The Application of Magnesium Fluoride Anti-Reflection Films to Glass Surfaces

JSGT 1947 V31 T170-T193

Previous to the war, anti-reflection films had not reached a stage of development sufficiently advanced to enable them to be used in military instruments. Their value, however, soon became evident and methods were devised as quickly as possible to produce, in quantity, films that combined optical efficiency with adequate durability. Without doubt the latter quality was the most difficult to attain. Magnesium fluoride, deposited by thermal evaporation, fulfils the foregoing requirements most satisfactorily, and it is with anti-reflection coatings of this material that this account is concerned. On the basis of simple theory the optimum film thickness and the refractive index necessary to attain maximum efficiency may be predicted. Reference to the chromaticity diagram indicates that the roost efficient films lie in the purple region when viewed by reflected white light. The vacuum equipment and the process by which magnesium fluoride films are produced are described in some detail. In order to realise maximum hardness and durability the work must be thoroughly heat-conditioned and the glass held at a temperature not lower than 250° during deposition of the fluoride. Adhesion between the film and the glass is further influenced by the kinetic energy of the incident molecules. Therefore by selecting a source which produces a high velocity molecular beam, much harder films can be secured. Spattering is reduced to a minimum by avoiding the formation of magnesium oxide during preparation of the fluoride.

S. Bateson