P3D Benefits of "frosted" light bulbs

["Dr. George A. Themelis" <DrT-3d@xxxxxxxxxxxxxxxxxxxxxxxxxxx>]

I have a friend who buys lots of light bulbs from me for his many 
viewers.  In the past he has ordered the clear shortened halogen 
bulbs.  Last time I accidentally sent him "frosted" bulbs.  He tried 
these bulbs and he was thrilled.  He told me that they soften or 
eliminate the shadows in his viewers.

There is a very good explanation for that:  In a halogen bulb the 
bright light is emitted from a small filament.  Because of the very 
small size of the light source, there is a tendency to form harsh 
shadows (like with our own sun).  With the "frosted" bulb the light is 
emitted by the glass envelope, which has a larger surface area.  This 
tends to soften shadows (like when a large cloud blocks the sun).

The halogen bulb is also responsible for two artifacts of its own:  

1) The wires holding the filament tend to cast a shadow.  
2) The bending of the glass at the top acts as a lens, focusing the 
   light and causing a bright ring.  

Sometimes these artifacts can be eliminated by orienting the bulb so 
that the shadows or bright ring are outside the viewing area.  The 
"frosted" bulb however will eliminate both artifacts.  I pointed out 
the bright ring in Jon Golden's Wollensak.  Since then he has been 
using "frosted" bulbs exclusively.

The quality of the light in a battery operated slide viewer is very 
important for me.  I want this light to be bright, uniform and white. 
Using frosted bulbs is one way to get uniform illumination.  But 
that's not the entire story.  Having a properly tuned viewer (clean 
contacts, bright white reflector) helps.  Also, having all the areas 
surrounding the bulb painted or covered with white reflectors to 
induce secondary reflections, helps too.

-- George Themelis


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