Owing to the high refractive index of TiO2, films of this material show a great difference in reflection between the spectral regions of constructive and destructive interference. A glass slide coated on one side with a film of such thickness as to give its first order reflection maximum at 11 000A and its reflection minimum at 5500A, will reflect approximately 30 percent at 11 000A and 8 percent at 5500A. If the glass is coated on both sides, it will reflect in the neighborhood of 40 percent of the integrated infrared from a tungsten lamp operated at 2700°K, while it reflects only 8 percent at 5500A. A series of experiments with incandescent lamps was undertaken to determine the effect of such coatings applied to the bulbs. Uniform TiO2 coatings of the proper thickness were deposited on the inside and outside of clear spherical bulbs, and the light output measured from compact filaments when carefully centered inside a coated bulb and when centered in an uncoated bulb. It was found that, for a given power input, the light output from the coated bulb was as much as 20 percent higher than from the uncoated bulb. For the same light output, 10 percent less power input was required in the coated bulb. Because this result was obtained only when spherical nonfrosted bulbs with accurately centered small filaments were used, it is questionable whether this process will have any practical applications in connection with incandescent lamps.
FRANK J. STUDER and D. A. CUSANO, "Titanium Dioxide Films as Selective Reflectors of the Near-Infrared," J. Opt. Soc. Am. 43, 522-522 (1953)