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Re: Fresnel lens image



On Mon, 23 Apr 2001, kyle forinash wrote:

Couple of students were just in my office- they are trying to get an
advertising business started and they want to use poster size images
which show different images from different angles (as you move
realative to the poster you see different images). They also have a
sample that has apparent depth (a real image floating in front of a
background image)- very striking to see. This technique is available
commercially but they want to make their own. The ones they showed me
have Frensel lens like grooves on the front surface only all parallel
(not concentric like a Fresnel magnifying lens). They want to know
exactly how to place the image(s) on the back so that you get the
double image effect. Anyone know how this works? I know this is a
diffraction effect but I don't see how to get different images.

Not diffraction. Search for "lenticular 3D" I once made a huge version
for the 3D show at the Boston Museum of Science. The serrated plate is a
"lenticular screen" where each line is a cylinder lens. Behind each of
these lenses you place a photograph taken of a 3D object from a particular
angle, but where the photograph is extremely compressed horizontally.
This could be computer-generated, but a pre-computer technique exists.
This entailed a multi-lens camera (between 5 and 12 lenses in a horizontal
array), and a glass lenticular plate held against the film.

I had a hard time understanding how this thing worked until I realized
that the optics of a wire window screen is similar: each little square
hole in the window screen looks out on a two-dimensional scene, but each
hole sees the scene from a different vantage point. If you look through a
single hole in the window screen, your view is 2D, but if you pull your
head back from the screen, all the little pieces of images glimpsed
through the square array of holes combines to form a 3D sciene. The
"lenticular postcard" 3D technology uses the same effect, but rather than
having holes looking out on the real world, each little hole is replaced
with a lens and a tiny but complete photograph (with each photograph taken
from the vantage point of that hole in the screen.) Then, rather than
taking tens of thousands of photos (one per "pixel") instead only take a
few hundred but make the "pixels" very tall. And finally, rather than
taking hundreds of photographs, instead only take a handfull, then slice
them up and shuffle the slices together to form hundreds of
strip-photographs each with much lower resolution.

Is that totally obscure? I think the modern version has you scan a bunch
of photographs into a computer (each photo taken from a different
horizontal position), then the software slices them up and combines them
into hundreds of extremely "anamorphed" narrow strip-images, then prints
them out so each composite image is behind one of the cylinder lenses in
the lenticular panel. A few years ago there were similar panels as
advertisments in airport terminals, but rather than using cylinder lenses,
they used an array of extremely narrow slits (the 'pinhole camera' version
of the same geometery.)


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William J. Beaty SCIENCE HOBBYIST website
billb@eskimo.com http://www.amasci.com
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