One of the biggest challenges in this experiment is to keep the optical equipment stable during the exposure. This can be very difficult because the exposure time is very long for reflection holograms. The film used in transmission hologram ( 10 E 75 ) do not have high enough resolution to be used in the recording process of reflection holograms. Reflection holograms need higher resolution of the filmplate, and we use AGFA 8 E 75 HD filmplates, which have a resolution of 5000 lines/mm. This filmplate need about 20 times more light energy than 10 E 75. It means that the exposure time will be about 20 times longer for recording reflection holograms than for recording transmission holograms, if the filmplate is illuminated with the same light power.
Figure 12-15 Optical set-up for recording 2-step reflection hologram ( Seen from the side ) .
We know from experiment 2 that the lens and the film must not
be parallel, because it can give rise to reflections and lead
to destructive interference of the film plate. The result can
be a loss of information, and a bad quality hologram.
The distance between H1 (master transmission hologram) and H2 (film plate for reflection hologram) is 13 cm. The focus of the real image is placed between the master hologram (H1) and the film (H2). The distance between the master hologram and the focus is 12 cm, and the distance between the focus of the real image in the master hologram and the film is also 1 cm.
Figure 12-16 Optical set-up for recording 2-step reflection hologram ( Seen from the side ).
Optical equipment used for the recordings :
He-Ne laser : Output power 24 mW. Wavelength 632.8 nm.
Spatial filters : Pinhole size 10 
m. Microscope objectives 45 x 0.65
Lenses : Diameter 100 mm. Focal length 175 mm.
Mirror : Flatness 
/ 10.
Filmplate : Type 10 E 75. Resolution 3000 l / mm.
Filter : Transmission 50 %.
Laser beam distance : Object beam = 205 cm.
Reference beam = 205 cm.
Image 1 :
The set-up for this recording process is the same as described
above.
Light power on the film : Object beam = 0.95 W.
Reference beam = 1.50 W.
Light power ratio = 1.50 W / 0.95 
1.6:1
Exposure time = 100 seconds.
Result : It is impossible to see the image in the hologram, and
so the hologram is unfit for use.
Comments : The bad quality of the hologram is probably caused by reflections or vibrations of the optical equipment.
Image 2:
There are some alternations in this recording, where the reference beam's incident angle on
the film is changed from 38 to 44. This alternation is made to
decrease the possibility of reflections between the lens and the
film.
Figure 12-17 Alternation in the recording of image 2 (Seen from above).
The distance between the master hologram ( H1 ) and the film plate
( H2 ) is still 13 cm, and the focus of the real image to the
master hologram is still 1 cm from the film.
Light power on the film : Object beam = 0.95 W.
Reference beam = 1.50 W.
Light power ratio = 1.50 W / 0.95 1.6:1
Exposure time = 100 seconds.
Result : The hologram is good, but the brightness can be still better.
Comments : There are several things which could
have gone wrong during the exposure, and lead to lack
of desired brightness.
Image 3 :
The optical set-up is nearly the same as that used for the holographic recording process of image 2. The only difference between this recording and the recording of image 2 is that the reference and object beams are tuned finer, and a better focusing of the beams in the spatial filter is obtained.
Light power on the film : Object beam = 0.95 W.
Reference beam = 1.50 W.
Light power ratio = 1.50 W / 0.95 1.6:1
Exposure time = 100 seconds.
Result : The quality of the hologram is very good. The brightness
of the image is also very good.
Comments : With this hologram it has been shown that it is possible to make good 2-step reflection holograms from computer images. An advantage with the 2-step method is that the vertical lines from the slit on the master transmission hologram disappear when it is copied to a new hologram.
An important parameter is the time from when the filmplate is taken out from the refrigerator to when it is used in the holographic recording process. The temperature in the refrigerator is
about 4o C and the temperature in the laboratory where the recording process is carried out is about 22o C. There is therefore a temperature difference on the film of 18 oC. The emulsion on the film is placed on a plate of glass, and this glass plate will expand when it is heated. This is explained in section 11.4 Temperature changes of the film during the exposure.
Image 4 :
The optical set-up in the recording process
of this image is nearly the same as used during the recording
process of image 3. The only difference is the short time the
film is tempered in the laboratory before the exposure. The temper
time is 30 minutes for this image.
Light power on the film : Object beam = 0.95 W.
Reference beam = 1.50 W.
Light power ratio = 1.50 W / 0.95 1.6:1
Exposure time = 100 seconds.
Result : The hologram is bright, but there are a area of the hologram that has no information.
Bilde slettet pga for stor størrelse på filen
Figure 12-18 Reconstruction of the reflection hologram.
Comments : The bad quality of the hologram is caused by the lack of thermal stability of the filmplate during the exposure.
Image 5 :
The optical set-up in the recording process of this image is the
same as used for the recording process of image 3 and 4. The
temper time is 24 hours for this image. The temper time was made
so long to ensure that there is no thermal instability on the
filmplate.
Result : The hologram is very good.
Bildet slettet pga for stor størrelse på filen
Figure 12-19 Reconstruction of the reflection hologram.
Comments : It is very important that the filmplate is tempered
is thermally stable during the exposure.
13 holograms were produced with this optical set-up. The recording data is the same as that used for the production of image 3. Too short a temper time lead to a bad quality hologram. This is solved by using a temper time of 4 hours before exposure. The quality of these holograms are very good.
Figure 12-20 Optical set-up for recording 1-step reflection hologram.
Optical equipment used for the recordings :
He-Ne laser : Output power 24 mW. Wavelength 632.8 nm.
Spatial filters : Pinhole size 10 m. Microscope objectives 45
x 0.65 and 63 x 0.85
Lenses : Diameter 100 mm. Focal length 175 mm.
Mirror : Flatness / 10.
Filmplate : Type 8 E 75 HD. Resolution 5000 l / mm.
Filter : Transmission 50 %
LCD : Transparent LCD
Laser beam distance : Object beam = 218 cm.
Reference beam = 209 cm.
Image 1 :
The optical set-up for the exposure of this image is generally
the same as shown in figure 12-20. The only difference is that
this image is recorded without the holographic printer and with
only one exposure. It is an advantage to use only one image exposure
when the purpose is to find the right exposure time and to test
the optical set-up. Another advantage is the total recording time
is for one exposure only instead of for 70, which also reduces
the element of unsteadiness like vibrations from the printer.
The ground glass is mounted close to the LCD, and the distance between the LCD and the filmplate is 10 cm. The
reference beam illuminates the film from the upper level, and
has a incident angle of 50 . To get the same polarisation direction
of the reference and object beam, a half-wave plate is mounted
between the mirror and the spatial filter on the reference beam.
Figure 12-21 Location of optical equipment ( seen from the side).
Light power on the film : Object beam = 0.5 W.
Reference beam = 3.0 W.
Light power ratio = 3.0 W / 0.5 W = 6:1
Half-wave plate = 22 o.
Exposure time : 50 seconds.
Result : The quality of the hologram is good.
Comments : There are no embarrassing reflections on the hologram and so the set-up is used for further image recording. The brightness of the hologram is good, and the exposure time appears to be correct for this light power.
Image 2 :
The aim of this recording process is to make 1- step reflection
hologram of computer pictures. To make a holographic image in
3 dimensions, the use of the holographic 3-D printer is necessary.
The optical set-up is shown in figure 12-20, where the picture
gets exposed with horizontal parallax.
Light power on the film : Object beam = 0.6 W.
Reference beam = 1.9 W.
Light power ratio = 1.9 W / 0.6 W 3.2:1
Slit width = 1.70 mm.
Distance the film is to be moved = 1.60 mm.
Half-wave plate = 22 o.
Exposure time : 80 seconds.
The exposure time is longer for this exposure than for image 1,
because the light power is lower.
Result : The brightness of the image is good, but the hologram
is disfigured by vertical lines from the slit.
Comments : It seems to be impossible to get rid of the vertical lines from the slit.
