Re: Color temp of lamps in projection

[P3D Dr. George A. Themelis <DrT-3d@xxxxxxxxxxxxxxxxxxxxxxxxxxx>]

Paul Talbot is looking for a comparison of the viewer halogen bulb
and the lamp of the projector.  He is interested in this subject
because he has noticed that certain pictures work better in a
viewer vs. in projection and he wonders if the color temperature is 
the problem.

The problem might not be so much the color temperature but the light
intensity.  The eye tends to get used to the color of the light
to which it is exposed.   Any bright source of light appears "white" 
to the eye if there is no direct comparison with something else.  
For example, the color of the halogenized viewer appears white, just 
because the intensity of the light is high.  If you compare this 
with the color of a fluorescent bulb or direct sunlight (outdoors) 
you will conclude that this halogen bulb is actually very orange.

One factor complicating the comparison of the effects of light 
intensity vs. light temperature is that when you reduce the light 
intensity of a light bulb by reducing the voltage (as it is done in 
a Kodaslide viewer with a rheostat) you are also reducing the color 
temperature.  Experiments show that light with higher color 
temperature appears brighter to the eye that light of lower
color temperature and higher intensity.  For example, putting a 
blue filter in a Kodaslide viewer will make the viewer appear
brighter even though the intensity is slightly reduced.  But the
fact remains that the eye gets used to the light source and
any sufficiently bright light appears white when there is nothing
to compare it against.

Stereo projection has the disadvantage that the polarizing filters
reduce the intensity of the light.  The result of this reduced
intensity is that some slides do not have the snap that they have
in the viewer.  If you experiment and remove the polarizing
filters from the projector you will have the impression that the
the light gets "whiter" and the slides look better.

I personally like to use bright light in my viewer because I think 
this makes the good slides look even better (the bad ones look worse 
BTW).  And that's why I am trying to find ways in increase the light 
output in my projection system.

George Themelis



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