From: vicorp!ron@uunet.UU.NET (Ron Peterson)
Subject: Re: TECH: Projecting LCD panels
Date: Fri, 17 Jul 1992 21:40:32 GMT
Organization: V. I. Corporation, Northampton, Massachusetts



In article <1992Jul16.000252.9401@u.washington.edu> S.M.Clark@lut.ac.uk writes:

>I would like to construct a 'video projector' for a VIDEOPLACE/MANDALA
>system I am developing. I have heard that it is possible to project the
>image from a LCD panel (such as that found in a pocket TV) by integrating
>it with a slide projector and adding a cooling fan. Has anyone tried this?
>>From your collective experience in taking pocket TVs apart for HMDs do you
>think that is possible? What would be the best panel to use?

I've tried this.  I used a cheap ($30) black and white Casio lcd tv.
The tv was designed to be backlit so I shined the light from a slide
projector on the back and put a surplus copy lense in front and
presto!---projection TV.  I was able to project a sharp image about
eight feet wide onto a flat white screen.  There are several
considerations when trying to construct a device like this:

 1) Resolution - the tv had only about a 150x150 resolution.  So although
    I had an eight foot wide image, each pixel was large enough to see
    as a square rectangle on the screen (each about a centimeter square
    if I remember correctly.)  So you want to use the highest resolution
    tv you can get.

 2) When you send an image through a lense it gets inverted and left/right
    swapped.  Easy enough to turn the tv upsidedown to get the image
    upright but the left/right swap makes all text on screen backwards.
    Fortunately, the tv I used was designed to bounce the image off a
    mirror for viewing so the circuitry was designed to scan backwards
    and so appeared correctly on the screen.  You could try modifying
    the circuitry in a tv that doesn't use a mirror, but surface
    mount technology and IC's can make that tough.  Adding another
    lense to the system is another option but the easiest solution is
    to just bounce the image you are projecting off a front surface
    mirror.

 3) The projected image was not very bright even in a darkened room.
    If I had placed the tv right next to the light condensor (where
    a slide would normally go) it probably would have been much brighter.
    There are brighter grades of projection lamps that you can get for
    some slide projectors also.  I was using an EXY in a Kodak
    Carousel projector while an FHS is about 30% brighter I think.

 4) Heat is a problem because some lcds are sensitive to it.  Mine tended
    to fade to blank after about half an hour, after which I had to let
    it cool for a few minutes to get the image to reappear.

You can buy B&W lcd panels made to sit on top of overhead projectors.
These are usually for connection to IBM pc's though and I don't know
if they would handle video.  Also they are not all that cheap.
Also, some (all? the ones Radio Shack sells anyway) of the lcd tv's 
today seem to use a different type of lcd and so are constructed
without a backlight.  If they can be backlit it means cutting a hole
in the case of the tv (at the least) or rearranging its internal
components when removed from the case (at worst.)  Even given these
engineering problems I don't understand why we don't have high
quality color projection lcd tvs available to the consumer today
for under $300.  If you have any success, please send me a note.
I'd be interested in the construction details.
ron@vicorp.com or uunet!vicorp!ron