[tech-3d] Re: holographic screen?? (was: fresnels & episcopes)

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--- In tech-3d@xxxx, "sergio baldissara" <bobongo@xxxx> wrote:
> I confess I can hardly understand what you (& other AA) mean by
> "holographic screen"...

As i understand the screen is made using holography.In this case one 
has to use traditional holographic setups , split a laser beam into an 
object and reference beam.By using appropriate beam shaping lenses and 
mirrors one lets the beams interfere in space and records the 
interference pattern in a high resolution plate or film.This is a 
common technique used for the production of screens for pilot HUDs in 
which case the photosensitive material is dichromated gelatin.
By controling the two beams one can make holographic optical elements 
(HOEs) that behave as traditional lenses or systems but with the added 
advantage of less weight and better optical optimization.The best of 
all is that you can produce easily off axis systems.
The disadvantage of HOEs is that they are pure diffraction systems so 
they dont work well with white light.They are best suited for 
manipulation of laser light.In the case of a holographic screen one 
should be very carefull in design as to reduce image color splitting 
due to different diffraction angles at different wavelenghts.The best 
way to avoid such dispersion would be to focus at exactly the screen's 
effective focal plane and use a very well collimated reference beam 
during holography.



 

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