Re: Single lamp twin projetcor.

[T3D Peter Homer <P.J.Homer@xxxxxxxxxxxxxxxxx>]

>on Tech3D digest 200 P.J. Homer included interesting information

Jose Lunazzi wrote

>It seems that he states that there is no wavelength dependance on
>dividing a beam by a prism polarizer.
>I agree with that, because the angle of total reflection should have
>enough tolerance as to include all visible wavelengths.  The paper
>catalog of Newport Research Co. offers a Glan Prism acting from 320 -
>2300nm wavelength.
>But is expensive.  The internet catalog does not include the item yet
>(<http://www.newport.com>).

That sounds like a Glan-Focault prism which has the two halves of the
prism held together externally so that no cement is used.
Because it is pure calcite its transmission is extended from the visible to
the infrared and ultraviolet as suggested by the above figures, but it has
an acceptance angle of only 7 degrees. The two halves can be cemented
together to increase the angle and it is then known as a Glan-Thompson
prism but the cement tends to reduce its transmission of UV which would be
an advantage for your purpose. They both differ from a Nicol in that they
don't retain the original shape of a Calcite cleavage fragment as the ends
are ground flat but like a Nicol they don't normally produce two beams, the
ordinary ray being absorbed by the blackened sides . Perhaps this could be
removed but maybe it's there to prevent the beam "Totaly internally
reflecting" back in just as it is reflected out by an air interface in the
Glan-Focault.

>The tolerance on the critical angle will allow not parallel light, but
>it would be convenient to put the condenser before the prism.

I am not sure what you are saying here.  Do you mean the critical angle for
total or partial internal reflection or do you mean polarisation angle and
are you saying that the light must be parallel or not?

>Regarding the Anderton vs. an anaglyphic system, it is clear that the
>anaglyphic is better in having only one projector and not needing metal
>screen, but renders no colors.

This is certainly true of Theodore Brown's system where the the pairs of
lantern slides were bound together along with secondary colour filters and
projected in a single lantern. But Newton and Co's system introduced
earlier 1896 but after Anderton's 1893 used two lanterns or a double Biunial
normally used for dissolves as did Anderton's system.

>Regarding my proposal of upgrading the Anderton system with a polarizing
>sheet, the advantage remains in the calculation's result: less absorbed
>energy and less parasitic image.  But I remain interested in the less
>absorbed energy case. The critical angle for polarization (Brewster's)
>has good tolerance to angle variation, considering the condenser before
>the plates.  Fragility of glass plates would be a critical problem if
>they must be thin in order to not include lateral displacement on the
>reflections, it seems to me not important.

Although it would seem to be logical to have the plates before the slide
and after the condenser as you suggest and which is what I originaly
assumed. Apparently the devices were attatched to objective projection
lenses of the manafacturer's biunial. They would also make them for other
manafacturer's biunials or pairs of lanterns.

>The problem of the parasitic image seems to me now less important
>because of a report I received from Peter Abrahms. I understood that,
>even if you polarize perfectly at the projection step (linear
>polarization case), tilting of the observer's head introduces parasitic
>(ghost) images.

The first time I saw polarised projection I realised was not such a problem
as I thought it would be as I seemed to have to turn my head quite a lot to
get ghosting and it was not a comfortable position to remain in anyway. So
I think it worthwhile to try to eliminate ghosting from other causes as much
as possible this is where the use of opposite circularily polarised light
might be an advantage.

>Is not possible that it is just because of the facility of mass
>production of a single projector that Anderton's system is not made
>anymore?.  Is there any Anderton's system available for testing? (it
>would not be difficult to put a polarizer sheet inside). It seems that
>Anderton, cited during the 1880s , died before Edwin Land (died 1992,
>aprox.) presented available cheap polarizers.

I do not know of an Anderton system still in existence and I have been
looking.  It was on the market in 1893 manafactured by R.Field and Co of 142
Suffolk street Birmingham Britain. This is quite close to me.  Suffolk street
still exists but the oldest building left is a church and that is only from
1905.  All the rest have been replaced. An article I wrote for the British
Magic Lantern Society newsletter included the American patent diagram for
the device but so far no one has recognised it. There was a advertisment in
stereo world a little while back which I did not notice at first until
someone on Photo-3D commented on it. Someone was selling a Bausch and Lomb
device described as an antique which projected lantern slides. It was said to
have its original polarisers although it was not mentioned what these were.
When I questioned how long Bausch and Lomb had been in existence on
photo-3D I was told they were around in the last century so perhaps they
were the American manafacturers of Anderton's system.
  I have a book "Dates and Sources" published by the Magic Lantern society
which has information on Anderton including a photograph.  It shows quite a
young man so if it dates from around 1893 he could still have been alive
when Polaroid was developed in the 1920s.
                                   P.J.Homer


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End of TECH-3D Digest 202
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