International projectionist (Jan 1963-June 1965)

production of yellow-image dye soundtracks, but is unfit for use with tinted-base films. Fortunately, the cesium-silver-oxygen S-4 photoemissive cell in common use is most sensitive to those red and infrared wavelengths which form the bulk of exciter radiation. Maximum response occurs at about 800 millimicrons. Small variations in exciter voltage are well tolerated bv S-4 photocells. They are also suitable for the reproduction of regular silver-image tracks on both clear and tinted-base film. The silicon photodiode is perhaps the most troublefree and satisfactory of all optical-sound pickups be Vlsible Infrared Visible Infrared t X \ >N / \ 'an l» V i. Of \ V \ > \ 10 \ £ i i +00 500 fcOO 700 800 900 1000 1100 1200 Wavelength (ny*) Response of S'4 photocell under gxciter illumination FIG. 4 — The integrated response curve of a photocell illuminated by an exciting lamp amounts to the simple mathematical product of photocell sensitivity and exciter emission at each wavelength all along the spectrum. Here we have the response of the common S-4 photocell to exciting light having a color temperature of 2500° K. Note that tne strong ultraviolet peak of S-4 photocell response (Fig. 3) is markedly diminished by the weakness of the ultraviolet in exciter radiation (Fig. 2). A similarly integrated curve for the silicon photodiode would be much the same as this one, except for sharp cutoffs at 455 millimicrons (blueviolet) and 1010 millimicrons (infrared). cause of its high signal output, low noise level, extreme ruggedness, and indefinitely long life. Its response to the various wavelengths of radiation is very similar to that of the S-4 photoemissive cell and to the general characteristics of a large family of phototransistors. Fig. 4 is an "integrated curve" — the response of the S-4 photoelectric cell multiplied all along the spectrum by the radiant emission of a tungsten-filament exciter operated at a color temperature of 2500° K ( 10 lumens watt). This curve indicates the response of a standard type S-4 photocell under actual operating conditions. A comparison of this integrated curve with the S-4 curve in Fig. 3 reveals little alteration in shape except for marked attenuation of the secondary response peak in the ultraviolet. This attenuation — quite unimportant — is caused by the very feeble emission of ultraviolet by the exciter lamp. Dyes Transparent to Infrared Now we come to a most important matter — the transmission characteristics of tinted film bases. It is evident that such materials absorb certain parts of the visible spectrum, for that is what causes the color. Are there any absorption bands in the infrared which might conceivably decrease S-4 photocell response? Happilv. the answer is negative. The three colors of tinted base chosen for illustration in Fig. 5 are widely separated in absorption character 1 ' 1 ' 700 800 900 1000 1100 1200 Wavelength (ny*) Transmission of three colors of tinted -film stock FIG. 5 — The spectral transmittances of clear film base and of three selected colors of tinted film base, amber, bluish green (aquamarine), and deep blue. Observe that the clear acetate base transmits all wavelengths freely except for an insignificant falloff in the extreme violet and near ultraviolet. The colors of the tinted-base samples are produced by selective absorptions in the visible spectrum, as the curves show. All samples nevertheless transmit the infrared spectrum down to about 1500 millimicrons as freely as the clear base does — a transmissivity of about 90 per cent. istics and representative of film tints available in the days of nitrate film and silent pictures. They are amber I soft orange I . bluish green I vivid aquamarine ) , and deep blue, corresponding roughly to the tinted films made by Eastman Kodak many years a~o under the names "Peachblow." "Aquagreen," and '"Nocturne." The curve in the upper left-hand corner of Fig. 5 is for clear acetate film base. This material freely transmits all of the wavelengths under consideration: and the slight "dip" in the far violet is not sufficiently pronounced to produce either visual or sound-reproduction effects. The transmission and absorption bands of the three colored film bases are distributed only in the visible part of the spectrum I and also in the unimportant ultraviolet] : and it is these selective absorptions, as we said, which are responsible for the colors exhibited by the tinted base materials. There are no absorption bands in the infrared down to 1200 millimicrons, however. Is this merely a fortunate coincidence? Not at all. The organic chemical dyes employed to impart color to the filmbase material, in common with most organic dyes, transmit freely, without any absorption bands, in the all-important infrared region of the spectrum. The top horizontal line of Fig. 5 may be taken to represent a radiant-energy transmissivity of 90% (a transmittance of 0.9). Visible Infrared S t 3 L. C // > J? // -i 1 i -^^" 600 700 800 90C Wavelength U Tungstenexcited S~4 cell"" — Amber-tinted film FIG. 6 — This diagram is extremely important to the subject under discussion. See text for details. International Projectionist December. 1963