International projectionist (Jan 1963-June 1965)

r greater importance than these rather incidental aesthetic contributions of color is its great potential power to enhance, b) either objective or subjective association. the emotional significance of the scene with which it is assocated." Technical Considerations The pressing technical problem attendant upon the use of tinted film is. of course, satisfactory reproduction of the optical soundtrack. If the tinted base interposes an appreciable optical densit) to the photoelectric cell or phototransistor, significant!) reducing response, it will be necessary for the projectionist to increase the fadei setting in order to obtain a normal level of sound volume. \nd no matter how much '"reserve power" the amplifier nun bave, compensation bj means of the volume control invariablv increases "ground noise;" and if the photoelectric densit) ol the film is excessive, the ini rease in gain required for adequate volume mav he so great as to introduce overload distortion. These fears were well founded in the days of bluesensitive photocells. Tests confirmed the fact that certain red. orange (amber), yellow, and green films had excessive photoelectric densit) to potassium cells. The main difficulh however, was not >n much the groundnoise as the rather drastic and frequent changes in fader setting required for the projection of reels in which different colors of film were intercut. \ stated at the outset, this difficult no longer exists. The possibility of interference with photocell response b\ the film-base dyes has now been completely eliminated l>\ the almost universal use of red — and infrared-sensitive photoemissive cells (type S1 rseponse), silicon photodiodes, and germanium transistors. This statement can be proved. Tig. 1 is the tvpe of chart used for plotting the response of photoelectric devices as well as the radianl emission of exciting lamps and the transmission characteristics of tinted film-base material-. It is, in effect, a graph having rectangular co-ordinates extended from the horizontal scale of radiation wavelengths and the vertical scale of response, radiant emission, or transmittance in terms of relative energy. Fig. 1 has no "curve"" to indicate am of the data which we shall presentlv examine. The chart has purposelv been left blank in order to shov more clearlv the nature of the spectrum wavelengths which mosl concern us — the wavelengths from 350 millimicrons in the near ultraviolet down to 120(1 millimicrons in trie infrared. Note that the visible portion of the spectrum, with its characteristic bands of colors (violet, hlue. green, yellow, orange, and red), extends onlv from -100 to 700 millimicrons.* Beyond the shortwave extreme of the visible spectrum i v iolet I are the chemicallv active, but invisible, ultraviolet wavelengths: beyond the longwave Although the human eye responds only very feebly to wavelengths shorter than 100 millimicrons or longer than 700 millimicrons, the most painstaking recent research proves that there is some visual response, albeit extremely slight, to about 360 millimicrons in the violet-ultraviolet and to about 830 millimicrons in the red-infrared. The commonly stated limits. 400 and 700 millimicrons, are nevertheless good round figures to remember. Ordinary crown window glass is opaque to ultraviolet beyond 330 millimicrons and to infrared beyond 5000 millimicrons. Clear acetate film base is opaque beyond 300 millimicrons and has strong absorption bands in the far infrared I beyond 3000 millimicrons I . International Projectionist December. 1963 v;s;bi Q?l 400 500 600 700 800 900 1000 1100 1200 Wavelength (ny*) Equalenergy response of various photoelectric cells FIG. 3 — The sensitivity of three different types of photoelectric cells to the range of wavelengths under consideration. The heavy line charts the response of the common cesiumsilver-oxygen photoemissive cell (type S-4). Observe that there are two peaks of strong response, one in the ultraviolet and a broader one in the infrared. The S-4 photocell is alsa sensitive in greater or lesser degree to all visible wavelengths. The response curve drawn as a broken line indicates the sensitivity of the S-l blue-sensitive potassium cell. Formerly used in America, the blue-sensitive photocell is now confined to a few European equipments. It has the disadvantage of excessive sensitivity to variations in exciter voltage and ensuing changes in the color of the light, causing undesirable variations in sound volume. The light solid line shows the response of the modern silicon photodiode or phototransistor. It closely matches the S-4 curve in the important yelbw-red-infrared band of the spectrum. extreme I red I are the heat-producing, but also invisible, infrared wavelengths. Because the photoelectric cells commonly used in soundheads at the present time are red — and infraredsensitive devices, it is the longer wavelengths, both visible and invisible, which most concern us. Exciter Emission, Photocell Response Fig. 2 presents the radiant emission curve of a tungsten-filament exciting lamp operated at 10 lumens per watt — a "color temperature" of 2500° on the absolute. or Kelvin, thermometer scale (=2227° C=4532° F). Light of this color temperature may be described as amber-yellowish in appearance. It is of particular interest that most of the radiant energy emitted by a soundhead exciter occurs in the invisible infrared, with its maximum close to 1000 millimicrons when the lamp is burned at 10 L/W. Fig. 3 shows the response characteristics of three photoelectric devices employed in optical soundheads. The blue-sensitive type S-l photoemissive cell, used in the early days of sound-on-film reproducers, is totally blind to the red and infrared wavelengths which predominate in exciter radiation. This cell has been brought back for use in a few equipments of European manufacture, but it may justly be condemned because it aggravates the bad effect of incidental variations in exciter voltage. When exciter voltage decreases, the light not only becomes dimmer, but also redder. The S-l cell is insensitive to red light. In consequence, sound volume falls off more markedly than it would with an infrared-sensitive cell, thus annoying the projectionist with the necessity of "riding the gain" to maintain a reasonably level volume of sound output. The blue-sensitive photocell is suitable for the re