Supersensitized Silver Halide Photographic Emulsions

Abstract

A silver halide photographic emulsion containing a supersensitizing amount of the combination comprising at least one of the sensitizing -naphthoxazole represented by the following General Formula (I). ##SPC1## wherein Z.sub.1 represents a non-metallic atomic group necessary for forming a benzthiazole nucleus or a benzselenazole nucleus; Z.sub.2 represents a non-metallic atomic group necessary for forming a benzene nucleus; R.sub.1 and R.sub.2 each represents an alkyl group having from one to about six carbon atoms, a substituted lower alkyl group conventionally used in cyanine dyes, or an aliphatic hydrocarbon group having an unsaturated bond; X.sub.1 .sup.- represents an anion and m is 1 or 2, said m being 1 when the dye forms an intramolecular salt; and at least one of the sensitizing dyes represented by the General Formula (II) ##SPC2## wherein Z.sub.3 represents a non-metallic atomic group necessary for forming a benzoxazole nucleus, a .beta., .beta.naphthoxazole nucleus, or a .beta.-naphthoxazole nucleus; Z.sub.4 represents a non-metallic atomic group necessary for forming a benzene nucleus; R.sub.3 and R.sub.4, each represents an alkyl group having from one to six carbon atoms, a substituted alkyl group conventionally used in cyanine dyes, or an aliphatic hydrocarbon group having an unsaturated bond; at least one of said R.sub.3 and R.sub.4 being an alkyl group having at least one of a sulfo group, a carboxyl group or a hydroxyl group; R.sub.5 represents an alkyl group having from one to about six carbon atoms, a substituted alkyl group conventionally used in cyanine dye, an aryl group, or an unsaturated aliphatic hydrocarbon group; X.sub.2 .sup.- represents an anion; and n is 1 or 2, said n being 1 when the dye forms an intramolecular salt is disclosed.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a gelatino silver halide photographic emulsion which is spectrally sensitized by at least two sensitizing dyes exhibiting a super sensitizing effect on each other and in particular, it relates to a silver halide photographic emulsion having an increased spectral sensitivity in the green wave length region. 2. Description of the Prior Art

In the production of light-sensitive materials, it is well known to expand the light-sensitive wave length region, of a silver halide photographic emulsion, that is to say, to apply a spectral sensitization to the silver halide photographic emulsion. In particular, the sensitization in the green wave length region among the spectral sensitization techniques is important since the maximum sensitivity of the human eye is near 545 nm. and the human eye is quite sensitive to the light in the green wave length region.

It is known, in natural color sensitive materials, that even slight differences in the light sensitivity and the spectral sensitivity distribution of the green-sensitive emulsion layers influence to a great extent the color reproducibility of color images and as a result much activity exists in this area. If the maximum sensitive wave length in the green wave length region of from 500 nm. to 600 nm. is shifted too much to a longer wave length, the yellow tones become too strong, which reduces the color reproducibility for green, whereas if the sensitivity in the shorter wave length region of from 500 nm. to 540 nm. is low, a clear green color reproduction is difficult to obtain. Therefore, the approach in spectral sensitization techniques for the green-sensitive silver halide emulsion for a natural color sensitive material is to locate the maximum sensitive wave length in the wave length region of from 560 nm. to 570 nm. and to increase the sensitivity at the shorter wave length side of from 520 nm. to 530 nm.

In X-ray recording materials, intensifying screens or fluorescent screens are frequently used together with silver halide photographic films for increasing the recording sensitivity to X-rays. Because excessive exposure to X-ray radiation is hazardous to humans various attempts have been made for increasing the recoding sensitivity to X-rays to improve the reproducibility of details by using X-rays in an amount as low as possible and by expanding the X-ray recording range. For example, not only techniques for highly sensitizing silver halide photographic emulsions but also systems for using an X-ray image intensifier and for using a solid-state light amplifier have been developed. In any of these cases, however, the X-ray image is recorded finally on a gelatino silver halide light-sensitive material as a fluorescent image. As the fluorescent substances used for this purpose, there are illustrated blue fluorescent substances such as calcium tungstate, zinc sulfide activated by silver, and barium sulfate activated by silver and green fluorescent substances such as zinc sulfide cadmium activated by silver. The maximum emission energy of the latter fluorescent substance is about 540 nm. and the intensity distribution of the emission energy is as shown in FIG. 7 of the accompanying drawings. Also, as is clear from the above explanation, the human eye is particularly sensitive to light emitted from the latter type of fluorescent substance. Therefore, such a fluorescent substance is suitable as a fluorescent substance which is used for observing X-rays directly with the naked eye. That is to say, it is advantageous to use a fluorescent screen in which the fluorescent substance for reproducing an X-ray image on a fluorescent screen and observing the reproduced image with the naked eye or taking X-ray photograph of the image.

In fact, a fluorescent screen employing a green fluorescent substance is used in such an X-ray photographic method. On the other hand, it is desirable that an X-ray light-sensitive material, which is usually a so-called indirect X-ray photographic material, be treated easily in processings such as development and fixing and in particular it is desirable that such a lightsensitive material be handled in a room in which the level of light is as high as possible. Such an X-ray photographic material is, in fact, handled under a safety light by using, for instance, Filter No. 7 made by the Fuji Photo Film Co., Ltd. The spectral transmittance curve of Filter No. 7 is illustrated in FIG. 8 of the accompanying drawings. In other words, it is desirable that an X-ray photographic material having a silver halide photographic emulsion layer be highly sensitive to the radiation emitted from a green fluorescent substance excited by X-rays and not very sensitive to ordinary light.

In this information conscious age, the development of faster information communicating systems has become necessary and various systems have been developed. For instance, there are the press facsimile system, the high speed photo-typesetting system, the cathode ray tube display system, the high speed photographic recording system employing high speed photography, in particular, photography, and the printing method of photo-masks using in an IC plate producing step conducted at a high speed.

For the cathode ray tube display system, the emissions on the order of micro-seconds from various kinds of fluorescent substances are used. The fluorescent substance, P-2 has at least one maximum emission energy at about 545 nm., P-4 at about 560 nm., P-22D at about 525 nm., P-31 at about 520 nm., and P-20 at about 560 nm.

The developing and fixing steps of light-sensitive materials using silver halide photographic emulsions are shortened to 60 to 120 seconds each. Thus, the sensitizing dyes contained in the photographic emulsion layers of the light-sensitive materials are hardly washed out and then the photographs obtained by the processings are apt to be stained by the remaining dyes, which is one factor by which the photographic images are degraded.

The spectral sensitization in the green wave length region is also conducted by a merocyanine dye, a hemicyanine dye, or a trinuclear cyanine dye. However, such a technique is unsuitable for sensitization in a narrow and specific wave length region and the spectral sensitivity distribution obtained by such a sensitizing means is too broad. Also, with such a technique, it is difficult to obtain a high sensitivity and in particular, such a technique is disadvantageous in the spectral sensitization of high-sensitive silver iodo-bromide photographic emulsions due to its low sensitizing power. Furthermore, the aforesaid spectral sensitization has the disadvantage that it is not easy to find a super-sensitizer for the sensitizing dye.

For this purpose the application of a J-aggregate type cyanine dye is profitable and many such cyanine dyes are known. For instance, using imidazolocarbocyanine dyes as disclosed in the specifications of U.S. Pat. Nos. 2,701,198; 2,945,763; 2,973,264; 3,173,791; 3,364,031; 3,397,060; and 3,506,443, the specification of Japanese Patent Publication No. 4936/1968, the specifications of German Offenelgungsschriften Nos. 1,944,751; 2,011,879; 2,018,687; and 2,030,326 using oxacarbocyanine dyes as disclosed in the specification of Japanese Patent Publication No. 32753/1969 and the specification of German Offenlegungsschriften No. 1,928,295, and using pseudocyanine dyes as disclosed in the specification of German Offenlegungsschriften No. 1,936,262 and the specification of French Pat. No. 1,488,057 are known techniques.

However, although such prior art techniques may give high sensitivity, many of them are useless for practical purposes since the wave length to which such prior art techniques sensitize extends to the longer wave length side and color stains are formed to too great an extent.

Techniques of using styryl type dyes as the super-sensitizer are known by, for instance, as disclosed in the specification of British Pat. No. 498,031, the specification of German Pat. No. 1,051,116, and the specifications of U.S. Pat. Nos. 2,313,922; 2,316,268; 2,533,426; and 2,852,385. However, when such a technique is applied to the case in which the exposure period of time is on the order of a microsecond which is far shorter than an ordinary exposure period of time, only a low sensitivity is obtained. In other words, the above-mentioned sensitization techniques are accompanied by the difficulty in obtaining high sensitivity when the exposure period of time is short.

An object of this invention is, therefore, to provide a silver halide photographic emulsion which is spectrally sensitized, which has a high green sensitivity and which gives rise to less color stains.

A second object of this invention is to provide a silver halide photographic emulsion which is suitable for making medical X-ray photographic light-sensitive materials having a high sensitivity to the radiation emitted from a green fluorescent substance and having a high stability to a safety light.

A third object of this invention is to provide a silver halide photographic emulsion having a high sensitivity in, particularly, in the wave length region of 520 - 540 nm. and giving less color stains.

A fourth object of this invention is to provide a silver halide photographic emulsion having a spectral sensitivity which is suitable for recording in a cathode ray tube type display system.

A fifth object of this invention is to provide a spectrally sensitized silver halide photographic emulsion which is suitable for use in a rapid processing system yet which is resistant to the formation of color stains as well as the harmful actions of antifoggants and a developing promotors.

The above objects and additional objects of this invention will become apparent from the descriptions contained in this specification in detail.

SUMMARY OF THE INVENTION

The above-described objects of this invention can be attained by incorporating in a silver halide emulsion a supersensitizing amount of the combination comprising at least one of the sensitizing dyes represented by the General Formula (I). ##SPC3##

wherein Z.sub.1 represents a non-metallic atomic group necessary for forming a benzthiazole nucleus or a benzselenazole nucleus; Z.sub.2 represents a non-metallic atomic group necessary for forming a benzene nucleus; R.sub.1 and R.sub.2 each represents an alkyl group having from one to about six carbon atoms, a substituted lower alkyl group conventionally used in cyanine dyes, or an aliphatic hydrocarbon group having an unsaturated bond; X.sub.1 .sup.- represents an anion and m is 1 or 2, m being 1 when the dye forms an intramolecular salt; and at least one of the sensitizing dyes represented by the General Formula (II) ##SPC4##

wherein Z.sub.3 represents a non-metallic atomic group necessary for forming a benzoxazole nucleus, a .beta., .beta.-naphthoxazole nucleus, or a .beta.-naphthoxazole nucleus; Z.sub.4 represents a non-metallic atomic group necessary for forming a benzene nucleus; R.sub.3 and R.sub.4 each represents an alkyl group having from one to about six carbon atoms, a substituted alkyl group conventionally used in cyanine dyes, or an aliphatic hydrocarbon group having an unsaturated bond; at least one of said R.sub.3 and R.sub.4 being an alkyl group having at least one of a sulfo group, a carboxyl group or a hydroxyl group; R.sub.5 represents an alkyl group having from one to about six carbon atoms, a substituted alkyl group conventionally used in cyanine dyes, an aryl group, or an unsaturated aliphatic hydrocarbon group; X.sub.2 .sup.- represents an anion; and n is 1 or 2, n being 1 when the dye forms an intramolecular salt.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

FIGS. 1 to 4 show the spectrograms obtained using the combination of the sensitizing dyes of the invention.

FIGS. 5 to 6 show spectrograms for comparison sensitizing dyes.

FIG. 7 is a graph showing the spectral energy distribution emitted upon excitation by X-rays of a fluorescent screen usually used in X-ray photography.

FIG. 8 is a graph showing the spectral penetration coefficient curve of a dark room safty light filter used for developing X-ray photographic films.

DETAILED DESCRIPTION OF THE INVENTION

As described above, in the General Formula (I) Z.sub.1 represents an atomic group necessary for forming a benzthiazole nucleus or a benzselenazole nucleus, the benzene nucleus of Z.sub.1 may be unsubstituted or may be substituted by a lower alkyl group having from one to about six, preferably from one to four, carbon atoms (such as a 5-methyl group, a 6-methyl group, a 5-ethyl group, a 6-propyl group, etc.,), an alkoxyl group (such as a 6-methoxy group, a 4-ethoxy group, etc.,), a hydroxyl group (such as a 5-hydroxyl group, a 4-hydroxyl group, etc.,), a halogen atom (such as a 5-chloro substituent, a 6-bromo substituent, etc.,), an aryl group (such as a 5-phenyl group etc., a sulfoaryl group (such as a 5-sulfophenyl group, etc.,), a carboxy aryl group, a N-alkyl substituted amino group (such as a N, N-dimethylamino group, etc.,), or an alkoxycarbonyl group (such as a 5-methoxy carbonyl group, etc.,).

Z.sub.2 represents a non-metallic atomic group necessary for forming a benzene nucleus; the benzene nucleus may be unsubstituted or may be substituted by lower alkyl group having from one to about six, preferably from one to four, carbon atoms (such as a 5-methyl group, a 4-ethyl group, etc.,), a halogen atom (such as a chlorine atom, a bromine atom, an iodine atom, etc.,), or an alkoxyl group (such as a 5-methoxy group, etc.,).

R.sub.1 and R.sub.2 each represents a lower alkyl group having from one to about six, preferably from one to four, carbon atoms (such as an ethyl group, a propyl group, a butyl group, etc.,) or such a substituted alkyl group as is conventionally used in cyanine dyes such as an aralkyl group (e.g., a benzyl group), an alkyl group having a sulfo group (e.g., a sulfoalkyl group such as a sulfobutyl group, a hydroxy sulfo alkyl group such as a .beta.-hydroxysulfoethyl group, a sulfoalkoxyalkyl group such as a sulfopropoxyethyl group, a sulfoarylalkyl group such as a p-sulfophenylethyl group, a sulfoalkyl aminoalkyl group such as a sulfopropylaminoethyl group, a sulfoalkylamidoalkyl group such as a sulfobutylamidoethyl group. a sulfoalkylthioalkyl group such as a --C.sub.2 H.sub.4 --S--C.sub.3 H.sub.6 SO.sub.3 H group, etc.,) and an alkyl group having a carboxyl group, and an alophatic hydrocarbon group having an unsaturated bond (e.g., allyl).

X.sub.1 represents an anion conventionally used for forming a cyanine dye such as a bromide ion, an iodide ion, a p-toluene-sulfonic acid ion, a perchlorate ion, a methyl sulfate ion, an ethyl sulfate ion, or a benzenesulfonate ion. m is 1 or 2; and where m is 1 the sensitizing dye of the General Formula (I) forms a intramolecular salt (betaine like structure), i.e., in such a case no extra anion need be present.

As described above, in the General Formula (II) Z.sub.3 represents an atomic group necessary for forming a .beta. ,.beta.-naphthoxazole nucleus, a .beta.-naphthoxazole nucleus, or a benzoxazole nucleus, in which the nucleus may be unsubstituted or may be substituted with a lower alkyl group having from one to six, preferably from one to four, carbon atoms (such as a 6-methyl group, a 5-ethyl group, etc.,) a halogen atom (such as a 5-chloro substituent, a 5-bromo substituent, etc., ) an alkyl group (such as a 6-methyl group, etc.,), an alkoxyl group (such as a 5-methoxy group, 7-ethoxy group, etc.,), a hydroxyalkyl group (such as a 5-hydroxymethyl group, etc.,), an alkoxycarbonyl group (such as a 5-methoxycarbonyl group, etc.,), an aryl group (such as a 5-phenyl group etc.,), a sulfoaryl group (such as a 5-sulfophenyl group, or a carboxyaryl group etc.,).

Z.sub.4 represents a non-metallic atomic group necessary for forming a benzene nucleus, in which the benzene nucleus may be unsubstituted or may be substituted by a substituent conventionally known as a substituent of the benzimidazole nucleus in a cyanine dye nucleus. For example, the substituent can be a halogen atom such as a chlorine atom, a bromine atom, an iodine atom; a cyano group; an alkoxycarbonyl group; a trifluoromethyl group; a carbamoyl group, an alkyl substituted carbamoyl group, such as a methylcarbamoyl group or a dimethylcarbamoyl group; a sulfamoyl group, an alkyl substituted sulfamoyl group, such as methyl sulfamoyl and dimethyl sulfamoyl, etc.

R.sub.3 and R.sub.4 each represents a lower alkyl group having from one to six, preferably from one to four carbon atoms e.g., a methyl group, an ethyl group, a propyl group, a butyl group, an isobutyl group, an unsaturated aliphatic hydrocarbon group for instance, an allyl group, or a substituted alkyl group conventionally used as a substituent on a nitrogen atom in the cyanine nucleus. Such nucleus N-substituted groups can be groups such as a hydroxyalkyl group, an alkyl group having a sulfo group (such as a .gamma.-sulfobutyl group, a .gamma.-sulfobutyl group, a sulfopropoxy ethoxy ethoxy ethyl group etc.,), an alkyl group having a carboxy group (such as a .beta.-carboxyethyl group, a 2-(2-carboxyethoxy) ethyl group etc.,), or an aralkyl group (such as a benzyl group etc.,). In addition, R.sub.3 and R.sub.4 may further represent a .beta.-methoxyethyl group, a .beta.-ethoxyelthyl group, a .beta.-methallyl group, a benzyl group, a .beta.-phenylethyl group, a .beta.-acetoxyethyl group, a .gamma.-acetoxypropyl group, a carbomethoxymethyl group, a .beta.-carbomethoxyethyl group, a carbethoxymethyl group, a .beta.-carbethoxyethyl group, a phenylmercaptomethyl group, a phenoxymethyl group, a .beta.-phenylmercaptoethyl group, a .beta.-phenoxyethyl group, and a 2-(2-hydroxy-3-sulfopropxy) ethyl group and a carbamylmethyl group. Groups R.sub.3 and R.sub.4 may be substituted by the substituents described in the specifications of, e.g., German Pat. No. 929,080; U.S. Pat. Nos. 2,537,880 and 2,776,280; British Pat. No. 1,001,480; and Japanese Patent Publication No. 4843/1965; Japanese Patent Publication No. 5829/1960; Japanese Patent Publication No. 14112/1965; Japanese Patent Publication No. 23,467/1965; Japanese Patent Publication No. 27,164/1967; Japanese Patent Publication No. 27,165/1967; Japanese Patent Publication No. 27,166/1967; Japanese Patent Publication No. 27,167/1967; Japanese Patent Publication No. 2607/1968; Japanese Patent Publication No. 15,894/1970; Japanese Patent Publication No. 27,672/1970; Japanese Patent Publication No. 27,673/1970; and Japanese Patent Publication No. 32,740/1970; at least one of R.sub.3 or R.sub.4 is alkyl group having at least one of sulfo group, carboxyl group and hydroxyl group.

R.sub.5 represents a lower alkyl group having from one to about six, preferably from one to four, (such as a methyl group, an ethyl group, a propyl group, etc.,), an unsaturated aliphatic hydrocarbon group (such as an allyl group, etc.,), a substituted alkyl group and an aryl group (such as a phenyl group). The substituted alkyl group can be a group such as an aralkyl group (such as a benzyl group, etc.,), a hydroxyalkyl group (such as a .beta.-hydroxylethyl group, etc.,), or an acetoxyalkyl group (such as a .beta.-acetoxyethyl group, a .gamma.-acetoxypropyl group, etc.,).

X.sub.2 .sup.- is an anion and has the same significance as X.sub.1 .sup.- in the General Formula (I); and n is 1 or 2, where n is 1 the sensitizing dye forms an intramolecular salt.

Preferred embodiments of combinations of the sensitizing dyes of the above general Formulas are as follows: the combination of at least one of the sensitizing dyes represented by the General Formula (I') ##SPC5##

wherein R.sub.1 and R.sub.2 each represents a methyl group, an ethyl group, or an allyl group; X.sub.1 .sup.- represents an anion; and R represents a hydrogen atom, a methyl group, or an ethyl group; and at least one of the sensitizing dyes represented by the following General Formula (II') ##SPC6##

wherein W.sub.1 and W.sub.2 each represents a hydrogen atom, a chlorine atom, or a methoxycarbonyl group and R.sub.3 and R.sub.4 each represents a sulfoalkyl group or an alkyl group; at least one of R.sub.3 and R.sub.4 being a sulfoalkyl group.

An important feature of this invention is the discovery of the imidaoxacarbocyanine dye of a so-called asymmetric nucleus represented by the General Formula (II) and the pseudocyanine dye represented by the General Formula (I).

By the discovery of the super-sensitizing action of these sensitizing dyes, the difficulties caused by the use of the sensitizing dyes individually can be overcome and the advantages of these dyes can be utilized effectively. That is to say, when the pseudocyanine dye of the General Formula (I) is used individually, it gives rise to a low sensitivity only. However, it is known that pseudocyanine dye described in the specification of some German Offenlegungsschrift gives an exceptionally high sensitivity but in order to obtain such a high sensitivity using the sensitizing dye of the German Patent application, the dye must be used at 4 to 8 times on a molar basis the amount usually employed in this art. Thus, the use of such a large amount of the dye gives rise to disadvantages in that the formation of color stains increases, the spectrally sensitized region extends to the longer wave length side, and the light-sensitive material containing the sensitizing dye tends to be fogged even under a safety light.

On the other hand, when the pseudocyanine dye of the General Formula (I) is used together with the carbocyanine dye of the General Formula (II), the maximum sensitivity can be obtained by employing the pseudocyanine dye of the General Formula (I) at a level of only a few percent of the amount required where the pseudocyanine dye is used alone. Consequently, the aforesaid difficulties are overcome by the present invention.

The sensitizing dye of the General Formula (II) sensitizes essentially in the wave length region substantially corresponding to the objects of this invention in giving a strong and sharp J-aggregate, i.e., a J-band. When the sensitizing dye of the General Formula (II) is used together with the pseudocyanine dye of the General Formula (I), the J-aggregate is properly partitioned and the spectral absorption region is shifted toward the blue to give a spectral sensitivity distribution corresponding better the emission energy distribution shown in FIG. 7 of the accompanying drawings. Moreover, in the present invention, the maximum sensitivity is obtained in a small range of the amount of the dye and the formation of color stains is less. In general, the formation of color stains tends to increase rapidly when the amount of the same dye added is increased and the stronger aggregates are formed. Accordingly, the fundamental features of this invention will be understood on consideration of these circumstances.

Specific examples of the sensitizing dyes used in this invention are illustrated herein below although the sensitizing dyes of this invention are not to be interpreted as limited to these specific dyes. Examples of the sensitizing dyes represented by the General Formula (I) are as follows: ##SPC7##

Also, the specific examples of the sensitizing dyes represented by the General Formula (II) are illustrated below: ##SPC8##

The spectral sensitizing method of the present invention is particularly advantageous for the spectral sensitization of gelatino silver halide photographic emulsions. However, the spectral sensitizing method of this invention is also usefully applied to a photographic emulsion containing other hydrophilic polymers than gelatin, such as agar-agar, collodion, water-soluble cellulose derivatives, polyvinyl alcohols, copolymers of polyvinyl pyrrolidone, synthetic hydrophilic resins, natural hydrophilic polymers, and gelatin derivatives.

As the photographic emulsion used in this invention, a mixed silver halide emulsion containing silver chloride, silver bromide, or silver iodide or a mixed silver halide thereof such as silver bromoiodide and silver iodochlorobromide is suitable. Furthermore, conventional methods of sensitization may be employed on a silver halide photographic emulsion which is spectrally sensitized by the sensitizing method of this invention, for instance, chemical sensitization such as sulfur sensitization, reduction sensitization, gold sensitization, sensitization by a metal belonging to the Group VIII of the periodic table and Noble metals. Combinations of these methods may also be used.

The photographic emulsion spectrally sensitized by the present invention may be prepared by incorporating the sensitizing dyes represented by the General Formula (I) and the General Formula (II) in the photographic emulsion using conventional techniques.

Each of the sensitizing dyes of the general formulae is generally added to a photographic emulsion as a solution thereof in a water-miscible solvent such as methanol, ethanol, water, cellosolve, or a water-miscible ketone such as methylethyl ketone, methyl ketone and ethyl ketone. Also, the sensitizing dye may be added to a photographic emulsion as a dispersion of a solution in a weakly soluble oil in water or a hydrophilic colloid.

The addition ratio of the sensitizing dye of the General Formula (I) to the sensitizing dye of the General Formula (II) to be incorporated in the photographic emulsion may be varied over a wide range, for example, from 9:1 to 1:10 on a molar basis, in accordance with the effects or advantages desired. Also, it is preferred that the amount of each of the sensitizing dyes employed in this invention range from 1 .times. 10.sup.-.sup.6 mol to 1 .times. 10.sup.-.sup.3 mol per mol of silver depending upon the characteristics of the photographic emulsion.

The photographic emulsion of this invention may further be subjected to conventional supersensitization techniques. Moreover, the spectrally sensitized photographic emulsion of this invention may further contain additives conventionally employed in the art, such as sensitizers, stabilizers, color toning agents, hardening agents, surface active agents, antifoggants, plasticizers, developing promotors, color couplers, and fluorescent brightening agents.

The photographic emulsion of this invention may be applied to an appropriate support such as a glass sheet, a cellulose ester derivative film, a polyethylene terephthalate film, a synthetic resin film, a barayta-coated paper, a resin-coated paper, or a synthetic paper using conventional techniques.

Furthermore, as described before, the silver halide photographic emulsion of this invention may be used as a light-sensitive material suitable for cathode ray tube display.

This invention will now be explained in greater detail by reference to the examples given below.

EXAMPLES 1 - 29

A silver halide photographic emulsion was prepared by incorporating the sensitizing dyes represented by the General Formula (I) and the General Formula (II) shown in the tables below in the amounts set forth in a gelatino silver iodo-bromide emulsion (AgI: AgBr = 7 mol%: 93 mol%, gelatin (g)AgNO.sub.3 = 1.3: 0.26 mol silver salt/kg emulsion). Also, for comparison, a silver halide photographic emulsion was prepared by using individually each of the sensitizing dyes of the General Formula (I) and the General Formula (II) and also a comparison silver halide photographic emulsion was prepared by using each of the comparison sensitizing dyes represented by the Formula (T) and Formula (U) shown below.

Each of the silver halide photographic emulsions thus prepared was applied to a triacetyl cellulose film base. The light-sensitive photographic films thus prepared were each exposed to day light radiation of 64 lux (corresponding to 5,400 K.degree.) through a yellow filter (made by the Fuji Photo Film Co., a filter passing light of wave lengths longer than 460 nm.), a green filter (made by the Fuji Photo Film Co., a filter passing light of wave lengths of 480-620 nm. and having a maximum penetration coefficient at a wave length of 530 nm.), or a blue filter (made by the Fuji Photo Film Co., a filter passing light of wave lengths of 400-490 nm. and having a maximum penetration coefficient at a wave length of 450 nm.). They were then developed for 2 minutes at 24.degree.C in a developing solution haveing the following composition: Metol 2.2 g Sodium Sulfite 96 g Hydroquinone 8.8 g Sodium Carbonate (mono-hydrate) 56 g Potassium Bromide 5 g

Water added to make the total volume 1 liter. In the following table, the yellow sensitivities and the green sensitivities obtained by adding the sensitizing dyes illustrated above individually and the comparison sensitizing dyes shown below and the yellow sensitivities and the green sensitivities obtained by adding the sensitizing dyes as combinations thereof are shown together with the intrinsic sensitivities of the silver halide photographic emulsion containing the sensitizing dye or dyes relative to the intrinsic sensitivity of the original silver halide photographic emulsion which did not contain a sensitizing dye.

In the following table, the yellow sensitivity and the green sensitivity are the relative values when the sensitivities of the silver halide photographic emulsion containing 4 .times. 10.sup.-.sup.5 mol/kg emulsion of the sensitizing dye (A) shown above exposed to the above-mentioned day light radiation through the yellow filter or the green filter respectively were defined as 100 each.

The relative sensitivity is shown relatively when the sensitivity of the original silver halide photographic emulsion exposed to the day light radiation through the blue filter is defined as 100.

The comparison sensitizing dyes used for the comparison tests have the following structures; ##SPC9##

Table __________________________________________________________________________ Ex. Sens.Dye Sens.Dye Max. Rel. Rel. Intrinsic Spectro- (amount*) (amount*) sens. yellow green sens. gram (nm) sens. sens. __________________________________________________________________________ A (1) 550 57 69 100 A (2) 550 88 88 100 A (4) 550 100 100 100 A (8) 550 125 120 100 7 (2) 540 100 115 100 7 (4) 540 120 120 100 7 (6) 540 120 125 94 __________________________________________________________________________ A (1) 7 (6) 544 125 140 100 1 A (2) 7 (6) 548 140 165 90 A (4) 7 (6) 550 155 170 90 __________________________________________________________________________ C (1) 546 52 48 104 C (2) 546 66 66 94 C (4) 550 70 70 94 C (8) 550 78 78 90 __________________________________________________________________________ C (1) 7 (6) 548 120 135 100 2 C (2) 7 (6) 548 150 170 100 __________________________________________________________________________ 8 (2) 540 80 80 94 8 (4) 540 88 90 90 8 (6) 540 90 92 76 __________________________________________________________________________ A (1) 8 (6) 540 96 115 78 3 A (2) 8 (6) 540 112 115 78 A (4) 8 (6) 548 125 135 85 __________________________________________________________________________ L (4) 532 30 25 96 FIG.1(1) L (8) 532 38 30 94 6 (1) 556 71 62 94 6 (2) 556 80 72 94 6 (4) 560 92 90 90 FIG.1(2) 6 (8) 560 104 112 85 __________________________________________________________________________ L (8) 6 (1) 532/ 140 164 94 556 4 L (8 6 (2) 535/ 152 178 94 556 L (4) 6 (4) 535/ 185 194 94 FIG.1(3) 560 __________________________________________________________________________ M (4) 540 70 82 88 M (8) 540 76 85 83 __________________________________________________________________________ M (8) 6 (1) 540/ 130 152 85 550 5 M (8) 6 (2) 540/ 140 172 85 560 M (6) 6 (4) 560 135 158 85 __________________________________________________________________________ 1 (1) 560 100 90 94 1 (2) 560 130 136 90 1 (4) 560 158 168 87 1 (8) 560 193 200 83 __________________________________________________________________________ L (8) 1 (1) 535/ 170 192 98 558 6 L (8) 1 (2) 535/ 200 208 96 560 L (4) 1 (4) 540/ 216 225 96 560 __________________________________________________________________________ E (4) 544 148 156 100 E (8) 544 164 170 103 14 (4) 552 130 140 98 14 (8) 540- 146 185 98 556 14 )12) 540- 170 205 96 560 __________________________________________________________________________ E (8) 14 (4) 545 200 210 104 E (4) 14 (4) 550 200 208 104 __________________________________________________________________________ F (4) 538 82 90 100 F (8) 538 90 96 100 __________________________________________________________________________ F (4) 14 (4) 540- 180 180 100 8 550 F (8) 14 (4) 540- 185 180 100 550 __________________________________________________________________________ C (4) 515- 46 30 100 520 C (8) 515- 48 35 104 520 __________________________________________________________________________ C (8) 1 (1) 550 135 125 104 C (8) 1 (2) 550 146 160 104 __________________________________________________________________________ D (4) 530 46 34 96 D (8) 530 52 37 94 __________________________________________________________________________ D (4) (1) 525- 180 185 104 10 555 D (8) 1 (2) 525- 200 208 104 555 __________________________________________________________________________ G (4) 540 25 18 104 G (8) 540 31 21 104 21 (1) 560 102 90 96 21 (2) 560 134 140 94 FIG.4(11) 21 (4) 560 158 172 94 21 (8) 560 196 205 90 __________________________________________________________________________ G (8) 21 (1) 555 102 110 104 11 G (8) 21 (2) 555 152 150 104 __________________________________________________________________________ P (4) 538 30 22 104 P (8) 538 38 28 104 FIG.4(10) __________________________________________________________________________ P (8) 21 (1) 540- 178 194 104 12 560 P (8) 21 (2) 540- 210 216 96 FIG.4(12) 560 __________________________________________________________________________ J (2) 556 107 100 98 J (4) 558 135 135 98 J (8) 558 158 164 98 4 (2) 548 92 55 100 4 (4) 548 100 60 100 4 (6) 548 107 66 100 __________________________________________________________________________ J (2) 4 (6) 550 170 170 100 13 J (4) 4 (6) 550 224 215 104 __________________________________________________________________________ K (2) 560 135 135 100 K (4) 560 170 170 100 K (8) 560 178 185 96 __________________________________________________________________________ K (2) 4 (6) 550 164 164 104 14 K (4) 4 (6) 550 208 208 104 __________________________________________________________________________ H (2) 540 42 28 100 H (4) 540 50 46 100 H (8) 540 66 60 100 3 (1) 562 76 64 96 3 (2) 562 107 85 96 __________________________________________________________________________ 15 H (2) 3 (1) 560 76 78 96 H (2) 3 (2) 562 110 96 94 __________________________________________________________________________ I (2) 540 65 62 96 I (4) 540 76 73 96 I (6) 540 69 66 94 2 (1) 550 73 70 100 2 (2) 552 85 85 100 __________________________________________________________________________ I (6) 2 (1) 552 94 104 90 16 I (6) 2 (2) 552 112 116 88 __________________________________________________________________________ N (4) 560 158 152 96 N (8) 560 200 200 96 FIG. 2 (4) N (12) 560 224 208 96 16 (4) 520- 103 110 110 545 16 (8) 550 152 170 110 16 (12) 550 193 200 110 __________________________________________________________________________ N (1) 16 (8) 552 193 208 110 17 N (2) 16 (8) 552 210 240 110 N (4) 16 (8) 554 234 260 110 __________________________________________________________________________ 5 (4) 540- 130 156 110 545 5 (8) 540- 158 178 110 FIG. 2 (5) 545 5 (12) 545 192 200 115 __________________________________________________________________________ N (2) 5 (8) 560 240 233 120 18 N (4) 5 (8) 560 260 280 120 FIG. 2 (6) __________________________________________________________________________ Q (2) 520 30 19 100 Q (4) 520 38 25 100 Q (6) 520 42 29 98 17 (2) 550 88 85 94 17 (4) 550 110 110 90 __________________________________________________________________________ Q (6) 17 (2) 520- 140 152 100 19 550 Q (6) 17 (4) 520- 170 194 100 550 __________________________________________________________________________ R (4) 522 32 20 100 R (6) 522 37 27 100 __________________________________________________________________________ R (6) 17 (2) 540 152 152 104 20 R (6) 17 (4) 540 172 185 104 __________________________________________________________________________ 18 (2) 530 69 56 90 18 (4) 542 83 73 88 __________________________________________________________________________ R (6) 18 (2) 530 90 83 100 21 R (5) 18 (4) 530 110 100 100 __________________________________________________________________________ B (4) 536 42 30 104 B (8) 536 45 40 104 FIG. 3 (7) 11 (4) 550 85 83 94 11 (8) 550 96 94 94 11 (12) 550 103 100 88 __________________________________________________________________________ B (4) 11 (4) 548 103 110 100 22 B (4) 11 (8) 548 110 110 100 __________________________________________________________________________ 20 (0.5) 560 76 69 94 20 (1) 560 99 96 90 20 (2) 560 110 128 85 FIG. 3 (8) 20 (4) 560 130 152 80 __________________________________________________________________________ B (8) 20 (1) 540/ 152 88 108 23 558 B (8) 20 (2) 530/ 185 215 108 FIG. 3 558 (9) B (8) 20 (4) 558 194 224 100 __________________________________________________________________________ 19 (2) 555 85 100 94 19 (4) 555 106 125 88 19 (8) 555 130 146 88 __________________________________________________________________________ B (8) 19 (1) 540 103 110 108 B (8) 19 (2) 530/ 130 152 108 552 B (4) 19 (4) 554 136 152 100 __________________________________________________________________________ 9 (2) 548 88 90 100 9 (4) 548 125 120 98

9 (8) 548 125 135 98 __________________________________________________________________________ N (1) 9 (4) 546/ 130 135 100 548 25 N (2) 9 (4) 550/ 135 152 96 560 N (4) 9 (4) 550/ 176 176 96 560 __________________________________________________________________________ 10 (4) 562 90 80 76 10 (8) 562 90 80 76 10 (12) 564 85 76 70 __________________________________________________________________________ B (2) 10 (4) 560 96 96 76 26 B (4) 10 (4) 560 100 96 76 __________________________________________________________________________ 12 (4) 566 116 100 76 12 (8) 566 116 107 71 12 (12) 568 100 96 48 __________________________________________________________________________ B (4) 12 (4) 564 178 185 94 27 B (2) 12 (8) 564 158 158 85 __________________________________________________________________________ 15 (2) 540 70 78 92 15 (4) 540 86 90 88 15 (8) 540 90 96 83 __________________________________________________________________________ L (2) 15 (8) 540 128 130 90 28 L (4) 15 (8) 540 135 152 90 __________________________________________________________________________ 13 (4) 540 120 140 104 13 (8) 540 145 158 104 __________________________________________________________________________ N (2) 13 (8) 560 228 216 100 29 N (4) 13 (8) 560 246 250 100 __________________________________________________________________________ T (4) 582 180 110 85 T (8) 584 185 120 76 FIG. 5 (13) T (12) 584 185 110 72 __________________________________________________________________________ U (4) 574 178 106 90 U (8) 574 182 114 66 FIG. 6 (14) U (12) 574 180 110 62 __________________________________________________________________________ * amount = 10.sup.-.sup.5 mol/kg emulsion

EXAMPLE 30

Two kinds of spectrally sensitized silver halide photographic emulsions were prepared by adding the dye (2) show above alone and the dye (B) and the dye (20) in the combination according to this invention to a silver iodo-bromide emulsion (AgI : AgBr= 1.5 mol : 98.5 mol%, gelatin (g)/AgNo.sub.3 (g) = 0.4, 0.75 mol silver salt/kg emulsion). Each of the photographic emulsions was applied to a polyethylene terephthalate film base and after drying, sensitometery was conducted on the light-sensitive films thus obtained.

The light-sensitive films was exposed to a day light source (corresponding to 5,400.degree.K) through a blue filter (made by the Fuji Photo Film Co., a filter passing light of wave lengths of 400-500 nm. and having a maximum penetration coefficient at a wave length of 450 nm.) or a green filter (made by the Fuji Photo Film Co., a filter passing light of wave lengths of 480-650 nm., having a maximum penetration coefficient at a wave length of 530 nm.) and then developed for 4 minutes at 20.degree.C in the same developing solution as used in the above Examples 1 - 29. The results obtained are shown in the following table.

In the following table, the sensitivity is shown by the relative sensitivity when the sensitivity of the silver halide photographic emulsion containing the dye (20) shown above in an amount of 4 .times. 10.sup.-.sup.5 mol per kg of the emulsion is defined as 100.

Table 3 __________________________________________________________________________ Example Sens. Dye Sens. Dye Maximum Relative Relative (amount)* (amount)* Sensitization Green Sens. Blue Sens. __________________________________________________________________________ 20 (4) 560 nm 100 100 20 (8) 560 125 100 20 (10) 560 130 90 B (1.7) 20 (4) 560 150 95 30 B (3.4) 20 (8) 560 191 95 B (4.0) 20 (10) 560 238 95 __________________________________________________________________________ * = 10.sup.-.sup.5 mol/kg emulsion

From the results shown in the tables, it can be seen that the green sensitivity is super-additively increased, which is one of the objects of this invention, by using at least one of the sensitizing dyes of the General Formula (I) and at least one of the sensitizing dyes of the General Formula (II).

The results obtained by the above examples are also shown in the accompanying drawings. That is to say, FIG. 1, FIG. 2, FIG. 3 and FIG. 4 are spectrograms obtained by using the combinations of the sensitizing dyes of the General Formula (I) and the General Formula (II) according to the present invention. FIG. 5 and FIG. 6 are the spectrograms obtained by using the comparison sensitizing dyes of the Formula (T) and the Formula (U) respectively. In addition, the numbers in the parentheses the examples shown in the above table to which the curves correspond.

FIG. 7 is a graph showing the spectral energy distribution of light emitted by exciting by X-rays the fluoresecnt substance of a fluorescent screen usually used for X-ray photography.

FIG. 8 is a graph showing the spectral penetration coefficient curve of a safety light filter usually used in a dark room for obtaining safety light in the case of developing direct X-ray photographic films.

Now, as clear from the results of Example 18, when the dye (N), which is one of the sensitizing dyes represented by the General Formula (I) is added in such a large amount so as to give the maximum green sensitivity, the sensitized region of the light-sensitive film extends to 596 nm. as shown by the curve (4) of FIG. 2 but when the dye (N) is used together with the sensitizing dye (5) of the General Formula (II), the sensitized wave length end of the light-sensitive film shifts to the shorter wave length side and also the sensitivity reduction in the longer wave length side is sharp as is shown by the curve (6) of FIG. 2. Besides the above-described advantages the green sensitivity of the light-sensitive film prepared by using the combination of the sensitizing dyes according to the present invention is about 1.6 times higher than that of the light-sensitive film obtained by using the sensitizing dye individually.

From the results shown above, it will aslo be understood that the photographic emulsion of this invention is less fogged by exposure to a safety light and also the spectral sensitivity distribution of the photographic emulsion fits the energy distribution of the light emitted from the green fluorescent substance of the X-ray fluorescent screen as shown in FIG. 7.

Furthermore, on investigating the results of the table obtained in the Examples 1 - 29 and the results shown in FIG. 1 to FIG. 6 of the accompanying drawings, it will be understood that the objects of this invention are well attained by using the sensitizing dye of the General Formula (I) together with the sensitizing dye of the General Formula (II).

Also, from the results of the examples, it will be understood that the present invention can be applied effectively to various fields in a practical manner.

Claims

What is claimed is:

1. A silver halide photographic emulsion containing a supersensitizing amount of the combination comprising at least one of the sensitizing dyes represented by the following General Formula (I) ##SPC10##

wherein Z.sub.1 represents a non-metallic atomic group necessary for forming a benzothiazole nucleus or a benzoselenazole nucleus; Z.sub.2 represents a non-metallic atomic group necessary for forming a benzene nucleus; R.sub.1 and R.sub.2 each represent an alkyl group having from one to about six carbon atoms, a substituted lower alkyl group conventionally used in cyanine dyes selected from the group consisting of a sulfoalkyl group, a hydroxy sulfoalkyl group, a sulfoalkoxyalkyl group, a sulfoarylalkyl group, a sulfoalkyl amino alkyl group, a sulfoalkylamido alkyl group, and a sulfoalkylthioalkyl group, or an aliphatic hydrocarbon group having an unsaturated bond; X.sub.1.sup.- represents an anion and m is 1 or 2, said m being 1 when the dye forms an intramolecular salt; and at least one of the sensitizing dyes represented by the General Formula (II) ##SPC11##

wherein Z.sub.3 represents a benzoxazole nucleus, Z.sub.4 represents a non-metallic atomic group necessary for forming a benzene nucleus; R.sub.3 and R.sub.4 each represents an alkyl group having from one to about six carbon atoms, a substituted alkyl group conventionally used in cyanine dyes selected from the group consisting of an alkoxyalkyl group, an alkallyl group, an aralkyl group, an acetoxyalkyl group, a carboalkoxyalkyl group, an arylmercaptoalkyl group, an aryloxyalkyl group, a hydroxysulfoalkoxyalkyl group and a carbamylalkyl group, an aralkyl group for an aliphatic hydrocarbon group having an unsaturated bond; at least one of said R.sub.3 and R.sub.4 being an alkyl group having at least one of a sulfo group, a carboxyl group or a hydroxyl group; R.sub.5 represents an alkyl group having from one to about six carbon atoms, a substituted alkyl group conventionally used in cyanine dyes, selected from the group consisting of an aralkyl group, a hydroxyalkyl group, and an acetoxyalkyl group, an aryl group, or an unsaturated aliphatic hydrocarbon group; X.sub.2.sup.- represents an anion; and n is 1 or 2; said n being 1 when the dye forms an intramolecular salt, and said emulsion having a high sensitivity to the radiation emitted from a green fluorescent substance and having a high stability to a safety light.

2. An photographic light-sensitive material comprising a support having thereon at least one layer of the silver halide photographic emulsion as set forth in claim 1.

3. A silver halide photographic emulsion containing at least one of the sensitizing dyes represented by the General Formula (I) ##SPC12##

wherein Z.sub.1 represents an atomic group necessary for forming a benzothiazole nucleus or a benzoselenazole nucleus, in which the benzene ring moiety of said nucleus may be unsubstituted or substituted by a lower alkyl group having from one to about six carbon atoms, an alkoxyl group, a hydroxyl group, a halogen atom, an aryl group, a sulfoaryl group, an N-alkyl substituted amino group, a carboxyaryl group or an alkoxycarbonyl group; wherein Z.sub.2 represents a non-metallic atomic group necessary for forming a benzene nucleus, which may be unsubstituted or substituted by a lower alkyl group having from one to about six carbon atoms, a halogen atom, or an alkoxy group; wherein R.sub.1 and R.sub.2 each represents an alkyl group having from one to about six carbon atoms, an aralkyl group, an alkyl group having a sulfo group, an allyl group, or an alkyl group having a carboxyl group; wherein X.sub.1.sup.- represents an anion and m is 1 or 2, in which when m is 1 an intramolecular salt is formed, and at least one of the sensitizing dyes represented by the General Formula (II) ##SPC13##

wherein Z.sub.3 represents a benzoxazole nucleus, which may be unsubstituted or substituted with a lower alkyl group having from one to about six carbon atoms, a halogen atom, an alkoxyl group, a hydroxyalkyl group, an alkoxycarbonyl group, an aryl group, a carboxyaryl group or a sulfoaryl group; wherein Z.sub.4 represents a non-metallic atomic group necessary for forming a benzene nucleus, in which said nucleus may be unsubstituted or substituted with a halogen atom, a cyano group, an alkoxycarbonyl group, a trifluoromethyl group, a carbamoyl group, an alkyl-substituted carbamoyl group, a sulfamoyl group, or an alkyl-substituted sulfamoyl group; wherein R.sub.3 and R.sub.4 each represents an alkyl group having from one to about six carbon atoms, an allyl group, a hydroxyalkyl group, an alkyl group having a sulfo group, an alkyl group having a carboxyl group, or an aralkyl group, and wherein at least one of R.sub.3 and R.sub.4 is an alkyl group having at least one of a sulfo group, a carboxy group or a hydroxy group, R.sub.5 is a lower alkyl group having from one to about six carbon atoms, an allyl group, a hydroxy-alkyl group, an aralkyl group, an acetoxyalkyl group, or a aryl group wherein X.sub.2.sup.- represents an anion and n is 1 or 2, in which when n is 1, an intramolecular salt is formed, and said emulsion having a high sensitivity to the radiation emitted from a green fluorescent substance and having a high stability to a safety light.

4. The silver halide photographic emulsion of claim 3, wherein said alkyl group having a sulfo group is selected from the group consisting of a sulfo alkyl group, a hydroxy(sulfo) alkyl group, a sulfo alkoxy alkyl group, a sulfo phenyl alkyl group, a sulfo-alkyl amino alkyl group, a sulfo-alkyl amino alkyl group and a sulfo-alkyl thio-alkyl group.

5. An photographic light-sensitive material comprising a support having thereon at least one layer of the silver halide photographic emulsion as set forth in claim 4.

6. A silver halide photographic emulsion containing a supersensitizing amount of the combination comprising at least one of the sensitizing dyes represented by the following General Formula (I') ##SPC14##

wherein R.sub.1 and R.sub.2 each represents a methyl group, an ethyl group, or an allyl group; X.sub.1 .sup.-represents an anion; and R represents a hydrogen atom, a methyl group, or an ethyl group; and at least one of the sensitizing dyes represented by the following General Formula (II') ##SPC15## wherein W.sub.1 and W.sub.2 each represents a hydrogen atom, a chlorine atom, or a methoxycarbonyl group and R.sub.3 and R.sub.4 each represents a sulfoalkyl group or an alkyl group; at least one of R.sub.3 and R.sub.4 being a sulfoalkyl group.

7. The silver halide photographic emulsion of claim 1 wherein the molar ratio of the sensitizing dye of the General Forumula (I) to the sensitizing dye of the General Formula (II) ranges from 9:1 to 1:10 and wherein the amount of each of said dyes to the silver halide ranges from 1.times.10.sup.-.sup.4 to 1 .times. 10.sup.-.sup.3 mole per mole of silver.

8. The emulsion of claim 1 wherein R.sub.3 and R.sub.4 if substituted alkyl are selected from the group consisting of a .beta.-methoxyethyl group, a .beta.-ethoxyethyl group, a .beta.-methallyl group, a benzyl group, a .beta.-phenylethyl group, a .beta.-acetoxyethyl group, a .beta.-acetoxypropyl group, a carbomethoxymethyl group, a .beta.-carbomethoxyethyl group, a carbethoxymethyl group, a .beta.-carbethoxyethyl group, a phenylmercaptomethyl group, a phenoxymethyl group, a .beta.-phenylmercaptoethyl group, a .beta.-phenoxyethyl group, and a 2-(2-hydroxy-3-sulfopropxy) ethyl group and a carbamylmethyl group.

9. A silver halide photographic emulsion as set forth in claim 1 wherein Z.sub.1 represents a non-metallic atomic group necessary for forming a benzthiazole nucleus.

10. A silver halide photographic emulsion as set forth in claim 9 wherein R.sub.1 and R.sub.2 each represents an alkyl group having from one to about six carbon atoms, and wherein both of R.sub.3 and R.sub.4 have at least one sulfo group.