H. I. B jelkhagen
Recording Materials

Acridine orange

3,6"-(dimethyl amino)-acridine hydrocIoride hydrate
Auramine 0 (Basic yellow 2)
4,4' -(imidocarbonyl)-"-(N,N ,-dimethylaniline) hydrochloride
Dithiozanin iodide
3,3' -diethyl-thiacarbocyanine iodide
Eosin Y (Eosin yellowish)
2' ,4' ,5' ,7' -tetrabromofluorescein disodium salt
Erythrosin B (Iodesine)
tetraiodfluoresine disodium salt
Isocyanine iodide
1,1' -diethyl-2,4' -cyanine iodide
Kryptocyanine (Rubrocyanine)
1,1' -diethyl-4,4' -carbocyanine iodide
Orthochrom T
1,1' -diethyl-6,6' -dimethylisocyanine
1,1' -diethyl-6-ethoxy-6' -methoxyisocyanine bromide
Pinacyanol bromide
l,l'-diethyl-2,2' -carbocyanine bromide
Pinacyanol chloride (Quinaldine blue)
1,1' -diethyl-2,2' -carbocyanine chloride
Pinacyanol iodide (Sensitol red)
l,l'-diethyl-2,2'-carbocyanine iodide
1-methyl-2-p-dimethylaminostyryl pyridine
Pinaverdol (Sensitol green)
1,1' ,6-trimethylisocyanine iodide
Pseudocyanine iodide
l,I'-diethyl-2,2'-cyanine iodide
Rhodamine B
N,N,N' ,N' ,tetra ethylrhodamine hydrochloride

Sensitizing Dyes
The most difficult part of emulsion-making is to find suitable sensitizers
for, in particular, fine-grained materials. Commercial companies are usually
very secretful about sensitizers they apply to their emulsions, which is why
it is so hard to find any publication that gives comprehensive description of
dyes or sensitizers. In particular, sensitizers for the deep-red part of the
spectrum are difficult to find. There are two types of sensitization of photographic
I. Chemical sensitization which refers to methods to obtain the highest
possible sensitivity of the silver-halide crystals. Here, sulfur, gold and reduction
sensitization are common.
2. Spectral sensitization which refers to methods to sensitize the silverhalide
grains to light in a region of the spectrum in which they would normally
not absorb. Here, special dyes are used.
In this section the second type of sensitization is of main interest.
Chemical sensitization of holographic silver-halide emulsions has, in general,
been discussed by Pantcheva et al. [2.68]. Emulsions intended for short
exposure times using pulsed lasers have been treated by Pangelova et al.

As already mentioned, pure silver-halide crystals are only sensitive to
light in the UV and violet parts of the spectrum. According to the Grotthus-
Draper law, only light which is absorbed can cause a chemical change.
The formation of a latent image is a chemical change. Up to a certain
photon energy, the silver-halide crystal alone can absorb the radiation energy
according to Planck's law (E = hll). The energy is high enough to raise
the silver-halide molecule from the ground state to a higher state to free an
electron which can combine with a silver ion and thus form a latent-image
speck. For light of longer wavelengths the energy of individual photons are
not high enough and no photon absorption takes place. Therefore, in order
to record at other wavelengths within the electromagnetic spectrum the
emulsion has to absorb the radiation in some way and transfer the energy to
the silver-halide grain to produce a latent image. There are special dyes that
can absorb light of different wavelengths. At certain wavelengths the molecules
of the dye can absorb the radiation and the molecule is then raised to
a higher state. This means that an electron is raised from the valence band
to the conduction band. Now two things could happen:
Either the electron is directly transferred to the silver-halide crystal to
form· metallic silver by combining with a silver ion (electron transfer); or
the electron formed by the dye will bring about an excitation of the silverhalide
crystal, causing a bromide ion to part with an electron (energy
transfer). The duration of the exited state of the dye molecule which is responsible
for the sensitization is very short, about 10-11 s. Both electron and
energy transfer can take place depending on type of dye. By such mechanisms
it is possible to affect the silver-halide crystals to form a latent image
at radiation of various wavelengths.
Not only must such a dye absorb the light of a certain wavelength but
also must it adsorb to the silver-halide crystal surface. Only if the dye is in
intimate contact with the grain can it transfer the effect of the light. The
force acting between the dye and halide molecules is of the Van der Waal
type. If the dye is not in good contact with the grain or if it is located in
the gelatin matrix the effect of it is a light-filtering function which actually
reduces the intensity of the illumination in this region.
The dyes used for the sensitization are mainly the cyanine dyes. They
can be divided into two classes: the true cyanines and the merocyanines.
Often the chalkocarbocyanines are used. More about these dyes can be
found in Duffin's book [2.36]. Some of the dyes employed in Lippmann
emulsions and holographic emulsions have already been mentioned and are
listed in Table 2.4.
A special sensitizer mentioned in one of the Russian papers that is
claimed to be very good for holograms at the ruby-laser wavelength
(694 nm) is the: 3-allyl-3' - ethyl-4' ,5' -diphenyl-4-keto-5(I "-ethyl-dihydroQuinolilidene-
4"-ethylidene) thiazolinothiazolocyanine bromide [2.70].

2.68 M. Pantcheva, T. Petrova, N. Pangelova, A. Katsev: Chemical sensitization of
fine-grain silver halide emulsions for holographic recording, in H%graphy'89,
ed. by Y.N. Denisyuk, T.H. Jeong. Proc. SPIE 1183, 128-130 (1990)
2.69 N. Pangelova, T. Petrova, A. Katsev, M. Pantcheva: Silver halide materials for
pulsed holographic recording, in Holography'89, ed. by Y.N. Denisyuk, T.H.
Jeong. Proc. SPIE lIS3, 131-133 (1990)
2.70 N.S. Gafurova, L.G. Logak, Kh.Kh. Fassakhova, R.K. Khakimova, R.K. Tep-
10va, LN. Zelinsky, V.T. Chernikh
: Silver halide photographic material having a
flexible base for the use in pulse holography. Proc. 14th Int'! Congr. on High
Speed Photography and Photonics, ed. by B.M. Stepanov (Moscow, October
19-24, 1980) pp.437-439
2.36 G.F. Duffin: Photographic Emulsion Chemistry (Focal, London, New York