Boxoffice barometer (1941)

beauty of color features. The high-intensity arc, emitting essentially equal intensities of all the spectral colors, reproduces all hues and tints with remarkable accuracy. Even though some theatres of small seating capacity may not have felt the need for the greater volume of screen light that has been experienced by houses of greater capacity, they are feeling the need for a better quality of projection light than low-intensity lamps provide. The snow-white light of the high-intensity arc means just as much in the way of satisfied patronage and increased attendance to these small theatres as it does to the large down-town houses, and the latest development in projection equipment, the new low-wattage high-intensity arcs, puts high-intensity projection right in the lap of this smallest member of the theatre family. Both a-c and d-c lamps are now on the market in which the power consumed at the arc is of the order of one kilowatt, and the cost of operation correspondingly low. The operating cost with these new lamps is less than that of the low-intensity lamp although the light output is 50 to 80 per cent greater and the efficiency of screen light production the highest yet obtained. Cost of operation is therefore no longer a justification for any theatre, however small, doing without the increasingly important advantages of high-intensity projection. The new a-c high-intensity lamps avoid the flicker sometimes observed when the a-c high-intensity arc is operated on 60cycle alternating current by operating through a frequency changer which supplies 96-cycle alternating current to the arc. The cut-off frequency of the twoblade shutter at standard projection speed is 48 cycles per second. With a 60-cycle light-source, this results in a 12-cycle beat or fluctuation in the screen light which, under certain conditions, may be disturbing to the observer. By using a frequency of 96 cycles at the arc one full cycle of current occurs during each 90-degree shutter opening, and disturbing flicker is eliminated. Regardless of the phase relation between the current and the shutter, the same amount of light is passed during each period the shutter is open. The new d-c high-intensity lamps are operated at 30, 35 and 40 amperes arc current with 27.5 volts or less across the arc. This low arc voltage has been made possible by the development of an improved negative carbon which permits operation at short arc length without the formation of a carbide tip. The optical principle in these new low-wattage highintensity lamps is the same as that used in the simplified high-intensity lamps which have achieved such popularity during the past four or five years. Reviewing on a more specific basis the foregoing narrative of progress, five lines of improvement will be noted. These are (1) intrinsic brilliancy, (2) light quality, (3) volume of screen light, (4) efficiency of light production, and (5) economy of operation. Q) Intrinsic Brilliancy. — In the early days of motion picture projection, methods of light measurement now available had not come into use, but it seems probable that in the original low-intensity lamp the brilliancy of the positive crater was much lower than the values now attained. Later improvements in vertical trim lamps may have brought this value up to 150 cp/mm2 and, in the low-intensity d-c reflecting arc a crater brilliancy of 175 cp/mm2 is attained. This, as has been stated, is about the limit of brilliancy for the low-intensity d-c arc under stable operating conditions. The application of the high-intensity d-c arc to projection, about 1919 or 1920, removed this fixed limit to crater brilliancy which is an inherent characteristic of the low-intensity arc. High-intensity arcs are operated at crater brilliancies in excess of 800 cp/mm^ and a recently developed super-high-intensity carbon for process projection can be operated at a brilliancy of 1200 cp/mm^, an 8:1 improvement over the early types of projection arcs. (2) Light Quality. — Improvements in quality of projection light have been along two lines, improved steadiness and improved color. The earliest projection lamps used solid carbons for both positive and negative electrodes. Due to a tendency for the arc stream to shift its position over the tip of the positive carbon, considerable unsteadiness in light output was experienced. This was reduced by the use of cored positive carbons. The effect of the core is to stabilize the arc stream at the center of the positive carbon face and thus improve the steadiness of burning. Further improvement in steadiness was effected by introducing a core in the negative carbon and later by substituting, for the plain negative carbon, equal to or near the diameter of the positive, a metal-coated negative carbon considerably smaller in diameter than the positive. This metal-coated negative carbon, called the “Silvertip” carbon, was introduced about 1916 or 1917. An improved metal-coated negative, the “Orotip” carbon, was developed several years later. Steadiness of the a-c low-intensity arc was improved in 1917 by the introduction of certain rare-earth materials in the core of the carbons. This material, by its arcsupporting properties, greatly improves the steadiness of burning on alternating current. It has the further advantage of giving a snow-white projection light. The neutral cored low-intensity arc, either a-c or d-c, gives a light of yellowish tint. The higher the crater temperature, the whiter the light produced, but at the maximum temperature attainable in the low-intensity arc the color composition on the basis of energy distribution is approximately 18 per cent violet and blue, 32 per cent green and yellow, and 50 per cent orange and red. The adaptation of the high-intensity arc to projection about 1919 or 1920 made a marked improvement in the quality of light. The striking difference in color composition between the low-intensity and the high-intensity arc is shown in Fig. 6. It will be noted that the light from the high-intensity arc contains approximately equal proportions of all the primary colors. This quality of light proved much more pleasing for monochromatic pictures than the yellowish light from the low-intensity arc. With the introduction of color photography and the need for accurate color reproduction on the screen, the importance of snow-white projection light was increased, since all 35-mm film is processed for projection with light having approximately equal proportions of all the spectral colors. 34% 35% 31% VIOLET GREEN ORANGE AMD AHD AND BLUE YELUm RED High Intek-:,ity Light 18% VIOLET BLUE 32% GREEN AND YELLm 50% ORARBE AND RED Fig. 6. Comparison of color composition of light from high-intensity and low-intensity carbon arcs. Sixteen-mm color-film, on the other hand, is usually processed for projection with incandescent light, the type of light at present most frequently used in 16-mm projectors. This light is even yellower than that of the low-intensity arc. Since 16-mm projectors are often equipped with carbon arc lamp>s to permit their use before gatherings of considerable size, a new carbon trim, known as the “Pearlex” trim, was developed in 1937 especially for 16mm projection. The color composition of the light from this carbon, as now made, is proportioned to give accurate color reproduction with film processed for incandescent projection. (3) Volume of Screen Light. — Although accurate records are not available, it seems probable that less than 200 lumens were projected upon the screen in the earlier motion picture theatres. This figure, as well as those on screen light which follow, are without shutter or film. When a shutter having 90-degree blades is used with no film, the figures will be reduced to one-half the values given. Further reduction, varying in amount, results from the density of the film being projected. Improvements in carbons and in optical equipment of the old-style, vertical-trim projection lamps raised the available screen light to about 1600 lumens. The adaptation of the high-intensity arc to projection about 1919 further increased the available screen light to about 5700 lumens and was a major factor in making possible the marked increase in seating capacity of motion picture theatres which characterized that period in the history of the industry. Later improvements in the condenser system of the high-intensity lamps give almost 8000 screen lumens from a 13.6-mm high-intensity positive operating at 125 amperes. A development of great importance to motion picture projection was the reflecting arc lamp, adapted to the d-c lowintensity arc about 1924 and to the highintensity arc, as the “Hi-Low” lamp, about 1926 or 1927. The optical principle of the reflecting arc lamp, by greatly increasing the angle of light picked up from the arc and projected upon the screen, made it possible to obtain 2000 screen lumens from the d-c low-intensity arc, and subsequent improvements in carbons have increased this figure to 2400 screen lumens. At 32 amperes of arc current the d-c low-intensity reflecting arc gives 50 per cent more screen light than the condensertype low-intensity arc operated at 50 am 184 BOXOFFICE BAROMETER