Fine grain silver halide emulsions with polyheteronuclear sensitizing dyes
This is a Continuation of application Ser. No. 193,138, filed Oct. 27, 1971, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a photographic element useful for holographic recording and for reproducing images using coherent rays emitted from lasers, and most especially to photographic emulsions of finely-divided silver halide particles spectrally sensitized by certain kinds of sensitizing dyes which have a high sensitivity in the red-sensitive range of 615-660 mμ. 
     2. Description of the Prior Art 
     Sensitive elements used in holographic recording must have sufficient sensitivity to the laser beam used, i.e., they must have a high spectral sensitivity to the wavelength of the laser beam used. 
     Laser beams have an intrinsic wavelength which depends upon the kind of laser used; that is the substance which is the laser oscillation medium. The laser wavelength usually has a very high monochromatic characteristic. 
     Nowadays, the lasers most widely used include those using a ruby, a neon and helium gas, an argon gas, a krypton gas and a carbon dioxide gas as the laser oscillation medium. Among these, the laser beam obtained using a neon and helium gases as the laser oscillation medium is most widely used because of low cost and stable out-put. This type of laser is generally termed a neon-helium laser. In holography this neon-helium laser is now most preferably used, as the wavelength of the laser beam emitted from a neon-helium laser is 632.8 mμ in most cases. 
     It is necessary that a sensitive silver halide photographic element suitable for holographic recording using a neon-helium laser have a high spectral sensitivity to the 632.8 mμ wavelength of the neon-helium laser. Further, it is required that the sensitive element have a high resolving power because the space frequency of a holographic image to be recorded and reproduced usually exceeds 1000/mm. Accordingly, it is necessary that such a photographic emulsion have very low light scattering. Moreover, such a photographic emulsion should have an extremely low image granulation. In order to satisfy these conditions, it is necessary to use photographic emulsions containing extremely fine silver halide particles, namely those having an average particle size below 0.1 micron. 
     A problem is encountered in sensitizing such emulsions comprising extremely fine silver halide particles. Specifically, sensitizing dyes which afford an excellent spectral sensitization function to silver halide emulsions of a comparatively large particle size do not always afford an excellent spectral sensitization function to the silver halide emulsions of such small particle size. 
     Accordingly, for high resolution holographic recording and reproduction, it is necessary to discover sensitizing dyes which provide an excellent spectral sensitization function to silver halide photographic emulsions having a very small particle size. 
     SUMMARY OF THE INVENTION 
     The present inventors have found that high spectral sensitivity to monochromatic light of 632.8 mμ emitted from a neon-helium laser is obtained when a silver halide emulsion in which the average particle size of the silver halide particles by number is not greater than 0.18 micron in diameter, or wherein 95% by number of the silver halide particles are not greater than 0.2 micron in diameter, is spectrally sensitized with a certain type of sensitizing dye. Such fine particles as employed in the present invention are spherical. 
     Accordingly, one object of the present invention is to provide a silver halide photographic emulsion comprising fine silver halide particles having a high spectral sensitivity to monochromatic light of 632.8 mμ emitted from a neon-helium laser, more generally to such emulsions having a high spectral sensitivity to coherent neon-helium laser emissions of a wavelength of 615 mμ - about 660 mμ by spectrally sensitizing such fine particle silver halide emulsions using certain types of sensitizing dyes. 
     These objects have been attained by adding at least one sensitizing dye represented by the following formula to silver halide emulsions in which the average particle size of the silver halide particles by number is not greater than 0.18 micron in diameter or wherein at least 95% by number of the silver halide particles have particle sizes not greater than 0.2 micron in diameter. Alternatively, the preferable particle size is about 0.1 micron. 
     SENSITIZING DYE ##EQU2## wherein Z 1  represents the atoms necessary to form any 5-membered or 6-membered heterocyclic ring conventionally employed in cyanine dyes. Representative of such heterocyclic rings are the pyridine nucleus, thiazole nucleus, selenazole nucleus, benzothiazole nucleus, benzoselenazole nucleus, naphthothiazole nucleus, naphthoselenazole nucleus and quinoline nucleus, which may be substituted with any desired substituent which do not shift the spectral sensitization wavelength of the dye outside of the neon-helium laser wavelength and which is compatible with the silver halide emulsion, such as lower alkyl (e.g. methyl), alkoxy (e.g. methoxycarbonyl), trifluoromethyl, cyano, methyl sulfonyl ( --SO 2  CH 3 ), sulfamyl (e.g. --SO 2  NHC 2  H 5 ), carbamyl (e.g. --CONHC 2  H 5 ), etc; Z 2  represents the atoms necessary to form a nucleus selected from the group consisting of a thiazoline nucleus, an oxazole nucleus, a benzoxazole nucleus, a thiazole nucleus, a selenazole nucleus, a benzothiazole nucleus, a benzoselenazole nucleus, a napththothiazole nucleus and a naphthoselenazole nucleus, which may be substituted with any desired substituents having the same properties as those described in Z 1  ; R and R 1  represent a lower alkyl group having preferably up to four carbon atoms, such as methyl, ethyl, n -propyl etc., a hydroxy alkyl group such as β -hydroxyethyl etc., an alkoxy alkyl group such as β -methoxyethyl etc., an acetoxy alkyl group such as β  -acetoxy ethyl etc., a carboxy-alkyl group such as β  -carboxyethyl, γ-carboxypropyl, δ-carboxy butyl, ω-carboxypenyl etc., a sulfoalkyl group such as β-sulfoethyl, γ-sulfopropyl, γ-sulfobutyl, δ-sulfobutyl, ω-sulfopentyl, etc., an allyl group such as vinyl methyl etc., an aralkyl group such as benzyl, phenylethyl, p-carboxyphenylethyl, p-sulfophenylethyl etc., and equivalent groups, wherein the alkyl moiety has preferably up to four carbon atoms; 
     R 2  represents an alkyl group (such as methyl, ethyl, benzyl, vinylmethyl etc.) or an aryl group such as phenyl, the above-said alkyl moiety having preferably up to four carbon atoms; 
     L represents a methine group or ##EQU3## wherein R 3  represents an alkyl group (such as methyl, ethyl, etc., ) an alkoxyalkyl group (e.g. ethoxyethyl, etc.), or an aryl group (such as phenyl, o-carboxyphenyl, etc.), the above-said alkyl moiety having preferably up to four carbon atoms; L and R and L and R 1  may also be joined together by a methylene chain; 
     m represents 0 or 1; 
     X represents an anion conventionally employed in cyanine dyes, e.g., a chlorine ion, bromine ion, iodine ion, perchlorate ion, methyl sulfate ion, p-toluene sulfonate ion, etc; 
     p represents 0 to 1, with p being 0 when the compound is in the form of an inner salt. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description, typical sensitizing dyes used in the present invention are described. However, the sensitizing dyes as may be used in the present invention are not intended to be limited thereby. ##SPC1## 
     The sensitizing dyes used in the present invention are known and can be synthesized easily by persons skilled in the art, with the formation procedure being described in, e.g., &#34;The Cyanine Dyes and Related Compounds&#34; written by Frances M. Hamer; John Wiley and Sons Ltd. (1964). 
     The silver halide emulsions used in the present invention in which the average particle size of the silver halide crystal particles by number is not greater than 0.18 micron (in diameter) or wherein 95% by number of the silver halide particles have particle sizes not greater than 0.2 micron in diameter can be spectrally sensitized very effectively by the sensitizing dyes used in the present invention. 
     The spectral sensitization of the present invention is especially effective for gelatin-silver halide emulsions. However, it is also effectively employed with silver halide emulsions containing other hydrophilic colloids such as agar agar, collodion, water soluble cellulose derivatives, polyvinyl alcohol, polyacrylamides and other synthetic or natural hydrophilic resins. The amount thereof is in accordance with art recognized amounts. 
     As for the silver halide emulsions used in the present invention, any of those containing silver chloride, silver chloride-bromide, silver bromide and silver iodide-bromide-chloride may be used in addition to mixtures thereof. However, best results are obtained using a silver bromide or silver iodide-bromide emulsion. 
     In order to prepare the silver halide photographic emulsions which are spectrally sensitized in accordance with the present invention, one or more sensitizing dyes can be added to the emulsion by any known prior art process. The emulsions can be chemically sensitized by adding one or more sulfur compounds, noble metal complex salts and reducing compounds as are known to the art. 
     It is generally most convenient to add the sensitizing dyes as a solution dissolved in a suitable solvent, such as methanol and ethanol, to the emulsions. The amount of sensitizing dyes present in the emulsions can vary widely within from about 5 to about 500 mg of sensitizing dye (s) per kg of the silver halide emulsion including most useful commercial forms. 
     The silver halide photographic emulsions of the present invention may also be subjected to hypersensitization or supersensitization as these terms are understood to the art. 
     In preparing silver halide photographic emulsions in accordance with the present invention, additives commonly used in the art may be so used for their known functions, e.g., other chemical sensitizers, stabilizers, fog inhibitors, toning agents, hardening agents, surface active agents, plasticizers, antistatic agents, lubricants, development accelerators, color formers and fluorescent whitening agents. 
     The present invention will now be explained in greater detail by the following examples which are to be taken as only illustrative of the broad scope of the present invention. 
     EXAMPLE 1 
     A silver halide photographic emulsion was produced by adding sensitizing dyes as exemplified above (refer to Table 3) to a silver iodide-bromide emulsion (AgI:AgBr = 1 mole : 99 moles) in which the average particle size of the silver halide particles by number was 0.06 micron in diameter. The product was coated onto a cellulose triacetate film base and dried. 
     Sensitivity to a coherent neon-helium laser beam (wavelength : 632.8 mμ) can be determined from a curve of log (Amount of exposure) - D determined by applying the laser beam emitted from the laser tube to a revolving lightscattering plate, thereby exposing the sensitive element by the laser beam passed therethrough, developing and measuring the photographic density (D) by means of an optical densitometer. However, this method is not suitable for determining sensitivity of a large number of samples because it requires a great amount of the time as exposure takes place in a point by point manner. Thus, in this example, the above-mentioned sample was exposed to a tungsten source ray having an intensity of 8000 luxes (2854°K) by passing the tungsten ray through an interference filter (T max 633 mμ: Δλ1/2 2.5 m) which provides a red light of 633 mμ wavelength (corresponding to the monochromatic ray of a neon-helium laser), and thereafter developing the emulsions. A developer having the formulation shown in Table 2 was used. 
     
                       Table 2                                                     
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Metol                 2 g                                                 
Hydroquinone          8.8 g                                               
Anhydrous sodium sulfite                                                  
                      96 g                                                
Anhydrous sodium carbonate                                                
                     48 g                                                 
Potassium bromide     5 g                                                 
Water to make         1 liter                                             
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     For nearly equal wavelengths, sensitivity to a coherent laser ray and sensitivity to an incoherent tungsten ray are theoretically different in some degree. However, experimental date up to three figures is identical for both sources. This will be understood from the following experiment whether intensity of both coherent laser and an incoherent tungsten beam rays was controlled by a thermopile to be identical and microfilms were exposed therewith. The time of exposrue necessary to obtain identical density was 13.5 seconds for the tungsten light beam and 13.3 seconds for the laser beam, a difference within the range of experimental error. 
     In Table 3, the red sensitivity and the maximum sensitization obtained upon adding the sensitizing dyes of the present invention are shown. In Table 4, the red sensitivity of typical dyes is provided to form the basis of a comparison with the dyes of the present invention. The same AgBrI emulsion was used. 
     
                                           Table 3                                 
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 Dye Amount/                                                              
            Emulsion                                                      
                 Average                                                  
                      Red-   Maximum                                      
     silver 29g  particle                                                 
                      sensitivity                                         
                             sensitization                                
                 size        wavelength                                   
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1    3.2×10.sup.-.sup.5                                             
            AgBrI                                                         
                 0.06 μ                                                
                      500      624 mμ                                  
     gram mol                                                             
2    &#34;      &#34;    &#34;    1800   642                                          
3    &#34;      &#34;    &#34;    1850   644                                          
4    &#34;      &#34;    &#34;    1850   643                                          
5    &#34;      &#34;    &#34;    1700   640                                          
6    &#34;      &#34;    &#34;    610    625                                          
7    &#34;      &#34;    &#34;    1840   644                                          
8    &#34;      &#34;    &#34;    1200   642                                          
9    &#34;      &#34;    &#34;    880    615                                          
10   &#34;      &#34;    &#34;    890    638                                          
11   &#34;      &#34;    &#34;    1750   644                                          
12   &#34;      &#34;    &#34;    1200   654                                          
13   &#34;      &#34;    &#34;    1100   650                                          
14   &#34;      &#34;    &#34;    1250   650                                          
15   &#34;      &#34;    &#34;    1000   644                                          
16   &#34;      &#34;    &#34;    730    650                                          
17   &#34;      &#34;    &#34;    730    652                                          
18   &#34;      &#34;    &#34;    660    655                                          
19   &#34;      &#34;    &#34;    610    650                                          
20   &#34;      &#34;    &#34;    420    658                                          
21   &#34;      &#34;    &#34;    880    660                                          
22   &#34;      &#34;    &#34;    650    642                                          
23   &#34;      &#34;    &#34;    610    640                                          
24   &#34;      &#34;    &#34;    1500   638                                          
25   &#34;      &#34;    &#34;    1300   638                                          
26   &#34;      &#34;    &#34;    400    618                                          
27   &#34;      &#34;    &#34;    400    615                                          
28   &#34;      &#34;    &#34;    950    656                                          
29   &#34;      &#34;    &#34;    1600   640                                          
30   &#34;      &#34;    &#34;    1750   644                                          
31   &#34;      &#34;    &#34;    1800   642                                          
32   &#34;      &#34;    &#34;    900    655                                          
33   &#34;      &#34;    &#34;    800    654                                          
34   &#34;      &#34;    &#34;    600    658                                          
35   &#34;      &#34;    &#34;    550    655                                          
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     The value of red sensitivity is shown as the relative sensitivity to that of Comparison dye B (100) upon exposure of the same to the tungsten light beam heretofore described using an interference filter (T max 633 mμ, Δλ 1/2 2.5 mμ.) 
     
                                           Table 4                                 
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                Average                                                   
   amount/      particle                                                  
                     Red-    Maximum                                      
Dye                                                                       
   silver 29g                                                             
           Emulsion                                                       
                size sensiitivity                                         
                            sensitization                                 
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2   1/5000 mol                                                            
           AgBrI                                                          
                0.06 μ                                                 
                     1800   642 mμ                                     
   (methanol                                                              
   solution 4 cc)                                                         
3  &#34;       &#34;    &#34;    1850   644                                           
28 &#34;       &#34;    &#34;     950   656                                           
A  &#34;       &#34;    &#34;     50    658                                           
B  &#34;       &#34;    &#34;     100   638                                           
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     The chemical formulae of sensitizing dyes A and B exemplified as comparison dyes are as follows: ##SPC2## 
     EXAMPLE 2 
     Sensitive elements separately containing Sensitizing dyes 2 and 3 and Comparison dyes A and B, produced in accordance with Example 1, were exposed using a neon-helium laser (single mode) having a 50 mW output. They were then developed using a developer having the formulation described in Table 2. 
     The time of exposure necessary to obtain an identical density was as in Table 5. 
     
                       Table 5                                                     
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Sensitizing dye   Time of exposure*                                       
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2                   1 second                                              
3                   1 second                                              
A                   36 seconds                                            
B                   18 seconds                                            
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     As shown in Table 5, the time of exposure for sensitizing dye 2 or 3 is shorter than that in case of Comparison dye A or B, i.e., the former have a higher sensitivity to the neon-helium laser than the latter. 
     As a result of shortening the time of exposure, distinct holograms can be obtained since image dimness caused by minute vibration of the apparatus is minimized.