P3D Viewer variable illumination

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

>From: sbere@xxxxxxxxxxxxx (Steve Berezin)

>I made several other viewers based on the brighter Xenon bulb but felt I
>needed to add a dimmer to allow people to diminish the light if they
>were viewing slides in a dark room.  

That's a good point.  As I have said in the past, the main reason for
wanting variable illumination is the ambient light during viewing.
If you view slides in an "office illumination" environment then you
need plenty of light.  In a dark rook late at night you need much
less light.  I once brought my red button in a wedding reception.
The room was very dark.  People were almost blinded when they pressed
the red button... Boy, how I wished I had a way to reduce the reduce 
the brightness!

I think the easiest way to get two levels of brightness in a battery
operated viewer is to carry two bulbs.  Say a halogen 0.8A and a No.
14 and switch if needed.

>Although AC current would allow a
>simple commercially available Rheostat to be used for a dimmer, DC
>current required that I make a potentiometer, resister, transistor
>combination that would be expensive and time-consuming to produce...

Correction:  By "AC current" you mean 110 VAC from a house outlet.
It is the high voltage that makes possible to use simple rheostats
(like in Kodaslide and Realist AC viewers) not the AC current per se.
Also, I do make a series of voltage-regulated power supplies to power
2.5V light bulbs.  Yes, they are time-consuming for me to make but
they are not too expensive and they are well-worth it IMO.

>I found that graduations in brightness can also be achieved by moving
>the reflector closer to the light source.  

There is a danger here:  The closer the bulb is to the reflector, 
the steeper the illumination drop-off at the edges, and this
can be a problem if the viewer is supposed to support 7p formats or
wider.  The ideal would be to have the bulb located at equal distances
from all points of the reflector.  You lose intensity but you gain
uniformity.  What is more important for you?  (Such design does not
exist... the deWijs "Comby" light supply has two halogen bulbs, one
for each side of the slide).

George Themelis