Early art required a powerful light source because of low phototube and amplifier efficiency.
Developments extending back over the past ten to fifteen years (1924-1938) have provided increased phototube-amplifier efficiency, and, as a result, a marked reduction in the amount of light required. A standard sound-motion-picture tungsten-filament exciter lamp, 50-75 wafts, provides ample light for most systems employed today.
See Whitaker and Artzt references.
Withaker & Artzt (RCA Institutes Technical Press, Radio Facsimile, pp. 229-247, J. N. Whitaker & M. Artzt)
(C) Marius Rensen