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/snip/
Until the advent of concentrated arc lamps and then inexpensive LASERs,
Std. lab lamps were W ribbon filaments operating at 6 => 14 V. The
30A/T20/4 at 30A has so long a life that it's not given in that
table It's a 60 cd; T (color) 2300 K; 225 W lamp. If "over voltage" to
640 W; 700 cd; 3000 K, it still has a long life. However, another table
gives its life as 300 hr. at 3100 and 100 at 3200 (color temp.)
A tungsten table gives the evaporation rate of W as a function of temp.
(deg. Kelvin; color temp. and thermo. temp. differ by ~ 50 to 110 deg. in
this range). If one assumes that life is proportional to the rate, I find
the life is exponential with one order for ~ every 200 deg. Therefore, at
3,000 deg. the life should be ~ 1k hr. and at 2300 ~ > 100 k hr. Long
before this, the envelop will be darkened.
bc
p.s. I think it was I. Langmuir's * great invention to add an inert gas to
reduce the evaporation rate. Again a trade off: much longer life, but
reduced efficiency due to the thermal conductivity.
* Yes (1913), and I discover that H. Becquerel (1867!) was the inventor
of the fluorescent lamp!
/snip/
>A rational lamp selection parameter would be
> lumen.hours per watt.cent (of first cost)
/snip/
>Brian Whatcott Altus
>