Future Prospects of Silver Halide Photography ―T Owards the 21St Century―

J.Soc. Photogr. Sci. Technol. Japan, Vol. 53, No. 2, 1990

特別記事 Future Prospects of Silver Halide Photography ―T owards the 21st Century―

Tadaaki TANI

Ashigara Research Loboratories, Fuji Photo Film Co., Ltd. Minami-ashigara, Kanagawa, Japan 250-01

Abstract

It is pointed out that future progress in imaging technology for taking pictures will solely depend upon innovations in silver halide photography from now to the 21st century and will greatly benefit our cultural life. Analyses on the processes for image formation have revealed the existence of big room for future innovations in silver halide photography by conventional and unconventional ways. Extensive investigations of silver halide photography will contribute, not only to its progress, but also to the progress in the sciences and technologies underlying it.

their photographic sensitivity and format 1. Requirments for Innovation in Silver size.3) It is noted that color negative film Halide Photography over the 21st Century provides a system with wide variety of Silver halide photography has a long selection in sensitivity, image quality and history and still dominates imaging systems, format size. It is possible to take a picture since it has the merit of great capability of with ultra-high quality by a large format sensitivity and image quality. Then, future film, to take appropriate pictures depending prospect of silver halide photography in upon various conditions by selecting its sen- imaging technology is a matter of primary sitivity, and to take a picture by a small concern to imaging scientists and engineers. camera with small-format film. The number Silver halide photography has surpassed of pixels in a solid state device for a still all the other imaging systems in taking video camera with mass production is as pictures. Several papers1-5) have been re- small as 4•~105, and will not reach that of cently reported on the comparison between a color negative film in the future, since silver halide photography and electronic there are many difficult problems for an imaging system which has recently ap- electronic imaging to solve.1-5) peared. The general conclusion described It is therefore considered that silver halide in those papers is that the former is much photography will continue to dominate superior to the latter in the capability of imaging systems for taking pictures by its sensitivity and image quality for taking incomparable capability, and that future pictures. advancement in imaging technologies for Let's take a color negative film for exam- taking pictures will solely depend upon ple. Figure 1 shows the electron micrographs future innovations in the field of silver of cross sections of color negative films and halide photography. top view of a solid state device (CCD) used Let's consider anticipated results of the for electronic imaging. The number of innovations in the field of silver halide equivalent pixels in a film would be a con- photography, taking the case of the system venient measure of the image quality for its of color film and paper. As already stated comparison with electronic imaging. Figure in the previous paper, 5) there are the follow- 2 shows the number of equivalent pixels in ing three major categories in which big ad- various color negative films as a function of vancements will be achieved in the future.

―87― ―88― Tadaaki TANI J. Soc. Photogr. Sci. Technol. Japan

Top View of CCD Cross Section of Color Negative Film

Fig. 1 Electron micrographs of cross sections of color negative films and top view of a solid state device (CCD).

(1) Advancement in the capability of sensitivity and image quality. (2) Advancement in the hue and stability of color images due to the development in color forming chemistry. (3) Advancement in the adaptability to the requirements of the times. The advancement in the hue and stability of color images is fascinating for the system of color film and paper and is promising, since there is considerable room for future developments in functional organic compounds useful for the advancement. The advancement in the adaptability to the requirements of the times will be essen- tially important for silver halide photography to grow in the future. For example, the adaptability of silver halide photography to electronic technology and to new light sources will expand its functions. Further- more, improvements in the processing of silver halide photography are highly required. Especially, quick and simplified processing will be very valuable. There is considerable room for the improvement in this direction, for which extensive efforts will Fig. 2 The numbers of equivalent pixels in be made in the future. One of the ultimate color negative films as functions of their figure might be a slot machine for the pro- sensitivity and format size.3) The num- cessing and printing. The development of bers are three times the ones which dry processing6-8) as well as the improve- Kriss reported. Vol. 53, No. 2, 1990 Future Prospects of Silver Halide Photography ―89― ―Towards the 21st Century― ment in the conventional processing will be use of a camera which is small enough for promising for that purpose. us to carry all the time. Exposed films can As seen in Fig. 2, high capability of sensi- be quickly processed and printed by a slot tivity and image quality is especially unique machine. to silver halide photography. The increase It is also expected that future innovations in sensitivity itself is undoubtedly fascinat- in silver halide photography will provide ing merit for taking pictures. There are various kinds of photographic materials still many occasions for us not to be able which will be adapted deeply to our cultural to take pictures because of insufficiency of life. the sensitivity of the current films. As seen in Fig. 2, silver halide photog- 2. Innovations in Emulsion Technology in raphy has overwhelming superiority in image the 21st Century quality over the others. Further advance- As stated above, future developments in ment in the image quality of silver halide imaging technology for taking pictures will photography will be promissing and fascinat- greatly benefit our cultural life and solely ing for taking pictures. depend upon future innovations in silver Since the current cameras are too heavy halide photography. In this paper, discus- and bulky for us to carry all the time, we sions are] forcussed on future development miss many chances to take pictures. Future in emulsion technology, which is essential to improvement in the image quality will pro- silver halide photography. vide small-format films and small cameras (1) Processes for Image Formation which we can carry all the time and take Figure 3 schematically shows the processes good pictures. for image formation in a generalized form Now, one can draw a figure of some de- and in a conventional silver halide photog- sirable color film/paper system whcih will raphy. Some room for future innovations probably appear in the future. We can take in the above-stated forms are discussed good pictures anywhere and any time by below.

Ffg. 3 Illustration of the processes for image formation in a generalized form (a) and in a conventional silver halide photography (b). ―90― Tadaaki TANI J. Soc. Photogr. Sci. Technol. Japan

A light signal is absorbed by a photosensi- tional to the light absorbance of a photosen- tive element, and then transferred to a reac- sitive element (its absorption coefficient a tion area. in silver halide photography, a times size A), the efficiency of latent image blue-light signal is absorbed by a silver formation ƒÓ (including the efficiency of the halide grain, and green-light and red-light transfer of an absorbed light signal to a re- signals by sensitizing dye molecules on the action area), and a factor ƒÔ relating to the

grain surface. As a result of the absorption amplification. of a photon, a free electron appears in the ( 1 ) conduction band of the grain, and is trans- The factor ƒÔ is introduced to indicate the ferred to one of sensitivity centers where a discrepancy between the characteristic latent image center is to be formed. curves for the fraction of the photosensitive In the next stage, a latent image center is formed in a reaction area. In silver halide

photography, a free electron and an inter- stitial silver ion are alternately captured by a sensitivity center to form a latent image center (i.e., a metal cluster). Through an amplification process, a latent image center is amplified to give a picture element. In silver halide photography, a silver halide grain with a latent image center on its surface is reduced to give a metallic silver particle or a dye crowd. Fig. 5 Illustration of characteristic curves for Figure 4 illustrates a latent image center the fraction of the photosensitive ele- on a cubic silver bromide grain with edge ments with latent image centers (P) and length of 1ƒÊm, showing that the degree of the normalized optical density of the the amplification can be as large as 5•~109. image formed by some amplification According to the processes in Fig. 3, photo- process (D). graphic sensitivity S is in principle propor-

Fig. 6 Photographic sensitivity (S) of octahed- ral AgBr grains as a function of the Fig. 4 A latent imege center on a cubic AgBr diameter of the circle whose area is the grain. same as their average projective area. Vol. 53, No. 2, 1990 Future Prospects of Silver Halide Photography ―91― ―Towards the 21st Century―

elements which formed latent image centers tion the fact that the smallest latent image

(Curve a) and for the optical density of the center is composed of three or four metal image thus formed (Curve b), as exemplified atoms,10-14) ƒÓ of nearly unity has been in Fig. 5. Namely, it is principally possible achieved experimentally in smaller grains. to use any part of Curve a to obtain a pic- However, it is judged from the values of ture, when we can control the amplification larger grains in Fig. 7 that the state-of-the-

process. The increase in A and ƒÔ is usually art high-spaed photographic film with such associated with the deterioration of the image large grains forms latent image centers on

quality of the picture thus formed. those grains with ƒÓ of about 1/5. It has been The increase in ƒÓ can be used either for reported that the high-speed film uses about the increase in S of a photographic film 1/3 of the incident light.15) The increase in without change in its image quality or for the sensitivity of the high-speed film by a the improvement in image quality of a film factor of 10 through the improvements in ƒ¿, without change in S by decreasing A and/or A, and ƒÓ will be achieved in this century

χ. The increase in S caused by the increase or in the early stage of the 21st century5,10) in A, which is usually achieved by increas- By taking into consideration that the ing the size of silver halide grains, is there- number of photons absorbed by a silver fore associated with the deterioration in halide grain is proportional to its volume,16,17) image quality, and limited by the deteriora- the proportionality of its sensitivity to vol- tion in ƒÓ with the increase in the size of the ume as seen in Fig. 6 could indicate that grains, as seen in Fig. 6. and the size of the minimum latent image (2) Future Developments of Emulsion Tech- center were constant, and only one latent nology by Conventional Ways image center was required for the initiation The quantum sensitivity is usually ex- of the development of each grain. It is pressed by the average number of absorbed believed that the deviation from the propor- photons per grain to render half of the tionality begins to appear, when the grain grains developable, and used as the measure size reaches the diffusion length of free elec- of ƒÓ. Figure 7 shows the reported values trons.16,17) It is known that the diffusion of quantum sensitivities of various silver length of free electrons in a large crystal, halide grains, which Hailstone and coworkers which is as large as several tens ƒÊm,18) is have collected.9) By taking into considera- much larger than the grain size at which the deviation appeared. It is therefore considered that there is considerable room for the increase in S by creasing A. There is also room for some advancement relating to ƒÔ. The technologies to control

χ by controlling the size of an picture element in Fig. 3, are highly required for the improvements in image quality as well as in sensitivity. For example, the technologies to release some inhibitor during the development process could improve the image quality,19-24) and will be further developed in the future. (3) Future Developments of Emulsion Tech- nology by Unconventional Ways In addition to the conventional way as stated above, unconventional ways are highly Fig. 7 Quantum sensitivities of various emul- required for big developments of emulsion sions as expressed by the number of technology in the future, although it will be absorbed photons per emulsion grain. difficult and will need a long time to achieve ―92― Tadaaki TANI J. Soc. Photogr. Sci. Technol. Japan them. It is also quite difficult to predict Efficient light-harvesting techniques, as the possibility of those developments. How- seen in photosynthetic systems, are highly ever, it seems that there are already some desirable for innovative improvements in trials which might lead to those develop- the above-stated problem. Namely, all the ments in the future. It might be therefore incident light signals are absorbed by a large meaningful to make some considerations on amount of antenna dye molecules arranged the direction of those developments, by around each emulsion grain with sensitizing picking up some of those trials for example. dye molecules on its surface. Then, energy In the former paper, the author has de- transfer takes place among the antenna dye scribed an opinion that the superiority of molecules, allowing the light signals to reach silver halide photography for taking pictures and excite the sensitizing dye molecules. mainly depends upon the formation and The excitation of the sensitizing dye molec- property of a latent image center (i.e., a ules is followed by their efficient electron mesoscopic metal cluster) on a silver halide injection into the conduction band of the

grain.10) It is therefore considered that, grain. among the processes in Fig. 3a, only the Several attempts have been made by sev- latent image formation is peculiar to and eral groups of works29,32-35) to look for such need a silver halide grain. It is noted that a light-harvesting technique. However, they φ of about unity could be already achieved could not have any efficient light-harvesting in small silver halide grains.12,13) technique. The control of the position and Then, it is considered that the function of orientation of antenna-dye molecules might a silver halide grain to absorb a light signal be required for the efficient energy transfer and transfer it to a reaction area can be among them. replaced by non-silver material. Levy and Although a silver metal image is replaced coworkers prepared emulsion grains with by a dye image in a color film, it seems that heterojunction of AgCl and orthorhombic there is still some room for innovations re- PbO. In such a grain, free electrons were lating to ƒÔ. As seen in Fig. 5, the introduc- given by the light absorption of PbO and tion of some acceleration process (e. g., transferred to AgCl,25-27) where latent image infectious development) to the development

centers were formed. House prepared emul- process could increase effective sensitivity sion grains with heterojunction of AgX and of a photographic film36) with deterioration merocyanine dye.28) Free electrons, which of its image quality. The deterioration of were given by the light absorption of the the image quality could be restricted by dye, were transferred to AgX, where latent dividing emulsions into many small units.37) image centers were formed. Although the The arrangement of each emulsion grain in above-stated trials have not yet been sucess- regularly arrayed microcavities one by one ful enough to give high-sensitive emulsions, has been tried to get a picture with improved they should be extended to get new emulsions with ultra-high sensitivity in the future. In conventional spectrally-sensitized emulsions, the amount of sensitizing dyes added is less than that required for their mono- molecular layer coverage of emulsion grains, and far less than the amount to absorb all the light incident to each grain. It is reported that the absorbance of a monomole- cular layer of a J-aggregating dye on AgBr is only several percentages.29) Gilman prepared a multimolecular layer of sensitizing dyes and a supersensitizer on emulsion Fig. 8 Regularly arrayed cubic AgBr grains grains.30, 31) (T. Heki, ref. 39) Vol. 53, No. 2, 1990 Future Prospects of Silver Halide Photography •\ 93•\ ―Towards the 21st Century― image quality.38) A trial has been also made is highly dependent upon the formation and to regularly array emulsion grains (Fig. 8).39) property of latent image and sensitivity centers. Those centers are regarded as 3. Innovations in Silver Halide Photography microclusters, to which many scientists are in Relation to Basic Science and Tech- paying keen attention. The efficiency of the nology latent image formation mainly depend upon Photographic science and Technology are the properties of silver halide grains, which based on various kinds of basic sciences and might be one of the most interesting mate- technologies and have been deeply investi- rials in solid state physics.40,41) Spectral gated for a long time. It is considered that sensitization is a quite interesting example future innovations in silver halide photo- of the light-induced electron transfer pro- graphy as anticipated above will depend on cesses,42) which are hilighted phenomena in those sciences and technologies. It is also photochemistry. expected that future investigations of silver Photographic development is an interesting halide photography will contribute to the example of catalytic reactions. Moreover, progresses of those sciences and technologies. various chemical reactions takes place dur- It is expected that silver halide grains ing photographic development to make prepared for photography, which are already image processing. It is noted that the image finest and most well-defined among various processing performed during the photo- grains, will make further progresses includ- graphic development is parallel, 5) in con- ing the heterojunctions as stated above with trast to the fact that only series image the aid of crystallization technology. The processing is possible at the present in elec- progress of molecular arrangement technol- tronic imaging, as shown in Fig. 9. Parallel ogy is highly required for the improvement image processing is in general superior to in J-aggregate formation of sensitizing dyes series one, in that the former can work and for the development of efficient antenna promptly without any image transfer process dye. The functional organic compounds for which need special devices and is unavoid- photographic materials are of great import- ably associated with image deterioration. ance and will make further progresses with Parallel image processing as developed in the aid of synthetic chemistry. silver halide photography will be an impor- The capability of silver halide photography tant subject in the next century.

Fig. 9 Illustration of pallarel image-processing in silver halide photography and series image-processing in electronic imaging. ―94― Tadaaki TANI J. Soc. Photogr. Sci. Technol. Japan

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