old Hughes video projector - test-drb@ccmp.vtda.org

29 Jun 2006

...
 I've just got hold of an old (probably late-80s)
Hughes 800 projector.
It's a couple of feet long and very very heavy.  Inside is a big xenon
lamp, a CRT of some kind, and some optics.  It appears to work by
shining the lamp (through mirrors) onto the face of the CRT.  The big
problem is, it doesn't appear to produce any video.  I can hear the CRT
scan coils going.  The scan pitch changes when I remove the video
source.  In fact, it does appear to do all the things I'd expect when I
press buttons, but no picture comes out :-/ 
I think this might be an Image Light Amplifier (ILA). This used small CRT's coupled to
a liquid crystal light valve.
The ILA was a sandwich of materials that included a photosensor and the liquid crystal
material. The CRT wrote it's image onto the photosensor which converted the light into
a varying voltage. This voltage was transferred to the liquid crystal material which could
now reflect light from the Xenon source. This is not a pixel based system, the liquid
crystal material was a continuos sheet so would be able to image the full resolution of
the CRT. They can have an almost film like image because there is no pixel structure. I
believe the CRT's were infrared and so you could not see an image on the CRT face
directly.

Bob