Camera/Projector lens question.

(very naive understanding of camera lenses)

So what, the post was interesting. And it does sound like an interesting
project.

Eric Webster wrote:


My current thoughts are to replace the lcd's standard back light with
a
metal halide lamp. They are very bright have 10k+ hour lifespans and
reasonable color. There are several problems. They emit light from a
small cylinder not a point and produce lots of heat.


Metal halide lamps these days have very small arc sizes, and are
essentially a point source, especially for an LCD as large as 17"
diagonal. Bear in mind that DMD projectors use similar light sources
and must deal with drastically smaller object sizes.

Try to get a lamp with an ellipsoidal reflector, which forms a very
intense spot of light. If you place one end of a rectangular rod of
fused silica at the focal spot, then light will exit the other end in a
very uniform pattern. The rectangular rod must be polished on all six
faces, and is usually called a scrambler or beam homogenizer. It works
by total internal reflection, and should be as long as possible. A
simple lens system can then be used to image the output end of the
scrambler onto your LCD panel to provide efficient and very uniform
illumination. Make sure that the the rectangular cross section of the
scrambler rod matches the aspect ratio of the LCD. You will need a
very large field lens near the LCD, and you might try using the Fresnel
lenses from a junked overhead projector.
Brian
www.caldwellphotographic.com

Mike Owens wrote:
I didn't bother mentioning color drift due to viewing angle since the
normal backlight from a LCD wouldn't work. If one designed this like a
regular projection system, the colors would certainly change from the
center to the edge of the resulting projection due to the differing
angles the light passes through the LCD. How much color shift is
present would depend on the viewing angle of the LCD compared to the
distance from the projection lens. Newer LCDs with wider viewing angles
would definitely help.

The only way to prevent the color shift would be to have all the light
pass through the screen perpindicular to it. Of course, you'll need a
rather large parabolic reflector to focus the light properly on a 17"
panel, then another one to direct it towards the camera lens.

However, if you have an overhead projector handy, you might try placing
the disassembled LCD BELOW the glass that you normally place the
transparencies on. This glass is a large lens which takes the parallel
light generated by the reflector and focuses it (to some extent anyway)
on the projection head. Of course the heat of the lamp might destroy
the panel...
The whole thing would be an interesting science experiment.


I'm not so sure that putting the lcd sans backilght under the fresnel
panel that the transparecies normaly sit is such a good idea.

A few years a go, before digital projectors, our department used an LCD
panel that sat on the over-head projector in place of a transparency and
it looked not too bad.
It had a slightly smaller screen size (15 inches I think) and had no
problems with different density across the image (that I could see), the
only problem is that an OHP get pretty warm and the panel started to go
wierd colours after about 10~15 minutes of continuous use.