From: rcd@ico.isc.com (Dick Dunn)
Subject: Re: Polarized light 3D systems: info wanted
Date: Wed, 29 May 1991 02:49:01 GMT
Organization: Interactive Systems Corporation, Boulder, CO



Thanks to testarne@athena.mit.edu (Thad E Starner) for info on the
Tektronix stereoscopic system.  A few comments and questions...

> ...The shutter switches between acting as
> a right circularly polarized filter and a left circularly polarized
> filter at rates of up to 120 Hz (60 Hz per eye)...

Later in the description, there are various figures of merit for turn-on
time and extinction ratios, all of which are better for the right eye than
the left.  Can you explain this?  Is there some mechanism such as the
shutter being actively driven for one polarization but allowed to "relax"
for the other polarization?  I'm curious about the mechanism; I'm also
curious whether the response asymmetry will be noticeable (probably not
overtly in the short term, but perhaps over the longer term or at a sub-
liminal level?).

> Interesting Manufacturer's Specs (again from 1988 User's Manual):
...
> Average light transmission    12%  (?????  seemed much better than
> that)

Light levels can be deceptive...12% is about 3 f-stops.

> The display used needs to have fast phosphors (~1.5 msec for decay)
> for this system to work well.

This is where I start getting very interested in how it feels to use the
display.  Phosphors for typical color monitors seem to be selected without
a lot of attention to matching persistence among the colors.  The differ-
ences aren't dramatic, but they can be made noticeable.  This could add to
(or partially compensate for) the effect of the "shutter" itself, which
Thad notes:

> ...The advantage of red objects is
> probably due to the good extinction ratio of red as opposed to green
> and blue as shown in the manufacturer's specs...

Actually, I'm confused about whether the color-dependent variation in the
extinction ratio is all due to the shutter itself (polarizers tend to be
more effective at longer wavelengths), or if some phosphor characteristic
is also factored in.

>...There are several ways
> to address this issue.  The first is to get a display with faster
> phosphor decay.  It is possible that the display used has a decay rate
> of > 1.5msec...

My concern with that would be flicker.  An interlaced monitor at 60 Hz will
be objectionable to a lot of people.  (I once tried a monitor at 56 Hz; I
started feeling physically ill after a few minutes.)  Longer-persistence
phosphors help cover the flicker problems of existing monitors.  So, are
you on the horns of a dilemma here, where faster phosphors will just trade
ghosting for flicker?  Thad already mentioned that
>...Another drawback was the flicker observed in our 
> system.

>...Another solution may be to increase the frame rate...

That gets you out of the ghosting/flicker dilemma.  It does come back to
my earlier question about the effect of the asymmetry of switching time
(about a factor of 10) for the shutter...if you double the switching rate,
does it become noticeable?
-- 
Dick Dunn     rcd@ico.isc.com -or- ico!rcd       Boulder, CO   (303)449-2870
   ...Simpler is better.