Light-sensitive photographic material

A photographic material with at least one light-sensitive silver halide emulsion is disclosed. The emulsion has been prepared by carrying out the precipitation of the silver halide in the presence of certain imidazoles.

This invention relates to a photographic material with at least one 
light-sensitive silver halide emulsion layer, in which the silver halide 
grains have been precipitated in the presence of imidazole and/or 
imidazole derivatives. It also relates to a process for the preparation of 
such photographic materials and to a process for the production of 
photographic images. 
When preparing photographic materials in an aqueous solution of gelatine 
and halide ions, it is known to produce small silver halide crystals by 
the addition of silver ions (single jet) or the simultaneous addition of 
silver ions and halide ions (double jet) and then to leave these crystals 
to grow. The small silver halide crystals initially precipitated will 
hereinafter be referred to as silver halide nuclei. 
The necessity to allow only a portion of the silver halide nuclei to grow 
is due to the fact that the number of silver halide nuclei formed is 
generally much greater than the number of silver halide crystals which 
should be produced from the given quantity of starting materials. If all 
of the silver halide nuclei produced from a given quantity of starting 
materials were allowed to grow, the silver halide crystals obtained would 
generally be too small and therefore insufficiently sensitive. 
Various methods are known by which the number of silver halide nuclei 
initially formed by precipitation can subsequently be reduced. 
In the earliest known process, the number of nuclei is reduced by heat 
treating the emulsion in the presence of excess halide ions after the 
precipitation (Charles Bennet, The British Journal of Photography, 1878, 
page 146). In this so-called physical ripening or Ostwald ripening, the 
larger crystals continue to grow at the expense of smaller crystals, and 
some of the smaller crystals dissolve. A detailed description of this 
method of ripening is given in P. Glafkides, Chemie et physique 
photographiques, 3rd Edition, 1967, pages 339-344. 
In another known process, the number of silver halide nuclei produced 
initially is subsequently reduced by precipitating a silver halide 
emulsion, removing a small portion of the precipitate and dispersing this 
small portion in a gelatine solution, and then allowing additional silver 
halide to grow on this small portion by means of double inflow until the 
crystals have the desired size. This process has been described, for 
example, in German Offenlegungsschrift No. 2,107,118 and in British Pat. 
No. 1,170,648. 
A well known variation of the last mentioned process consists of separating 
a small portion from an emulsion precipitate, dispersing it in a gelatine 
solution, adding a fine grained emulsion (grain diameter&lt;0.3 .mu.m) and 
stirring the mixture in the presence of excess halide ions at an elevated 
temperature until the more finely grained emulsion has dissolved and its 
silver halide has grown on the silver halide crystals provided. 
All the processes mentioned above have the disadvantage that a larger 
quantity of silver halide nuclei per mol of silver is initially formed 
than is desired and special measures must subsequently be taken to reduce 
the number of silver halide nuclei to the required amount. 
From U.S. Pat. No. 3,574,628 it is known to prepare monodisperse emulsions 
containing relatively large silver halide grains in the presence of a 
thioether. One of the main disadvantages of using thioethers in the 
precipitation of photographic emulsions is that they are difficult to 
obtain in the pure state and may contain undesirable impurities. The 
instability of thioethers, especially in the presence of silver ions, is 
equally disadvantageous. The products resulting from the decomposition of 
thioethers may cause an undesirably high fog, especially if the emulsions 
are subsequently chemically ripened. 
The problem therefore exists of finding a photographic material which 
reduces or substantially obviates the disadvantages of known materials. 
Another problem is to provide an improved process for the preparation of 
such materials, particularly one in which only the required number of 
silver halide nuclei are formed from the start so that no subsequent 
measures are required for reducing the number. 
We now have found a photographic material having at least one 
light-sensitive silver halide emulsion layer of silver halide grains, 
which grains have been precipitated in the presence of certain imidazoles. 
Particularly advantageous imidazoles are those corresponding to the 
following general formula I 
##STR1## 
in which R.sup.1, R.sup.2, R.sup.3 and R.sup.4 which may be the same or 
different, represent hydrogen and/or substituted and unsubstituted alkyl, 
alkenyl, aryl and/or aralkyl groups. 
These substituents may combine with the imidazole nucleus tc form a 
multiple ligand for silver ions, for example the particularly suitable 
1-allyl-imidazole. 
Preferred alkyl groups contain from 1 to 8 carbon atoms, particularly from 
1 to 4 carbon atoms, e.g. methyl and ethyl. Alkenyl groups may contain 
from 3 to 8 carbon atoms, e.g. allyl, butenyl, hexenyl or octenyl. 
Particularly preferred are alkenyl groups containing 3 or 4 carbon atoms, 
e.g. allyl. Aryl groups may contain from 6 to 12 carbon atoms, e.g. 
phenyl, biphenyl or naphthyl. Phenyl is particularly preferred. Aralkyl 
groups may have 1 or 2 carbon atoms in the aliphatic part and 6 to 12 
carbon atoms in the aromatic part, e.g. benzyl and phenylethyl. 
Suitable substituted alkyl groups include, for example, hydroxyl, cyano and 
alkoxy groups and free or esterified carboxyl and/or sulphoalkyl groups. 
The compounds used according to the invention are preferably 
water-soluble. 
Examples of particularly suitable compounds are set out in the following 
Table 1: 
TABLE 1 
______________________________________ 
Compound No. Compound 
______________________________________ 
1 Imidazole 
2 1-methylimidazole 
3 2-methylimidazole 
4 1,2-dimethylimidazole 
5 1-allylimidazole 
6 1-methoxymethylimidazole 
7 1-(2-carboxyethyl)-imidazole 
8 4(or 5)-methylimidazole 
9 2-ethyl-4-methylimidazole 
______________________________________ 
The following groups, for example, are not suitable as substituents: HS-, 
thioether-containing groups (e.g. CH.sub.3 --S--CH.sub.2 --) and 
heterocyclic groups such as 2-imidazolyl-(1)-ethyl and 
2-imidazolyl-(2)-ethyl. 
Examples of methods of preparation of imidazole and its derivatives may be 
found e.g. in Klaus HOFMANN, "The Chemistry of Heterocyclic Compounds, 
Imidazole and Its Derivatives", Interscience Publishers, New York-London, 
1953, and in the literature cited in this book. 
The materials according to the invention are particularly suitable for the 
production of photographic images by imagewise exposure, development and 
usual further processing. 
The imidazole compounds used according to the invention may be added to the 
emulsion at any stage of its preparation and either singly or in 
combination. According to the invention, they are preferably already added 
to the reaction mixture in which the silver halide is precipitated. They 
may also be added to 
(a) The starting solution which contains the silver ions, generally a 
silver nitrate solution; or 
(b) the starting solution containing halides. 
When precipitating the emulsion, the protective colloid, in particular 
gelatine, may be present not only in the original reaction mixture but 
also in the halide solution run into it. 
The compounds used according to the invention may be used over a very wide 
range of concentrations. The concentration to be employed in any 
particular case depends upon the desired ultimate size of the silver 
halide crystals and may vary from a 10.sup.-4 molar solution to the point 
of saturation of the compound. They are preferably used as 0.003 molar to 
1 molar solutions. The pH in the precipitation medium is generally above 
5, preferably from 5.5 to 6.5, although precipitations may also be carried 
out in alkaline solution. The pH value in the precipitation medium can 
also be altered during precipitation. Especially it is possible, to 
precipitate the silver halide nuclei at pH&gt;5 and to let them grow 
afterwards into larger silver halide crystals at pH &lt;5. 
Silver halide emulsions precipitated in the presence of imidazoles 
according to the invention are separated from water soluble salts and 
other water soluble substances after the precipitation step as known in 
the art, e.g. by flocculation and/or washing with water. Under such 
conditions the imidazoles are removed from the emulsion due to their 
solubility in water. Therefore, they cannot affect and impair the further 
processing of the photographic material since they are removed after 
precipitation. 
It had already been known in the art to use imidazoles in photographic 
materials, e.g. as antifogging agents. But in these materials the 
imidazoles are either added after washing and flocculation or the 
emulsions are not washed at all. 
The photographic emulsions of this invention generally comprise silver 
halide grains having a substantially uniform diameter. Preferred 
photographic emulsions of this invention comprise silver halide grains, at 
least 95%, of said grains having a diameter which is within 40%, 
preferably within about 30%, of the mean grain diameter. Silver halide 
grains having a narrow size distribution can be obtained by controlling 
the conditions at which the silver halide grains are prepared using a 
double jet procedure. Suitable methods for preparing photographic silver 
halide emulsions generally having uniform particle size are disclosed in 
an article entitled "Ia: Properties of Photographic Emulsion Grains", by 
Klein and Moisar, The Journal of Photographic Science, vol. 12, 1963, 
pages 242-251. 
In a preferred embodiment of this invention the emulsion comprises grains 
having a cubic or octahedral structure. 
The usual silver halide emulsions are suitable for the present invention. 
The silver halide contained in them may comprise silver chloride, silver 
bromide and silver iodide. They may be negative emulsions or direct 
positive emulsions. The emulsion grains may have ripening nuclei both 
internally and externally or they may have a layered grain structure. 
The material according to the invention may be used for the so-called 
silver dye bleaching process. The present invention is also suitable for 
the instant image colour process and colour transfer processes. In these 
processes, the dyes for the partial colour images diffuse into an image 
receiving layer where they become firmly fixed or the colour couplers 
diffuse into the image receiving layer where they are converted into the 
image dye after the usual colour producing development. 
The light-sensitive material generally contains three light-sensitive 
emulsion layers, each of which has a colour producing system associated 
with it. By "colour producing system" is meant a compound incorporated in 
a diffusion resistant form in the particular layer which compound is a dye 
or a dye precursor product which releases diffusible dyes, preferably dyes 
containing acid groups, under the action of oxidation products which are 
produced imagewise from the photographic developers when development is 
carried out in the presence of the alkaline processing mass. A wide 
variety of chemical compounds are available for use as such colour 
producing systems. The diffusion resistant colour producing substances 
according to U.S. Pat. No. 3,628,952, for example, are particularly 
suitable. These compounds release diffusible dyes in their reaction with 
oxidation products of black-and-white developers or colour developers. 
Another suitable class of compounds has been described in German Pat. No. 
1,095,115. The compounds mentioned in the said Patent react with oxidized 
colour developers to produce diffusible dyes which generally belong to the 
class of azomethine dyes. Another suitable colour producing system has 
been described in U.S. Pat. Nos. 3,443,939 and 3,443,940. In this system, 
the release of diffusible dyes is accompanied by ring closure under the 
action of oxidized developer substances. 
Colour transfer processes and couplers used in such processes which may 
also be used in the present invention have also been described in U.S. 
Pat. Nos. 2,983,606; 3,087,817; 3,185,567; 3,227,550; 3,227,551; 
3,227,552; 3,227,554; 3,253,915; 3,415,644; 3,415,645; and 3,415,646. 
The light-sensitive materials used for such instant colour image processes 
generally have the following structue: 
Blue-sensitive silver halide emulsion layers; 
Layer containing system which releases a yellow dye; 
Separating layer; 
Green-sensitized silver halide emulsion layer; 
Layer containing system releasing a magenta dye; 
Separating layer; 
Red-sensitized silver halide emulsion layer; 
Layer containing system releasing a cyan dye. 
Furthermore, the emulsions according to the invention provide better 
stability, especially a better stability of the gradation curve of both 
black-and-white and of color materials, as compared with known emulsions, 
e.g. emulsions prepared in the presence of Rh-compounds. The desired 
gradation can be achieved with just one of the emulsions according to the 
invention or, as is generally known by mixing several of these emulsions, 
the gradation becoming flatter the more these emulsions differ in their 
sensitivity. 
For example, the emulsions according to the invention may be used for a 
photographic recording material on a white, reflective supporting base for 
the production of positives. Such a material may contain three 
light-sensitive layers of which the red-sensitive layer, which contains 
the coupler capable of forming blue-green dye, is arranged farthest away 
from the supporting base. Beneath this layer are arranged and UV-absorbing 
intermediate layer, the green-sensitive layer containing the coupler for 
the formation of a purple dye and beneath this, separated by an 
intermediate layer the blue-sensitive layer containing the coupler for the 
yellow dye. Such materials may additionally contain compounds for 
stabilizing the white parts of the photographic images, e.g. 
n-octyl-hydroquinone, 2-ethyl-hexylhydroquinone and 
2,5-bis-(1',1',3',3'-tetramethyl)-butylhydroquinone. These compounds may 
have been emulsified and can be applied in varying amounts, e.g. from 1 to 
100 mg and particularly from 10 to 50 mg/m.sup.2. 
The materials prepared according to the invention may be developed with the 
usual colour developer compounds, e.g. 
N,N-dimethyl-p-phenylenediamine; 
4-amino-3-methyl-N-ethyl-N-methoxyethylaniline; 
2-amino-5-diethylaminotoluene; 
N-butyl-N-.omega.-sulphobutyl-p-phenylenediamine; 
2-amino-5-(N-ethyl-N-.beta.-methanesulphonamidoethyl-amino)-toluene; 
N-ethyl-N-.beta.-hydroxyethyl-p-phenylenediamine; 
N,N-bis-(.beta.-hydroxyethyl)-p-phenylenediamine and 
2-amino-5-(N-ethyl-N-.beta.-hydroxyethylamino)-toluene. 
Other suitable colour developers have been described, for example, in J. 
Amer. Chem. Soc. 73, 3100 (1951). 
The photographic material prepared according to this invention may contain 
the usual colour couplers which are generally incorporated in the silver 
halide layers. Thus, the red sensitive layer, for example, may contain a 
non-diffusible colour coupler for producing the cyan partial colour image, 
generally a phenol or .alpha.-naphthol coupler. The green sensitive layer 
may contain at least one non-diffusible colour coupler for producing the 
magenta particle colour image, usually a 5-pyrazolone or indazolone colour 
coupler. The blue sensitive layer unit may contain at least one 
non-diffusible colour coupler for producing the yellow partial colour 
image, generally a colour coupler which has an open chain ketomethylene 
group. Colour couplers of this type are known in large numbers and have 
been described in numerous Patent Specifications. References may be found, 
for example, in the publication "Farbkuppler" by W. Pelz in "Mitteilungen 
aus den Forschungslaboratorien der Agfa, Leverkusen/Munchen", Volume III 
(1961) and the publication by K. Venkataraman in "The Chemistry of 
Synthetic Dyes", Vol. 4, 341 to 387, Academic Press, 1971. 
2-Equivalent couplers may also be used as non-diffusible colour couplers. 
These contain a releasable substituent in the coupling position so that 
they require only two equivalents of silver halide to produce the colour, 
in contrast to the usual 4-equivalent couplers. Suitable 2-equivalent 
couplers include, for example, the well known DIR couplers, in which the 
removable group is released as a diffusible development inhibitor after 
the reaction with colour developer oxidation products. So-called white 
couplers may also be used to improve the properties of the photographic 
material. 
The non-diffusible colour couplers and colour producing compounds are added 
to the light-sensitive silver halide emulsions or other casting solutions 
by the usual methods. If they are water-soluble or alkali soluble 
compounds, they may be added to the emulsions in the form of aqueous 
solutions, optionally with the addition of water-miscible organic solvents 
such as ethanol, acetone or dimethylformamide. If the non-diffusible 
colour couplers and colour producing compounds used are insoluble in water 
or alkaline solutions, they may be emulsified in known manner, for example 
by mixing a solution of the compound in a low boiling organic solvent 
either directly with the silver halide emulsion or first with an aqueous 
gelatine solution and then removing the organic solvent in the usual 
manner. An emulsion of the given compound in gelatine obtained in this way 
is subsequently mixed with the silver halide emulsion. So-called coupler 
solvents or oil formers may also be used for emulsifying such hydrophobic 
compounds; these are generally organic compounds which have a relatively 
high boiling point and which enclose in the form of oily droplets the 
non-diffusible colour couplers and development inhibiting releasing 
compounds which are required to be emulsified in the silver halide 
emulsions. 
Information may be found, for example, in U.S. Pat. Nos. 2,322,027; 
2,533,514; 3,689,271; 3,764,336 and 3,765,897. 
The binder used for the photographic layers is preferably gelatine although 
this may be partly or completely replaced by other natural or synthetic 
binders. Suitable natural binders include, for example, alginic acid and 
its derivatives such as its salts, esters or amides, cellulose derivatives 
such as carboxymethylcellulose, alkyl celluloses such as 
hydroxyethylcellulose, starch or its derivatives such as ethers or esters, 
or carragheenates. Suitable synthetic binders include polyvinyl alcohol, 
partially saponified polyvinyl acetate and polyvinylpyrrolidone. 
The emulsions may also be chemically sensitized, e.g. by the addition of 
sulphur compounds at the chemical ripening stage, for example, allyl 
isothiocyanate, allyl thiourea and sodium thiosulphate. Reducing agents 
may also be used as chemical sensitizers, e.g. the tin compound described 
in Belgian Pat. Nos. 493, 464 and 568,687, or polyamines such as 
diethylene triamine or aminoethylsulphinic acid derivatives, e.g. 
according to Belgian Pat. No. 547,323. 
Noble metals such as gold, platinum, palladium, iridium, ruthenium or 
rhodium and compounds of these metals are also suitable as chemical 
sensitizers. This method of chemical sensitization has been described in 
the article by R. Koslowsky, Z. Wiss. Phot. 46, 65-72 (1951). 
The emulsions may also be sensitized with polyalkylene oxide derivatives, 
e.g. with a polyethylene oxide having a molecular weight of from 1000 to 
20,000 or with condensation products of alkylene oxides and aliphatic 
alcohols, glycols, cyclic dehydration products of hexitols, alkyl 
substituted phenols, aliphatic carboxylic acids, aliphatic amines, 
aliphatic diamines and amides. The condensation products have a molecular 
weight of at least 700, preferably more than 1000. These sensitizers may, 
of course, be combined to produce special effects, as described in Belgian 
Pat. No. 537,278 and British Pat. No. 727,982. 
The emulsions may also be optically sensitized, e.g. with the usual 
polymethine dyes such as neutrocyanines, basic or acid carbocyanines, 
rhodacyanines, hemicyanines, styryl dyes and oxonols. Sensitizers of this 
type have been described in the work by F. M Hamer entitled "The Cyanine 
Dyes and related Compounds", 1964, Interscience Publishers, John Wiley and 
Sons. 
The emulsions may contain the usual stabilizers, e.g. salts or homopolar 
compounds of mercury containing aromatic or heterocyclic rings, such as 
mercaptotriazoles, simple mercury salts, sulphonium mercury double salts 
and other mercury compounds. Azaindenes are also suitable stabilizers, 
particularly tetra- and penta azaindenes and especially those which are 
substituted with hydroxyl or amino groups. Compounds of this type have 
been described in the article by Birr, Z. Wiss. Phot. 47 (1952), 2 to 58. 
Other suitable stabilizers include heterocyclic mercapto compounds such as 
phenyl mercaptotetrazole, quaternary benzothiazole derivatives and 
benzotriazole. 
The emulsions may be hardened in the usual manner, for example with 
formaldehyde or halogen substituted aldehydes containing a carboxyl group, 
such as mucobromic acid, diketones, methanesulphonic acid esters and 
dialdehydes. 
The photographic layers may also be hardened with epoxide type hardeners, 
heterocyclic ethylene imine hardeners or acryloyl hardeners. Examples of 
such hardeners have been described, for example, in German 
Offenlegungsschrift No. 2,263,602, and in British Pat. No. 1,266,655. The 
layers may also be hardened by the process according to German 
Offenlegungsschrift No. 2,218,009 to produce colour photographic materials 
which are suitable for high temperature processing. 
The photographic layers or colour photographic multilayered materials may 
also be hardened with hardeners of the diazine, triazine or 
1,2-dihydroquinoline series as described in British Pat. Nos. 1,193,290; 
1,251,091; 1,306,544 and 1,266,655; French Pat. No. 7,102,716 and British 
Pat. No. 1,452,669.

The following are examples of such hardeners: Diazine derivatives 
containing alkylsulphonyl or arylsulphonyl groups, derivatives of 
hydrogenated diazines or triazines, e.g. 1,3,5-hexahydrotriazine, 
fluorosubstituted diazine derivatives, e.g. fluoropyrimidine, esters of 
2-substituted, 1,2-dihydroquinoline- or 
1,2-dihydroisoquinoline-N-carboxylic acids. Vinyl sulphonic acid 
hardeners, carbodiimide hardeners and carbamoyl hardeners such as those 
described in German Offenlegungsschriften Nos. 2,263,602; 2,225,230 and 
1,808,685, French Pat. No. 1,491,807, German Pat. No. 872,153 and DDR Pat. 
No. 7218 may also be used. Other suitable hardeners have been described, 
for example, in British Pat. No. 1,268,550. 
EXAMPLE 1 
The emulsions described in this Example are prepared as follows: 
1. A solution of the following composition, hereinafter referred to as the 
"reaction medium", is introduced into the reaction vessel: 
______________________________________ 
Reaction medium 
______________________________________ 
Water 1300 ml 
Gelatine 30 g 
Potassium bromide 0.16 g 
______________________________________ 
The reaction medium is adjusted to pH 6.0. The quantities of imidazole 
indicated in Table 2 are optionally added. 
2. The following are then added separately by the double inflow method at 
63.degree. C.: 
100 ml of a 0.3 molar silver nitrate solution and, at the same time, 
100 ml of a 0.3 molar potassium bromide solution. The rate of inflow is the 
same for both solutions, and is initially 88 ml per hour for 4 minutes and 
is then increased by 22 ml per hour every 15 seconds until it reaches 660 
ml per hour. 
3. The following are then added separately by the double inflow method at a 
rate of 330 ml/h each: 
85 ml of a 2-molar silver nitrate solution and, at the same time, 
85 ml of a 2-molar potassium bromide solution. 
4. The pAg is then adjusted to 9.6 with a 2-molar potassium bromide 
solution. At this pAg value, 400 ml of a 2-molar silver nitrate solution 
and the quantity of 2-molar potassium bromide solution required to keep 
the pAg constant are added by a pAg controlled process of double inflow, 
the rate of inflow of silver nitrate solution being 330 ml/h. 
5. The resulting emulsions are then cooled, flocculated and washed in the 
usual manner and finally redispersed in a solution of 55 g of gelatine and 
430 ml of water and, if desired, further processed as indicated. 
Emulsions A to E indicated in Table 2 are obtained. 
In all of the emulsions according to the invention, the diameter of the 
octahedric silver halide grains is greater than that of the comparison 
emulsion without imidazole. The grain diameter increases with increasing 
quantity of imidazole without any other conditions of precipitation having 
to be altered. There is no need to subdivide the emulsion precipitate nor 
is any physical ripening to increase the grain size required. 
TABLE 2 
______________________________________ 
Imidazole Crystal 
(g in reaction 
Crystal diameter 
Emulsion medium) form (.mu.m) 
______________________________________ 
A (com- 
parison) 0 octahedron 0.50 
B 0.9 octahedron 0.65 
C 2.7 octahedron 0.90 
D 9.0 octahedron 1.4 
E 27.0 octahedron 2.0 
______________________________________ 
The emulsion grains may be chemically ripened by known methods, as already 
mentioned above. As is will known, in the case of chemical ripening, the 
larger the emulsion grains, the shorter must be the ripening time or the 
lower the ripening temperature if usuable emulsions are to be obtained. 
Comparison emulsion A and emulsions C and D are then further processed as 
follows: 
1/10 of the resulting emulsion is adjusted to a pAg of 8.9 with a 1-molar 
potassium bromide solution at the ripening temperature indicated in Table 
3, 3.5 mg of KSCN and 0.48 mg of HAuCl.sub.4 are added and the reaction 
mixture is kept at the ripening temperature for two hours and then cooled. 
The indices 1, 2 and 3 are appended to emulsions A, B and C to indicate 
the temperatures at which the emulsions were ripened. To these ripened 
portions of the emulsions are then added 68 ml of a 0.001-molar methanolic 
solution of a spectral sensitizer corresponding to the following formula: 
##STR2## 
and the emulsions are cast on a substrate of cellulose acetate to form 
layer 0.18 mm in thickness. The amount of silver applied corresponds to 
2.8 g/m.sup.2. Strips of the resulting light-sensitive material are 
exposed behind a 3.cuberoot.2 wedge for 1/20 seconds. The exposed strips 
are then developed for 13 minutes at 20.degree. C. in the following 
developer (developer 1): 
______________________________________ 
Developer 1 
______________________________________ 
Water 800 ml 
p-methylammoniophenolsulphate 
3.5 g 
Ascorbic acid 9.0 g 
Sodium carbonate, anhydrous 
14.0 g 
Potassium bromide 2.4 g 
made up with water to 1 liter. 
______________________________________ 
The layers are then fixed, washed and dried in the usual manner. The 
sensitivity is measured at a density of 0.1 above fog. As can be seen from 
Table 3, after chemical ripening, the sensitivity of emulsions C and D 
according to the invention is higher than that of comparison emulsion A. 
TABLE 3 
______________________________________ 
Material 
containing 
Ripening 
emulsion temperature Image Relative sensitivity 
______________________________________ 
A 1 65.degree. C. 
Negative 68 
A 2 67.degree. C. 
Negative 71 
A 3 69.degree. C. 
Negative 100 
C 1 65.degree. C. 
Negative 166 
C 2 67.degree. C. 
Negative 166 
C 3 69.degree. C. 
Negative 170 
D 1 65.degree. C. 
Negative 347 
D 2 67.degree. C. 
Negative 389 
D 3 69.degree. C. 
Negative 468 
______________________________________ 
Doubling of the number in the column headed "relative sensitivity" 
corresponds to doubling of the sensitivity. 
EXAMPLE 2 
To precipitate the emulsion indicated in the following Table 4, the 
procedure is the same as indicated in Example 1 except that the quantities 
of the given imidazole derivative shown in Table 4 are added to the 
reaction medium. Emulsion A is a comparison emulsion for which no 
imidazole or imidazole derivative was added to the reaction medium for its 
preparation. Emulsions with octahedric emulsion grains are obtained in all 
cases. 
TABLE 4 
______________________________________ 
Quantity of 
imidazole 
derivative Diameter 
Em- (g in reac- of octa- 
ulsion 
tion medium) 
Imidazole derivative 
hedron (.mu.m) 
______________________________________ 
A -- -- 0.5 
F 10.9 1-methyl-imidazole 
1.5 
G 10.9 2-methyl-imidazole 
1.2 
H 12.7 1,2-dimethylimidazole 
0.9 
I 14.3 1-allylimidazole 2.0 
J 14.8 1-methoxymethylimidazole 
1.4 
K 18.5 1-(2-carboxyethyl)- 
0.9 
imidazole 
L 10.8 4(5)-methyl-imidazole 
1.0 
M 14.6 2-ethyl-4-methyl- 
0.9 
imidazole 
______________________________________ 
As can be seen in Table 4, the use of the given imidazole derivatives has 
the same effect as the use of imidazole in resulting in silver halide 
crystals with a larger diameter. 
EXAMPLE 3 
Emulsion N indicated below is prepared as described in Example 1 but with 
the following changes: 
1. The reaction medium contains in addition 9.0 g of imidazole. 
2. The double inflow described under 4. in Example 1 is not carried out at 
a pAg of 9.6; the pAg is adjusted to 8.5 with a 2-molar potassium bromide 
solution. Silver halide cubes having a diameter of 1.7 .mu.m (length of 
the side: 1,0 .mu.m) are obtained. 
Emulsion O (Comparison emulsion) 
Emulsion O is prepared by the same methods as emulsion N except that the 
reaction medium contains no imidazole. Tetradecahedrons with a diameter of 
0.50 .mu.m are obtained. 
Example 3 demonstrates that emulsions with cubical crystals can be prepared 
by means of the compounds used according to the invention. 
EXAMPLE 4 
Emulsion P indicated below is prepared by the same method as emulsion N 
except that the pH of the reaction medium is adjusted to 7.6. A 
heterodisperse emulsion with cubical crystals having a diameter of from 
1.4 to 3.3 .mu.m is obtained. 
Emulsion Q (comparison emulsion) 
Emulsion Q is prepared in the same way as Emulsion P except that the 
reaction medium contains no imidazole. Tetradecahedrons with a diameter of 
0.55 .mu.m are obtained. 
This Example shows that heterodisperse emulsions can also be prepared with 
the aid of the compounds to be used according to the invention. 
EXAMPLE 5 
For the preparation of Emulsion R, the procedure is initially the same as 
that used for preparing Emulsion D. After the pAg controlled double inflow 
of 400 ml of a 2-molar silver nitrate solution and the corresponding 
quantity of a 2-molar potassium bromide solution carried out for the 
preparation of emulsion D, the resulting emulsion is heated to 70.degree. 
C., 4.73 ml of a solution of 0.1 g of sodium dithiosulphato-aurate (I). 
2H.sub.2 O in 100 ml of water are added, and the reaction mixture is kept 
at 70.degree. C. for 45 minutes and then cooled to 63.degree. C. Silver 
bromide is precipitated on the resulting chemically ripened emulsion 
grains by adding, by a process of pAg controlled double inflow at pAg 9.6, 
1000 ml of a 2-molar silver nitrate solution and the quantity of 2-molar 
potassium bromide solution necessary to keep the pAg constant, the rate of 
inflow of the silver nitrate solution being 330 ml/h. 
The emulsion is then cooled, flocculated and washed in the usual manner and 
redispersed in a solution of 195 g of gelatine in 1350 ml of water. The 
silver halide crystals of the emulsion form octahedrons with a diameter of 
1.8 .mu.m. 
Portions of the emulsion prepared in this way, which contains internal 
nuclei, are then chemically ripened as follows: 
1/10 of the emulsion precipitate is heated to 57.degree. C. and adjusted to 
pAg 9.4 with a 1-molar potassium bromide solution, and 0.37 ml of a 
solution of 0.1 g of sodium dithiosulphato-aurate (I). 2H.sub.2 O in 100 
ml of water is then added. The emulsions are maintained at 57.degree. C. 
for 30 minutes (Emulsion R1), 45 minutes (Emulsion R2) or 60 minutes 
(Emulsion R3) and then cooled. 
102 ml of a 0.001-molar methanolic solution of the spectral sensitizer 
indicated in Example 1 are added to the ripened portions of emulsion which 
are then cast on a 0.18 mm thick cellulose acetate substrate. The silver 
application is 3.3 g/m.sup.2. Strips of the resulting light sensitive 
material are exposed behind a 3.cuberoot.2 wedge for 1/20 seconds. The 
exposed strips are developed in the following fogging developer (developer 
2) for 10 minutes at 20.degree. C.: 
______________________________________ 
Developer 2 
______________________________________ 
Water 800 ml 
Sodium sulphite, anhydrous 
2.6 g 
p-aminophenol 3.3 g 
Sodium hydroxide 5.0 g 
Trisodium phosphate . 12H.sub.2 O 
40.0 g 
______________________________________ 
The strips are then fixed, washed and dried in the usual manner. The 
sensitivity as defined in Example 1 is tested on emulsions R1, R2 and R3 
at 0.9 times the maximum density. Table 5 shows that Emulsion R provides a 
light-sensitive photographic material which produces positive images. 
The grain size and hence sensitivity can again be varied by varying the 
concentration of imidazole (or of its derivatives) but in the direct 
positive emulsions of Example 5 the internal and external ripening must be 
carefully adjusted to the grain size. It is therefore inadvisable to vary 
the grain size under constant conditions of ripening if usable direct 
positive emulsions are to be obtained every time. 
TABLE 5 
______________________________________ 
Time of 
Material external 
containing 
chemical Relative 
emulsion ripening Image sensitivity 
______________________________________ 
A 3 120 mins. Negative 100 
R 1 30 mins. Positive 1500 
R 2 45 mins. Positive 790 
R 3 60 mins. Positive 600 
______________________________________ 
The material containing Emulsion A 3 is similar to that described in 
Example 1. 
EXAMPLE 6 
The photographic material described below contains Emulsion S which is 
prepared as follows: 
1. The reaction medium provided is a solution of: 
______________________________________ 
Water 1300 ml 
Gelatine 30 g 
Potassium chloride 1 g 
Imidazole 9 g 
______________________________________ 
The solution is adjusted to pH 5.8. 
2. The following are added separately by the method of double inflow to 
solution 1. above at 63.degree. C.: 
200 ml of 2-molar silver nitrate solution and at the same time 
200 ml of 2-molar potassium chloride solution. 
The rate of inflow is the same for both solutions. It starts at 110 ml/h 
for 4 minutes and is then raised by 22 ml/h every 15 seconds up to 220 
ml/h. 
3. The following solutions are then added separately by the double inflow 
method at a rate of 330 ml/h: 
100 ml of 2-molar silver nitrate solution and, at the same time, 
100 ml of 2-molar potassium bromide solution. 
4. The pAg is then adjusted to 9.6 with a 2-molar potassium bromide 
solution. 700 ml of a 2-molar silver nitrate solution and the quantity of 
2-molar potassium bromide solution necessary to maintain the pAg constant 
are added at this pAg by a method of pAg controlled double inflow, the 
silver nitrate solution being added at the rate of 440 ml/hour. 
The emulsion is cooled, flocculated and washed in the usual manner and 
finally redispersed in a solution of 110 g of gelatine in 860 ml of water. 
The silver halide crystals of the emulsion are distorted octahedrons with 
a diameter of from 1.3 to 2.0 .mu.m. 
68 ml of a 0.001 molar methanolic solution of the spectral sensitizer 
indicated in Example 1 are added to 1/10 of the emulsion prepared as 
described above and the emulsion is then cast on a 0.18 mm thick cellulose 
acetate substrate. The silver application is 3.3 g. 
Strips of the resulting light-sensitive material are exposed behind a 
.cuberoot.2 wedge in a sensitometer for 1/20 seconds. The exposed strips 
are developed for 13 minutes at 20.degree. C. in the developer described 
in Example 5. They are then fixed, washed and dried in the usual manner. 
The sensitivity of Emulsion S is measured at 0.9 times the maximum 
density. As can be seen from Table 5, the light-sensitive photographic 
material obtained with Emulsion S produces positive images. 
The grain size and therefore the sensitivity can again be varied by varying 
the concentration of imidazole or of imidazole derivatives but in direct 
positive emulsions obtained according to Example 6 the ratio of 
##EQU1## 
must be adjusted to the grain size. It is therefore inadvisable to vary 
the grain size at a constant percentage composition of the emulsion grains 
if usable direct positive emulsions are to be obtained each time. 
TABLE 6 
______________________________________ 
Material 
containing Relative 
emulsion Image sensitivity 
______________________________________ 
A 3 Negative 100 
S Positive 560 
______________________________________ 
The material containing Emulsion A 3 is similar to that described in 
Example 1. The sensitivity is as defined in Example 1. 
EXAMPLE 7 
The following layers are applied one after the other to a transparent 
polyester foil used as substrate: 
1. A mordanting layer of gelatine (6.0 g/m.sup.2) and a polyurethane (6.0 
g/m.sup.2) of 4,4'-diphenylmethane diisocyanate, N-ethyl-diethanolamine 
and epichlorohydrin; 
2. A reflection layer of titanium dioxide (24 g/m.sup.2) and gelatine (2.4 
g/m.sup.2); 
3. a layer of carbon black (1.9 g/m.sup.2) and gelatine (2.0 g/m.sup.2); 
4. A dye layer containing 1.0 g/m.sup.2 of the following compound: 
##STR3## 
and gelatine (1.0 g/m.sup.2); 
5. an emulsion layer consisting of the spectrally sensitized emulsion S 
indicated in Example 6 (1.2 g of silver/m.sup.2, 0.78 g of 
gelatine/m.sup.2) and the fogging agent, acetic acid-2-phenyl hydrazide 
(1.35 mg/m.sup.2); 
6. a protective layer of gelatine (1 g/m.sup.2) and formaldehyde (15 
mg/m.sup.2). 
A strip of the light-sensitive material produced in this way is exposed in 
a sensitometer behind a .cuberoot.2 wedge for 1/5 second. The exposed 
strip is then developed for 7 minutes with an alkaline developer paste of 
the following composition applied in a thickness of 0.180 mm: 
______________________________________ 
Water 495 ml 
Hydroxyethylcellulose 15 g 
Benzyl alcohol 5 ml 
Potassium hydroxide 20 g 
Ascorbic acid 0.125 g 
Methylbenzotriazole 1.5 g 
1-phenyl-4-methyl-4-hydroxymethyl- 
3-pyrazolidone 0.65 g 
Hydroquinone 0.05 g 
Paraformaldehyde 0.5 g. 
______________________________________ 
Placed along the longitudinal edges of the light-sensitive strips, are 
spacer strips which, together with the light-sensitive strips and a cover 
sheet, form a cavity which is filled with the developer paste. The paste 
is initially inside a rupturable container at one end of the cavity and is 
distributed in the cavity when the whole arrangement is passed between a 
pair of squeezing rollers. After development, the strip is stopped, washed 
and dried in the usual manner. 
A magenta coloured positive image having a maximum colour density of 1.90 
and minimum colour density of 0.23 is obtained.