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LAMP LIFE AND LAMP CONSERVATION 97
difference current 0. — amp."; if the new film requires less lamp current, enter in the log book : ' ' Subtract direct positive film difference current 0. — amp."
This procedure applies only to films of the same type but of different lot numbers. Films of different types cannot be properly compared by this method, since the slopes of density-exposure curves will not be alike.
Lamp Life and Lamp Conservation
In order to obtain sufficient exposure for high-resolving-power soundrecording films such as EK 5372, it is often necessary for lamp currents of 2.25 amps, or more to be used with a 2-amp. lamp. An accompanying result of high exposure currents is short lamp life; occasional comments have been made by users of recording equipment that short life has been an annoying maintenance problem.
When an original is recorded, it is essential that the effective width of the recording light beam be exceptionally small and that the film used be of the highest resolving power practicable. In everyday language, the recording line of light that is focused on the film shall operate like a very fine pencil point that is drawing the details of the sound waves being recorded. Similarly, the film used shall be capable of clearly rendering the very fine lines which the optical system of a good sound recorder can draw. The fidelity of the recorded sound is directly dependent upon the narrowness of the line of light in the sound recorder and upon the resolving power of the sound-recording film used.
The lamp used in the new Maurer sound-recording machine is rated at 8 volts-2 amps. ; it has a nominal life at this rating of over 25 hours. If the lamp is operated above 2.25 amps, it is obvious that its life is correspondingly reduced. The estimated life of the lamp may be readily calculated by a lamp engineer from the above data. Even at such short life as that encountered with 2.25 amps., a single lamp is still capable of recording several reels of film; the cost of the lamp per reel is still but a fraction of the cost of the film itself.
There is a very simple method of increasing effective lamp life: win n not actually recording, reduce the lamp current by turning the lamp rheostat to the lowest position {counter clockwise). Do not turn the lamp completely out; burning it at reduced current causes far less shock to the filament than turning it off and on. Turning the lamp down as a matter of habit will result in a great economy of lamp life ; by doing so