US 3655394 A
Photographic emulsions comprising cubic-regular grains free of reduction sensitization are prepared by running an aqueous solution of silver nitrate and an aqueous solution of a silver halide simultaneously into an agitated aqueous solution of a peptizer in a double-run precipitation procedure while maintaining the pH at no more than 4.0 and the pAg at a value within the range of 8.6 to 9.1. In one aspect of this invention, emulsions prepared by the above procedure are chemically sensitized with a compound containing a labile atom selected from the group consisting of sulfur, selenium and tellurium and spectrally sensitized with a cyanine dye having an anodic half-wave potential less than 1.0 volt and a cathodic half-wave potential less than -0.8 volt.
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Description (OCR text may contain errors)
United States Patent Illingsworth, deceased  PREPARATION OF SILVER HALIDE GRAINS  Inventor: Bernard D. Illingsworth, deceased, late of Rochester, N.Y. by Mary D. Illingsworth,
executrix  Assignee: Eastman Kodak Company, Rochester,
 Filed: Aug. 25, 1969 21 App]. No.: 853,236
Related U.S. Application Data  Continuation-in-part of Ser. No. 500,366, Oct. 21,
E. Klein in The Journal of Photographic Science, Ia: Properties of Photographic Emulsions Grains, Vol. 12, 1964, pages 242- 251 Effects of Environment on the Growth of Silver Bromide Microcrystals, by Berry et al., Photographic Science and Engineering, Vol. 5, No. 6, Nov.-Dec. 1961, pages 332- 336.
[ 3,655,394 [451 Apr.ll, 1972 Primary Examiner-Norman G. Torchin Assistant Examiner-Richard E. Fichter Attomey-W. H. J. Kline, Bernard D. Wiese and Gerald E. Battist [5 7] ABSTRACT Photographic emulsions comprising cubic-regular grains free of reduction sensitization are prepared by running an aqueous solution of silver nitrate and an aqueous solution of a silver halide simultaneously into an agitated aqueous solution of a peptizer in a double-run precipitation procedure while maintainirig the pH at no more than 4.0 and the pAg at a value within the range of 8.6 to 9.1. In one aspect of this invention, emulsions prepared by the above procedure are chemically sensitized with a compound containing a labile atom selected from the group consisting of sulfur, selenium and tellurium and spectrally sensitized with a cyanine dye having an anodic half-wave potential less than 1.0 volt and a cathodic half-wave potential less than 0.8 volt.
14 Claims, No Drawings PREPARATION OF SILVER HALIDE GRAINS This application is a continuation-in-part of U.S. Pat. Ser. No. 500,366 filed Oct. 21, 1965 and now abandoned.
This invention relates to a method of preparing silver halide grains of cubic-regular shape substantially free of reduction sensitization and sensitized photographic emulsions prepared from those grains.
In the conventional making of silver halide grains, grains are obtained of irregular shapes, the crystal faces of which are in general (1,1,1). These grains experience a certain amount of reduction sensitization in their preparation. When emulsions prepared from silver halide grains of conventional type are spectrally sensitized, the sensitizing dye is prone to desensitize the grains in the blue region of the spectrum. Moreover, sulfurand gold-sensitizing of emulsions which were reductionsensitized during grain preparation results in high fog.
One object of my invention is to provide a method of preparing silver halide grains of cubic-regular shape and free of the effects of reduction sensitization. Another object of my invention is to provide photographic emulsions prepared from silver halide grains of cubic-regular type which are readily susceptible to sensitizing by sensitizing agents without adverse effects upon certain regions of sensitivity. A further object of my invention is to provide cubic-regular grained silver halide emulsions sensitized with sulfur-type chemical sensitizers and optionally with noble metal-type and/or spectral sensitizer. Other objects of invention will appear herein.
Silver bromideor chloride is ordinarily prepared in the form of fine crystals or grains which have cubic structures. The faces of these crystals are generally (1,1,1), even though the lattice structure is cubic. Grains presenting cubic faces are not common and do not result from the usual conventional methods of making silver halide. Such grains in which the 1,0,0) faces are predominant will be referred to herein as cubic-regular. By carrying out the preparation of the silver halide grains under certain special conditions, cubic-regular grains are obtained.
The emulsions of this invention can contain some silver halide grains which are not regular; however, when such grains are present, they generally do not constitute more than about 20 percent, by weight, of the grains smaller than the mean grain size and/or more than about 20 percent, by number, of the grains larger than the mean grain size. Preferred emulsions prepared according to this invention comprise silver halide grains of which at least about 80 percent, often at least about 90 or 95 percent, by weight, are regular. Silver halide emulsions in which a major portion of the grains have a regular structure or shape are known. As disclosed in a paper entitled Fundamental Aspects of Growth and Shape of Photographic Silver Halide Crystals, published in The Proceedings of the 5th International Conference on Nuclear Photography held at Cern, Geneva, Sept. 15-18, 1965, edited by E. Dahl lensen, regular silver halide grains or crystals are obtained by isotropic growth occurring simultaneously and uniformly on all crystal faces. Such crystals develop (1,0,0) or (1,1,1) faces and are free of twin plane stacking faults, e.g., twin planes such as 1,1,1) twin planes. A (1,1,1) twin plane is a stacking fault which arises when a silver halide crystal grows in such a manner as to alter the previously established order of stacking of( 1,1,1) planes in forming the crystal.
Photographic silver halide emulsions comprising silver halide grains of regular shape or structure can be obtained by controlling the reaction conditions during the phase of crystal growth. Depending upon these conditions, the regular silver halide grains will be characterized by a certain crystal habit, e.g., cubic, cubo-octahedral and/or octahedral, and will exhibit certain planes, e.g., (1,0,0) or (1,1,1) planes, as crystal faces. For example, in an article entitled Precipitation of Twinned AgBr Crystals, by Berry and Skillman, Photographic Science and Engineering, Vol. 6, No. 3, May-June, 1962, it is shown that by a change in pAg it is possible to conduct a double-run precipitation of silver halide in such a manner that regular cubes or octahedra are formed. In a double-run procedure, the silver halide grains are prepared by simultaneously running an aqueous solution of a water-soluble silver salt, for example, silver nitrate, and a water-soluble halide, for example, an alkali metal halide such as potassium bromide, into a rapidly agitated aqueous solution of a silver halide peptizer, preferably gelatin, a gelatin derivative or some other protein peptizer. To obtain substantially regular grains, a large excess of halide ion is generally avoided. The conditions employed during the preparation of the silver halide grains are interrelated and a change in one variable such as pAg, pl-l, etc., while maintaining other conditions constant can change the silver halide crystal structure. In addition to previous references, a suitable method for preparing photographic silver halide emulsions having the required regular shape is also disclosed in an article entitled la: Properties of Photographic Emulsion Grains, by Klein and Moisar, The Journal of Photographic Science, Vol. 12, 1964, pages 242-251. A prepared class of photographic silver halide emulsions employed in the practice of this invention contains silver halide grains in which (1,0,0) faces predominate. Such silver halide grains are referred to herein as cubic-regular grains.
According to this invention, I have now found that if cubicregular grains are formed in a peptizer solution having a pH of 4 or less, reduction sensitization of the grains is avoided. In one embodiment, l have found that these grains when sensitized with sulfur-type and noble metal-type sensitizers show a greater increase in speed upon spectral sensitization than silver halide grains which are not of this type. Sulfur-type sensitizers include all compounds utilizing sensitization of photographic emulsions with an element from the sulfur group, e. g., sulfur, selenium or tellurium.
The method of preparing silver halide grains in accordance with my invention is carried out by running aqueous solutions of silver nitrate and of water-soluble halide simultaneously into a rapidly agitated aqueous solution of a peptizer such as gelatin or gelatin derivative, preferably at a temperature between 30 and C., the pH being maintained at 4.0 or less with sulfuric or equivalent acid, the reactants being so added that the pAg is maintained at a value within the range of 8.6 to 9.2. Quantities of halide ion greatly in excess of silver ion in the mass should be avoided.
The silver halide grains thus prepared are useful in preparing photographic emulsions. When a gelatin derivative is employed as the peptizer, the grains obtained can be conveniently washed by the grain-wash method described in Yutzy and Frame, U.S. Pat. No. 2,614,928, which grains are useful in making a washed emulsion. Conversely, the silver halide dispersion prepared as described can be used directly for making an emulsion by mixing with protective colloid for the silver halide grains.
To the emulsion may be added various addenda including sulfur-type sensitizers, i.e., sulfur sensitizer, selenium sensitizer or tellurium sensitizer as described in Sheppard, U.S. pat. Nos. 1,574,944 and 1,623,499, and Petz, German Pat. No. 1,151,174; noble metal-type sensitizers such as gold sensitizers as described in U.S. pat. Nos. 2,399,083, 2,448,060 and 2,642,361; coating aids; hardeners; antifoggants; antiplumming agents; etc.
Also added to the emulsion is a spectral sensitizer. Particularly good results are obtained with organic cyanine dyes having a high resistance to reduction but which are easily oxidized; that is, the dyes should have a cathodic polarographic half-wave potential (E less than 0.8 volt and an anodic half-wave potential (E less than 1.0 volt. Preferably E, is less than 0.9 and E is less than 1 .1. Cathodic measurements can be made with a 1 X 10 molar solution of the spectral sensitizer in a solvent, for example, methanol which is 0.05 molar in lithium chloride using a dropping mercury electrode with the polarographic half-wave potential for the most positive cathodic wave being designated E Anodic measurements can be made with l X 10 molar aqueous solvent solution, for example, methanolic solutions of the spectral sensitizer which are 0.05 molar in sodium acetate and 0.005 molar in acetic acid using a carbon paste of pyrolytic graphite electrode, with the voltammetric half-peak potential for the most negative anodic response being designated E In each measurement, the reference electrode can be an aqueous silver silver chloride (saturated potassium chloride) electrode at C. Electrochemical measurements of this type are known in the art and are described in New Instrumental Methods in Eleczrochemistry, by Delahay, Interscience Publishers, New York, New York, 1954; Polarography, by Kolthoff and Lingane, 2nd Edition, lnterscience Publishers, New York, New York, 1952; Analytical Chemistry, 36, 2426 (1964) by Elving; and Analytical Chemistry, 30, 1576 (1958) by Adams. Plus and minus signs are according to IUPAC (International Union of Pure and Applied Chemistry) Stockholm Convention 1953. A cyanine dye sensitizer, especially one containing an acid radition is taken and given no spectral sensitization and two other portions are given spectral sensitization, one being sensitized to green with an oxacarbocyanine dye as described in US. Pat. No. 2,688,545 and the other to red with a thiacarbocyanine dye as described in US. Pat. No. 2,704,714. Each of the twelve emulsions are coated on cellulose acetate supports at the rate of 100 mg. silver and 315 mg. ofgelatin per square foot. The coatings are exposed on an Eastman 1B sensitometer for one-fifth of a second to a Positive lamp plus a color chart and developed 6 minutes in Kodak Rapid X-ray Developer, after which they are subjected to a stop bath and a fixing bath and are washed and dried in that order. The speeds of each of the products are measured with the following results:
Cubic-regular bromoiodide emul ion Clear Blue Green" Red SulIurand gold-sensitized spectral sensitization Green Red ....III: 'I I .I "III Irregular-octahedral bromoiodide emulsion Sulfurand gold-sensitized spectral sensitization:
1 Blue sfieeds are obtained using Kodak Wratten Filters and BA and are considered to be a va d measure of the basic blue sensitivity of a bromoiodide emulsion.
I Wratten #61 and #16 Filters. J Wratten #2.) Filter.
cal, is preferred. Examples of useful spectral sensitizers are those described in US. pat. Nos. 2,688,545 and 2,704,714. I have found that emulsions prepared as described when chemically sensitized such as with sulfur and gold sensitizers give much better speeds upon spectral sensitization than is the case with either emulsions having other than cubic-regular silver halide grains or with emulsions composed of cubic-regular silver halide grains which have been subjected to reduction sensitization. I have found that desensitization in the region of blue sensitivity often encountered in spectrally sensitizing silver halide emulsions is inhibited when spectral sensitizers are added to emulsions composed of cubic-regular silver halide grains which have not been reduction-sensitized.
The following examples further illustrate preferred embodiments of the invention, although it will be understood that the examples are included merely for purposes of illustration and are not intended to limit the scope of the invention unless otherwise indicated.
EXAMPLE 1 An aqueous solution of potassium bromide and potassium iodide and an aqueous solution of silver nitrate in equal molar amounts are added simultaneously to a rapidly agitated aqueous gelatin solution maintained at a temperature of 70 C., the addition being carried out over a period of 35 minutes. The pH of the aqueous gelatin solution is maintained at 2.0 by occasional additions of sulfuric acid and the pAg is maintained at 9.0 by adjusting the relative rates of addition of the halide and silver nitrate solutions. There is obtained a dispersion of cubic-regular grain silver bromoiodide crystals having an average grain size of about 0.2 micron. The emulsion is washed and a protective colloid is added in the form of an aqueous gelatin solution. One portion of the emulsion is sulfurand gold-sensitized as described in Waller, US. Pat. No. 2,399,083, and another portion is reduction-sensitized as described in Allen et al, US. Pat. No. 2,983,609v An irregular octahedral-grained silver bromoiodide emulsion is also divided into two portions, one of which is sulfurand gold-sensitized and the other reduction-sensitized. In each case a por- The sulfurand gold-sensitized cubic-regular silver bromoiodide emulsion shows a large increase in speed on spectral sensitization accompanied by an increase in blue speed, while the reduction-sensitized cubic-regular bromoiodide and the irregular-octahedral-grained bromoiodide emulsions, either sulfurand gold-sensitized or reduction-sensitized, show a smaller increase in speed, partly due to blue-speed loss on spectral sensitization.
EXAMPLE 2 A sulfur-gold-sensitized cubic-regular-grained silver bromoiodide emulsion is prepared as described in Example 1. The emulsion is divided into several portions which are spectrally sensitized with the following sensitizing dyes:
dirmt mso rr (dHmonsoa Me Me Anhydro-Bethyl-D-methyl-3-(3-sulfobutyl)-thlacarboeyanlne hydroxide (II) Bill: Ei,
K c1 or (oampnso rr (CHzhCfHSOg Me Me Anbydre-5,5,6.6-tetrachlore-1.1'-dlethyl-3.3-d1-(3-snlfobutyl)- benzlmidazolocarbocyanlne hydroxide Anhyd ro5,5-dichlor-9-ethyl-3,3'-di(3-sultopr0pyl)oxacarbocyanine hydroxide, sodium salt 3-carbox ymethyl-5-(3-ethyl-2(3)benzothiazolylidene)-rhodanine C=CH l. 18 2,2-diethylthiacyartine iodide 5-(2-ethyl-1(2)-benzoxazylidene)-3phenylrhodanir1e man (VIII) Et El; 1
2,3 diethyl-4methyloxathiazolocarbocyanine iodide Each portion is coated on a film support as described in Example 1. A sample of each coating is exposed on a sensitometer at 365 mu, processed for 4 minutes in Kodak Developer D-] 9, fixed, washed and dried with the following results:
(l) E,, is determined on a polarograph using a methanol solution of the dye and-a carbon paste electrode vs. a Ag/AgCl electrode in saturated KCl.
(2) E,. is determined on a polarograph using a methanol solution ofthe dye and a dropping mercury electrode vs. a Ag/AgCl electrode in saturated KCl.
it can be seen from the above comparison that highest blue speeds are obtained with cyaninc dyes having an anodic halfample l at pAg of8.8 and a wave potential (E less than 1.0 volt and a cathodic half-wave potential (E less than 0.8 volt; with the best dyes E is less than 0.9 and E, is less than -l .1 volts.
EXAMPLE 3 The presence of reduction sensitizing can be observed by comparing blue speeds of comparative emulsions. It is wellknown that reduction sensitizing increases emulsion speed as described in Allen and Lowe, US. Pat. No. 2,983,609.
Two emulsions are made according to Example 1 with a pAg of 8.8 and a pH of 2 and 5.6 respectively. The emulsions are coated and given a long exposure of 10 seconds, developed for 5 minutes in Kodak DK-SO Developer, fixed, washed and dried. The results in the table below demonstrate an increased emulsion speed for the emulsion made at a pH of 5.6 which is apparently due to reduction sensitizing, whereas the emulsion prepared at a pH of 2.0 has a lower speed.
Relative Blue Speed EXAMPLE 4 Cubic-regular emulsions prepared at a pH of 2.0 are spectrally sensitized more effectively than emulsions prepared at a pH of 5.6.
Comparative cubic-regular grain emulsions are made according to Example 1 wherein the pAg is controlled at 8.8 and the pH at 2.0 and 5.6 respectively. Samples of each of the respective emulsions serve as controls. Other portions of the respective emulsions are spectrally sensitized, one being sensitized to the green region of the spectrum with 400 mg. of a benzimidazolocarbocyanine dye as described in US. Pat. No. 2,945,763 and one other portion of each emulsion to the red region of the spectrum with 400 mg. of a thiocarbocyanine dye of the type described in US. Pat. No. 2,917,516. Each emulsion is coated, exposed for 10 seconds to blue and white light as in Example 1, processed in Kodak DK-SO Developer, fixed, washed and dried.
The results in the following table show less emulsion desensitization for the cubic emulsions prepared at a pH of 2.0 than for the emulsions prepared at 5.6 as shown by the relative blue speeds, with the unsensitized control given a relative speed of 100. It can also be observed that the pH 2.0 emulsions are spectrally sensitized more effectively than the pH 5.6 emulsions, probably due to less emulsion desensitization on addition of the dyes, by comparing relative speeds.
Emulsion Spectral Relative Speeds Preparation pH Sensitization Blue Clear 2.0 100 100 2.0 green 78 363 2.0 red 62 407 5.6 100 100 5.6 green 23 94 5.6 red 32 80 EXAMPLE 5 Spectrally sensitized cubic emulsions prepared at a pH of 5.6 and which have been sulfurand gold-sensitized exhibit high fog compared to similar emulsions prepared at a pH of 2.0.
Portions of separate emulsions prepared according to ExpH of 2.0 and 5 .6 respectively and which have been sulfurand gold-sensitized according to Example l are spectrally sensitized as in Example 4 with one sample serving as a control for each emulsion. The emulsions are coated, exposed for one-fifth second to blue and white light as in Example 1, processed in Kodak DK-SO Developer, fixed, washed and dried. The results are demonstrated in the following table:
Emulsion Preparation Spectral Relative Speeds pH Sensitization Blue Clear Fog 2.0 100 100 .04 2.0 green 129 575 .04 2.0 red 204 1150 .04
5.6 89 82 .91 56 green 170 313 .97 .6 red 135 725 .79
It is apparent that the spectrally sensitized sulfurand goldsensitized cubic-grained emulsions made at a pH of 5.6 provide heavy fog, whereas the same emulsion made at a pH of 2.0 is effectively spectrally sensitized without the formation of fog.
EXAMPLE 6 Cubic-grained emulsions which are made at a pH of 2.0 are sensitized more effectively to the infrared region of the spectrum than comparative emulsions prepared at a pH of 5.6.
Sulfurand gold-sensitized emulsion samples are prepared as described in Example 5, except that infrared Sensitizers are used in place of the spectral sensitizers of Example 5. To each sample of the emulsion is added the infrared sensitizer 3,3- diethylselenadicarbocyanine ethylsulfate at 400 mg./silver mole, 4 g. of 4-hydroxy-6-methyl-l,3a,7-tetraazaindene/silver mole and 2.5 ml. of 6-methoxyquinoline/silver mole. The emulsions are coated and samples are exposed one-fifth second to blue and white light as described in Example l and to an exposure modulated by a Wratten 89B filter (infrared), processed in Kodak DK-SO Developer, fixed, washed and dried. The results are as follows:
Emulsion Relative Speeds Preparation Blue Clear infrared Fog pH 2.0 pAg 8.8 100 257 148 .09 pH 5.6 pAg as 16 71 a0 .10
It is apparent that sensitization to the infrared region of sulfurand gold-sensitized cubic-grained emulsions made at a pH of 5.6 causes heavy fog, whereas the same emulsion prepared at a pH of 2.0 is effectively sensitized to the infrared region.
The silver halide employed in the preparation of the cubicgrain emulsions of this invention includes any of the photographic silver halides as exemplified by silver bromide, silver chloride and silver iodide, or mixed silver halides such as silver chlorobromide or silver bromoiodide, and the like. Particularly good results are obtained with silver halide emulsions in which the halide is predominantly bromide and the average grain size of the silver halide crystals is less than 0.5 micron.
Mean grain diameter, i.e., average grain size, can be determined using conventional methods, e.g., projective area and diameters of equivalent circles such as described in Methods of Particle-Size Analysis, AS TM Symposium on Light Microscopy, by Loveland, 1953, pages 94-122, and as disclosed in Chapter 2 of The Theory of the Photographic Process, by Mees and James, 1966, New York, The MacMillan Company.
Emulsion layers and other layers present in photographic elements made according to this invention can be hardened with any suitable hardener such as aldehyde hardeners, aziridine hardeners, hardeners which are derivatives of dioxane, oxypolysaccharides such as oxystarch, oxy plant gums, and the like.
The cubic-grain emulsions can also contain additional additives, particularly those known to be beneficial in photographic emulsions including, for example, stabilizers or antifoggants, particularly the water-soluble inorganic acid salts of cadmium, cobalt, manganese and zinc as disclosed in U.S. Pat. No. 2,829,404, the substituted triazaindolizines as disclosed in U.S. pat. Nos. 2,444,605 and 2,444,607, speed-increasing materials, plasticizers and the like. Sensitizers which give particularly good results in the photographic compositions disclosed herein are the alkylene oxide polymers which can be employed alone or in combination with other materials, such as quaternary ammonium salts, as disclosed in U.S. Pat. No. 2,886,437, or with mercury compounds and nitrogen containing compounds, as disclosed in U.S. Pat. No. 2,751,299.
The emulsions of this invention can be used in color photography, for example, emulsions containing color-forming couplers or emulsions to be developed by solutions containing couplers or other color-generating materials.
The emulsions of the invention are especially useful in color photography particularly where spectral sensitizers are incorporated in the emulsions, in which case a color coupler will be incorporated to correspond to the spectral sensitivity which has been imparted.
The emulsions of this invention can be used in diffusion transfer processes which utilize the non-developed silver halide in the non-image areas of the negative to form a positive by dissolving the undeveloped silver halide and precipitating it on a receiving layer in close proximity to the original silver halide emulsion layer. Such processes are described in Rott, U.S. Pat. No. 2,352,0l4; Land, U.S. Pat. No. 2,543,181; and Yackel et al., U.S. Pat. No. 3,020,155. The cubic silver halide emulsions described herein can also be used in color transfer processes which utilize the diffusion transfer of an imagewise distribution of developer, coupler or dye from a light-sensitive layer to a second layer while the two layers are in close proximity to one another. Color transfer processes of this type are described in Yutzy, U.S. Pat. No. 2,856,142; Land et al., U.S. Pat. No. 2,983,606; Whitmore et al., British pat. Nos. 904,364 and 840,731; and Whitmore et al., U.S. application Ser. No. 392,471. The cubic silver halide emulsions of this invention can be processed in monobath processes such as described in Haist et al., U.S. Pat. No. 2,875,048, or in stabilization-type processes.
Variable contrast in the cubic-grain emulsions can be ob tained by exposing through various Kodak Wratten filters a spectrally sensitized cubic-grain emulsion, whereby the level of the sensitizing dye used is sufficient to cover at least 60 percent of the grain surface. Appropriate filters are described in Kodak pamphlet Kodak Wratten Filters, data book B-3, 1965.
The invention has been described in considerable detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.
1. A method of preparing silver halide grains of cubic-regular shape free of reduction sensitization which comprises running an aqueous solution of silver nitrate and an aqueous solution of water-soluble halide simultaneously into an agitated aqueous solution of a peptizer in a double-run precipitation procedure while maintaining the pH at no more than 4.0 and the pAg at a value within the range of 8.6 to 9.2.
2. A method according to claim 1 wherein said silver halide is silver bromoiodide.
3. A method according to claim I wherein said silver halide grains are chemically sensitized with a compound containing a labile atom selected from the group consisting of sulfur, selenium and tellurium atoms.
4. A method according to claim 1 including the step of goldsensitizing said silver halide grains.
5. A cubic-regular grain silver halide emulsion, said grains being substantially free of reduction sensitization upon formation, said silver halide being thereafter chemically sensitized with a compound containing a labile atom selected from the group consisting of sulfur, selenium and tellurium and spectrally sensitized with a dye having anodic half-wave potential less than 1.0 volt and a cathodic half-wave potential less than -08 volt.
6. A cubic-regular grain silver halide emulsion in which the halide is predominately bromide, said grains being substantially free of reduction sensitization upon formation, said silver halide being chemically sensitized with a compound containing a labile atom selected from the group consisting of sulfur, selenium and tellurium, and spectrally sensitized with a cyanine dye having an anodic half-wave potential less than 1.0 volt and a cathodic half-wave potential less than 0.8 volt.
7. The emulsion of claim 6 in which the silver halide is silver bromoiodide having an average grain diameter of less than 0.5 micron.
8. The emulsion of claim 6 additionally being gold-sen sitized.
9. The emulsion of claim 6 wherein it is spectrally sensitized with a cyanine dye containing at least one heterocyclic ring selected from the class consisting of thiazole, benzimidazole, oxazole, selenazole and quinoline.
14. An emulsion according to claim 6 wherein at least percent, by weight or by number, of said silver halide grains are free of twin plane stacking faults.