[MF3D.FORUM:135] Re: Light for viewer question

[Tom Hubin <thubin@xxxxxxxxx>]

Hello Lincoln,
 
> I remember a while ago we were talking about using
> white LED's for the light source for a viewer.
> Did anyone end up trying them?

I got sidetracked by looking at various eyepieces. I have ordered some
lenses to make what I hope will be a pair of good eyepieces. I have an
assortment of white LEDs with which to experiment. Although, I have one
particular brand and model in mind.

Other distractions, like working for a living, have caused delays also.
But I have not quit. I am just slow.

> What about color balance with the white LED?

Some are soft or warm white like tungsten filament white. Some are cool
white and are somewhat bluish. I suspect that you could filter out the
blue spike and get close to a daylight white spectrum. Some appear white
over entire beam, both near and far field. Others have color that is
generally white but may be yellowish near the perimeter of the beam in
the far field. 
  
> I was thinking about using one LED for each eye and
> using thin plastic diffusion material.  This would be
> for my 8p 35mm viewer.  Do you think that would work?

It may work or you may need several white LEDs if the light exits the
diffuser over a full hemisphere.

Imaging the LED so that it ends up at your eye would be much more
efficient than using a diffuser. That requires that you place the LED
far enough behind the slide so that the beam illuminates the slide
evenly. Then a lens just before the light hits the slide. The lens
should image the LED through the eyepiece and end up at the exit pupil
of the eyepiece. That is where your eye is.

Place a piece of paper where your eye would be. Remove the slide. You
should see a projection of the LED itself on the paper. If so, then all
light passing through the slide will enter your eye. No light wasted.
 
> When I do it for a Med frmt viewer, will the light
> spread enough?

The amount of light should not depend on the format. However, the light
must spread enough to illuminate the entire slide evenly. That means
that the LED for a medium format slide will be farther away than for a
35mm slide because it needs the extra distance to spread over a larger
area.

Tom Hubin
thubin@xxxxxxxxx