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Tungsten-Halogen Lamps
Theory of Operation
Tungsten-halogen lamps are a subset of incandescent lamps,
which are based on a filament (a thin electrically-conductive wire) contained
within a glass bulb. The tungsten filament of the lamp provides electrical resistance
in an electrical circuit; this resistance generates heat in the filament when
the circuit is energized. This heat is sufficient to cause the filament to
become "white-hot" and emit visible light, i.e. to incandesce. This
incandescence produces luminous flux that is emitted by the underwater lighting
fixture, travels to the target, and reflects to the viewer, thereby
illuminating the target.
Tungsten is the primary material of choice for most
incandescent lamps, due to its high melting point (3655 K) and the high
strength and ductility of tungsten wire. Only a portion of the total radiation
from an incandescent source is in the visible region of the spectrum; most of
the rest is radiated as waste heat. As the temperature of the filament increases,
the source's radiation in the visible region increases faster than that in the
infrared; hence, the efficacy of an incandescent lamp is directly related to
the temperature of its filament. Maximum lighting efficacy (approximately 53
lumens/watt) would be achieved by operating a tungsten filament at its melting
point (3655 K), but lamp lifetime considerations necessitate operation at lower
filament temperatures (usually between 2950 K and 3450 K), at the expense of
lighting efficacy.
The tungsten-halogen cycle operating within the lamp is a
closed-loop regenerative one. Hot tungsten evaporates from the filament during
incandescent operation. In older-technology lamps, this tungsten would deposit
on the inner surface of the bulb, causing bulb wall blackening. However, within
the tungsten-halogen cycle, the tungsten does not deposit on the inner surface
of the bulb wall; rather, the evaporated tungsten gas chemically combines with
the halide gas fill and circulates inside the bulb, transported by convection
currents. Thereafter it redeposits onto the filament and separates again into
its tungsten and halide components, maintaining a clean bulb wall.
Operating Position
These lamps exhibit very little change in efficacy or electrical characteristics when operated in a horizontal (rather than vertical) position. They can consequently be operated in any position, with no significant effect on light output.