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Mar., 1935] LlGHT SOURCES FOR PROJECTION 199
class of projector user the most economical service for his particular requirements. These modern lamps have made it possible, except in the very largest units, to dispense with the cost and inconvenience of low-voltage operation for new equipments and to substitute standardvoltage lamps in the older projectors as well.
DISCUSSION
MR. PALMER: Is the advantage of air cooling principally to prevent blackening of the bulb, or to give a longer filament life?
MR. CARLSON: Air cooling seems to be an unimportant factor in both bulb blackening and filament life, but forced ventilation is essential where high wattages are employed in a small bulb, if the glass is not to fail. Lamps that must operate in projectors having only natural ventilation are necessarily either of lower wattage for a particular bulb size or, for a given wattage, of larger diameter.
MR. MITCHELL: The problem of the amateur is different from that of the theater. First, there is no question of box-office benefit involved. Then, the amateur requires a maximum of light source and a minimum of bulk. The effect of air cooling is three-fold: first, it decreases lamp blackening; second, it increases lamp life; third, and perhaps most important, it keeps the projector cool, which is definitely desirable in equipment used by amateurs.
The amateur, and more particularly the industrial user, has pressed the manufacturer for more light — extra light is desired without appreciable increase in bulk. The researches referred to by Mr. Carlson make it possible to give the maximum effective light in the most compact form, and users of the small 16-mm. projectors have been able to show pictures of theatrical quality to fairly large audiences. For instance, with a 750-watt lamp we quite regularly show a 12 or 14-ft. picture to 1000 or 2000 people with quite satisfactory results.
MR. GAGE: Some time ago I had occasion to study condenser design for these small projectors to see what could be done, and I made a certain assumption, namely, that the filament of the lamp was going to be in the center of either a tubular or globular bulb. That then seemed reasonable for what seemed to be a necessary size of filament for adequate illumination such as is desired by the amateurs. It led to a bulb of a certain diameter, and therefore the glass of the condenser had to be placed outside the bulb. That led to a certain condenser design.
Those who make the projectors like to have very small condensers. Now the whole thing was completely revised from our standpoint by what the lamp designers did, pushing the filament of the lamp close to one side of the bulb. Then the projector manufacturers found that they could use those 750-watt lamps if they were ventilated. I do not see any reason why that can not be done with the professional-size projectors if it is found that the optics of the system will work with the filament brightness available. We are not now limited as to the position of the filament in the lamp if we are willing to use forced ventilation.
MR. BEGGS: The factors that Mr. Carlson presented are in good agreement with our findings, and therefore may be taken as representative. One of the most important items encountered in recent years is the limitation of bulb glass tern