FR2534036A1 - Emulsion containing tabular silver bromide particles, photographic product containing it and process for its preparation. - Google Patents

Abstract

The invention relates to photography. The radiation-sensitive emulsion comprises a dispersing medium and thin tabular particles of silver bromide bounded by two parallel or practically parallel principal crystalline faces of index 111. These particles have a thickness of less than 0.3 mu m and represent at least 97 % of the total projected area of the silver bromide particles present in the emulsion. Application to the preparation of photographic products which have improved photographic properties.

Description

 The invention relates to silver halide photography and, more particularly, to a radiation sensitive emulsion containing tabular silver bromide grains, to the process for preparing it and to the photographic material containing it.

 It has been observed in photographic emulsions of.

silver bromide, different forms of grains, regular and irregular. Regular grains are often cubic or octahedral. The grain edges may be rounded due to maturation effects due to the presence of ripening agents, such as ammonia; the grains may even be spherical or have the form of thick, almost spherical tablets, as described for example in U.S. Patent No. 3,894,871 and by Zelikman and Levi in Making and Coating Photographic Emulsions, Focal Press, 1964, p. 223. Variable proportions of rod-shaped or tabular-shaped grains have frequently been observed with other-shaped grains, particularly when the pAg (i.e. logarithmic negative of the silver ion concentration) of the emulsions was modified during precipitation, as is the case for example in single-jet precipitation processes.

 The tabular grains of silver bromide have been studied extensively, but the grains thus studied were often large grains without photographic utility. In the present description, what is meant by tabular grain is a grain delimited by two parallel or substantially parallel crystalline faces which each have a substantially larger surface area than any other face of the crystal constituting the grain. The shape index, that is, the ratio of diameter to thick, of a tabular grain is well above 1: 1.

From 1937 until the 1950s, the firm Eastman-
Kodak Company sold an X-ray film which was referred to as No-Screen X-Ray Code 5133. This product cured, on each side of a film carrier, a tabular grain of silver bromide. prepared by a single jet precipitation process. The tabular grains had an average shape index of 5: 1 to 7: 1. Non-tabular grains accounted for more than 25% of the projected area. The emulsion whose grains had the lowest mean thickness, selected from several repetitions, included tabular grains having an average diameter of 2.5 μm. the average thickness of 0.36 μm and an average shape index of 7: 1. Other representations of these emulsions yielded thicker, smaller diameter tabular grains with a lower average shape index.

Gutoff, in his study "Nucleation and Growth Rates
During the Precipitation of Silver Halide Photographic Emulsions, published in Photographic Sciences and Engineering, Vol 14, No 4, July-August 1970, pp. 248-257, shows the preparation of silver bromide and silver bromideodide emulsions. a single jet precipitation process using a continuous precipitation apparatus.

 U.S. Patent 4,067,739 describes the preparation of silver halide emulsions in which most of the crystals have an octahedral interlocked form.

The process consists of forming seed crystals, causing the growth of these crystals by Ostwald maturation in the presence of a silver halide solvent and completing the growth of the grain, without renucleation or maturation of Ostwald, by controlling the pBr (that is, the negative logarithm of the bromide ion concentration)
It is stated in this patent that increased contrast and hiding power is achieved.

Berry and Skillman in "Fundamental mechanisms of
Silver halide precipitation ", Journal of Photographic
Science, Vol 16, (1968), p. 137-147, observed that Ostwald ripening could cause the formation of doubly twinned silver halide grains. FIG. 5 shows wedge-shaped grains doubly twinned, obtained by Ostwald ripening in the presence of ammonium hydroxide 0.35 NA FIG. 4 shows doubly twinned tabular grains obtained by Ostwald ripening carried out. as indicated by ## Jnes de Cugnac and Chateau in "Evolution of the Morphology of Silver Bromide Crystals during Physical Ripening", Science and Photogra- phic Industries, vol. 33, No. 2 (1962) p. 121 to 125. As can be seen in Figure 4, a significant portion of the projected total grain area is small non-tabular grains.

 The invention relates to emulsions containing silver bromide grains in which the thin tabular grains represent a significant percentage of the total projected area of the silver bromide grains present.

According to the invention, the radiation-sensitive emulsion comprises a dispersion medium and silver bromide grains: at least 97% of the total projected area of the silver bromide grains is comprised of thickness less than 0.3 vm and bounded by two parallel or substantially parallel principal crystalline crystal faces ("11lu
The invention also relates to a photographic product comprising a support and at least one silver bromide emulsion layer described above.

The invention also relates to a method for preparing the emulsion defined above. This process consists of preparing an emulsion containing silver bromide seeding grains and maturing them to obtain tabular grains. The process is characterized by the formation of a seed grain emulsion having a diameter less than 0.15 μm and bounded by {111} crystal faces and while maintaining the pAg of this emulsion at a value between 8.4 and 11, subject it to maturation in the absence of an agent
the complexing ion / silver other than a halide, to obtain thin tabular silver bromide grains having a thickness of less than 0.3 μm and representing at least 97% of the total projected area of the bromide grains; money
It was known prior to the present invention that emulsions comprising thin tabular silver bromide grains could have a variety of forward and backward properties, such as increased maximum density and coverage. , a better separation between the velocities in the region of the natural sensitivity spectrum and in the region of the spectrum to which the sensitivity has been extended by spectral sensitizing dyes, increased speed / granularity ratios and increased sharpness. However, emulsions comprising thin tabular silver bromide grains, heretofore known, are far from effective because a substantial part of all the silver bromide grains is not in the form of thin tabular grains. By increasing the percentage of the thin tabular silver bromide grains, relative to the total projected grain area, the present invention provides more efficient access to the advantages due to the presence of the thin tabular grains. The present invention thus provides thin tabular grain silver bromide emulsions which contain a smaller amount of non-optimally shaped grains necessary to achieve the photographic advantages known to be enhanced by the presence of the thin tabular grains.

The drawings illustrate the invention
FIGS. 1A and 2 show photomicrographs of emulsions according to the invention and
FIG. 1B shows the granulometric distribution curve of tabular grains of silver bromide according to the invention (the percentage of the number of grains is obtained as a function of the diameter of the grains in micrometers)
As indicated, in the radiation-sensitive emulsions according to the invention, the thin tabular grains represent at least 97% of the total projected area of the silver bromide grains, but preferably they represent at least 99% In the optimum case, practically all silver bromide grains present in the emulsion are thin tabular grains. The term "projected surface" is used in the same sense as the terms "projection area" and "area" Projective "commonly used in the art (see for example, James and Higgins, in Fundamentals of Photography Theory, Morgan and Morgan,
New York, p. 15.

 By tabular grain is meant a grain having two parallel or substantially parallel crystalline faces, each of which has a substantially larger surface than any other face of the crystal constituting the grain.

 "Thin" is understood to mean a grain which has a thickness of less than 0.3 μm; preferably the tabular grains have an average thickness of less than 0.15 μm. Typically, the average thickness is at least 0.03 μm; but the grains may in principle be thinner.

Even thin tabular grain silver bromide emulsions having a low average form factor have advantages over non-tabular silver bromide emulsions. It is preferred, however, that the tabular tabular grains according to the invention have an average intermediate form index, i.e. at least 5: 1. Preferred thin tabular grain emulsions are those which have a high form factor, i.e. those in which the grains have an average shape index greater than 8: 1. In a preferred case, the average shape index c1 is at least 12: 1. The average form indices typically have values that are between
From about 8: 1 to about 50: 1. Higher average index grains can be obtained by decreasing their average thickness and / or increasing their average diameter.

 The characteristics of the grains of the emulsions according to the invention can easily be demonstrated by methods well known in the art. By the term "shape index" is meant the ratio of the diameter of the grain to its thickness. The term "diameter" itself is defined as the diameter of a circle having an area equal to the projected area of the grain as it appears on a photomicrograph or electron micrograph of the emulsion sample. From the shadows of an electron micrograph of an emulsion, it is possible to determine the thickness and diameter of each grain and identify those of tabular grains whose thickness is less than 0.3 pm that is, the thin tabular grains. From these data, the shape index of each of these thin tabular grains can be calculated and the shape indices of all the thin tabular grains of the sample of which thickness is less than 0.3 Mm can be used to make an average which constitutes the average shape index. According to this definition, the average shape index is the average of the shape indices of each grain. In practice, it is generally simpler to obtain an average thickness and a mean diameter of the thin tabular grains and to calculate the average shape index, which is then the ratio of these two averages. chosen, and given the tolerances of the particle size measurements, the values obtained for the average shape index do not differ significantly. The projected areas of the thin tabular grains of silver bromide can be summed; then separately, the projected areas of the other silver bromide grains of the photomicrograph, if any, can be summed; from these two respective sums, the percentage of the total projected area occupied by the thin tabular grains of silver bromide can be obtained. For the above evaluations, a reference tabular grain was chosen which grain has a thickness less than 0.3 pm. The purpose of this choice is to distinguish thin tabular grains from thicker tabular grains with lower photographic characteristics.

In order to prepare emulsions according to the invention, a seed seed emulsion comprising a dispersion medium and silver bromide grains having a diameter of less than 0.15 μm (preferably less than 0) is first formed. 10 μm) and bounded by faces ùlÙ. These emulsions can be obtained by conventional precipitation methods, for example by double jet. The silver bromide seeding grain emulsion can be prepared in the following manner
A dispersion medium is introduced into a conventional reactor for the precipitation of silver halides, equipped with a suitable stirring device. In general, the dispersion medium, initially introduced into the reactor, represents at least about 10%, and preferably 80%, of the total mass of the dispersion medium which will be present in the emulsion at the end of the precipitation.

The dispersion medium can be removed from the reactor by ultrafiltration during the precipitation of the grains, according to the indications of French Patent 2,471,620; therefore, the volume of dispersion medium initially present in the reactor may be equal to or even greater than the volume of the emulsion which will be in the reactor at the end of the grain precipitation. The initially introduced dispersion medium is preferably water or a peptizer dispersion in water, optionally containing other substances, for example one or more metal dopants. When a peptizing agent is present initially, its concentration represents at least 10%, and preferably at least 20%, of the total of peptizing agent present at the end of the precipitation.

An additional amount of the dispersion medium is added to the reactor with the silver salt and the bromide and optionally by means of a separate jet. Usually, the proportion of dispersion medium is adjusted, in particular in order to increase the proportion of peptizing agent, once the addition of the salts is complete.

 In order to adjust the bromide ion concentration to the silver ion concentration at the onset of precipitation, the reactor contains a small amount, typically less than 10%, of bromide used to form the grains. The pAg of the reactor contents is monitored to promote the formation of silver bromide seeding grains bounded by such crystallographic faces. The pAg is preferably maintained between 8.4 and 11 when the precipitation takes place at a temperature of between about 400 ° C. and about 700 ° C. If desired, the temperature used during the precipitation may be from about 300 ° C to about 900 ° C, with a corresponding adjustment of pAg.

 During the precipitation phase, the silver salt and bromide are introduced into the reactor by well known double jet precipitation techniques. Generally, an aqueous solution of a soluble silver salt, such as silver nitrate, and an aqueous solution of a soluble bromide, such as an alkali metal bromide (for example, sodium), are simultaneously introduced into the reactor. or potassium) or alkaline earth metal (for example magne # sium or calcium).

Subject to the above pAg conditions, the concentration and flow rate of silver salt and bromide streams may be of any standard value.

The silver salt and the bromide are preferably introduced in concentrations of 0.1 to 4 moles per liter, although larger customary concentration ranges may be used, for example 0.01 moles per liter. until saturation. In order for the seeding grains to have a diameter of less than 0.15 items, it is preferable to rapidly introduce the silver salt and bromide solutions. It is possible to use very high flow rates for each of the solutions (for example 1000 ml / min or more); but with solutions of bromide and silver salt having concentrations in the preferred range, good results are obtained with flow rates of 30 ml to 150 ml per minute.

Particularly good results are achieved by precipitating, in about one minute, using flow rates of 130 ml to 140 ml per minute, solutions of silver salt and molar concentration bromide.

 During the precipitation of the seed grain emulsion, the reactor contents are kept acidic. The pH is preferably less than 5.5 to avoid spontaneous maturation of seed grains during their preparation. According to a particular embodiment, illustrated in the examples, it is advantageous to maintain the pH between 2 and 4.5. In order to adjust the pH to an acid value, any known acid such as sulfuric acid, nitric acid, citric acid or hydrobromic acid may be used during the precipitation of the silver halide.

 Modification agents may be present during the precipitation of seed grains, either initially in the reactor, or added together with one or more salts, according to conventional methods.

These modifiers may be compounds of copper, thallium, lead, bismuth, cadmium, zinc, middle chalcogen (i.e., sulfur, selenium and tellurium), gold and noble metals of group VIII, according to the indications given in U.S. Patents 1,195,432, 1,951,933, 2,448,060, 2,628,167, 2,950,972, 3,488,709, 3,737,313, 3,772,031, 4,269,927 and in Research Disclosure, Vol. 134, June 1975 Publication 13452. Research Disclosure and its predecessor Product Licensing Index are published by Industrial Opportunities Limited; Homewell, Havant; Hampshire
P09 1EF; United Kingdom.

The silver salt and the bromide can be added to the reactor by means of surface or sub-surface feed tubes, by gravity feed or by means of apparatus which make it possible to regulate the rate of addition as well as pH and / or pAg of the reactor contents, as described in U.S. Patent Nos. 3,821,002 and 3,031,304 and in Claes et al. in the journal Photographische Korrespondenz volute 102, No. 1 (#> 1967, p. 162. To obtain a rapid distribution of reagents in the reactor, specially constructed mixing devices such as those described in U.S. Patents 2,996,287, 3,342,605, 3,415,650, can be used. Nos. 3,785,777, 4,147,551 and 4,171,224, British Patent 2,022,431A, German Patent Applications 2,555,364 and 2,556,885 and Research.
Disclosure, volume 166, February 1978, publication 16662.

 To form the emulsions according to the invention, a peptizing concentration of between 0.2 and approximately 10% by weight relative to the total mass of the constituents of the emulsion in the reactor can be used. It is preferable to maintain the concentration by peptizing in the reactor to less than about 6% of the total mass, before and during the formation of the seed seeds and preferably also during the subsequent ripening and to adjust later, at higher values, the vehicle concentration of the emulsion (the vehicle including the binder and the peptizer) by additional vehicle additions, to obtain the optimum coating characteristics. The emulsion initially formed may contain from 5 to 50 g (and preferably 10 to 30 g) of peptizer per mole of silver bromide. A vehicle supplement may be added subsequently to bring the concentration up to 1000 g per mole of bromide. money.

Advantageously, the concentration of the vehicle in the finished emulsion is greater than 50 g per mole of silver bromide. When coated and dried in a photographic material, the vehicle represents about 30 to 70% of the mass of the emulsion layer.

 The vehicles, which include both binders and peptizers, may be selected from the substances usually employed as vehicles in silver halide emulsions. Preferred peptizers are hydrophilic colloids which can be used alone or in combination with hydrophobic substances. Suitable hydrophilic carriers include substances such as proteins, protein derivatives, cellulose derivatives, for example cellulosic esters, gelatin such as gelatin treated with an alkaline agent (skin or bone gelatin) or gelatin treated with an acidic agent (gelatin of pig skin), derivatives of gelatin such as acetylated gelatin and phthalylated gelatin.

These and other vehicles are described in Research Disclosure Vol.176, December 1978, Publication 17643, Section IX.

 The vehicles, and particularly the hydrophilic colloids, as well as the hydrophobic substances that can be used in combination with them, can be found not only in emulsion layers according to the invention, but also in other layers, such as overlays, layers intermediates, layers placed under the emulsion layers.

 Immediately after the precipitation of the silver bromide seeding grains, a physical maturation, also called Ostwald ripening, is carried out to obtain the desired thin tabular silver bromide grains.

During the maturation phase, some of the seeding seeds disappear and the other grains undergo transformation into a distinct tabular form. The maturation is carried out in the absence of virtually any complexing agent of the silver ion other than a halide (and preferably in the total absence) for example in the absence of ammonia, thiocyanates and thioethers . These complexing agents are particularly undesirable since they cause the thickness of the tabular grains contained in the emulsion to increase and their average diameter to change, thereby increasing the heterogeneous dispersity of the emulsion.

 It is known that the bromide ion is a complexing agent for silver. If the pAg is monitored during maturation, the bromide ion concentration in solution is effectively controlled. When the temperature is 400C. at about 700c during maturation the pAg is preferably maintained between 8.4 and 11. The maturation rate increases with increasing temperature. Therefore, accelerated maturation can be carried out at temperatures above 700C. If desired, a temperature known to be suitable during the double jet precipitation phase can be used during ripening. The pAg can be adjusted according to the ripening temperatures.

For ripening to take place, it is necessary for the pH to be greater than 5.5. The maturation can be carried out at an acidic pH, preferably at a pH of between 5.5 and 6.5.

Thin tabular grain emulsions according to the invention are preferably washed to remove soluble salts by known techniques such as decantation, filtration and / or by jelly and filtration, as described in Research Disclosure, Vol. 176, December 1978; publication 17643, section II. The washing is particularly advantageous in the present invention for terminating the maturation of the tabular grains after the end of the precipitation, to avoid increasing their thickness and reducing their shape index,
The emulsions, with or without sensitizer, can be dried and stored before being used.

 Preferred tabular grain emulsions according to the invention are directly obtained by the method described above. These emulsions have a relatively narrow particle size distribution. More particularly, the tabular grains have a coefficient of variation of less than 30 and preferably less than 20, values which correspond to a relatively narrow particle size distribution for the simplest grains, the photografts according to the invention comprise a coating layer. Thin tabular grain emulsion of silver bromide according to the invention, applied to a conventional photographic support. The photographic products may also contain other emulsion layers which may be conventional or in accordance with the invention.

 The present invention is applicable to photographic products forming positive or negative images.

Thus, the products can be of those which form, by exposure, a superficial or internal image and which produce a negative image by treatment. Photographic products can also be ones that form direct positive images as a result of a single development phase. When the tabular grains and the other silver halide grains present in the photographic product must form direct positive images, they may be veiled on the surface and used in combination with an organic electron acceptor as described, for example U.S. Patent Nos. 2,541,472, 3,501,305, 3,501,306, 3,501,307, 3,672,900, 3,600,180 and 3,647,643 to British Patent 723019 and Research.
Disclosure, vol. 134, June 1975, Publication 13452. The organic electron acceptor compound may be used in combination with a spectral sensitizing dye or the electron acceptor compound may itself be a spectral sensitizing dye, as described in US Pat.
United States of America 3,501,310. If emulsions with internal sensitivity are used, the grains can be veiled on the surface and associated with organic electron-accepting compounds, as described in United States patent 3,501,311. but neither the organic electron acceptors nor the veiling surface treatment are necessary to form direct positive images. This type of image can be formed by developing internally sensitive emulsions in the presence of nucleating agents contained in the developer or in the photographic product, as described in Research Disclosure, Vol. 151, Nov. 1976, Publication No. 15162. Preferred nucleating agents are those that adsorb directly to the surfaces of the silver halide grains, as described in United States Patents 3,615,615, 3,759. 901, 3,718,470, 3,719,494, 3,748,738, 4,030,925, 4,080,207, 4,115,122 and 4,139,387 and British Patents 2,011,391 and 2,012,443.
In addition to the grains defined in the present invention, radiation-sensitive emulsions and photographic products may use conventional elements such as those described in the Research Disclosure paragraphs cited below (176, December 1978, 17643).

Thus, vehicles previously described can be used in other layers of photographic products. Vehicles may be tanned as described in paragraph X.

The tabular grain emulsions can be mixed with conventional emulsions as described in paragraph I. The tabular grains can be chemically sensitized as described in paragraph III and / or spectrally sensitized or desensitized as described in section IV. The photographic material may contain optical brighteners, antifoggants, stabilizers, absorbing or diffusing agents, coating aids, plasticizers, lubricants and matting agents, as described in paragraphs V, VI, VIII, XI
XII and XVI. Constituent addition, coating and drying methods such as those described in paragraphs XIV and XV may be used. Usual photographic media such as those described in paragraph XVII may be used. The photographic products obtained may be products for black and white photography or for color photography, which form silver images and / or dye images by selective destruction, formation or physical removal of dye, as described in paragraph VII. Preferred color photographic materials are those which form dye images by use of color developers and dye couplers. In order to use these products, they can be displayed in a customary manner, as described in paragraph XVIII, and can be processed as described in paragraph XIX.

 The following examples illustrate the invention. In each of the examples, the reactor contents are stirred vigorously during the introduction of the silver salt and the bromide. Unless otherwise stated, the term "percentage" refers to a mass percentage, the letter "M" to a molar concentration and all solutions are aqueous solutions.

EXAMPLE 1
A solution of 80 g of inert gelatin is prepared in 4000 ml of distilled water; the pH of this solution is adjusted to 3.02 by addition of hydrobromic acid and maintained at 400 ° C. By the double jet technique, 140 ml of a 1M solution are introduced into this solution in one minute. of silver nitrate and 140 ml of a 1 M solution of potassium bromide, keeping the temperature at 40 C. At the end of precipitation, the pAg is 9.74 and the pH of 2.92 * average diameter of grains obtained is 0.06 μm.

 The physical maturation is then carried out by maintaining the emulsion for 1 hour at 600 ° C., after raising the pH to 5.76. The pAg is maintained at 9.57. The tabular grains obtained have an average diameter of 0.61 μm and an average thickness of 0.06 μm. The average shape index is 10: 1 and the thin tabular grains represent at least 99% of the total projected grain area. The coefficient of variation is 15.

 Figure B is a photomicrograph of the 11.1-face tubular grains obtained according to the procedure of Example 1, the magnification being 10,000. The particle size distribution curve of the tabular grains obtained is given in Figure 1B.

EXAMPLE 2
A solution of 80 g of inert gelatin is prepared in 4000 ml of distilled water. The pH of this solution is adjusted to 3.01 by addition of hydrobromic acid and maintained at 400 ° C. By the double jet technique, 130 ml of a 4M solution of silver nitrate and 130 ml of a 4M solution of potassium bromide are introduced into this gelatin solution over one minute. The pAg is 9.65. The pH is adjusted to 6 and the emulsion is heated for 1 hour at 60 ° C.

During maturation, the pAg is maintained at 9.7.

 Figure 2 is a magnified photomicrograph of 10,000 tabular grains (1113 obtained average diameter is 0.73 μm, average thickness 0.06 μm and average shape index 12: 1. The coefficient of variation is 27. Tabular grains represent 97% of the total projected grain area.

EXAMPLE 3
For the precipitation of the emulsion, the procedure is as in Example 1. The emulsion is then heated for 1 hour at 6000 (pH = 2.74 and pAg = 9.71). It is noted that because of the low pH, physical maturation is prevented. To obtain a maturation, the pH of the emulsion is adjusted to 6.05 and the emulsion is maintained for 3 hours at 600 ° C., the pAg being 9.71.

 Tabular grains having an average diameter of 0.66 μm, an average thickness of 0.055 μm and an average shape index of 12: 1 are obtained. The tabular grains represent more than 97% of the total projected area of the grains The coefficient of variation is 16.

EXAMPLE 4
A solution containing 340 g of inert gelatin in 15 liters of distilled water is prepared. The pH of the solution is adjusted to 2.97 by addition of hydrobromic acid. The solution is maintained at 4000. 100 ml of a 4M solution of silver nitrate and 100 ml of 4M potassium bromide solution are added by double jet in one minute. At the end of the precipitation, the pAg is 9.59 and the pH is 2.98.

 The physical maturation is carried out by maintaining the emulsion at 60 ° C. for 45 minutes, after raising the pH to 6.01 by addition of NaOH. The pAg is maintained at 9.60.

 Tabular grains having an average diameter of 1.54 μm and an average thickness of 0.06 μm are obtained.

The average shape index is 25.6 and the coefficient of variation 28.4. The tabular grains represent more than 97% of the projected total area of silver bromide grains present in the emulsion.

EXAMPLE 5
The emulsion obtained in Example 2 is chemically sensitized by adding, at 400 ° C., a pH of 6.4 and a pAg of 8.41.7 mg of potassium tetrachloroaurate per mole of silver and 22 mg of sodium thiosulfate pentahydrate per mole. money. The emulsion is maintained for 30 minutes at 700 ° C.

The emulsion is applied to a polyester support at a rate of 30 mg of silver per square square of support.

A sample of the product obtained behind a sensitometric scale at a light source of 28500K is exposed for 0.1 seconds.
The sample is developed for 4 minutes at 200 ° C. in an N-methyl-p-aminophenol-hydroquinone developer (Kodak D 19 R).

 The sensitometric curve shows no sail and a maximum density of 3.49.

Claims (16)

1 - Emulsion sensitive to radiation comprising a medium  of dispersion and grains of silver bromide  characterized in that it comprises tabular grains  silver bromide res limited by two crystal faces  main lines parallel or practically parallel  crystallographic index, which have a thick  less than 0.3 μm and which represent at least  97% of the total projected area of bromide grains  money. 2 - Emulsion according to claim 1, characterized  in that the tabular grains of silver bromide  account for at least 99% of the projected total area  grains of silver bromide present in the emulsion. 3 - Emulsion according to claim 1, characterized  in that the tabular grains of silver bromide  have an average shape index of at least 5: 1. 4 - Emulsion according to claim 1, characterized  in that the tabular grains of silver bromide have  an average shape index of at least 8: 1. 5 - Emulsion according to claim 1, characterized in  what tabular grains of silver bromide have a  average shape index of at least 12: 1. Emulsion according to claim 1, characterized  in that the dispersion medium comprises a peptizer 7 - Emulsion according to claim 6, characterized  in that the peptizer is gelatin or a derivative  gelatin. 8 - Emulsion according to claim 1, characterized  what the average thickness of the tabular grains of  silver bromide is less than 0.15 vm. 9 - Emulsion according to claim 1, characterized  in that it contains tabular grains of bromide  silver having a thickness of 0.03 pm  and 0.3 μm and an average form index between  - 8: 1 and 50: 1. 10 - Photographic product comprising a support and,  applied on this support, at least one layer  of silver bromide emulsion, characterized in that  that the emulsion conforms to any of the  Claims 1 to 9. 11 - Process for preparing an emulsion conforming to one  any of claims 1 to 9, characterized in that  an emulsion of seed grains is prepared  of silver bromide bounded by crystalline faces  t111}, having a diameter less than 0.15 vm and  subject these seed grains to ripening  physically, by keeping the pAg of the emulsion between  8.4 and 11, in the absence of ion complexing agent  silver other than a halide, in order to obtain  thin tabular grains of silver bromide having a  thickness less than 0,3 μm and representing at least  97% of the total projected area of  silver bromide. 12 - Process according to claim 11, characterized in  what seeding grains have a diameter  less than 0.10 vm. 13 - Process according to any one of the claims  11 and 12, characterized in that the grains are prepared  seeding by double jet precipitation of a  aqueous solution of silver nitrate and a solution  aqueous bromide, at a pAg between 8.4 and 11.  14 - Process according to claim 13, characterized  in that the aqueous solutions have a concentration  equal to or less than 4M. 15 - Process according to claim 11, characterized  in that, during the precipitation, the pH has a value  between 2.0 and 4.5. 16 - Process according to claim 11, characterized  in that the physical maturation is carried out at a pH  between 5.5 and 6.5. 17 - Process according to claim 11, characterized in  what is the physical maturation at a tempe  between 4000 and 70 C.