Re: A bit more on projectors...

[P3D John W Roberts <roberts@xxxxxxxxxxxxxxxxx>]

>Date: Thu, 19 Dec 1996 17:50:54 -0600
>From: "P3D  Dr. George A. Themelis"  <DrT-3d@xxxxxxxxxxxxxxxxxxxxxxxxxxx>
>Subject: A bit more on projectors...

>Well, when my 500W bulbs died, the
>change to 750W bulbs resulted in brighter images which leads me now 
>to conclude that bulbs lose brightness towards the end of their life.

The lifetime of some display devices is rated by the time to 50% reduction
in light output, rather than to total failure of the device.

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?

John R


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