Halogen bulbs and heat: another try to reduce the confusion-
Basics of light bulbs: early ones had a vacuum inside. Trouble
was, any filament molecule which evaporated travelled straight
line to the glass bulb and deposited there, darkening bulb and
reducing filament life. The vacuum was used because, with air
in the bulb, the filament would burn at operating temperature.
Later a non-reactive gas was added inside. Evaporating filament
atoms would then go into Brownian motion in the gas, and many
would preferentially condense on the filament again when turned
off. (But some lesser amount still condensed on the glass, and
the re-condensation on the filament was not perfectly even, so
the bulb lasted longer but not forever).
The main tradeoff to consider: higher filament temperature gives
higher efficiency ie. more light with less heat, but more
evaporation so shorter life. Adding a suitable gas enabled not
only longer life, but also some rise in filament temperature,
giving more light for the same power.
Concept of the quartz-halogen bulb: make the bulb out of
very-high temperature resistant glass; and make it very small,
so it gets very hot! so that any filament metal which might
try to settle on the bulb gets re-evaporated, and eventually
condenses back on the filament. Hence: longer life, and the
potential for even higher filament temperature and so more light.
Myths debunked: halogen (quartz-halogen) bulbs which replace
conventional bulbs are usually the same wattage, but brighter
due to higher efficiency. (It's a no-brainer to get more light,
and heat, by switching to a higher power bulb).
-the bulb is more efficient, ie more light less heat; yet
still the glass is hotter, but only because the bulb is much
smaller.
-the spectrum is also shifted: a "whiter" less yellowly light,
rather less infrared, and more UV!- not a lot, but a significant
increase compared to a conventional bulb. Some of the higher
wattage quartz-halogens have a caution to this effect on
the box they come in.
Examples of the small bulb: our 50W, 12V halogen potlights in
the kitchen are in a bulb about 1/4 in. across. The 300W
floorlamp has a bulb in a cylinder about 1/2 in across by
2in. long.
Now the confusion: many halogens are effectively the same size
as the conventional bulb they replace- eg. halogen "sealed beam"
headlights. However, look closely: there is a tiny quartz-halogen
bulb within the main bulb. (If not, it's not a high efficiency
bulb.) In this design, only the inner bulb is very hot; the
outer bulb is cooler than a regular bulb, because of the higher
light efficiency of the bulb, meaning less waste heat to be
dissipated from the outer bulb surface.
HOWEVER: around the base (socket) area of the bulb, it is
possible for the bulb to get hotter than that part of a regular
bulb. The hot inner bulb is joined to the outer bulb with glass,
or at least by the power wires to the filament, around the base.
So any extra heat in such a bulb is only a concern around the bulb
socket. Even there, likely only a concern with some fully
enclosing plastic sockets: they don't carry away the little
bit of extra base heat, and may have a low melting point.
Hope this helps...
Dave Agnew crm24@bnr.ca
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