Figure 13. Common incandescent filament forms and their designations.
The standard incandescent source utilizes a tungsten filament in a gas-filled enclosure of commercial glass. These basic lamp types have been available for many years of motion-picture production. It has been traditional to produce two ranges of Kelvin temperature for professional use in these types of lamps. Typically, at the rated voltage (i.e., 120 volts), a 3200K and a 3350K design have been available. 3350K lamps are close to the Photoflood balance of Type
A color film and 3200K lamps are used for all professional color motion picture films.
The tungsten-halogen lamp is an incandescent lamp. Its radiant energy output is based strictly on the temperature of its filament, but it offers an important difference in operating principles when compared to the standard incandescent type.
The addition of a halogen gas in the fill plus the use of high temperature materials in the envelope of the lamp (quartz or fused silica, and recently hard glass), has resulted in a design which does not experience the blackening effect with age that is characteristic of the standard incandescent types. Due to the presence of the "halogen cycle" within the lamp, the tungsten is not permitted to deposit on the bulb walls (as long as the wall temperature is above 250 degrees C). It is, in fact, re-deposited on the filament (See Figure 14). The results of this development have been manifold:
1. Tungsten-halogen lamps have minimal loss in lumen output and no significant shift in color temperature during their entire life.
2. Tungsten-halogen lamps with similar configurations, wattages and initial lumen outputs as standard incandescent types are now produced with substantially longer useful life.
3. Because of the requirement for high bulb wall temperatures, it has been necessary to shrink the envelope size of these lamps, resulting in completely new families of lamps with much smaller external dimensions than the standard incandescent equivalent.
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