This invention relates in general to photography and in particular
to a photographic donor material that is useful in the silver salt diffusion transfer
The principle of the silver salt diffusion transfer process is described
in British Patent No. 614,155 filed November 2, 1939. This process comprises the
steps of exposing a photosensitive element containing a silver halide emulsion
layer, developing the exposed photosensitive silver halide emulsion layer and
forming a soluble silver complex of unexposed silver halide by treating the said
photosensitive silver halide emulsion layer with an alkaline processing fluid
in the presence of a developing agent and a silver halide complexing agent, transferring
said soluble silver complex by diffusion to the silver receptive layer of an image-receiving
element in superposed relationship with said silver halide emulsion, forming at
said silver receptive layer an image incorporating silver from said silver complex
under the action of development nuclei, and separating said image-receiving element
from said photosensitive element. As explained in British Patent No. 614,155, the
development nuclei are uniformly distributed throughout the silver receptive layer
of the image-receiving element.
A more detailed description of the silver salt diffusion transfer
process is provided in Chapter 16 (Author: G.I.P. Levenson) of "The Theory of
the Photographic Process", Ed. T. H. James, 4th Edition, Macmillan, New York.
The silver salt diffusion transfer process has long been used in
the field of graphic arts. In most applications in this field, images of high
contrast are required, but there is also a need to produce continuous tone images
by use of the same processing conditions used for the high contrast images.
One method of reducing the contrast of a photographic silver halide
layer is by using silver halide grains of different photographic speed. These
grains can be blended and coated in a single layer or two separate layers can be
coated, comprising a faster layer and a slower layer. This approach is taken in
the materials described in European Specification 0 187 879.
U. S. patent 4,873,181 describes a photographic material comprising
a support, a light-sensitive silver halide emulsion layer that contains silver
iodide overlying the support, and an auxiliary layer containing non-light-sensitive
silver halide grains having an average grain size of not more than 0.5 µm. The
purpose of the auxiliary layer is to provide high covering power, improved graininess
and improved development stability.
The present invention provides a photosensitive donor material for
the silver salt diffusion transfer process which can provide continuous tone images
in the receiver because of the novel construction of the photosensitive donor material.
According to the present invention, there is provided a photosensitive
donor material which comprises a support bearing a photosensitive silver halide
emulsion layer substantially free of iodide, at a silver laydown of from 140 to
240 mg/m² and, located between the photosensitive emulsion layer and the support,
a layer of a silver halide emulsion which is non-photosensitive under conditions
In use, the photographic donor material of this invention is exposed
to an image and processed by bringing it into face-to-face contact with a receiving
layer in the presence of an alkaline processing solution. As with conventional
diffusion transfer donors, the exposed areas of the photosensitive emulsion layer
develop to metallic silver. Meanwhile, in the unexposed areas of the photosensitive
emulsion and throughout the whole of the non-photosensitive emulsion, undeveloped
silver halide is solubilized by the silver halide solvent in the processing solution.
The soluble silver complexes so formed then begin to diffuse to the receiving
layer, which contains silver precipitating nuclei where a metallic silver image
In all diffusion transfer materials, the ratio of the silver complexes
to silver is high at the start of development. However, in areas where there has
been exposure to light, silver centers will be produced fairly rapidly on development.
The silver centers act as nuclei for physical development in the donor before
the silver complexes can diffuse to the receiver layer, and this gives rise to
the high contrast found in diffusion transfer materials.
In the present material, the ratio of silver complexes to silver
is increased in areas where there has been light exposure because of the use of
a non-photosensitive silver halide layer. Thus, some of the silver complexes are
able to diffuse to the receiver layer in areas of intermediate exposure, giving
a lower contrast than in conventional diffusion transfer materials.
In the photographic donor material of this invention, the photosensitive
silver halide emulsion layer is at least 80% silver chloride, and preferably it
is substantially pure silver chloride. It is particularly important to avoid any
significant iodide content in the photosensitive silver halide emulsion layer
employed in the present donor material, i.e., it is substantially free from iodide.
This is because iodide content in the photosensitive silver halide emulsion will
slow down the rate of development, and this will interfere with the diffusion
transfer image-forming process. In this process, development needs to be rapid
so that it will be completed before any dissolution and transfer of unexposed
and undeveloped silver halide occurs.
The present invention further provides a method for forming an image
by the silver salt diffusion transfer process in which a donor of the present
invention is imagewise exposed, placed in face-to-face contact with a receiving
sheet comprising a layer containing silver precipitating nuclei, in the presence
of an alkaline processing solution, and thereafter separated to provide an image-bearing
The non-photosensitive emulsion layer is insensitive to at least
the degree that it forms no image on exposure and processing under conditions of
use. If its speed was measured, however, it could be found to have a speed of
2 log E, preferably at least 3 log E, less than the photosensitive emulsion.
A continuous tone image can be obtained under a range of processing
conditions, such as high and low temperatures or fresh or near exhausted processing
solutions, and still provide acceptable results. There is, however, a degree of
control excercisable over the contrast by varying the time the donor and receiver
are held in contact.
The non-photosensitive emulsion layer may or may not have the same
silver halide content as that of the photosensitive emulsion. As a matter of practice,
it is sufficient to omit any sensitizing dye from the emulsion to make it non-photosensitive.
Emulsions which may be employed in both the photosensitive and non-photosensitive
layers are generally described in Research Disclosure Item 308119, December 1989,
Industrial Opportunities, Dudley Annexe, 21a North Street, Emsworth, Hampshire
PO1O 7DQ, United Kingdom.
The photosensitive emulsion layer may be coated at silver laydowns
of from 140 to 240 mg/m², preferably from 160 to 190 mg/m² and gelatin laydowns
of from 1 to 3 g/m². As is usual, a layer of the donor material, e.g., the emulsion
layer or an underlayer, may have incorporated therein a developing agent or developing
agent combination. The non-photosensitive emulsion layer may be coated at silver
laydowns of from 260 to 450 mg/m², preferaby from 324 to 405 mg/m² and gelatin
laydowns of from 3 to 5 g/m². An antihalation dye can be incorporated therein
to improve the sharpness of the image. The ratio of non-sensitive to sensitive
emulsion silver halide will affect the contrast of the image obtained.
The support, method of coating, additives, etc, may be as described
in the Research Disclosure item above.
The following Example is included for a better understanding of the
A silver chloride emulsion of grain size 0.34 micrometers was prepared.
Using this emulsion, Melt (A) was prepared by adding a potassium bromide solution
at the rate of 1.8 g per mole of silver. An orthochromatic sensitizing dye (peak
absorption of 509 nm) was added at 0.4 g per mole of silver. To this, benzothiazolium
iodide was added at 0.08 g per mole silver. One percent (1%) of TRITON X100™
was then added, followed by sufficient gelatin to make a 10% gelatin solution.
A second Melt (B) was prepared using the same basic silver halide
emulsion as above, adding 1.2 g potassium bromide per mole of silver. Sufficient
gelatin was added to make a 6.5% gelatin solution.
Solution (C) was prepared consisting of an aqueous solution of hydroquinone
(71.5 g), sodium formaldehyde bisulfite (23.6 g) and 1-phenyl-3-pyrazolidone (0.64
g) in 1 liter.
Melt (B) was coated on a resin-coated paper support at a laydown
of 0.405 g/m² silver and 5.0 g/m² gelatin. On top of this layer was coated the
solution obtained by mixing Melt (A) and Solution (C) at a laydown of 0.162 g/m²
silver and 3.0 g/m² gelatin. During coating, the hardener bisvinylsulfonylmethyl
ether (BVSME) was added to Melt (B) at a rate of 0.008 g/m² silver.
Strips of the final coating were exposed to a step wedge of incremental
density 0.11 log E and processed using Kodak™ PMTII Activator in a Kodak
Imagemate™ 43DT diffusion transfer processor and laminated to a Kodak™
PMTII paper receiver sheet. Contrasts of from 0.6 to 1.8 were obtained by varying
the strip time between 15 seconds and 1 minute.