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786 HANDBOOK OF PROJECTION FOR
tres formerly used the ordinary arc, and as high as 120 ampe: with very little if any better results.
We therefore recommend to projectionists and theatre mar agements that the ordinary arc be discarded in favor of eithf the high intensity arc, the reflector arc lamp or the incande cent — the high intensity only in cases where the maximum possible intensity of screen illumination is demanded.
THE HIGH INTENSITY ARC LAMP.— In the High Intensity lamp the positive carbon rests in a horizontal position, with its center exactly upon the optical axis of the projector optical train. This gives it an enormous advantage as against the ordinary arc, because its crater faces the collector lens squarely, and in exactly the correct position, whereas the crater of the ordinary arc canot possibly be maintained in good form at an angle of less than thirty-five degrees with the face of the collector lens ; also it is not perfect as to form, or necessarily central upon the optical axis of the projector optical train.
From this we understand that whereas the high intensity crater directs its strongest light flux straight toward the center of the condenser, the ordinary arc, due to the position of its crater, sends forward its strongest light flux toward the lower half of the condenser and the front, lower end of the lamphouse wall.
Another point in favor of the high intensity is that the brilliancy of the high intensity arc crater per unit area is very much higher than is that of the ordinary arc crater. The brilliancy of the ordinary arc crater when operating with cored carbons is given by Blondel as 132 c. p. per square m. m., but by reason of modern improvement in projection carbons this is claimed to be increased to about 160 c. p.
Engineers claim a brilliancy of 500 c. p. per square m. m. for the high intensity arc. We have not verified this claim, but certainly the high intensity arc crater has a very much higher brilliancy per unit area than has the crater of the ordinary arc. That last statement is not a subject for questioning.