Abstract

In making an optical filter to provide any desired attenuation between 0.9 and 0.001 (0.5–30 dB) in the visible and infrared, the absorbing material of choice was carbon because of its environmental stability. To obtain the desired gradation of thickness from 0 to ~1.5 μm, separated-aperture masking¹ was used; motion of the source as evaporation progressed provided the necessary gradation in thickness. The maximum thickness was controlled by the source–substrate distance; stringent efforts had to be made to avoid excessive substrate heating due to the proximity of the hot source. Finally, because the stresses in evaporated carbon layers can be controlled in part by choice of the angle at which the atoms strike the surface,² the substrate was tilted at the angle calculated to optimize the performance of the resulting filter.

© 1986 Optical Society of America