[ATM] Low Pressure Sodium Ballast

[Mark Holm]

Atlanta Light Bulbs, Inc. sells low pressure sodium ballasts.  Not so 
cheap though.
http://www.atlantalightbulbs.com/ecart/10Browse.asp?Category=B-HID+LPS+0-180W

Low pressure sodium lamps take a while to heat up.  It is several 
minutes from switch on to full output.

The green lines in fluorescent tubes come from the mercury arc 
discharge.  There are several other narrow lines in the mercury 
spectrum, but the green is brightest, not the least because human eyes 
are most sensitive there.  Mercury discharge makes a lot of UV.  That is 
why some can be used as germicidal lamps.  If you use a germicidal lamp, 
make sure to go overboard on the UV filtering, because the tube hasn't 
any and is pumping out a whole lot of UV, enough to kill things.  You 
don't want it killing your retina.  Actually, I would go overboard on UV 
filtering any uncoated mercury tube, just to be safe.  I like to see.

Most people think the low pressure mercury discharge is way too green 
for use as a general purpose illuminant, and also the energy in the UV 
goes to waste.  Hence the phosphor coating in ordinary fluorescent 
tubes.  The phosphor absorbs UV and emits in various bands, depending on 
the type of phosphor.  The bands are still narrow compared to sunlight, 
but much wider than the narrow lines from the mercury discharge.  
Fortunately, manufacturers do not use green emitting phosphors.  The 
mercury green lines already pump out more than enough green.  So, if you 
filter green, with rather wide band, inexpensive filters, you can 
reasonably isolate the green lines, because the manufacturers use 
phosphors that make red, yellow and blue to fill out the spectrum, but 
not green and because the nearest strong lines in the mercury spectrum 
are sufficiently far away.  I say green lines because there is a 
doublet, but for our sort of interferometry they are close enough 
together to often be used as if there were only one line.

-- 
Mark Holm
markholm at verizon.net
holmmarkd at gmail.com