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In order to "enhance" the projected image, the prism was introduced. First, it was a static
device that clipped over the focussing lens, but surely, it would be better to introduce kinetic energy and rotate them.
Above are some prisms of varying types from different manufacturers. Enlarge the picture to see the different number
of facets.
In static mode, it's easy to see what they do.
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First, we take a basic cassette.
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Then add a three facet prism, produces rather good results.
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Next, a four facet is added. It's looking dimmer!
This illustration is produced in a small area, with short focal length, projected onto a purpose made screen.
If used over a long distance on a venue, the detraction of the image would obviously be exaggerated.
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By the time we get to five facet, the image is both faded and losing definition.
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The first type of motorised prism to gain popularity was the twin upright type, produced by various manufacturers,
they were extremely similar. Obviously the blue one (minus motor at present) was Meteor.
Next came the flat disc type of prism, with diffraction produced by angled faces, rotated either by circumference
or axis.
Above is the brilliant Opti flop prism, rotated both by circumference (Motorised) and axis (Gravity). As
the prism face is off balance, it spins on axis in an erratic manner as balance is shifted.
Unfortunately, the prism was prone to dropping out. Sadly, they're no longer produced.
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