1. Field of the invention.
The present invention relates to a mono-sheet silver salt diffusion
transfer material and method for producing direct-positive images therewith.
2. Background of the invention.
In silver halide photography a photographic method, according to
which a positive image is made without the use of a negative image or an intermediary
process producing a negative image, is called a direct-positive method and a photographic
light-sensitive element is called a direct-positive element.
Two main types for producing direct-positive image are known. According
to a first class use is made of a photographic element containing a direct positive
silver halide emulsion. Such types of photographic elements are disclosed in for
example US-P-3,364,026, US-P-3,501,305, US-P-2,456,953 and US-P-3,761,276. These
types of photographic material are of rather low speed.
According to the second class use is made of the silver salt diffusion
transfer method. The principles of the silver complex diffusion transfer reversal
process, hereinafter called DTR-process, have been described e.g. in US-P-2,352,014
and the book "Photographic Silver Halide Diffusion Processes" by André Rott and
Edith Weyde - The Focal Press -London and New York, (1972).
According to the DTR process, a silver complex salt is image-wise
transferred by diffusion from the image-wise exposed silver halide emulsion layer
into the image receiving layer, where it is converted to a silver image usually
in the presence of physical development nuclei. For this purpose, the image-wise
exposed silver halide emulsion layer is developed in the presence of a developing
agent and non-developed silver halide is converted by means of a silver halide
complexing agent into a soluble silver complex salt while in contact with an image
At the exposed areas of the silver halide emulsion layer the silver
halide is developed (chemical development) and thus cannot be dissolved any more
to diffuse to the receiving layer.
At the unexposed areas of the silver halide emulsion layer the silver
halide is converted to a soluble silver complex salt and is transferred to the
receiving layer, where it forms a silver image usually in the presence of physical
The DTR process includes (a) a so-called "two-sheet" type where the
silver halide emulsion layer and the image receiving layer are contained on separate
supports and are brought in contact with each other at the time of diffusion transfer
development to produce the silver image in the receiving sheet and (b) a so-called
"mono-sheet" type where both the emulsion layer and the layer containing the physical
development nuclei are contained on the same support.
According to the DTR process a direct positive image can thus be
obtained either with a "mono-sheet" or "two-sheet" type. Mono-sheet type DTR-materials
are well known for producing lithographic printing plates. However for producing
images this type of DTR-material is less suited because of a rather large density
in the non-image areas. A DTR-material of the two-sheet type can yield direct
positive image having a high gradient and low density in the non-image areas but
is inconvenient for applications such as e.g. Computer Output to Microfilm (COM).
It is furthermore desirable that the image is of high resolution
3. Summary of the invention.
It is an object of the present invention to obtain images of high
density in the image areas, low density in the non-image areas (preferably less
than 0.4), improved resolution, sharpness and contrast using a mono-sheet DTR-material.
According to the present invention there is provided a photographic
material comprising on a support a layer combining photosensitive and image receiving
properties, said layer containing (i) light sensitive silver halide particles (ii)
a substantially light insensitive silver salt having a speed of at least a factor
10 less than said light sensitive silver halide particles and (iii) physical development
According to the present invention there is also provided a method
for obtaining direct-positive images with the above defined photographic material.
4. Detailed description of the invention.
It has been found that information-wise exposure and subsequent development
according to the DTR-process of a photographic material as defined above yields
images of high densities in the image areas and low densities in the non-image
areas generally of not more than 0.4. Furthermore images of improved sharpness
and resolution can be obtained so that the photographic material can be used in
It is believed that the working of the present invention in case
of negative working light sensitive silver halide particles is as follows. Upon
information-wise exposure a latent image will be formed by the light sensitive
or photosensitive silver halide particles and during subsequent development in
the presence of developing agent(s) and silver halide solvent(s) the light sensitive
silver halide particles will be developed to silver at the exposed parts. At the
same time the substantially light insensitive silver salts will be dissolved by
the silver halide solvent(s) and deposited as silver in the exposed areas on the
silver particles resulting from the development of the light sensitive silver halide
and on the physical development nuclei contained in the photosensitive layer in
the non-exposed areas. In the non-exposed areas the light sensitive silver halide
will not be developed to silver and may also be dissolved by the silver halide
solvent(s) and deposited as silver on the physical development nuclei.
Preferred features of the photographic material according to the
invention for obtaining images of good quality therewith are as follows. Firstly,
the coverage power of the light sensitive silver halide is preferably as low as
possible to obtain images of low density in the non-image areas. This can be achieved
by using a light sensitive silver halide having an average diameter of at least
0.5µm, preferably at least 0.6µm and most preferably at least 0.8µm and by keeping
the ratio of the coverage of light sensitive silver halide (in g/m² expressed as
AgNO&sub3;) to said average diameter (in µm) below 2. However the coverage of light
sensitive silver halide is preferably not less than 0.2g/m² expressed as g AgNO&sub3;/m².
By the diameter of a silver halide grain is meant the diameter of
a hypothetical sphere with an equivalent volume as the corresponding silver halide
grain. The average diameter of the silver halide grains thus being the average
of all these diameters. The average diameter can be measured according to e.g.
the method described by G. Möller disclosed on the International Congres of Photographic
Science (ICPS) held in Moskou from July 29th to August 5th 1970.
Secondly the speed of the substantially light insensitive silver
salt is preferably at least a factor 10 less than the speed of the light sensitive
silver halide under the same conditions of exposure and development. By keeping
this difference in speed the formation of a latent image by the substantially light
insensitive silver salt can be avoided and thus the build up of density in the
non-image areas due to development of said latent image.
A third feature is that the light sensitive silver halide emulsion
is preferably well stabilized in order to avoid fogging of said emulsion.
In order to obtain images of high density and good contrast it is
preferred that the weight ratio of substantially light insensitive silver salt
to photosensitive silver salt particles is between 1:2 and 10:1 and more preferably
between 1:1 and 5:1 and most preferably between 1:1 and 3:1.
Preferred substantially light insensitive silver salts for use in
accordance with the present invention are preferably water insoluble silver salts
e.g. a silver halide, bromate, molybdate, oxalate, chromate, iodate, isocyanate,
thioisocyanate, cyanide, citrate, phosphate, oxide etc.. Said water insoluble silver
salts may be prepared using the precipitation reaction of the water soluble salt
of the desired anion of the insoluble silver salt with a water soluble silver salt,
e.g. silver nitrate, in the presence of a hydrophilic binder.
Preferably said silver salt particles are readily transferable by
diffusion to the physical development nuclei during the DTR-process. For this purpose
silver salts containing at least 70 mole% of chloride are preferred in the present
invention. The substantially light insensitive silver salt particles in connection
with the present invention are preferably not chemically and not spectrally sensitized.
It is furthermore advantageous that the particle size of said silver salt is small
i.e. an average diameter of less than 0.3µm is preferred. Said silver salt is preferably
also doped with Rh³&spplus;, Ir&sup4;&spplus;, Cd²&spplus;, Zn²&spplus; or
Pb²&spplus; to reduce the light sensitivity of the silver halide. The said silver
salt particles may further be desensitized on the surface with a desensitizing
agent well known to those skilled in the art. Examples of desensitizing agent
are disclosed in e.g. the US Patents 2,930,644, 3,431,111, 3,492,123, 3,501,310,
3,501,311, 3,574,629, 3,579,345, 3,598,595, 3,592,653, 4.820.625, 3.933.498, and
GB 1.192.384. Further desensitizing agents suitable for use in accordance with
the present invention are described e.g. by P. Glafkides in "Chimie et Physique
Photographique", Paul Montel, Paris (1967).
The photosensitive silver halide particles can be prepared from soluble
silver salts and soluble halides according to different methods as described e.g.
by P. Glafkides in "Chimie et Physique Photographique", Paul Montel, Paris (1967),
by G.F. Duffin in "Photographic Emulsion Chemistry", The Focal Press, London (1966),
and by V.L. Zelikman et al. in "Making and Coating Photographic Emulsion", The
Focal Press, London (1966).
The photosensitive silver halide particles used according to the
present invention can be prepared by mixing the halide and silver solutions in
partially or fully controlled conditions of temperature, concentrations, sequence
of addition, and rates of addition. The silver halide can be precipitated according
to the single-jet method or the double-jet method.
The photosensitive silver halide particles of the photographic material
used according to the present invention may have a regular crystalline form such
as a cubic or octahedral form or they may have a transition form. They may also
have an irregular crystalline form such as a spherical form or a tabular form,
or may otherwise have a composite crystal form comprising a mixture of said regular
and irregular crystalline forms.
The photosensitive silver halide particles are preferably of high
speed and readily transferable by diffusion during the DTR-process. It is furthermore
advantageous that the photosensitive silver halide particles show a rapid chemical
development i.e. silver halide emulsions that show a complete chemical development
within at least 15s. The rate of chemical development can be easily determined
with the following method. The silver halide emulsion layer of which the rate of
chemical development is to be measured is coated to a transparent support in an
amount equivalent to 2g AgNO&sub3;/m² and 2.1g gelatin/m². The thus obtained element
is exposed to a suitable light-source and subsequently placed in a cuvette in a
spectrophotometer and thermostated at 25°C. A developing solution is brought in
the cuvette and the absorption at 800nm is followed with time. From the plot of
the absorption at 800nm against time, the time necessary to obtain a complete development
of the sample can be determined.
According to the present invention the photosensitive silver halide
particles preferably consist principally of silver chloride while a fraction of
silver bromide may be present ranging from 1 mole % to 40 mole %. The particles
may be of the core/shell type well known to those skilled in the art in the sense
that substantially all the bromide is concentrated in the core. This core contains
preferably 10 to 40 % of the total silver halide precipitated, while the shell
consists preferably of 60 to 90 % of the total silver halide precipitated.
The size distribution of the photosensitive silver halide particles
to be used according to the present invention can be homodisperse or heterodisperse.
A homodisperse size distribution is obtained when 95% of the grains have a size
that does not deviate more than 30% from the average grain size.
Preferably during the precipitation stage Iridium and/or Rhodium
containing compounds or a mixture of both are added. The concentration of these
added compounds ranges from 10&supmin;&sup8; to 10&supmin;³ mole per mole
of AgNO&sub3;, preferably between 10&supmin;&sup7; and 10&supmin;&sup4; mole per
mole of AgNO&sub3;. This results in the building in the silver halide crystal
lattice of minor amounts of Iridium and/or Rhodium, so-called Iridium and/or Rhodium
dopants. As known to those skilled in the art numerous scientific and patent publications
disclose the addition of Iridium or Rhodium containing compounds or compounds containing
other elements of Group VIII of the Periodic System during emulsion preparation.
The photosensitive silver halide particles can be chemically sensitized
e.g. by adding sulphur-containing compounds during the chemical ripening stage
e.g. allyl isothiocyanate, allyl thiourea, and sodium thiosulphate. Also reducing
agents e.g. the tin compounds described in BE-P-493,464 and 568,687, and polyamines
such as diethylene triamine or derivatives of aminomethane-sulphonic acid can
be used as chemical sensitizers. Other suitable chemical sensitizers are noble
metals and noble metal compounds such as gold, platinum, palladium, iridium, ruthenium
and rhodium. This method of chemical sensitization has been described in the article
of R.KOSLOWSKY, Z. Wiss. Photogr. Photophys. Photochem. 46, 65-72 (1951).
The photosensitive silver halide particles of the photographic element
of the present invention can be spectrally sensitized according to the spectral
emission of the exposure source for which the photographic element is designed.
Suitable sensitizing dyes for the visible spectral region include
methine dyes such as those described by F.M. Hamer in "The Cyanine Dyes and Related
Compounds", 1964, John Wiley & Sons. Dyes that can be used for this purpose
include cyanine dyes, merocyanine dyes, complex cyanine dyes, complex merocyanine
dyes, homopolar cyanine dyes, hemicyanine dyes, styryl dyes and hemioxonol dyes.
Particularly valuable dyes are those belonging to the cyanine dyes, merocyanine
dyes, complex merocyanine dyes.
To enhance the sensitivity in the near infra-red region use can be
made of so-called supersensitizers in combination with infra-red sensitizing dyes.
Suitable supersensitizers are described in Research Disclosure Vol 289, May 1988,
The spectral sensitizers can be added to the photosensitive emulsion(s)
in the form of an aqueous solution, a solution in an organic solvent or in the
form of a dispersion.
The silver halide emulsions may contain the usual stabilizers e.g.
homopolar or salt-like compounds of mercury with aromatic or heterocyclic rings
such as mercaptotriazoles, simple mercury salts, sulphonium mercury double salts
and other mercury compounds. Other suitable stabilizers are azaindenes, preferably
tetra- or penta-azaindenes, especially those substituted with hydroxy or amino
groups. Compounds of this kind have been described by BIRR in Z. Wiss. Photogr.
Photophys. Photochem. 47, 2-27 (1952). Other suitable stabilizers are i.a. heterocyclic
mercapto compounds e.g. phenylmercaptotetrazole, quaternary benzothiazole derivatives,
and benzotriazole. Preferred compounds are mercapto substituted pyrimidine derivatives
as disclosed in US-P-3,692,527.
More details about the composition, preparation and coating of silver
halide emulsions can be found in e.g. Product Licensing Index, Vol. 92, December
1971, publication 9232, p. 107-109.
According to the present invention, the layer comprising said light
sensitive silver halide particles, and said substantially light insensitive silver
salts also contains the physical development nuclei.
Suitable physical development nuclei for use in accordance with the
present invention are e.g. colloidal silver, heavy metal sulphides e.g. silver
sulphide, nickel sulphide, cobalt sulphide, silver nickel sulphide etc.. Preferred
physical development nuclei are colloidal silver and silver nickel sulphides.
The photographic layer containing said light sensitive silver halide
particles, said substantially light insensitive silver salts and said physical
development nuclei may contain pH controlling ingredients. Preferably said photographic
layer is coated at a pH value below the isoelectric point of the gelatin to improve
the stability characteristics of the coated layer. Other ingredients such as antifogging
agents, development accelerators, wetting agents, and hardening agents for gelatin
may be present. Said photographic layer may comprise light-screening dyes that
absorb scattering light and thus promote the image sharpness. Suitable light-absorbing
dyes are described in i.a. US-P-4,092,168, US-P-4,311,787, DE-P-2,453,217.
Development acceleration can be accomplished with the aid of various
compounds, preferably polyalkylene derivatives having a molecular weight of at
least 400 such as those described in e.g. US-P-3,038,805 - 4,038,075 - 4,292,400.
The photographic material of the present invention may contain additional
hydrophilic layers in water permeable relationship with said photographic layer.
For example a hydrophillic layer may be applied as an outermost layer as a protecting
or anti-stress layer.
In a preferred embodiment of the present invention an intermediate
hydrophilic layer, serving as antihalation layer, is provided between the support
and the silver halide emulsion layer. This layer can contain the same light-absorbing
dyes as described above for the emulsion layer ; as alternative finely divided
carbon black can be used for the same antihalation purposes as described in US-P-2,327,828.
On the other hand, in order to gain sensitivity, light reflecting pigments,e.g.
titaniumdioxide can be present. Further this layer may contain hardening agents,
matting agents, e.g. silica particles, and wetting agents.
The hydrophilic layers comprised in a photographic material according
to the invention usually contain gelatin as hydrophilic colloid binder. Mixtures
of different gelatins with different viscosities can be used to adjust the rheological
properties of the layer. The additional hydrophilic layers are coated preferably
at a pH value below the isoelectric point of the gelatin. But instead of or together
with gelatin, use can be made of one or more other natural and/or synthetic hydrophilic
colloids, e.g. albumin, casein, zein, polyvinyl alcohol, alginic acids or salts
thereof, cellulose derivatives such as carboxymethyl cellulose, modified gelatin,
e.g. phthaloyl gelatin etc.
The hydrophilic layers of a photographic element according to the
invention, especially when the binder used is gelatin, can be hardened with appropriate
hardening agents such as those of the epoxide type, those of the ethylenimine type,
those of the vinylsulfone type e.g. 1,3-vinylsulphonyl-2-propanol, chromium salts
e.g. chromium acetate and chromium alum, aldehydes e.g. formaldehyde, glyoxal,
and glutaraldehyde, N-methylol compounds e.g. dimethylolurea and methyloldimethylhydantoin,
dioxan derivatives e.g. 2,3-dihydroxy-dioxan, active vinyl compounds e.g. 1,3,5-triacryloyl-hexahydro-s-triazine,
active halogen compounds e.g. 2,4-dichloro-6-hydroxy-s-triazine, and mucohalogenic
acids e.g. mucochloric acid and mucophenoxychloric acid. These hardeners can be
used alone or in combination. The binders can also be hardened with fast-reacting
hardeners such as carbamoylpyridinium salts of the type, described in US 4,063,952.
The photographic element used according to the present invention
may further comprise various kinds of surface-active agents in the photographic
emulsion layer or in at least one other hydrophilic colloid layer. Suitable surface-active
agents include non-ionic agents such as saponins, alkylene oxides e.g. polyethylene
glycol, polyethylene glycol/polypropylene glycol condensation products, polyethylene
glycol alkyl ethers or polyethylene glycol alkylaryl ethers, polyethylene glycol
esters, polyethylene glycol sorbitan esters, polyalkylene glycol alkylamines or
alkylamides, silicone-polyethylene oxide adducts, glycidol derivatives, fatty
acid esters of polyhydric alcohols and alkyl esters of saccharides; anionic agents
comprising an acid group such as a carboxy, sulpho, phospho, sulphuric or phosphoric
ester group; ampholytic agents such as aminoacids, aminoalkyl sulphonic acids,
aminoalkyl sulphates or phosphates, alkyl betaines, and amine-N-oxides; and cationic
agents such as alkylamine salts, aliphatic, aromatic, or heterocyclic quaternary
ammonium salts, aliphatic or heterocyclic ring-containing phosphonium or sulphonium
salts. Preferably compounds containing perfluorinated alkyl groups are used. Such
surface-active agents can be used for various purposes e.g. as coating aids, as
compounds preventing electric charges, as compounds improving slidability, as
compounds facilitating dispersive emulsification and as compounds preventing or
The photographic element of the present invention may further comprise
various other additives such as e.g. compounds improving the dimensional stability
of the photographic element, UV-absorbers, spacing agents or matting agents and
plasticizers. Preferred spacing agents are SiO&sub2; particles having an average
size of from 0.8µm to 15µm. These spacing agents may be present in one or more
layers comprised on the support of the photographic material.
Suitable additives for improving the dimensional stability of the
photographic element are e.g. dispersions of a water-soluble or hardly soluble
synthetic polymer e.g. polymers of alkyl (meth)acrylates, alkoxy(meth)acrylates,
glycidyl (meth)acrylates, (meth)acrylamides, vinyl esters, acrylonitriles, olefins,
and styrenes, or copolymers of the above with acrylic acids, methacrylic acids,
Alpha-Beta-unsaturated dicarboxylic acids, hydroxyalkyl (meth)acrylates, sulphoalkyl
(meth)acrylates, and styrene sulphonic acids.
The support of the photographic material used in accordance with
the present invention can be any of the support materials customarily employed
in the art. They include paper coated on one or both sides with an Alpha-olefin
polymer, e.g. polyethylene, glass or film, e.g. cellulose acetate film, polyvinyl
acetal film, polystyrene film, polyethylene terephthalate film etc.. Polyethylene
terephthalate film coated with a subbing layer to improve the adhesion of the thereon
deposited layers of the invention is preferred. When the photographic material
of the present invention is intended for use in COM-applications it is desirable
that the support is highly antistatic and should therefore be highly electroconductive.
According to the method of the present invention the photographic
material is information-wise exposed and subsequently developed in an alkaline
processing solution in the presence of (a) developing agent(s) and (a) silver halide
The photographic material of the present invention is exposed in
an apparatus according to its particular application, e.g. a conventional process
camera containing a conventional light source or a laser containing device.
Suitable developing agents for developing the photographic material
in connection with the present invention are e.g. hydroquinone-type and 1-phenyl-3-pyrazolidone-type
developing agents as well as p-monomethylaminophenol and derivatives thereof.
Preferably used is a combination of a hydroquinone-type and 1-phenyl-3-pyrazolidone-type
developing agent whereby the latter is preferably incorporated in one of the layers
comprised on the support of the photographic material. A preferred class of 1-phenyl-3-pyrazolidone-type
developing agents is disclosed in the EP-A-449,340. It was found that most advantage
of the present invention is taken when at least one of the there disclosed developing
agents is present in the photographic material of the present invention preferably
in the layer comprising the physical development nuclei, the light sensitive and
substantially light insensitive silver salt particles. Other type of developing
agents suitable for use in accordance with the present invention are reductones
e.g. ascorbic acid derivatives. Such type of developing agents are disclosed in
The developing agent or a mixture of developing agents can be present
in an alkaline processing solution and/or in the photographic material. In case
the developing agent or a mixture of developing agents is contained in the photographic
material, the processing solution can be merely an aqueous alkaline solution that
initiates and activates the development.
The pH of the alkaline processing solution is preferably between
9 and 13. The desired pH of the processing solution can be reached by incorporating
alkaline substances in the processing solution. Suitable alkaline substances are
inorganic alkali e.g. sodium hydroxide, potassium carbonate or aminoalkohols or
mixtures thereof. Preferably used alkanolamines are tertiary alkanolamines e.g.
those described in EP-A-397,925, EP-A-397,926, EP-A-397,927, EP-A-398,435 and
US-P-4,632,896. A combination of alkanolamines having both a pKa
or below 9 or a combination of alkanolamines whereof at least one has a pKa
above 9 and another having a pKa of 9 or less may also be used as disclosed
in the Japanese patent applications laid open to the public numbers 73949/63, 73953/61,
169841/61, 212670/60, 73950/61, 73952/61, 102644/61, 226647/63, 229453/63, US-P-4,362,811,
US-P-4,568,634 etc.. The concentration of these alkanolamines is preferably from
0.1 mole/l to 0.9 mole/l.
Preferably used silver halide solvents are water soluble thiosulphate
compounds such as ammonium and sodium thiosulphate. Other useful silver halide
solvents (or "complexing agents") are described in the book "The Theory of the
Photographic Process" edited by T.H. James, 4th edition, p. 474-475 (1977), in
particular sulphites and uracil. Further interesting silver halide complexing
agents are cyclic imides, preferably combined with alkanolamines, as described
in US 4,297,430 and US 4,355,090. 2-mercaptobenzoic acid derivatives are described
as silver halide solvents in US 4,297,429, preferably combined with alkanolamines
or with cyclic imides and alkanolamines.
The silver halide solvent(s) may be partly or completely present
in the photographic material. When the silver halide solvent is incorporated in
the photographic material it may be incorporated as a silver halide solvent precursor
as disclosed in e.g. Japanese published unexamined patent applications no. 15247/59
and 271345/63, US-P-4,693,955 and US-P-3,685,991. Developing of the information-wise
exposed photographic material is preferably carried out using a single processing
liquid. However use can be made of two processing liquids whereby only the second
liquid that is applied comprises a silver halide solvent. The DTR-development step
may also be followed by a fixing step.
The present invention is illustrated with the following examples
without limiting it thereto. All parts are by weight unless otherwise specified.
Preparation of the substantially non-light sensitive silver chloride emulsion
Solution A (35°C):
2.94 M (water)
Solution B (35°C):
3.94 M (water)
Solution C (35°C):
Solution C was brought to 45°C and 65ml of a solution containing
0.136% Na&sub3;RhCl&sub6; was added. After adjusting the pH to 3.5 solution A and
B were added simultaneously to solution C in ten minutes. The resulting emulsion
was precipitated by adding polystyrene sulphonic acid. The precipitate was rinsed
several times and redispersed by adding 180g of gelatin to a final content of 200g
of AgNO&sub3; per kg of emulsion. The thus obtained emulsion was physically ripened
for 2 hours. A substantially non-light sensitive silver chloride emulsion was
thus obtained. The average diameter of the grains was 0.178µm. This emulsion was
stabilized with 1-phenyl-5-mercaptotetrazole (23.6µmole per gram of AgNO&sub3;)
at pH=4 (1 hour stirring at 38°C).
Preparation of the light sensitive silver chloride emulsion (b).
Solution D (50°C):
2.94 M (water)
Solution E (50°C):
2.94 M (water)
Solution F (60°C):
3.94 M (water)
Solution D and solution E were added to solution F at a constant
rate of 1.8 ml per minute for 300 seconds. Solution D was further added in 3398
seconds at an accelerating rate from 5 to 30 ml per minute, while solution E was
further added at a rate so as to keep the pAg at 75mV. This emulsion was chemically
sensitized at 60°C with potassium iodide (1.51 mmole), para-toluene thiosulphonic
acid (40 µmole), gold chloride (80 µmole), potassium iodide (1.33mmole) and 1-phenyl-5-mercaptotetrazole
(210µmole). The chemically ripened emulsion was precipitated by adding polystyrene
sulphonic acid. The precipitate was rinsed several times and redispersed by adding
129g of gelatin to a final content of 200g of AgNO&sub3; per kg of emulsion. A
silver chloride emulsion with an average diameter of the grains of 1.01µm was
thus obtained. This emulsion was further spectrally sensitized with an ortho sensitizer
(1.73 µmol per gram of AgNO&sub3;) at pH=4.
The different samples were prepared as follows. To a polyethylene
terephthalate film support coated with a hydrophilic adhesion layer was coated
a layer comprising a mixture of the substantially light insensitive silver chloride
emulsion and of the photosensitive silver chloride emulsion to wich were added
physical development nuclei from the type (NiS)0.8(Ag&sub2;S)0.2
Each tested layer contained 0.8 g AgNO&sub3;/m² of emulsion (a), 0.4
g AgNO&sub3;/m² of emulsion (b) and 7.6 mg/m² of the nuclei with a total of 2.6
g gelatine/m². Different amounts of 1-phenyl-5-mercaptotetrazole (PMT) were added
to the physical development nuclei before they were mixed with the two emulsions.
The amounts are given in table 1.
The samples were exposed (EG&G; 10&supmin;&sup5;sec; U460 filter)
through a discontinuous wedge (wedgeconstant=0.15). Only the light sensitive silver
halide emulsion is sensitive for this exposure.
After the exposure the samples were subsequently developed using
an alkaline developer liquid CP297b (commercially available from Agfa-Gevaert N.V.)
containing hydroquinone and 1-phenyl-4-methyl-3-pyrazolidone as developing agents
and thiosulphate as a silver halide solvent.
For each of the samples the minimum and maximum transmission density
and the speed were measured. These results are shown in table 1.
*The speed is expressed as the number of wedgeconstants for which
the density equals Dmin + 0.1. A higher number indicates a higher speed.
To a polyethylene terephthalate film support coated with a hydrophilic
adhesion layer was coated a layer comprising a mixture of the substantially light
insensitive silver chloride emulsion (a) and of the photosensitive silver chloride
emulsion (b) as described in example 1 to wich were added physical development
nuclei from the type Ag°.
Each tested layer contained 0.8 g AgNO&sub3;/m² of emulsion (a), 0.4
g AgNO&sub3; of emulsion (b) and 11.4 (1) or 22.8 (2) µmole of Ag°nuclei/m² with
a total of 2.6 g gelatine/m². Different amounts of 1-phenyl-5-mercaptotetrazole
(PMT) were added to the physical development nuclei before they were mixed with
the two emulsions. The amounts are given in table 2 together with the sensitometric
speed is expressed as the number of wedgeconstants for which the density equals
Dmin + 0.1. A higher number indicates a higher speed.