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Primary Color Filters With Interference Films
By H. H. SCHROEDER and A. F. TURNER
A set of vacuum-deposited thin-film multilayer transmission filters has been developed for use as highly efficient primaries in additive color projection. Light which is not transmitted is reflected. Consequently the filters can be used in high-illumination beams without overheating and changing color. Modification of spectrophotometric curves can be effected as desired in specific problems.
JL HROUGH THE USE of vacuum-deposited multilayer interference films it is now possible from a practical and useful standpoint to design and manufacture transmission filters capable not only of supplanting colored glasses or gelatin in many cases, but also of producing color to specifications impossible with these older types of filters. Particularly the filters discussed herein correspond in hue to the Wratten three color projection primaries (Kodak Wratten Filter No. 47, blue; No. 59, green; No. 24, red). The Wratten filter curves are seen in Fig. 1 with theoretical curves of the interference coatings shown in Figs. 2a and 2b, illustrating two types of green-transmitting filters. The new filters are suitable for use as highly efficient primaries in additive color projection.
Presented on October 8, 1953, at the Society's Convention at New York by H. H. Schroeder (who read the paper) and A. F. Turner, Bausch & Lomb Optical Co., Rochester 2, N. Y. (This paper was received August 31, 1953.)
Generally the filters are composed of alternating high and low-index optical films deposited on a suitable substrate such as glass. Depending on particular requirements these types may have as many as 1 5 alternating layers. Through appropriate design and control of the relative thicknesses of the individual films predetermined optical specifications of wavelength and transmission may be achieved.
The materials used are all colorless, forming transparent films with practically no absorption in the visible and infrared. In the filters themselves reflectance plus transmittance totals 100% with the reflected portions being unused in the applications described.
In order to provide the best description of the properties of the filters from an engineering viewpoint a set of engineering spectrophotometric curves has been prepared, Figs. 3, 4, 5 and 6. These are based on actual experimental curves but have been simplified by smoothing so that nonessential irregularities are elimi
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November 1953 Journal of the SMPTE Vol. 61