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Surface Treatment of Lenses
When a beam of light i$ incident normally upon a glass surface, or emergent from a glass surface, about 5% to 6% is lost by reflection. As the angle of incidence increases the percentage of reflected light increases, also the higher the refractive index of the glass the bigger the loss. Fig. 5, which shows light incident upon a block of glass before and after blooming, will illustrate these harmful reflections, and how they can be eliminated, the eliminated reflected light now passing through.
There are two big advantages of surface treatment. The first is in the increase in transmission. The second is in the increase in contrast which is
Fig. 6. Equipment for Blooming Optical Components
obtained because the light reflected from the surfaces is eliminated and does not appear as stray light in the image.
The bloom is a film a quarter of a wavelength of light in thickness, i.e., about 1/10,000 mm., and having an optimum refractive index equal to the geometric mean of glass and air. These optimum conditions for thickness of film and refractive index of film arise from mathematical consideration of the interference of light reflected from the top and bottom of the film. In order that the light reflected from the top and bottom of the film shall destructively interfere so that there is no resultant reflected light, the conditions are that the light reflected from the top of the film should be equal