and this was done in three different manners that represent three different points of view as to what average reflectance means. A number of alloys of aluminum were made and tested in an endeavor to improve on pure aluminum, and the entire group of mirrors was retested a number of times to determine the degree of permanence of the various metals.

© 1942 Optical Society of America

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Figures (14)

F. 1
F. 1

The rotatable part of the gonioreflectometer is seen to consist of a metal drum with a peripheral slit covered with a hanging strap of black velvet. The thumb tacks on the stationery base mark 15-deg. steps in mirror incidence and reflection and 30-deg. steps in rotation of the drum.

F. 2
F. 2

This view of the gonioreflectometer shows the lamp house at the right within the drum, the block of magnesium carbonate that serves as a target and in the center of the drum, the mirror holder. The instrument is here set for an angle of incidence of 60 deg.

F. 3
F. 3

This sketch shows how the magnesium carbonate target receives constant illumination and is always viewed along a radial line. The mirror is turned by hand at each setting to bring the center of the target into the center of the photometer field.

F. 4
F. 4

These reflection curves were computed from measured values of index and absorption of a microscope slide similar to the slides used to make the mirrors. A careful test with the gonioreflectometer was made to detect possible systematic errors in the instrument. The agreement between computed and test values indicate that the errors are negligibly small.

F. 5
F. 5

Measurements on the metal or first surface of eight kinds of metals gave the above curves for the angle-reflectance characteristics. Silver and aluminum show minima near 80 deg. and all curves are extraplated toward perfect reflectance at 90° incidence. The numbers in the circles indicate the number of sample mirrors used to get an average value.

F. 6
F. 6

These curves are for the mirrors of Fig. 5 tested as second surface mirrors. The characteristics are similar for the two sides, but there are several interchanges in relative positions, notably between nickel and chromium.

F. 7
F. 7

These 14 reflectance curves are for a group of plated and solid mirrors (see Table I for details) that were polished to the best obtainable brightness without regard to optical flatness. Twelve metals showed at least traces of a minimum reflectance near 80 deg. and only two, copper and gold, were without this minimum.

F. 8
F. 8

A group of magnesium-aluminum alloy mirrors were tested on both surfaces with the results shown above. There is no obvious relationship between the two surfaces, and one group of 4 containing 2 percent magnesium was close to the best as a first surface mirror and decidedly the poorest as a second surface. See test for percentages in charge placed on the evaporating filament and percentages of magnesium found on the mirrors.

F. 9
F. 9

This is a group of the best samples obtained with various alloys of copper, magnesium, and antimony with aluminum. As first surfaces they showed deep minima at 80 deg. and no minima as second surface mirrors.

F. 10
F. 10

These maintenance curves show how seven first surface metals maintain their 45-deg. reflectance with age when standing exposed in a laboratory room.

F. 11
F. 11

The same metals as in Fig. 10 are here tested as second surfaces. The protection afforded by the glass helps the maintenance of silver, nickel, and molybdenum but has slight value in preventing copper from depreciating.

F. 12
F. 12

These 14 metals show all degrees of stability with age, and in several cases a small appreciation in 250 days is probably due solely to photometric errors.

F. 13
F. 13

The Rθ characteristics of a repolished silver mirror show that the reflectance at angles near the normal depreciates faster with age than the reflectance near grazing. As a result the characteristic curves change shape and they illustrate the danger of making assumptions as to shape of the Rθ curve in any particular case.

F. 14
F. 14

These aluminum magnesium alloys show good reflectance and good maintenance as first surface mirrors, but the second surface has low reflectance combined with a definite appreciation. This increase is presumably due to an initial excess of magnesium next to the glass and as this layer migrates into the aluminum the composition approaches that of the first surface.

Tables (1)

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Table I Summary of reflectances.

Equations (6)

Equations on this page are rendered with MathJax. Learn more.

F a b = cos a cos b
S 0 90 = 0 90 R ( a + b ) / 2 ( cos a cos b ) ,
S 0 60 = 1 0.75 0 60 R ( a + b ) / 2 ( cos a cos b ) ,
P 0 60 = 1 0.75 0 60 R ( a + b ) / 4 ( cos a cos b )
Al + 1 2 % Sb
Al + 1 2 % Sb