Re: A bit more on projectors...

[P3D Gabriel Jacob <jacob@xxxxxxxxxxxxxx>]

John W Roberts writes

>In the case of incandescent bulbs, I think a major mechanism is often
>deposition of material from the filament on the envelope. I believe sealed
>beam automobile headlights were said to do that. I suppose that if the 
>filament gets thinner in a fairly uniform manner, it could decrease the 
>amount of power drawn by the bulb by raising the resistance.
>
>Does anyone have more information on this? Are the aging characteristics of
>halogen bulbs different from others? Also, do incandescent bulbs tend to 
>exhibit any color shift as they age?

Yes halogens are significantly different from regular tungsten lamps. In 
halogen lamps, the halogen gas effectively removes the deposited tungsten
from the envelope and returns it back to the filament. This helps provide
constant light compared to regular tungsten lamps which decrease
appreciably in their light output with age. Also other benefits of the 
halogen lamps is that they consequently have longer lifetimes and brighter
output because of their higher operating temperature. So to summarize,
in regular tungsten lamps, the filament evaporates and becomes thinner as
you mention and thus consumes less power. This lowering power consumption
decreases the operating temperature and will shift the color to red. The
light output decreases because of bulb blackening and also the lowering
filament temperature as mentioned. One final note, on halogen lamps, since
they use the high temperatures help to perpetuate the halogen cycle, it 
is not advisable to operate them much lower than their rated voltage.

On a slightly different note, it's interesting that the Stereomatic
uses one 500W bulb. Intuitively one would think that if it had used
two 500W bulbs, the output would be twice as much in each side, but
this is not the case. A pratical example of this is, one 100W light
bulb is more efficiant than two 50W light bulbs and will give more
total light output than the two. Of course in the stereomatic there
are probably other light losses introduced by the mirrors and light
path. 

Gabriel





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