The effect of speed of evaporation, pressure, thickness, and age on the reflectance of evaporated Al films was investigated in the extreme ultraviolet from 900 A to 2200 A. High speed of evaporation was found to be the most important factor in producing Al films of highest ultraviolet reflectance. By changing the evaporation time required to produce an 800 A thick coating from 2 sec to 130 sec at a pressure of 1×10−5 mm Hg, the reflectance is reduced from 90% to 67% at 2000 A, from 74% to 50% at 1600 A, and from 38% to 30% at 1200 A. For films produced at higher pressures such as 1×10−4 mm Hg the effect of evaporation speed on the reflectance is even more pronounced. Film thickness has very little effect on the reflectance as long as films are not thinner than 400–500 A and not thicker than 2000 A. The effect of aging on the reflectance increases strongly with decreasing wavelength. Between 2 hr and 1 month exposure to air the reflectance of good Al films decreases from 91.5% to 90.0% at 2200 A, from 75% to 69% at 1600 A, and from 44% to 27% at 1200 A.
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