CONFERENCE ON THE MAIN PROBLEMS OF SCIENTIFIC PHOTOGRAPHY

In the period from June 29 to July 4, 1951, an All-Union Conference on the main problems of scientific photography was held in Odessa, convened by the Commission on Scientific Photography and Cinematography under the Division of Chemical Sciences of the Academy of Sciences of the USSR.

This Conference was the sixth conference on scientific photography in our country. It was devoted to three main problems of scientific photography—the nature of photosensitivity and the latent photographic image, optical sensitization of photographic layers, and the process of photographic development.

The convening of the Conference in Odessa was due to the fact that this year marks 25 years of existence of the Scientific Research Institute of Physics of the Odessa State University named after I. I. Mechnikov, within whose walls the activity of one of the oldest workers in scientific photography and the photochemistry of the solid state, Professor E. A. Kirillov, and of his pupils, who are engaged in research on the action of light on silver halide crystals, proceeds.

At the Conference 28 papers were read, of which 13 papers were devoted to the nature of photosensitivity and the latent photographic image, 8 papers to optical sensitization of photographic layers, and 7 papers to the process of photographic development.

The first paper, on the problem of the nature of photosensitivity and of the latent photographic image, delivered by Prof. E. A. Kirillov, was devoted to new investigations of the absorption of light by the latent image in silver halides. The speaker, together with his pupils, succeeded in showing that, upon weak exposure of photographic layers of the Lippmann type or of silver halide single crystals, a very weak coloration arises, characterized by an absorption curve with a fine structure, the existence of which had previously been unknown. The author was able to detect this structure thanks to the use of a specially made and exceptionally accurate method of spectrophotometric measurements. As the action of light proceeds, a redistribution of the absorption intensity in the fine bands takes place, which subsequently leads to the appearance of broader absorption bands that had been observed earlier by a number of authors.

The second paper was delivered by E. A. Kirillov’s pupil M. E. Fonkin, who observed the appearance of the same fine structure of light absorption by the thinnest films of metallic silver deposited on the surface of silver halide crystals and even on glass. The position in the spectrum of these fine bands coincides with the position of similar bands arising during the photochemical coloration of crystals, which indicates that, under weak photochemical coloration, the smallest particles of silver are also formed, consisting, in all probability, of several atoms.

The next paper was delivered by F. F. Volkenshtein, who subjected the existing theory of color centers and, in particular, the nature of \(F\)-centers to critical analysis. The speaker showed that at present preference should not be given to any one of the two existing models of \(F\)-centers: the Frenkel–Poole theory, according to which the \(F\)-center is a silver atom located in the same agreement with the known experimental facts as the de Boer–Pekar model, according to which the \(F\)-center is an electron located in a vacant anion site. Further, the speaker described his work on taking into account the interaction between electro-

... in a crystal lattice, which led him to the conclusion that there is a possibility of the existence of doublets—formations consisting of two electrons bound to one another.

K. V. Shalimova reported on her experiments on the luminescence of silver-halide crystals. The author came to the conclusion that the activator of the luminescence is atomic silver, which the author succeeded in detecting also by spectrophotometric measurements.

P. V. Meyklyar spoke about the results of a spectrophotometric and photoelectric investigation of silver-halide crystals, carried out by him, in part jointly with E. K. Pushenko. These investigations led to the conclusion that in silver-halide crystals there are \(F\)-centers analogous to the same centers in alkali-halide crystals. From the work reported it also follows that the presence of \(F\)-centers in silver-halide crystals determines both the photoelectric and photochemical sensitivity of silver-halide crystals and, to a significant extent, the light sensitivity of photographic layers.

Thus, the spectral position of the band of photoelectric sensitivity of single crystals of silver halide of different composition coincides with the spectral position of the band of the intrinsic light sensitivity of photographic layers with a solid phase of the corresponding composition.

A. I. Kostarev gave a report on the connection between the system of optical levels of ionic crystals and their X-ray absorption spectrum. From the structure of the X-ray absorption spectrum the speaker succeeded in determining the position of the upper electron levels and in connecting them with optical transitions for alkali-halide crystals.

Corresponding Member of the Academy of Sciences of the USSR K. V. Chibisov, in his report, subjected Sheppard’s theory of the silver sulfide centers of light sensitivity to criticism. Numerous experiments carried out by the author jointly with A. A. Titov and A. A. Mikhailova showed that the formation of centers of light sensitivity is a restorative process, the kinetics of the change in light sensitivity being connected not with the kinetics of the formation of silver sulfide, but with the formation of metallic silver. Experiments carried out further on the influence, upon the process of second ripening, of sulfur compounds—thiosinamine and other substances—showed that these compounds are merely accelerators of chemical ripening, and not sensitizers in the sense of increasing the limiting value attained by the light sensitivity. Therefore it must be considered that in the process of chemical ripening silver sulfide plays not a determining role, but is only an accelerator, while the determining factor is the formation of centers of metallic silver.

The report by I. M. Ratner, made on work carried out jointly with A. A. Titov, was devoted to the role of internal silver centers in emulsion crystals. The authors illuminated a photographic emulsion intensively during the first ripening and observed what change in the light sensitivity of the emulsion this produced. It was found that, with increasing illumination under such exposure, the light sensitivity of the layer obtained first falls sharply, and then, having reached a certain minimum value, increases again. The observed decrease in light sensitivity can readily be explained by the competing action of internal silver centers. The increase in light sensitivity with a further increase in illumination appears unclear.

A report on increasing the light sensitivity of a photographic layer by means of its weak preliminary exposure was made by J. L. Brown. The author studied in detail the conditions of preliminary exposure lying below the threshold of blackening of the layer, as well as the conditions of a second

exposure, under which the greatest increase in light sensitivity is achieved.

A report on the properties of the photographic layer under various conditions of continuous and intermittent illumination, based on work carried out by the speaker jointly with P. V. Meyklyar, was delivered by A. L. Kartuzhanskii. The investigation required the construction of two special instruments. Data were obtained on deviations from the law of reciprocity when the illumination time of the layer was varied from \(10^{-7}\) sec to \(10^{+3}\) sec and at various temperatures. Data on intermittent illumination of the layer were also obtained for highly varied illumination conditions. The results obtained indicate that Talbot’s law is not fulfilled even when corrections for deviations from reciprocity are taken into account.

A short communication was made by I. I. Roikh on work carried out by him jointly with D. M. Rafalovich. These authors studied the action of hydrogen peroxide on photographic layers.

A report by S. G. Grenishin was devoted to the action of an electron beam on photographic layers. Photographic layers with different sensitivity to electrons were investigated. It was shown that irradiation of low-sensitivity photographic layers by electrons leads to an increase in their sensitivity to the subsequent action of visible light. The spectral distribution of this phenomenon is analogous to the corresponding distribution for brief, intense illumination of the layer by light of the visible part of the spectrum. Such a parallelism in the action on the layer of electrons and of light of high intensity was also observed in the kinetics of development of the latent image produced by both kinds of radiation.

B. U. Barshevskii read a short report on the quantum yield in the photoeffect in crystals of silver halide. The author reviewed the data known on this question and emphasized the need for further investigations.

On the problem of optical sensitization of photographic layers, the first report was read by E. K. Putseiko on behalf of himself and Academician A. N. Terenin. In this report, devoted to sensitization by dyes of the internal photoeffect in semiconductors, the authors described experiments on studying the photoeffect in thallium halide salts with various dyes adsorbed on them and used as photographic sensitizers. It was shown that the spectral distribution of the photoeffect corresponds to the distribution of light absorption by the dye, and also that the intensity of the photoeffect is connected with the presence of F-centers in the crystal.

The next report from the same group was made by Yu. N. Gorokhovskii on the topic “The relation between adsorption of sensitizers, light absorption, and the photosensitivity of sensitized photographic layers.” The report covered three questions mutually related to one another. In the first part, the results of V. V. Barbarin’s experiments on the study of the absolute density of adsorption of sensitizing dyes on halide-silver crystals of definite sizes separated from a photographic emulsion were reported; a dependence was established of the absolute level of adsorption on the sizes of the emulsion crystals, on the type of halide, and on the conditions of emulsion ripening, which explains many features of the sensitizability of emulsions. In the second part of the report, the results of E. I. Ozherel’eva’s experiments on establishing the relation between the conditions under which a dye is introduced into an emulsion and the character of the spectral distribution of the additional photosensitivity produced by this dye were set forth. It proved that, under the very same conditions of introduction, it is possible, by combining certain elements of them (concen-

tration of the dye, the type of solvent, the duration of bathing, etc.), to achieve one and the same spectral distribution, i.e. one and the same character of structure of the adsorption layer. Finally, the third part of the paper was devoted to establishing quantitative relations between light absorption and the photosensitivity of sensitized layers. It turned out that there is an analogy with what had previously been found in the region of the intrinsic photosensitivity of silver halide: as the absorption of light by the layer increases, the spectral photosensitivity of the latter first increases and then, after passing through a maximum, falls. This is caused by the strong screening action of the overlying rows of crystals on the underlying ones and by the concentration of blackening at the surface of the layer.

I. I. Breido reported the results of work carried out jointly with P. Kh. Pruss on the relation between light scattering in a photographic layer, its optical sensitization, and resolving power. The authors studied the spectral distribution of light scattering in layers with a solid phase of different dispersity and found that it depends on the latter: the greater the dispersity, the more the scattering band is located in the short-wave region. In the sensitization of such layers toward the long-wave part of the spectrum, low-dispersity layers exhibit a comparatively low resolving power in this region (caused, as is known, by the degree of light scattering of the layer), whereas highly dispersed layers exhibit a high one, since they scatter long-wave rays only slightly.

The next paper was presented by S. V. Natanson, on the subject of the quantum yield of the sensitized photolysis of silver bromide. For a number of dyes, under conditions of suspensions of silver bromide, the quantum yield was determined in the absorption band corresponding to the molecular state of the crystal. It turned out that, in the presence of dyes, photolysis in the region of the intrinsic photosensitivity of silver halide proceeds the less effectively, the greater the desensitizing action of the dye, and that in the region of added photosensitivity the quantum yield is related to the effectiveness of the photographic action of the dye.

Our leading specialist in the field of the synthesis of optical sensitizers, I. I. Levkoev, presented, jointly with S. V. Natanson, a paper entitled “The Structure of Sensitizing Dyes and Their Photographic Activity.” In this paper the authors, on the basis of the enormous experimental material of the synthetic laboratory of the All-Union Scientific Research Cinema and Photo Institute, examined in detail the role of the type of heterocyclic and substituent groups, and also the length of the vinylene chain in dyes of three most important classes—cyanines, merocyanines, and rhodacyanines. It was found, for example, that in most cases the introduction of substituents into the middle of the conjugated chain of carbocyanine dyes leads to an increase in their adsorbability and in the effectiveness of their action, provided that steric factors do not prevent this (violation of the flatness of the molecule). In low- and medium-basic dyes, the introduction of electropositive groups acts favorably, while the introduction of electronegative groups acts suppressively. In highly basic dyes the picture is often the reverse. An increase in the basicity of a dye (its affinity for the proton) leads at first to an increase in the sensitizing action, and then to its decline and to an increase in fogging capacity. Separately considered was the question of the harmful effect, on sensitization, of the dye-forming components of the multilayer color photographic process; it was shown that this harmful effect is due to adsorption competition between the sensitizer and the components.

In the paper by Academician A. I. Kiprianov, read by E. D. Sych, the results were reported of investigations of the same kind, carried out at the Institute of Organic Chemistry of the Academy of Sciences of the Ukrainian SSR and at Kiev University. In the paper, special attention was devoted to considering the role of steric obstacles in the relation both of the physicochemical and of the photographic properties of sensitizing dyes.

A short paper by Z. L. Petrushkina was devoted to the study of the adsorption of sensitizers on pure silver bromide. Isotherms of adsorption and desorption of dyes were obtained, and the influence of bromine ions on these processes was studied.

The last paper in the section on optical sensitization was given by A. V. Borin. The speaker presented in detail the results of a spectrophotometric study of the interaction of gelatin with dibenzothiacarbocyanine dyes which, in aqueous-gelatin solutions, possess several absorption bands. It was established that, when a water-soluble fraction of gelatin is stained with the dye, the absorption band of the dimer is formed predominantly, while the fraction insoluble in cold water, when stained, gives predominantly the absorption band of the highly aggregated form. The casting conditions of the gelatin film affect the nature of the orientation of the protein molecules at the surface of the film and thereby affect the ratio of the intensities of both absorption bands of the dye staining such a film.

On the problem of the process of photographic development, the first paper was read by G. P. Faerman, who gave a thorough review of the development of the ideas underlying the diffusion theory, and of the latest advances in the electrochemical theory of development over the past 20 years, and who elucidated the principal tasks that must be solved subsequently. The speaker subjected to critical consideration other theories of development that developed in parallel with the electrochemical theory, and showed wherein lie the advantages of the latter.

The second paper was given by K. S. Lyalikov on his own behalf and on behalf of V. N. Piskunova, and was devoted to a microphotographic study of the development process in visible rays and in an electron beam; it was established that, in the development of individual microcrystals of silver halide in developers very different in their ability to dissolve silver halide, one and the same picture is observed: the part of the crystal directly adjoining the development center dissolves. This casts doubt on the view that sulfite and other solvents manage, during the normal time of development, to dissolve the crystal to any significant extent. Apparently, development proceeds not at the expense of silver ions in solution, but at the expense of the silver ions of the lattice, and moreover not of interstitial ions, but of the principal ions of the lattice.

A paper on the subject “The mechanism of the catalytic action of development centers” was given by G. P. Faerman. Experiments carried out under his direction by E. D. Voykova showed that, upon introducing into an acidic solution of silver nitrate and a developer salts of metals (silver, copper, mercury), the reduction reaction was sharply accelerated; upon introducing salts possessing electronic conductivity of insoluble salts of heavy metals (silver sulfide, copper and mercury sulfides, vanadium pentoxide, etc.), the reaction was likewise accelerated, but the induction period did not disappear, as in the first case, but only shortened. Salts whose salts did not possess electronic conductivity (silver halides, barium sulfate) exerted no action. All this indicates that electron-conducting particles play the role of transmitters of electrons from the ions of the developing substance to silver ions, i.e., catalyze

oxidation-reduction reactions, including the reaction of photographic development. This work clarifies much in the understanding of the nature of the selectivity of action of photographic developers.

V. I. Sheberstov read a report on the study of the kinetics of photographic development. The speaker subjected to processing extensive experimental material on the kinetics of development by alkaline-sulfite developers. The complete inapplicability of the monomolecular-reaction equation for the analytical expression of the growth of the optical density of blackening with development time was shown, as was the comparatively satisfactory applicability of this equation to the expression of the kinetics of change of the contrast coefficient. Analysis of the kinetics curves of veil formation revealed several types of curves that do not fit the monomolecular-reaction equation. The author gave, in preliminary form, a physicochemical interpretation of the regularities he had observed.

The report by N. N. Shishkina was devoted to considering, from the standpoint of the electrochemical theory of development, the little-studied question of the temperature coefficient of development. Indeed, for two types of developers in the temperature interval \(+10^\circ \mathrm{C}\)—\(+25^\circ \mathrm{C}\) it proved possible to show that, when the equality of the oxidation-reduction potentials of the system is maintained, the results of development turn out to be identical.

S. G. Bogdanov, in the report presented by him jointly with N. V. Polyakova on the significance of the buffering of the developing solution for the development process, illuminated the latter from an entirely new point of view. On convincing experimental material the authors showed that the course of the development process is determined by the composition of the developer not around the emulsion layer, but inside it. Developers identical in the remaining electrochemical properties may give completely different development results depending on the degree of their buffering with respect to each of the components of the solution. Thus, two entirely identical fresh developers possessing one and the same active concentration of hydrogen ions (\(\mathrm{pH}\)) may develop quite differently if they contain different buffer capacities with respect to alkali. This difference is especially significant in strongly illuminated areas of the photographic layer, where, as a result of development, the pH falls particularly sharply. As a result, a decrease in the acid-base buffer capacity leads to the so-called “leveling” of the blackening densities, and, when development is carried to the same value of the contrast coefficient, to an increase in the photosensitivity of the developed photographic layer. The slower the development, the smaller, owing to the diminishing role of diffusion processes, is the significance of differences in buffer capacity. There is no doubt that the practical significance of the views and facts set forth in this report is very great.

In the last report N. M. Zyoskin spoke about the study of the process of dissolution of emulsion crystals during development of the photographic layer. The research method consisted in preparing combined photographic layers consisting of two layers brought into contact and separated at different stages of the development process. It was found that the dissolution of unexposed crystals located adjacent to exposed ones proceeds faster than that of the other unexposed crystals. The same acceleration of crystal dissolution is produced by the deposition of metallic silver in neighboring crystals. The results obtained indicate that the developing photographic layer cannot be regarded as an isolated system of exposed and developing crystals distributed among unexposed crystals.

More than 170 people from various cities of the Soviet Union took part in the Conference. The high level of the Conference was greatly aided by the active participation in it of most of the major figures in domestic scientific photography and the photochemistry of the solid state, including three full members of the Union and republican Academies of Sciences and two corresponding members of the Academy of Sciences of the USSR.

A number of representatives of industry and persons engaged in scientific-applied work in the field of photography took part in the work of the Conference.

At the close of the Conference a resolution was adopted, in which major successes in the development of domestic photographic science were noted, evidence of which was the high level of the Conference that had taken place. At the same time, shortcomings that had occurred in the development of individual areas of scientific photography were pointed out. Special mention was made of the insufficient development of work in the field of the mechanism of optical sensitization and in the field of theoretical interpretation of the phenomena occurring during the photochemical coloration of silver-halide crystals. The resolution noted the most important tasks facing workers in the field of scientific photography in the immediate future.

The proceedings of the Conference will be issued as a separate volume of Advances in Scientific Photography, published by the Academy of Sciences of the USSR.

Yu. N. Gorokhovskii, P. V. Meiklyar