modulation due to index changes dominates any residual amplitude
modulation. Phase modulators prove to be more efficient in terms of
the portion of incident illumination that is diffracted to form the
desired correlation. A sinusoidal hologram using absorption or
amplitude modulation can theoretically diffract only 6.25% of the
incident energy into an image. Experimentally, the number is about
4%.38 A phase-modulated hologram transmits all of the light (ignoring
the emulsion, substrate, and reflection losses). A sinusoidal phase
hologram can diffract as much as 33.9% of the incident light into the
first order.
The bleaching process converts the real function F(u,v), recorded
in silver on the film, to a phase delay.
H(u,v) = exp j[ F(u,v) ] (4.5)
To produce a kinoform, the film is exposed to the phase function
e(u,v) of the image transform. Upon subsequent bleaching, the film
contains the response
H(u,v) = exp j[ 6(u,v) ]. (4.6)
The kinoform, produced in this fashion, records the phase-only
information of the image transform. The bleaching process is not
restricted to phase-only information. Rather, the absorption hologram
created from equation 2.35 can also be bleached.
H'(u,v) = exp j[ H(u,v) ] (4.7)
= exp j[1 + IF(u,v)12 + F(u,v)exp j2irav + F*(u,v)exp -j2rav]