METALLIC FLUORIDE FILMS OF LOW REFLECTIVITY
Rept. for 1 Jul 1940-13 Jan 1941
NAVAL RESEARCH LAB WASHINGTON DC
Pagination or Media Count:
The method of reducing the reflection losses at glass surfaces by depositing thin films of magnesium fluoride, calcium fluoride, or sodium aluminum fluoride on the glass by evaporation in high vacuum has been investigated in detail. The action of the films depends on two things first, their refractive indexes are much lower than the refractive index of glass so that even a thin layer of the film material reduces the reflection and second, when the films are made to be one-quarter wave-length thick for any chosen wave- length, a minimum of reflected light intensity results from interference of the waves reflected from the front and back surfaces of the film. The effect of film thickness is illustrated by a calculation of the transmission for monochromatic light of a glass plate of refractive index 1.65 coated on both sides with cryolite films assumed to have a refractive index of 1.35, a reasonable value. The films are most effective when their optical thickness is one-fourth the wave-length of green light, about 5500 AU, in the middle of the visible spectrum where the eye has maximum sensitivity.
- Inorganic Chemistry