🚀 go-pugleaf

In article <johnhDnCwsp.1zu@netcom.com> johnh@netcom.com (john haskey) writes:

>It may be an old wives tale but I had heard that astronomers were very
>enthusiastic about pitching the Sodium Vapor lamps to cities since they
>were mono-chromatic ( which the cities didn't care about) and that they
>used a lot less energy than Mercury Vapor lamps (which the cities did
>care about).  But then cities discovered low-pressure Sodium Lamps which
>used even less energy but aren't mono-chromatic so astronomers are again
>bothered with 'light polution.'  Anyone else heard this???

I have! It is not apocryphal.

Originally, mercury vapor and sodium vapor street lamps were all of
the low pressure variety, which are almost monochromatic. The light
from these low pressure gas discharge lamps has discrete spectral
lines in it, rather than being the continuous blend of frequencies
that you get from an incandescant lamp.

There is a problem with low pressure lamps, though. Color rendition in
their light is terrible. I have read of police forces complaining
because you cannot tell the color of the perpetrator's car or
clothing. And people look cadaverous in their light.

High pressure gas discharge lamps remedy this color rendition problem.
The light is still yellow, but instead of the just a few frequencies
of light being emitted, the spectral lines are "smeared", yielding a
mix of frequencies and hence better color rendition. Unfortunately, it
is bad for the astronomers. This smearing, or continuous spectrum,
makes high pressure gas discharge light much harder to filter out.

It is part of a larger tragedy of light pollution. Most people don't
realize the value of darkness until they don't have any. I hate it
when city people bring their fear of the dark with them to the
country. There are other ways to protect your property, other than
flooding it (and the much greater surroundings) with light.

Dan L
-- 
Dan Liddell   dan@netcom.com