Halogen lamps
edited 2/6/01
The standard incandescent lamp is made with a tungsten filament.
- electrical power heats the filament to about 3000 K
- the filament becomes incandescent (emits light)
- the radiation is continuous (from infrared to edge of the ultraviolet)
At 3000 K the light is slightly yellow in color
- sunlight appears to be "whiter"
- light from the lamp contains relatively little light in the blue/violet end of the spectrum
We can shift the color by raising the filament temperature
- (increasing voltage and thus the current to the bulb)
- unfortunately, at these temperature tungsten metal slowly vaporizes
- over periods as short as 4-10 hours the inner surface of the lamp becomes coated with a thin layer of tungsten metal
- this causes the output of the lamp to decrease significantly
- even worse, it thins the filament and it causes the bulb to burn out very quickly
- commercial photoflood lamps are made this way
- one expects to replace the lamps every 4-5 hours
The tungsten halide lamp differs in several details
- a small trace of Iodine vapor is included in the bulb
- the filament is designed to run hotter (3200 K typically)
- since glass would soften at the higher temperatures, the envelope of the bulb is made from quartz
-
The chemistry is simple, but fascinating
- at modest temperatures tungsten (W) reacts with iodine to form a WI3
- WI3 has a modest vapor pressure
- this process can attack and remove the metal coating on the inner surface of the envelope
- (but eventually the process would stop)
- at higher temperatures WI3 is unstable and reverts to W-metal and I2 vapor
- this occurs at the hot filament
- the net result is the evaporated tungsten metal is removed from the walls and returned to the filament
- thus, the filament suffers little, if any, metal loss at elevated temperatures
Consequences
- halide lamps have relatively long life
- they produce whiter light, with considerably more blue
- in fact, they produce a modest UV output
- most lamps use special quartz designed to capture the UV portion for safety reasons
- the lamps are much more efficient, producing perhaps twice the usable light per watt
- the lamp surface is much hotter
- lamp housings must be designed to prevent accidental contact (and burns)
- extra care is required that the lamp does not ignite flammable materials like paper or cloth (a lamp that's knocked over can cause a fire.)
- some halide bulbs have a double envelope, with a larger outer glass envelope that stays cooler.
In the laboratory, tungsten halide lamps are generally better light sources than standard incandescent bulbs
- they are brighter, for the same heat dissipation
- the lamps are smaller, with smaller filaments
- the lamps are richer in the blue and violet end of the spectrum
- most will produce usable light to about 350 nm (near ultraviolet)
Alternative names
- halogen lamps, tungsten-halide lamps, quartz lamps
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