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 sensi- tivity 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 com- pounds 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 requir- ed. Especially, quick and simplified process- ing will be very valuable. There is consid- erable 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. a b 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.