P3D Projection and mounting

["Kenneth Luker" <kluker@xxxxxxxxxxxxxxxx>]

Sorry about the spacing on my previous post.  Here's the 
same thing with better line-length:
------
If a "standard" alignment slide were created such that the 
supposed infinity points were identified in each frame, and 
the projected image of the alignment slide were to be 
manipulated using the projector's horizontal control so 
that the two points were physically separated on the 
screen by the nominal inter-lens spacing of the projector, 
then it wouldn't matter how far away the screen is, or how 
large the screen is.  A mile-high screen a mile away 
would intercept the parallel lines extending from the 
projector toward those infinity points in the same way as a 
two-foot high screen in the same room:  the infinity points 
will be at the same separation on ANY screen, if the lines 
from the projector to those points are parallel to one 
another.   If the inter-lens spacing of the projector is about 
the same as the inter-eye spacing of the viewer, then 
there is no problem.  

It's true that the mile-distant screen would have the two 
infinity points so close together that one couldn't resolve 
them separately without a telescope, but the rays from 
those points into the eyes of the viewing person would still 
be parallel.  

My point is that all the projectionist must do is make sure 
that at setup time, the infinity points at the screen are at 
human interocular separation.  Then, any slides that have 
been mounted with infinity separation that match the 
standard calibration slide's infinity separation will be 
viewable with no need for adjustment and with no 
discomfort.  The problem is that slides mounted "to the 
window" without consideration of the infinity points will 
sometimes have faulty infinity placement.  As a safe 
compromise, I'd say, mount to the window but ensure that 
the infinity separation isn't violated.   The "far-point" line on 
the mounting gauge let's one do just that.  

Ken Luker


_______________________________________________________________
Kenneth Luker
Marriott Library Administration
KLUKER@xxxxxxxxxxxxxxxx