The motion picture projectionist (Nov 1929-Oct 1930)

lune, 1930 Motion Picture Projectionist 37 Screen Light and Projector Heat A CINEMA screen providing a picture showing detail in the shadows, and so acceptable to an audience, must be supplied with light sufficient to allow it to reflect a certain and definite amount of luminosity. Beyond that necessary amount, and today a picture only given that luminosity would be considered badly presented, experienced projectionists differ regarding the brilliance to be attained. Some authorities consider too much light is bad for the eyes and affirm a bright screen causes headaches and is prone to keep patrons away from the theatre. On the other hand, it is definitely established that the more light there is thrown back by the screen the more plastic are the moving figures, the more steroscopic the views. High luminosity appears to be a step in the direction of showing pictures in relief, and provides a show more satisfying to the critical portion of the audience. The amount of light reflected from the screen can be controlled. The projector arc is the source of the light, but, as I will show, other factors determine the capacity of the arc. The screen itself has, necessarily, a great influence on the amount of light returned to the audience. Screen Luminosity This reflected light, which, so far as the audience is concerned, is the only light that matters, can be measured. The standard of measurement is the foot candle, which is the illumination produced by the light of a standard candle on a normal surface one foot square and a foot away from the source of light. The usual illumination of screens in the old days was about 4% -foot candles, which was about sufficient to show all the details of the picture. The mirror arc was introduced with the object of keeping the same luminosity on the screen, but economizing the current; but the greatly enhanced value of the brightly lit screen made possible by the new invention altered the objective of its use, and now it is adopted solely for the amplification of the light. Today the luminosity of screens is from 7y2 to 17% -foot candles, and the average amount of light from the screen is about twice that of the early days. Light— More Light! When auditoriums of greater capacity were built, much larger screens became necessary. These, in order to keep to the standard of luminosity, entailed an enormous call upon the projector for more light. To appreciate the magnitude of the dc By J. McD. Burnett mand, we may assume a screen measuring 4 ft. by 4 ft. reflecting 4% -foot candles; the area is 16 square feet and the light reflected from the screen is equal to 72 candles. If the illumination is of the order of 10-foot candles, the reflection equals 160 candle power. But when we double the linear dimensions of the screen, making it 8 ft. by 8 ft., it measures 64 ft. square, and it reflects at the 10-foot candle rate 640 candle power. Screens now in use measure 24 ft. by 32 ft.; with the same reflected luminosity their 768 square feet give a light of 7,680 candle power. What candle power the arc has to supply depends on another factor. Large auditoriums require large screens, but they usually entail a much increased distance between the film and the picture. Doubling the distance of projection means quadrupling the candle power at the source in order to have the same intensity of light thrown back from the screen. Going back to our example of the 8 ft. by 8 ft. screen reflecting 640 candle power, we can assume that the projector was 100 ft. away and reflected all the light that left the lens; but if the projector was moved back to 200 ft. from the screen it would have to develop 2,560 candle power to provide light enough for efficient screening. How Light Is Absorbed Sound pictures require screens permeable to sound. The most efficient of this type do not reflect more than 60 per cent of the incident light rays. They have, therefore, placed a demand upon the projector for an increase of 30 to 50 per cent more light than that required by the solid screen. With the coming of the colored film, the call for more light will be insistent. In this case, the loss of light is felt at the source. The color in the film absorbs 20 to 40 per cent of the light thrown upon it by the condenser and, when the color films are combined with sound and talking effects, and projected from the front, the light will have to be greatly increased or the value of the picture diminished to that of the old lithographed pictures of the magic lantern days — -pictures that were as flat as the screen. When we consider that all this light has to be generated in the crater of the arc, we are not surprised at the continued growth of the current consumption. Amperages of 12 grew to 18; this rose to 24, to 36; now an arc taking 80 amps, is quite common and in the near future we may expect to see this exceeded, and 150 or 250 amps, for a colored talking film may possibly be necessary. This will probably entail entirely new wiring in most halls, probably new generating plants in many. Arc an Electric Furnace Now a projector with such an arc is nothing less than a small electric furnace; the waste heat from it is sufficient to warm efficiently a small hall. An hour's run on such a machine generates heat enough to boil 17 gallons of water. It will heat, provided there is no radiation, a ton of cast-iron to a temperature at which the film melts! Now the light from the arc is concentrated by the condenser lenses to a circle of only 36 mm. diameter upon a surface of one of the most inflammable substances known; so it is fortunate that glass is a substance with a great reluctance to allowing the passage of heat. Were the heat rays able to pass through it as freely as do the light rays, no film manufactured from any suitable substance we know today could be used ; there would be no cinematography. As it is, enough heat passes through to cause, when concentrated, a continual menace of fire and often serious accidents. Absorption and Radiation All substances absorb heat when near a source of it until equilibrium is reached; they also radiate it again until equilibrium is attained. A bar of iron placed in a heat ray will rise in temperature until the radiation from it into the air balances the amount of heat it is taking from the heat ray. Raise the temperature of the surrounding air, and temperature of the iron rises; lower it and the temperature drops, or send a blast of air across the bar and carry the absorbed heat away by what I may term forced radiation, and the iron drops to little above room temperature. On the other hand, place the iron in a niche in a fire brick, which conducts heat badly, and it will be found that the heat of the iron increases regularly with the minutes that have elapsed, until its temperature approaches that of the heat ray itself. By this we see time is a function in the absorption of heat. With no radiation, a substance in a heat ray absorbs twice as much heat in two seconds as it does in one; to which fact is due the trouble when a film sticks in the gate. Passing through at its normal speed it does not absorb enough heat to fire; passing out it radiates what heat it has absorbed to Ihe air. But when it sticks, or when ;i small piece torn from the film