Archived posting to the Leica Users Group, 1997/02/06

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Subject: Re: Leica slide projector
From: pgs@pa.dec.com (Patrick Sobalvarro)
Date: Thu, 6 Feb 1997 03:35:18 -0800

I don't know whether all lamps dissipate about the same proportion of
energy in the IR range, but that seems to be the base assumption here
(it actually surprises me a lot about fluorescents).  That said, I
realized that what has confused me while reading this discussion is
that David is talking about lumens, which are a very weird unit that
was made up just for humans.  Lumens are a measurement of light at 5.4
x 10^14 Hz, which is a cool yellow that we happen to perceive as
particularly bright.  Even if all lamps dissipate about the same
amount of energy in the IR range and the rest in the visible spectrum,
it is possible for two lamps that dissipate the same total energy to
have very different luminous efficacies because they can produce
different-colored light.  Which I think is what David was saying but I
didn't understand it because I was thinking of lumens as measuring all
visible light and was wondering where the rest of the energy was going
(gamma rays? sound?).