LASER HOLOGRAPHY

CAROL ZIMMERMAN                HYDE PARK CAREER ACADEMY
                               6220 SOUTH STONY ISLAND AVENUE
                               CHICAGO, ILLINOIS 60637
                               1-312-947-7237

OBJECTIVES: 
     To make a Laser holograph and to understand why it works.

MATERIALS: 
           1 3-milliwatt Neon Helium Laser
           3 rubber inner tubes (18 inch diameter) from a trailer
           1 slab of marble (20 inches by 23 inches) possibly a tombstone
           1 wooden  sandbox (20 inches by 23 inches) filled with sand
           1 -4mm focal length lens in a lens holder
           1 -8mm focal length lens in a lens holder
           1 white die to use as an object
           1 object support
           1 film holder consisting of 2 glass plates and 2 clips 
           1 white screen
           1 black screen
           non AH holographic film
           green light
           developer for holographic film
           bucket and continuously running water
           6 plastic clothespins with hooks
           rubber gloves
           2 wooden blocks, slightly smaller than the film holder
           1 rubber squeegee,  of the type window washers use

STRATEGIES: 
     The laser must first be turned on since at least fifteen minutes must be 
allowed to permit the laser sufficient time to warm up.  During this time the 
frequency of the light is changing.  Arrange the inner tubes on the table so that 
the marble slab may be securely balanced upon them.  Place the sandbox securely 
upon the marble slab.  Because of size constraints the lab will be set up on a 
diagonal inside the sandbox.  Set the laser in the sandbox with the back in one 
corner and the light aimed at the diagonally opposite corner.  Place the two 
lenses in front of the laser so that the light shines through the center of both.  
They must be as close as possible to each other without actually touching.  Allow 
enough space between the laser and the nearer lens holder so that the black screen 
may be inserted or removed without jarring either.  Place the object support in 
the corner diagonally opposite the laser with the object on it so that it is 
completely bathed in the light from the laser.  At any time while the materials 
are being aligned in the sandbox the sand may be rearranged to facilitate their 
proper placement.  Place the white screen in the film position directly in front 
of the object, between the object and the light.  Focus the light so that the 
sharpest possible circle of light is displayed on the screen.  Remove the white 
screen.  Place the black screen between the laser and the near lens holder.  DO 
NOT TURN OFF THE LASER.  Open the film package and place the package, the pieces 
of the film holder, a plastic clothespin, and the opened container of developer 
where they may be conveniently found in the dark.  Start the water running into 
the bucket.  Turn on the green light and focus it on the sandbox.  Make sure that 
all windows are completely covered.  Turn off all of the lights except the green 
light.  Allow several minutes for your eyes to become accustomed to the dark.  
Take out one film and place it in the film holder.  Press the film holder between 
2
the wooden blocks to remove all air, which could cause movement of the film and 
ruin the holograph.  Carefully place the film holder in front of the object in 
exactly the same spot where the white screen had been.  There will be a mark in 
the sand.  Carefully lift the black screen out of the sand, being sure that it 
still covers the light from the laser.  Wait approximately one minute, so that all 
vibration in the sandbox ceases.  Completely raise the black screen for two 
seconds, exposing the film, then lower the screen into the sand.  Put on the 
rubber gloves.  Take the film out of the holder, clip one corner with the 
clothespin, and place the film in the developer for one to two minutes.  You may 
take the film out of the development solution to check on its progress.  The film 
will darken as it develops.  When the developing is completed, place the film in 
the bucket of water with the hook over the side.  It should stay there 
approximately five minutes.  Squeegee the film dry and a preliminary viewing may 
be taken by shining the laser through the film and viewing the  holograph from the 
same side of the film as the laser.  The object will appear to be behind the film, 
in its original location. 
     The image is formed when the light rays directly from the laser and the light 
rays bouncing off the object strike the film simultaneously, forming a pattern of 
constructive and destructive interference.  The light coming through this 
interference pattern is viewed by the eye exactly as the eye would view the light 
bouncing off the original object.  In addition, if one views the holograph from a 
different angle one will appear to be viewing the original object from that 
different angle.  A green light is used because the holographic film, designed to 
be sensitive to the red laser light, appears not to recognize the wavelength of 
the green light.