ghosting

[P3D Peter Abrahams <telscope@xxxxxxxxxx>]

>>>>

>there was ghosting during the shows in Bellevue (worse than when

> viewed at home with a TDC.)  This was not because the film depolarized,
but

> because the most effective extinction with crossed Polaroid filters 
is

> about 99 per cent. 

I was standing near Bob Brackett when he (Bob) commented on the
projection of 

the x-ray shots.  He was convinced that the problem was that the 

transparencies were produced using a film base that de-polarizes the
light..... Peter dismisses this 

explanation, as quoted above.  

Ken Luker

<<<<<<<<

When I noticed the obvious ghosting during the projection, I did not
dismiss film depolarization by any means, but investigated it at some
length, and in fact almost alienated John Dennis (who shot the slides on
a Fuji E-6 type film from my x-ray originals) when I mentioned that Bob &
others had asked about film depolarization. I asked several other
knowledgeable people at the convention about E-6 films, and all stated
that these films should not depolarize. 

I had a long conversation with Bob (on many interesting subjects,) and
when I told him the film base was E-6, he felt that it would not have
depolarized the light.  (I am somewhat uncomfortable with all this
quoting of a man who is not on the list to correct anybody.)  I am not
faulting his projectors, although I believe that more complete extinction
could be achieved with a sandwiched polaroid.  I do not know whether all
Bracketts use the mounting for the Polaroids that the display model at
the trade fair used, which caused a slight bulging of the plastic.

The lesser ghosting from a TDC could have been from a less reflective
screen, a room that was not as dark, or other causes.


Regardless, it is a fact that crossed polars will transmit a small
percentage of light, one percent is the minimum and it is often higher.
Titles and other high contrast, hard edged objects often have ghosting
during stereo projection for that reason.  This effect alone is
sufficient to explain the ghosting in the x-rays.

It is possible that these slides are not suitable for projection.  I
purposely sought high contrast at every stage of the project, but may
have eliminated sharing them with a group as a result (until those
electronic goggles are more ubiquitous).

_______________________________________

Peter Abrahams   telscope@xxxxxxxxxx

the history of the telescope, the microscope,

    and the prism binocular



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