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center of the crater; however, the core is necessary to furnish arc-supporting materials, and to maintain the crater position as centered as possible with respect to the carbon. Without it the crater would travel around the end of the carbon and give unsteady operation and light. Regardless of how much current is used on this type of carbon, no greater brilliancy could be obtained. The carbon tip would merely “boil away” faster.
CARBON COMPOSITION
In the high-intensity arc, the core of the positive carbon is much larger in proportion to the diameter of the carbon than with the low-intensity positive. This high-intensity core contains special rare-earth metals (such as cerium, lantanum, neodymium and praesodymium) which under the bombardment of the electrical current produce a deep crater containing extreme amounts of light energy radiation, especially in the visible part of the spectrum.
The upper part of Figure 1 shows the intrinsic brilliancy across the diameter of a 7mm and an 8mm positive suprex carbon operating at 50 amperes and 65 amperes, respectively. It can be seen that with these high-intensity carbons, because of their special cores, the brilliancy at the center of the crater reaches the high value of 630 candles
per square millimeter of crater surface. This brilliancy is easily over four times greater than the similar point on a 13mm low-intensity carbon operating at 40 amperes.
Because of this higher brilliancy available at the crater of the suprex high-inten
sity carbons, it has been possible to greatly increase the amount of light delivered to the projection screen. Although the crater area of the 7mm positive suprex carbon is much smaller than that of the 12mm lowintensity carbon (see Figure 1) the amount
of light delivered to the screen by the suprex carbon is from two to four times greater.
Other factors which help to increase the total amount of light are the larger reflectors in the new lamps, and the higher optical speed of both lamp and projector
lenses. Figure 2 shows a comparison of light output from low-intensity carbon lamps and from some sizes of the suprex high-intensity systems. It can be seen here that the total amount of light delivered to the screen can easily be increased from 1250
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FIGURE 2 — Comparison of operating characteristics for low-intensity and suprex carbon systems.
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28
MOTION PICTURE HERALD, DECEMBER I, 1951