Here's how to understand hand-drawn holograms. Get a strand of spiderweb
(or use smooth, polished fishing line). Go outside on a sunny day. Stand
so that you face the sun. Stretch the strand horizontally before you. If
the sun is low, then hold the strand at a low position so the sunlight
bounces off the top of the strand. See the little "highlight" reflecting
off the strand? Close one eye, then open it and close the other instead.
See what happens? The little highlight jumps sideways, because each of
your eyes sees it from a different angle. The horizontal strand is acting
like a mirror which reflects the sun. The "highlight" is actually the
image of the sun.
Now open both eyes and relax them so that you see a double image of the
strand. Now align the horizontal strand so that it is perfectly
horizontal. From your viewpoint, you should see just one strand, even
though your eyes are making double images. Now gaze at the highlight in
the strand, and you will seem to see just one highlight! Because you are
gazing off into the distance, you can see the distant image of the sun
which is reflected in the strand. Each of your eyes sees the image from a
different angle, and if both of your eyes gaze off into infinity, then
they see just one image. It SEEMS as if the highlight is not on the strand
of silk or nylon at all, but instead is far, far away. Why? Because the
sun is far, far away, and so is the 3-D reflection of the sun.
The strand of silk or nylon is acting like a "scratch hologram". It is a
"hologram" of a single glowing dot. The depth of the dot is infinitely
deep within the strand. Suppose that you stretched a bunch of horizontal
spider webs across some black velvet cloth. If each strand of silk was in
a different place, and if each one was at a slightly different angle, what
would you see? Stars! The "highlights" would form a collection of bright
stars. They would SEEM to be deep within the black cloth, and not on its
surface. This is the simplest version of the scratch-hologram.
It uses straight scratches. The images seem to be infinitely deep
It is easy to make the bright dots of light move closer to the surface of
the hologram. Just use curved strands of silk instead of straight ones.
Curve them like a rainbow shape, so that they hump upwards. The tighter
the curve, the closer the dot of light will appear. The curved strand of
silk is actually spreading the sunlight like a concave lens. It forms a
virtual image of the sun behind the curved strand of silk. Your two eyes
see light coming from the deep location of this virtual image. If the
radius of curvature of the strand is 3cm, then the bright dot of light
will seem to be 3cm behind the strand. That bright dot of light is
a small, distorted image of the sun.
Now finally, instead of using strands of spiderweb, make scratches on
plastic. If the scratches are smooth and polished, and if the inside of
the scratches is round, then the reflections will be the same as with the
spiderweb. Smooth scratches scatter light in the same way that spiderwebs
do. If they are curved and horizontal, then they will create bright dots
of light which float in the region behind the scratches.
To make holograms that float in front of the scratches instead of behind,
just turn the scratches upside-down. Make them bowl-shaped, humping down
in the middle, rather than rainbow shaped and humping upwards. The bright
dots of light will float in space in front of the plastic plate. You
could also make a normal scratch hologram, then turn it upside-down. This
creates a "pseudoscopic image" which is inside-out. All holograms create
pseudoscopic images when illuminated and viewed from reversed angles.
(Does this mean that scratch-holograms are real holograms?)
DIFFERENCES BETWEEN VARIOUS KINDS OF HOLOGRAMWith conventional holograms, each point on the object will form a kind of "bullseye" or "target-shaped" pattern on the holographic film. The pattern looks like millions of concentric circles. Because it is made from closely-spaced lines, it is an interference pattern. In holo- graphic science it is called a "Gabor zone plate." Each "Gabor zoneplate" is a hologram of a single glowing dot. A conventional hologram is composed of millions of overlapped Gabor Zoneplates. The zoneplates use optical diffraction to create the 3-D image, and for this reason they must be illuminated by a laser, or by some other monochromatic point-source of light. (Sometimes a tiny spotlight with a red filter can be used to illuminate a conventional hologram.)
With the One-step version of Benton's Whitelight Holograms, the image of the object
is passed through a lens and focussed upon the holographic film. A
black plate covers this lens, and the plate has a narrow horizontal
slot in it. When the hologram is made, this slot chops off most of
the Gabor zoneplates. They look like horizontal stripes, and each
stripe is made from millions of curved, parallel lines. They are
"truncated zoneplates" in the shape of long, narrow, horizontal
rectangles. The "chopping" or "truncation" greatly alters the physics
of the hologram. When the finished hologram is viewed, the image is
reproduced ONLY because of the angles of the curved lines. The lines
reflect light to your eyes and form the 3-D image. The parallel lines
are interference patterns, but their narrow spacing merely creates the
rainbow colors. Benton's "Rainbow Holograms" do not use optical
diffraction in order to create the 3-D image. Instead they reflect
light from parallel lines of various angles. For this reason they can
be viewed using white light.
The "scratch hologram" uses the same physics as the Rainbow Hologram
above. However, rather than optically creating the truncated Gabor
zone plates (horizontal stripes which are made of millions of curved
lines,) instead we draw single curved scratches. Each curved scratch
reflects the light in the same was as the truncated Gabor Zoneplate of
a Rainbow Hologram. A "scratch hologram" is a Benton Rainbow Hologram
with no lasers, no camera film, and no diffraction. Drawn by hand!