Orthoseat and window, retry

[P3D John Bercovitz <bercov@xxxxxxxxxxx>]

> I'll try to think of how to say this stuff more clearly 
> and post again.  (What's that groaning sound?)

Let me try giving you some examples.  I learn better by examples; 
maybe you do too.

The 7P and 5P systems have slightly different specs so I'll just make 
up an imaginary system (with numbers you can do in your head) and then 
we won't be talking about either one.   Focal length of the system is 
36 mm.  Infinity spacing is 66 mm.  Mask aperture spacing is 66 - 1.2 
= 64.8 mm.  (The 1.2 comes from the focal length of the system, which 
is 36 mm, divided by 30 because 1 in 30 is about the maximum offset 
you can put in a stereograph and have any and all people say it's 
viewable regardless of the scene or conditions.)  The window is 
located at a distance of (66/1.2)36 = 1980 mm.  The apertures in the 
mask (image areas) are 24 mm square.  So the orthoseat is located 
36/24 = 1.5 times the image height from the image.

Anything spaced at 66 is at infinity and anything (including the mask 
apertures) which is spaced at 64.8 is "at the window".  Generally 
everything else will lie in between.

A projector's job is to magnify the image.  If the image is magnified 
100 times, it will now be 2400 mm square.  The offset will also be 
magnified, from 1.2 mm to 120 mm.  What happens if you make the mask 
aperture images coincide on the screen?  The window will be located at 
the screen alright, but the infinity spacing will be 120 mm and you'll 
be 1.5*2400 = 3600 mm from the screen.  If your eye spacing is the 
same as the system's (66 mm) you'll have to make your eyes diverge 
almost a degree for infinity.  Since you're an old hand at freeviewing 
stereocards from 8", you won't even notice.  But what about the guy 
who's not used to it?  He may think something is wrong and won't be 
able to figure out what it is.  (The guy at the 5th percentile has an 
eye spacing of 58 mm which makes his divergence also about one degree.)

Now what happens if we space the infinity points correctly at 66 mm?  
The window will be spaced at 66-120 = -54 and it will pop off the 
screen.  (The - sign means your lines of sight cross between you and 
the screen instead of behind the screen.)

What happens if we magnify the image 50 times?  The offset is now 
50*1.2 = 60 mm and the image height is 50*24 = 1200 mm and the 
orthoseat is 1200*1.5 = 3600 mm from the screen.  If you space 
infinity at 66, then the window spacing will be 66-60 = +6 mm and the 
window will be slightly behind the screen.

How are we gonna put the window at the screen so we can operate this 
thing in subdued light instead of in a perfectly black theater?  Well, 
we have to make the window spacing = 0 of course and that means we 
have to magnify the offset from 1.2 to 66 which is a magnification of 
66/1.2 = 55X.  The image height is now 55*24 = 1320 mm and sorry, it 
won't fit on your 4 foot screen.  That's the trouble with my made-up 
system.  8-)

Does this make sense?  Wanna try it with 5P or 7P?  Your numbers are 
63.6 and 63.3, as far as I know, and the offsets are in both cases 1.2 mm.

John B


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