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Re: Projection displays

We have definitive data on the retinal damage caused by scanning lasers. 
Have a look at http://www.hitl.washington.edu/publications/r-98-42/
for the abstract. Basically in the VRD retinal scanner the laser is such
low power that it can shine on the same rod or cone in the retina for 8
hours before doing permanent damage. Needless to say, you might notice
that the scanner has stopped working before then !

Of course for a projection display you want a lot more power, but the
paper gives some pointers at what the damage caused can be ..

Hope helps,
		Mark

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Mark Billinghurst 		| Human Interface Technology Laboratory
	| University of Washington, Box 352-142
fax: +1-206-543-5380		| Seattle, WA 98195
On Sat, 2 Jan 1999, John Flanagan wrote:

> At 09:55 AM 1/1/99 -0500,  wrote:
> 
> >So, the idea is shine a laser through the first Bragg cell for X
> >dimension scanning and through the second Bragg cell for Y dimension.
> >Voila', a solid state, no moving parts scanner.  About 15 degree FOV
> >we hope.
> >
> >Problems:  
> >
> >	1) Getting a bright enough laser
> >	2) If the scanning fails, not blinding someone with it
> 
> On the topic of #2, I have a question.  All right, so if your scanner
> fails, whatever the laser's pointing at will fry.  *But*, how extensive
> will the damage be?  The whole idea is to have the laser be focused enough
> to illuminate only one pixel's-worth of the subject's retina.  So if the
> heat of the laser doesn't start damaging adjacent tissue, you'll only burn
> out one "pixel" of the subject's eye, before whatever safety features of
> the system turn the laser off.  Given how human vision works, my bet is
> that the subject would either not notice *anything*, or the subject would
> be able to rapidly compensate for the loss of vision.  And this is all
> assuming that the hypothetical safety features are too slow to prevent the
> damage (which they might or might not be).  Still, I doubt that people
> would be all that happy about dead pixels cropping up on their retinae.
> Look how much of a big deal they make of dead pixels on laptops!
> 
> Another way of reducing the probabilities of damage would be to have the
> "home point" of the laser (where it would point if no deflection were
> performed) point to the retina's natural blind spot.  That wouldn't help if
> the laser got stuck at some set divergence, of course. 
> 
> Does anyone have any hard data on how much light is necessary to cause
> permanent damage?  If it's say, 50x what the maximum brightness of a normal
> pixel is, then you've got 50x the scan time of one pixel to detect the
> system failure and shut off or emergency-divert the laser.  For a
> 1000x1000x60Hz system, that would be about 0.83 microseconds before damage.
> 
> John Flanagan
> 
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