American cinematographer (July 1933)

90 American Cinematographer 0 July 1933 Super- Photofloods For the Studio by William Stull, A. S.C. Member of Research Committee, American Society of Cinematographers F OLLOWING in the footsteps of the well-known ‘Pho- toflood” bulbs announced a few months ago for non- professional photographic use, the General Electric Company has brought forth a similar type of globe in larger units for studio use, known as the ‘‘Super-Photoflood.” Al- though produced especially to meet the requirements of modern natural-color cinematography, the new lamps should be at least equally useful in the more common field of black-and-white camera work, since they offer a more in- tense, whiter light with a very marked reduction of infra- red (heat) radiation. The principle of the photoflood is probably too well known to require additional exposition here, especially since many cinematographers already employ the smaller photo- floods in ‘‘Baby Spots” and in “practical” set light-fixtures. Suffice it to say that they are high-efficiency bulbs, designed to operate with great intensity on currents considerably in excess of their rated voltage, giving an intense white light. It is well known that if the voltage applied to a lamp is increased, the light-output increases at a much faster rate than either voltage or wattage, while the amount of blue- violet radiation increases faster than does the red-orange. A 10% increase in voltage, for instance, will give a 16% increase in wattage, but a 40% increase in light output. In the case of the small photoflood lamp, rated at 65 volts but normally operated at from 110-115 volts, an 80% in- crease in volts increases the wattage two and one-half times, but raises the light volume five and one-half times. The new Super-photofloods, when compared to standard types now in general use in the studios, are found to give an increase of 270% in the violet region (4000-4500 Ang- strom) but only 55% increase in the red (6300-7000 Angstrom). Based on an equal red radiation for the two lamps, the increase in the violet is 140%. The overall increase in intensity is approximately 100%. Used in a standard studio unit—the Mole-Richardson “Rifle” type, which concentrates 59% of the light within a 60-degree angle—we find that whereas a standard 1 500-watt PS-52 type bulb directs 17,200 lumens in the sixty-degree useful angle, the new 2000-watt PS-52 (Super-Photoflood I gives 34,400 lumens in the same angle, an increase of exactly 100 %. There is, however, the familiar drawback to all photo- flood lamps—shorter life. The smaller photofloods, as is well known, have an average life of between two and three hours at full intensity. This life is materially increased in many installations, however, by the use of shunt circuits which permit the operator to burn the lamp at a reduced voltage while lining up, and then to switch to full voltage and intensity for actual photographing. The same expedient is recommended for conserving the larger photofloods, which normally have a life of from 1 5 to 1 8 hours at full intensity. It is entirely feasible to reduce the line voltage to approxi- mately 90 volts, by means of either a field control at the generator, or by the use of grids outside of the set, keeping this voltage while preparing, and operating the lamps at full voltage only when actually shooting. With all of the lamps operating at a uniformly-reduced voltage during the preparatory work, lighting balances, contrasts, shadows, etc., should not be altered when the lamps are brought up to full voltage, while the life of the bulbs would be materially * increased. As these bulbs darken quickly, a cleaning-pow- der is provided in the globes, and they should be cleaned after every five or six hours of full-intensity operation. These new lamps will take care of the floor-lighting units; but how about the larger units used overhead? It has been found unnecessary to develop new bulbs for these, since the already available 5 kw. and 10 kw. types are already designed for extreme efficiency. It is necessary only to use the bulbs of this size rated for 105 volts on 120- volt circuits to get the desired effect. Of course, when some new development such as this is announced, the inevitable question in the minds of its pro- spective users is, “Very nice—but what will it actually do?” To answer this, we must examine the relationship between existing lamps and the SuperSensitive film now in general use. Essentially—despite all of the recent progress in pan- chromatization—we have a film, highly sensitive at the blue-violet end of the spectrum, and somewhat less sensitive toward the red-orange end. With normal Mazda lamps, we have a light having its greatest radiation in the orange- red and beyond, but tapering down toward the blue-violet end of the spectrum—a very fortuitous combination. More than a few of the outstanding masters of the camera, how- ever, have found that the best results are obtained by using a very light bluish silk over their lights, passing the blue- violet and green components freely, but slightly curtailing the yellow, orange and red frequencies. The photoflood bulb, though not making so great a correction, follows this lead to a certain degree, as its radiation is markedly in- creased in the blue-violet region, and decreased in the orange-red. It is, in fact, a much closer approximation of sunlight than has hitherto been achieved in clear-glass Mazda bulbs. The Super-photoflood should, therefore, give a decidedly marked improvement in quality—in the pleas- ing rendition of tone and texture. This has already been acknowledged by many cinematographers in their practice of using the smaller photofloods in baby spots and practical set lighting-fixtures. Continued on Page 114