Calculated reflectance of alurminurm-overcoated iridium in the vacuum ultraviolet from 500 A to 2000 A.

The effect of thin overcoatings of aluminum on the normal and oblique reflectance of opaque iridium has been calculated with a digital computer for the wavelength region from 500 A to 2000A. It is shown theoretically that iridium coated with 100-150 A of aluminum has a high reflectance throughout this entire wavelength range and that, at wavelengths below 800 A, the iridium-aluminum film combination is considerably more efficient than plain iridium or aluminum. However, the high vacuum uv reflectance of such a mirror coating is not likely to be utilized under ordinary conditions because the formation of an oxide film on the aluminum surface decreases its reflectance drastically. The most likely application of iridium-aluminum reflecting coatings is in high resolution satellite spectrographs for studying the emission lines of stars and of the sun. In order to utilize fully their high reflectance in such satellite experiments,the aluminum film has to be deposited after he satellite has been placed in an orbit high enough to eliminate surface oxidation.