The purpose of these experiments is to develop a method for producing high quality holographic multi-stereogram from a computer image sequence.22 23 For this to be possible, a good deal of optical equipment had to be obtained, in addition to that used for Olav Birkeland's thesis work.10 Some of the equipment was made in the workshop of the Institute of Physics and the rest was bought from different optical companies. Equipment bought for this experiment was a transparent LCD, a red 24 mW He-Ne laser, optical breadboard, mirrors, lenses, and different types of optical holders. A lot of equipment was made and some was rebuilt in the workshop. Equipment made in the workshop was the glass cage mounted on the optical table and various special optical holders. The printer's slit was changed from a fixed slit with an angle of 60 to an adjustable slit with an angle of 110. An other important change was the adjustable screws on the spatial filter. The old screws were changed to micrometer screws that make it possible to adjust the microscope lens precisely in front of the pinhole.
This optical set-up is for recording transmission holograms from
a computer image (computer image is shown in figure 12-2). The
optical equipment is placed at two different levels, as this is
a more practical way to use the optical table. It is also more
natural to have the reference beam coming from above, as this
is the same (or nearly the same) wave that is used during the
reconstruction of the hologram.
Figure 12-1 Set-up for experiment 1
The ground glass is placed close to the LCD, and there is 20 cm between the ground glass and the filmplate.
The function of the computer is to control the exposure and display a picture on the LCD. The exposure time is set on the computer, and the shutter will open to let the laser beam pass through the shutter and illuminate the film. Before the shutter is opened, the computer calls up an image and displays it on the computer screen. The LCD is connected in parallel with the computer screen and the image is shown on the display during the entire exposure.
Figure 12-2 Computer image drawn on AutoCAD.
Optical equipment for the recordings :
He-Ne laser : Output power 12 mW. Wavelength 632.8 nm.
Spatial filters : Pinhole size 25 
m. Microscope objectives 40 x 0.65
Lenses : Diameter 100 mm. Focal length 175 mm.
Mirrors : Flatness 
/ 10.
Filmplate : Type 10 E 75. Resolution 3000 l / mm.
Filter : Transmission 50 %.
LCD : Transparency LCD
Laser beam distance : Object beam = 233 cm.
Reference beam = 235 cm.
Image 1 :
Light power on the film : Object beam = 1.7 W.
Reference beam = 4.5 W.
Light power ratio = 4.5 W / 1.7 W 2.6 : 1
Exposure time = 1 second.
Result : There is some noise in the hologram, but the image is not bad. The brightness of the hologram could have been better.
Comments : The noise in the hologram is probably caused by depolarising effects and/or vibrations.
Image 2 :
The optical set-up for this exposure is the same as used in the recording of image 1, but with the use of a vertical polarizer in front of the film. If the noise on the (image 1) hologram is caused by depolarizing effects, this noise will now be reduced or deleted from the hologram.
The polarizer transmits 38 % of the incident light. Therefore,
the exposure time should be about 3 times as long as the recording
of image 1.
Exposure time = 3 seconds.
Result : The quality and the brightness of the transmission hologram
is very good.
Comments : There seems to a be problem with the depolarizing of
the light during the process of exposure. This problem
seems to be solved with the use of a polarizer.
After a close study of the optical set-up, the cause of the depolarizing was found. The mirror which reflected the reference beam from the lower table to the upper table changed the direction of polarisation. This mirror changed the depolarizing direction by 24o, and the result was problems with interference on the film. \
Image 3 :
The optical set-up for this exposure is the same as used for earlier recordings in this experiment. The mirror which change the depolarizing direction is adjusted, and the polarizing direction of the reference and the object beams is vertical.
Light power on the film : Object beam = 0.2 W.
Reference beam = 0.7 W.
Light power ratio : 0.7 W / 0.2 W = 3.5 :1
Exposure time = 4 seconds.
Result : The quality of the hologram is very good.
Comments : It is very important to control the polarizing direction of the laser beams, since an unfortunate placing of mirrors can change the polarizing direction.
Figure 12-3 Parts of the optical set-up for 2-step reflection hologram recording.
The distance between the transmission master hologram and the
filmplate is 14 cm. The master hologram has the focus of the real
image 11 cm behind the hologram. (The left side of the master
hologram is shown in figure 12-3).
Optical equipment used for the recordings :
He-Ne laser : Output power 12 mW. Wavelength 632.8 nm.
Spatial filters : Pinhole size 25 m. Microscope objectives 45
x 0.65
Lenses : Diameter 100 mm. Focal length 175 mm.
Mirror : Flatness / 10.
Filmplate : Type 8 E 75 HD. Resolution 5000 l / mm
Image 1 :
Laser beam distance : Object beam = 190 cm.
Reference beam = 190 cm.
Light power on the film : Object beam = 1.2 W.
Reference beam = 2.6 W.
Light power ratio = 2.6 W / 1.2 W
2.2:1
Exposure time = 100 seconds.
Result : The quality of the hologram is very bad. It is impossible
to see the image in the hologram, but it is possible to see
the lens used in the reference beam on the hologram.
Comments : The bad quality of the hologram is probably caused by reflections from the lens. (The lens is placed at the ground level on the optical table, as shown in
figure 12-3).
Image 2 :
To solve the problem of reflections on the film, the master hologram and the filmplate are turned. Figure 12-4 shows the changes in the optical set-up made to avoid problems with reflections.
Figure 12-4 Changes in the optical set-up.
The master hologram and the filmplate are turned through 74o with
respect to the incident light.
Laser beam distance : Object beam = 190 cm.
Reference beam = 190 cm.
Light power on the film : Object beam = 1.2 W.
Reference beam = 2.2 W.
Light power ratio = 2.2 W / 1.2 W
1.8:1
Exposure time = 100 seconds.
Result : The brightness of the hologram is bad, but it is possible
to see the image in the hologram. It is also still possible
to see the lens in the hologram.
Comments : There are still some reflections from the lens onto
the hologram. The problem must be solved before any
high quality holograms can be made.
Image 3 :
The problem with reflections on the hologram can only be solved
by change the position of the lens. It is important that the lens
is not parallel to the filmplate.
Figure 12-5 Changes on the optical set-up ( seen from above).
Laser beam distance: Object beam = 210 cm.
Reference beam = 210 cm.
Light power on the film : Object beam = 0.9 W.
Reference beam = 1.4 W.
Light power ratio = 1.4 W / 0.9 W
1.6:1
Exposure time = 100 seconds.
Result : The brightness and the quality of the hologram are very
good.
Comments : The problem of reflections seems to be solved, and it is also possible to hold the optical equipment stable for 100 seconds.
