From: rspeirs@javelin.sim.es.com (Ron Speirs)
Subject: Re: Lamp intensity vs. lifetime (was Re: 486 CPU KOOLERS FOR SALE)
Organization: Evans & Sutherland Corp.
Date: Tue, 17 Nov 1992 18:36:34 GMT

In article <1992Nov16.164101.20150@bnr.ca> mwandel@bnr.ca (Markus Wandel) writes:
>In article <Bx93z4.n2B@cs.dal.ca> legrady@ug.cs.dal.ca (Tom Legrady) writes:
>>
>>I don't know about the exact relationship of heat and lifetime for
>>electronics comnponents, but there is an inverse exponential effect on
>>the life of high-intensity lamps, such as the ones used for stage
>>lighting. I have to fudge the exact numbers, since I've been out of
>>theatre for a number of years, but dropping a lamp to 90% full
>>intensity generally doubles the expected life of a lamp. Dropping to
>>80% results in an unlimited expected life. Since our reaction to light
>>is non-linear, dropping to 90% has little effect on preceived
>>brightness. Since the lamps used cost $30 - $50  or more apiece and
>>have an expected life ranging from 300 - 2000 hours, doubling the life
>>is a desirable things to do. Of course, if you drop it, it still dies. 
>
>What about photographic floodlights?  These things have expected lifetimes
>of just 2-8 hours.  I thought if I ever use the lamps and bulbs that I have
>I would build a control box that allows the lamps to be dimmed until the
>moment they are needed (for a still photo), then increased to full intensity,
>then dimmed again.  That would save on heat, lamp lifetime, power
>consumption, etc.  Several ways to dim a lamp come to mind:
>
>  1. Two identical ones in series
>  2. Diode in series (halfwave rectification)
>  3. Electronic dimmer.
>
>But there are a couple of things I am not sure about.  Lamp dimmers cause
>light bulbs to buzz, surely not a good thing for a fragile, overheated
>filament you are trying to extend the life of.  The diode approach also
>messes with the AC waveform.  Two in series sounds pretty good, but when
>I switch over, the lamps will be briefly off, then fired up to full
>intensity.  Would this switching transient and associated rapid heating
>or cooling of the filament be just as bad for the bulb as leaving it on
>high intensity in the first place?  I'm guessing this because most bulbs
>I've seen burn out did so when turned on.
>
You are right about lamps dimmers causing lamps to buzz.  This is because
the dimmer chops up the waveform (I won't go into details here).  Perhaps
someone else would have some data comparing lamp life on a non-linear
waveform with lamp life on a pure sinusoid.

Operating two identical lamps in series would indeed extend their life,
but they wouldn't be very bright (and their color temperature would also
be lower).  Perhaps you could employ a trick used in slide projectors:
get an appropriately-sized ballast resistor and put in series with the
lamp.  Also install a switch to short out the resistor to give the lamp
full power when needed.  If you want continuously variable pure sinusoidal
output, you will have to buy a variable transformer (trade name "Variac).
This could get a little pricey if you need a large unit.

Ron Speirs, Evans & Sutherland Computer Corp., Salt Lake City, Utah

Newsgroups: rec.photo