mJ/cm2In our holographic multi-stereogram development we use silver-halide materials even if in the future photopolymer materials could be the most common.
High resolving power and high speed are often incompatible properties,
which makes it necessary to arrive at a compromise of the highest
possible efficiency. The nature of the subject will determine
whether the ideal solution of this problem will be slanted towards
high speed or high resolving power.
High speed film means that the film is very sensitive to light,
and we can take a picture with low intensity of light. This means
physically that the size of grain of emulsion must be big and
that the resolution will be low. The resolution is expressed in
lines pr. millimetre.
There are a number of different types of filmplates and filmplates from different companies.
The filmplates that are chosen in this thesis is AGFA-GEVAERT Holotest, and have the number 10 E 75 and 8 E 75 HD.
These types of filmplate are made to be used with a red light
emitting laser.
| Film type | Size of grain | Resolution | Emulsion thickness | Sensitivity at 633 nm |
| nm | l/mm | m |
J/cm2 |
|
| 10 E 75 | 90 | 3000 | 7 | 1 |
| 8 E 75 HD | 35 | 5000 | 7 | 10 |
Figure 5-1 Film data from Holotest photographic materials
The emulsion thickness for these films is about 11 times the wavelength
of the Helium-Neon laser ( 632,8nm). This type of hologram is
a thick or volume hologram . What characterises a volume hologram
is the depth of the image produced on the film plate. Thin holograms
with emulsion thickness less than the wavelength of the laser
have little or no depth.
Figure 5-2 Spectral sensitivity for 10 E 75 Figure 5-3 Spectral sensitivity for 8 E 75 HD
The holographic emulsion 8 E 75 HD and 10 E 75 are specially sensitised
for wavelengths between 600 and 750 nm, and are intended for use
with the He-Ne laser (633nm) and the ruby laser (694nm). The sensitivity
for light of wavelengths around 500 nm (green light) is relatively
bad. Thus we may use green light with low intensity during the
hologram developing process.
Amplitude transmission is defined as the ratio between the amplitudes of a monochromatic plane wave before and after passing through the photographic emulsion.
This quantity T, is expressed as a function of the exposure shown
in figure 5-4.
The light intensity is chosen so that the mean value of the transmission
is in the linear region.
The optical density D is 0.6.
The blackening of photographic emulsion can be expressed with
help of the optical density, D.
The speed or sensitivity of the film is given by DIN or ASA. Both
of these are based on the linear part of the curve.
Figure 5-4 The characteristic curves for AGFA Holotest materials.
The relation between transmission T and optical density D is given
by the equation
T = e -1.15 * D (5.1)
From the amplitude transmission curve we choose T from the linear
part of the curve and get T= 0.5
With some manipulation we have
D = ln T /(-1.15) = 0.6
From the characteristic curve, shown in figure 5-4, we can see that D = 0.6 is in the non-linear part of the curve. The exposure of the filmplate will then be bad and the brightness of the image will not be very good.
What we want is a brighter image, and so we need a more exposed image. The result of a more exposed image is a over-exposed film plate. To remove the over-exposed material from the filmplate, we have to use a bleacher. The bleaching process transforms the hologram from an amplitude to a phase hologram. The phase hologram gives the best experimental results with D = 2.
Figure 5-5 Amplitude transmission T versus log exposure curves for AGFA Holotest materials
( It ) is the light intensity multiplied against the exposure
time.
From figure 5-5 we get the approximate values of
light intensities for T = 0.5 J/cm2 for
a 10 E 75 filmplate, and for 10 J/cm2 for a 8 E 75
HD filmplate.
We can see from these results that the 8 E 75 HD need 20 times as much energy during the exposure as the 10 E 75 filmplate. That means that the exposure time for a 8 E 75 HD has to be 20 times longer.
The relationship between these parameters is relatively simple.
If the reference and object beams cross the plate at angles v1
and v2 , we expect them to generate
fringes of width d where
(5-2)
Thus the resolution of the system is of order d.
Figure 5-6 Line density for the film
After d is decided, one requires a photographic plate capable of recording fringes of this width, even though they may not have a very high contrast. Uncertainty in the contrast is a factor which must be allowed for, and these criteria then allow one to choose a suitable filmplate without an actual test. But it is possible to indicate a range of useful emulsions. Once the emulsion is chosen, its speed is fixed.
It is common to characterise emulsions by a factor p which is the number of lines pr. millimetre that the plate will record.
The exposed time goes up as p2 for moderate p, in accordance with the theory, since the number of resolvable points per unit area in two dimensions also goes up as p2.
In the case of very fine grain plates the penalty in practice may be even higher than indicated in the theory the exposure time can goes up as p4. There is also a heavy penalty in the form of stricter vibration stability requirements.
(5.2a)
With the use of the trigonometric identity :
(5.2b) where v1+v2=U
and v1= v2
If v1 and v2 is equal, the distance between two interference lines can be written as:
(5.2c)
Spatial frequencies are defined as : 
, and for U= 90o we get
(5-3)
The film chosen for the recording of transmission and rainbow
hologram is AGFA-Gevaert holotest 10 E 75, which has a resolution
of 3000 lines/mm, which is adequate for these holograms.
When recording reflection holograms, the reference and object
beams illuminate the film on opposite sides. According to the
theory for reflection holograms, the distance between the layers
is 1/2 as shown in figure 5-7.
Figure 5-7 Interference lines in the film
emulsion produced by plane waves.
The distance between the interference lines or planes is given
by :
, where 
(5.4) using eq. (5.3)
and
, where the spatial frequencies
is defined as : 
The film chosen for reflection holograms is AGFA-Gevaert Holotest
8 E 75 HD, which has a resolution of 5000 lines/mm.
Dichromated gelatine is currently in widespread use because of its excellent holographic properties, including low scattering and high index modulation. The drawbacks of dichromated gelatine include the raw material's variability, complex wet processing, poor shelf-life, and environmental instability requiring hermetic sealing. In common with other non-silver halide materials, dichromated gelatine is usually sensitive only to UV and blue light. Recent research has made it possible to sensitise it to red laser light, though exposures are still long. The dichromated gelatine material has a rather low sensitivity of about 100 mJ/cm2 . Silver halide films are used when high exposure sensitivity and/or wide spectral sensitivity is needed, and when lower resolution and greater light scattering can be tolerated.
