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flicker disappears and the arc is completely steady during the rest of the burning period. One possible explanation is that the high ignition voltage produces minute irregularities on the surface of the cathode. As the cathode heats up, these bubbles smooth out. The problem is not considered to be serious as it is usually possible to pre-heat the arc before screenings. This is good practice with the carbon arc as well.
Because the xenon arc lamp radiates its light in all directions, an auxiliary mirror is placed in front to reflect light radiated in a forward direction back to the main mirror. The superimposition of this real but reversed image of the arc requires extremely careful adjustment if distribution on the screen is to be uniform. Further, the discharge between the electrodes is bell-shaped. If the major portion of the arc is to be utilized, some optical rectification is necessary for good screen distribution. One such optical rectifier or corrector is the Zeiss-Ikon Honeycomb lens. A single element corrective lens designed to spread the light evenly across the aperture is also used. That these devices are practical is evidenced by reports of up to 90% side-to-side screen light distribution, compared to the center of the screen.
It was discovered in the first the
7S-Q
ater installations of xenon lamps thyt the violent igniter discharge caused a brief cracking in the sound system. To overcome this both lamps weu ignited before the performance and the "off" lamp current was decreased. Such operation ended the sound problem without materially affecting the life of the lamps. Shortly before the changeover, the current was increased to normal and the change from one projector to another effected without a noticeable change in screen brightness. Later, a relay was inserted in the igniter circuit which shorted the amplifier input, interrupting the sound for about 0.1 sec. during ignition. A sound interruption cannot be tolerated in a review room. Films being viewed are subject to very critical visual and aural examination. A recorded foreign noise, a poorly looped splice or a projector sound system defect will in most cases prompt intensive investigation. Most of the installations surveyed reported igniter noise problems. Of course, if both lamps are left on during the entire performance, there is no problem. A more sophisticated approach might be to investigate the effectiveness of improved shielding around the first stage of the preamplifiers, the addition of a suitable low-pass filter in the first stage grid, or a small r-f (Please turn to page 6)
Get all the facts! Ask your equipment dealer for the new 1964 Kollmorgen Projection Lens Catalog or write us direct.
KOLLMORHEN
CORPORATION'
NORTHAMPTON. MASSACHUSETTS
International Projectionist September, 1964