At 14:22 3/23/2000, Stuart MacMillan wrote:
>Same reason lifetime light bulbs aren't on the market, event though the
>technology has been around for decades.

Er...(assuming you mean filament bulbs)...you can make a bulb to last
arbitrarily long by simply running it at lower voltage.  That's why people
can build 130-volt bulbs and sell them to 120-volt customers as "5-year"
bulbs.  The trouble is that (aside from the color of the light, which could
be rectified at additional loss in efficiency) the energy costs rise
dramatically b/c even more of the energy is being given off as
infrared.  The standard (750-hour) bulb is a very deliberate choice of
factors in the operating region where light output goes up with the 4th
power of the voltage, but life goes down with the 15th power of the
voltage.  Photoflood bulbs, for example, give lots of light which is
comparatively white -- but they last an hour.  Indicator bulbs tend to be
run much cooler and give life of as much as 100,000 hours in special
cases.  The 750-hour bulb is a decent balance btw life, light quality, and
energy costs.

Recent techniques (xenon fill, Krypton fill and so forth) change the
balance somewhat.  Halogen techniques, where the evaporating filament is
caused to redeposit back on the filament instead of on the bulb capsule
walls, have to run very hot so that the halogen cycle works, so there's a
limit to how cool you can run them and still get improved life.  However,
the basic halogen bulb works pretty well with a design lifetime of 2,000
hours...

david

David Beierl - Providence, RI
  http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage"
'85 GL "Poor Relation"