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Re: [APD] replacement for metal halide lamp

In a very tiny nutshell:

a halogen lamp is an incandescent lamp that works by
running electric current through a tungsten wire
(filament), which gets it so hot that it gives off visble
light. The heat slowly evaporates the filament. The
presence of the halogen in the bulb makes the filament last
longer and the bulb stay brighter longer than ordinary
incandesent bulbs because it causes some of the evaporated
filament material to be redeposited on the filament. About
95%-97% of the input energy is given off as heat rather
than visilble light. This worsens the longer the bulb is
used. You can conect them directly to an current source --
they are self-limiting in the amount of current they will
carry. Because they can be made with a small, strong quartz
envelope, the filament can burn at a higher temp, giving
off somewhat more bluish light than a regular incandescent.

A Metal Halide is an electric arc lamp that works by
sending a high voltage through conductive gas that acts as
a plasma -- which means the gas gets weird and radiates
visible light. Diff combinations of gases and pressures
makes for different frequencies of light output. About 65%
of the input energy is given off as heat instead of visible
light. This worsens the longer the bulb is used. They
require a special circuit to provide higher voltage, to get
them started, and to limit the current they draw -- they
are not self-limiting and will go into a runaway condition
if a limiter is not provided.  In a runaway condition they
would burn out quickly, probably with what the light folks
call and "active failure" -- what yhou and I would call an
exploding bulb. To contain an active failure, a outer
transparent envelope usually surrounds the bulb. 

That's a very small nutshell, and if you try to read too
much into it, you'll crack it ;-)

But I hope that helps,
Scott H.

--- ktlau <ktlau at netvigator_com> wrote:
> Can anyone tell me the difference between a metal halide
> lamp and normal
> halogen lamp (those 35W small bulbs)?
> Metal halide lamps are expensive and generate a lot of
> heat. I wonder if
> halogen can be a close substitute. They also give out a
> lot of light
> (albeit the spectral density will be different), and
> consume less space.

S. Hieber

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Amano Returns
to the AGA Annual Convention
Nov 2004 -- Baltimore

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