Share this article with friends
by Boris SHAPIRO, Dr. Sc. (Chem.), President, Russian Union of Scientific and Applied Photography
Now, at the turn of the century, it would be no exaggeration to say that most, if not all of us, are shearing a common temptation of taking a daring look into the future. Into our common future in general and into the future of our personal pursuits and occupations. For me, personally, the question is what will happen to the technology and art of photography after 160 years of its development. In order to answer such questions one should, of course, try and trace the main stages of this progress so as to make some viable assumptions for at least the next 3 to 4 decades.
The prominent Russia geneticist, Nikolai Timofeev-Resovsky (1900-1981) once said: "Why make it simple, if you can make it more complex?". What he had in mind, of course, was the process of natural evolution, but in my own interpretation this striking wisdom can be applied to the progress of photography. And, come to think of it, this analogy is really not surprising because photography is a human product and as most things human, it has been evolving in the direction of growing system complexity. Having said that, let us take a closer look at this situation on the example of the two basic "components" of photography: light-sensitive elements-micro-crystals of silver halides (AgHal) and organic chemistry.
The development of above monocrystals started from random experiments in which they were produced in random forms and combinations. These results were obtained by lone-wolf enthusiasts and kept in secret. Microcrystals of the last generation are anisotropic structures of very complicated nature and unusual shape, and the men who produced them deserve credit as microarchitects. As it was, the initial particles of homogeneous silver halide participles, of cubic, spheric or octahedral shape, were replaced, first, with similar ones, but of onion shape (with layers of different properties), and then with microcrystals having epitaxial(*) nodes on their apex and ribs and, later still, with flat (or tabular) ones.
The development of what we call silver halide photography is a
* Epitaxy-oriented growth of one monocrystal on the surface of another (substrate).- Ed.
vivid example of the fact that a system's perfection is not always best. The ideal lattice ofAgHal is useless from the point of view of a light-sensitive detector. Only the defects-internal and surface ones-bring into play the unique properties of this material. That is why the whole history of evolution of photo materials centers on the desire to "spoil" their crystals. Without the formation of surface defects in the form of what are called "impurity centers" (3-4 molecules of silver sul-fide) it would be simply impossible to produce highly light-sensitive photographic layers. The maximum distortions of the AgHal lattice were achieved in the case of tabular crystals.
Figuratively speaking, they long seemed to be a kind of mutants, and it was only in 1994 that US scientists came to the conclusion that their mysterious growth is not so much a crystallochemical as a colloidal process. In the process of synthesis of monocrystals-because of the fluctuation of additives (potassium bromide and silver nitrate)-there appear in the system oppositely charged particles of AgHal. These opposite charges of colloidal particles provide for their adhesion and the formation, as a result, of extremely defective crystalline structures with surfaces of adhesion. The excessive energy of the latter dictates the further anisotropic growth of monocrystals.
Later on flat AgHal monocrystal provided the basis for the development of crystal structures with lateral shells of different halide composition. Such structures are a vivid example of an extremely "spoiled" crystal lattice. The most aesthetically perfect architecture from the above crystals is the one with inner hollows (hollow crystal) synthesized in 1992 by Dr. M. Irving of Great Britain. He used the epitaxial growth of flat monocrystals of silver bromide (iodide) on the faces of adherent bipyramidal AgJ nuclei of tetrahedral symmetry. What was produced as a result is a hollow polyhedral structure.
Naturally enough, the microarchitecture of crystals has not been what the researchers were looking for. A complex organization of microcrystals makes it possible to control the process of separation of electrons and holes(*)-these charge carriers generated by light-and thus optimize the process of formation of centers of latent photographic image. And that means that the disorder produced in the AgHal lattice is not chaotic, but strictly organized. Complex microcrystals of our photo films are, essentially, physical solid-state devices for the processing of optical data.
In terms of crystallography, the most unusual ones are the absolutely new silver bromide structures which we have obtained jointly with Alexander Popoiznikov (Cand. Sc., Chem.) in 1989 at the State R&D Institute of Chemi-co-Photographic Industry. These are polyphorous microcrystals with different numbers of beams (or phores in Greek) fanning out from tops of the seed particles. These can be produced by using appropriate organic compounds, like bromoacetic acid, as the
* Hole (quantum physics)-quantum state not occupied by electron.-Ed.
source of bromide ions. Depending on synthesis conditions, octaphores-structures with eight beams emanating from the eight tips of the seed cube-can be produced; hexaphores-with 6 beams from the vertices of an octahedron, and even more complex structures, like didodecaphores with 24 beams from the vertices of a cube with truncated corners. Some such formations are shaped like snowflakes, but not as flat.
The main feature of these crystals is their intricate surface which absorbs much more light than the traditional volumetric microcrystals. In structures of this kind the beams act like light-collecting antennas. In their appearance polyphores approach the dendrite structures of green plants and look rather like cactuses or conifer needles. The complex shapes of the latter serve but one obvious objective-to have a maximum surface area to absorb as much as possible solar energy for further growth and development. In the case of silver halides, which are not living matter, there is no such obvious objective, though AgBr is also among light-sensitive systems. And their lattice provides for a maximum energy gain in the formation of complex shapes with a minimum surface.
Now, let us turn to the second integral part of the photo process-organic chemistry. At the initial stages it was simply ignored, for photography used to be a realm of inorganic chemistry only. It was only in the 1850s that photographers turned to organic substances and biopolymers (albumins, nitrocellulose, gelatin) in the form of binding film-forming compounds.
With the subsequent improvement of materials and, especially, with the transition to color photography what we call products of fine organic synthesis began to acquire greater, and sometimes decisive, importance. The main characteristics of photographic films are the value and spectral distribution of photosensitivity, structural-and sharpness-properties, shelf life, color reproduction, physico-mechanical parameters, processing speed, sharpness and stability of the image. All of these parameters depend on the structure of the special organic substances used in film manufacture and processing, such as color components, chemical and spectral sensitizers (more about it below), modifiers of monocrystal growth, stabilizing and anti-hazing pigments, developers and toners.
Color films we use today are a very complex chemical product. Its negative can be described as a bank of substances obtained by means of fine organic synthesis. As for the processes involved in its manufacture and subsequent processing, they are out of the ordinary to such an extent that one can speak of a new area of science-organic photochemistry-the latest stage of development of photography.
The evolution from the black-and-white to color photography is vivid proof of the growing complexity of the system. This involves, in the first place, what I would call a spatial separation of the photochemical (photographing) and the subsequent chemical (development) stages. In the classical black-and-white process the effect of light and image formation are combined, or superposed in space. In the modern color version using hydrophobic color-generating components light and image are separated: the light impacts AgHal microcrystals and the image in the form of a dye is formed elsewhere-in a micro-drop of an organic solvent containing the given component. This separation of the two stages in space and time makes it possible to regulate image formation in a more flexible and versatile way. But the main advantage of such organization is the possibility of self- or autoregulation of the process, since the sensor (AgHal microcrystal) is separated from the receptor (drop of solvent with the color-forming component), but with all that there is a possibility of a "backfeed" between them.
What is its role in the processes under consideration? Let us look at this from the angle of the central problem of photography- photosensitivity. Here the basic question is the regulation of development or boosting of the latent image. The problem consists in the priority identification of the useful signal, which determines the sensitivity of the photo material, from "noise" which produces fogging or hazing (grayish image). To solve the problem it is necessary first to sharply accelerate the development of latent image centers (3-4 silver atoms) and, once the speed
is sufficient for reaching the required photographic parameters, to activate a braking mechanism preventing hazing or fogging. And it is in the color process with its more complex organization that such regulation becomes possible.
At the present time it is performed with the help of DAR and DIR components. The former (DAR-development accelerator releasing agent) boosts the rate of growth of the optical density of the image. Without going into details, let me just say that more of the quinonediimines-a product of oxidation of the developer substance-is produced and more of the image pigment as a result.
If the DAR action is not stopped in time, the resulting image intensification will be accompanied by increased fogging. To prevent that a DIR component (development inhibitor releasing agent) is added to the drop of color-forming material. And due to the feedback, the more intense is the development (i.e. the more of the quinonediimine is formed), the stronger is the retardation which means that the automatic regulation of the process is intensified.
Photographic intensification control has already made it possible to boost the sensitivity of negative color films to ISO 1,000 and ISO 3,200 values (ISO- international unit of sensitivity).
One should also mention one remarkable property of color photography resulting from its growing complexity. I have in mind a new phase separation interface between water solution and drops of hydrophobic organic solvent in which the chromatogenic, or color-forming, component is dissolved. In a rough simile one can say that the process of color photography is organized at the "cell" level, like the most intricate processes in living nature, because dye formation occurs in separate microdrops. And what is the effect of this interface? Long ago, the formation of such an interface between the protoplasm and the environment led to the appearance of a living cell. The interfaces increase above all the number of the possible versions of chemical processes (catalytic ones in the main). This cellular organization of the photographic process also opens up new opportunities, for example, in twin-cascade (stage) amplification which we shall be discussing later. So what we are dealing with are two kinds of sensitization: chemical-by enhancing photo-sensitivity of photo materials-and spectral-expanding the same characteristic over the whole range of the visible and invisible spectrum. Silver halide itself is light-sensitive in the ultraviolet and blue bands of the spectrum. To broaden this range up to the entire visible and near infrared bands organic chromatogenic substances-spectral sensitizers-are used. In a similar process during photosynthesis the natural chlorophyll pigment is used which, as had been demonstrated by the French physicist Alexandre E. Becquerel (1820-1891), is a spectrum stabilizer also for AgHal. In the latter case, however,
polymethine pigments proved most suitable.
The first stage of sensitization in both processes occurs from a photoexcitation stage of a molecule in about 10'11 s. This, on the one hand, provides for "energy saving" in the photochemical process, and on the other-reduces the probability of any side reaction, that is provides for a greater selectivity of the process. It is also important that the first stage is a phototransition of electrons from a photoexcited pigment to the chemical substance not exposed to light-another common aspect of these processes.
For a very short time the photoexcited state of the pigment puts stringent demands on the process of effective space separation of photo-generated charges-electrons and holes. This is achieved, above all, through the formation of pigment aggregates, containing two or more molecules in which the primary separation of electrons and holes takes place. But the main feature of the process consists in the strict organization of the subsequent charge separation.
In perfecting photosynthesis Mother Nature has put in a lot of efforts into a very complex structure which provides for the maximum use of the absorbed quantum light in chemical reactions. Similar charge separation occurs in the spectral sensitization of silver halides thanks to their unique properties. A light quantum is absorbed by the pigment and a photoelectron passes from it into the AgHal crystal, producing a latent image center. In this way the required effect is achieved. In photographic materials with microcrystals of a traditional structure, like in photosynthesis, this parameter approaches 100 percent.
The organization of charge separation processes in spectral sensitization largely depends on defects in the AgHal lattice. As has been noted, the evolution of microcrystalls proceeds in the direction of their "degradation". The exceptional "defectiveness" of tabular crystals, especially those with lateral shells, obstructs
aggregation and often the absorption of cyanide pigments. Spectral sensitization of these has been so far of sporadic and random nature which makes it not effective enough. One of the reasons for that is the presence upon the defective surface of micro-crystals of a large number of electron-acceptor impurity centers which act as "collecting points" for pigment aggregations. As a result there is an increased probability of recombinations of the photoelectrons and holes formed in the photoexcited aggregate at the impurity centers. This is the price that has to be paid for the defects of microcrystals. With the growing defectiveness of the AgHal crystal lattice the problem of optimal regulation of surface absorption and pigment aggregation regulation gains ever greater importance with the aim of effective separation of photocharges in spectral sensitization. Purposeful regulation of the packing of pigment molecules into aggregations upon the actual surface of AgHal microcrystals is, in our view, the substance of new promising developments in the spectral sensitization of new photographic materials.
And that means that with the progress of organic chemistry, photographic processes look more and more like those occurring in living nature.
Yet it is beyond the scope of this article to discuss ways of producing even more complicated film structures and their processing. Suffice it to stress that in the development of photography we have passed from single-layer black-and-white films to multilayer color ones, with the number of the basic, or main, and auxiliary layers growing by 15 to 18 and more times. Present-day chemico-photographic processing of layers includes high-temperature boosted processes effected in an automatic mode.
Thus, making the system more complex is really the basic law of
perfecting of what we call the photographic process. The main development trend for the future consists in boosting the data-storage capacity of photo materials without impairing their sensitivity. What I would call the main paradigm of photography consists in maintaining the right ratio between the sensitivity and resolution of photo materials. In order to enhance their data storage capacity we need higher resolution which is linked with a reduced size ofAgHal monocrystals and-consequently-of photo-sensitivity. Thus the central problem which has been on the agenda over the 160 years of development of photography, and which will remain so also in the next century, is boosting the photosensitivity of AgHal monocrystals. Having said that, what are the likely solutions of this problem we can anticipate?
The first one consists in reducing the number of quanta impacting the microcrystal and generating latent image centers. At the present time this parameter amounts to 10- 20 quanta on the average. This value can obviously be improved by 2 to 3 times through better organization of electron-hole and ionic processes. And that calls for an improved microcrystal architecture and better processes of chemical and spectrum sensitization on their surface.
The second possibility consists in using the oxidation energy of the holes. What we have in mind are the photo holes of AgHal and of spectrum sensitizer. So far there has prevailed what I would call a "prohibitive" principle of combatting holes capable of recombining with photoelectrons. But they can be used for a release of the chemical energy stored in the defective, or flawed, AgHal microcrystals. As a result of oxidation of certain compounds, such as sub-centers of the latent image (silver molecules), there can occur a subsequent decay of cation radicals of these compounds, bound by interconnecting electrons, with injection of an extra electron into AgHal. This can be visualized as a kind of latensification(*). According to estimates, the above techniques can boost the photosensitivity of silver halide materials by about an order of magnitude.
One should dwell in particular on ways of boosting, or amplification of latent images. If we assume that a certain catalyst can split off from the color-forming component- one capable of selective initiation of oxidation of the developer substance down to quinonediimine on the surface of a drop-then we can expect to have much greater optical image densities. And the oxidizer will not be silver halide, but some other electron acceptor introduced into the system which, in itself (because of kinetic reasons) can not oxidize the developer substance. Thus what we are dealing with in this case will be two-stage catalytic amplification of the primary photochemical effect on the system. And it is this principle that is used in visual reception which is much
* Intensification-intensification of a latent photographic image by chemical treatment or exposure to light of low intensity-Ed.
more sensitive that the photomaterials.
Taking a comparative look at the growth of light sensitivity of photomaterials since the start of photography, one will see that during the first one hundred years it increased by about 10 thousand times and by 100 times more over the past 50 years. Overall, by one million times. Portraiture on colloidal films in the middle of the past century lasted for minutes in order to obtain a clear image. At the present level of sensitivity one can take sharp pictures of a short-distance runner. The sensitivity of color films today exceeds that of black-and-white films.
The growth of photosensitivity in the last century went through two distinct jumps, whereas today it has acquired a monotonic nature. The jumps were associated with the introduction of new processes and phenomena. The first was linked with the transition from the daguerreotype to a "wet" colloidal process with the development (amplification) of the latent image. The second jump coincided with the discovery in 1873 by the German scientist H. Vogel of spectral sensitization and the use of gelatin. The more gradual growth of photosensitivity at the start of the 20th century to this day must be linked with the development of the earlier discovered methods of chemical sensitization, improvements in the structure of AgHal emulsion micro-crystals and their spectral sensitization.
Extrapolating the photosensitivity curve into the next century, we can expect photo materials of ISO 10,000 and 20,000 by the year 2040. Possibly some new phenomena will be involved into the process of photography, such as twin-stage amplification which can lead to a new-third-jump in photosensitivity. Now, let me give one example.
Used in photography at the present time is what we call single-cascade magnification in the development of AgHal micro-crystals. The amplification factor at the first catalytic stage is about 109 (one photon amounts to about 109 silver atoms). If we assume that this value at the second stage will be within lOMO4, then the photosensitivity of our photographic materials can be increased by as many times also. With the initial sensitivity of 100 GOST (Russian) units the second stage of amplification can boost it up to 100 thousand or one million GOST units, which approaches the sensitivity of the eye. An analysis of the photographic process in terms of the signal-to-noise ratio indicates that for a practically acceptable grain-size of the image (about 30, which means that on a 13 by 18 cm photo grains will not be visible to the eye) such parameters are quite within our reach. What is more, prognostications have been published in the literature, though without mention of concrete ways of reaching high sensitivity, of the following schedule for the development of such materials: 100,000 ISO-by the year 2010, 5 min. ISO-by 2040. These, however, are nothing but expectations.
There remains the very important question of the interplay of silver halide and electronic photography. In other words-does chemical photography have a future, or will it perish under the wheels of a "digital express"?
Electronic photography on the basis of photosensitive light receptors is a rapidly developing area. Considerable progress has been achieved in recent years in boosting the data-storage capacity of digital matrixes containing up to 16xl06 picture elements, or pixels. So far the quality of images produced by electronic cameras is inferior to photographic ones. The main problem here is a high cost of equipment which runs into thousands of dollars. This being so, this kind of equipment can
only be afforded in professional photography, studios and printing shops.
What is more, electronic cameras require additional memory for digital image storage. In this respect photo films are truly versatile being "receivers" of images and the storage thereof-having "super-high" storage capacity. A negative frame of a standard film is not just a sensor of some 20 mm. pixels (100 ISO), but a "data" storage of 60 megabytes available at a price of about 10 cents only. A 24-36 color film of 24 frames can store about one gigabyte of data. That being so, one is bound to be skeptical about claims of electronic photography fully replacing the traditional one in the future.
The way we see it, chemical photography is a product of natural evolution. But the question is whether or not it has a future? Is there a future for chemical processes in general, including biological ones, or shall science fiction robots squeeze out the living beings? Looking back at the evolution of life, computers were invented by "chemical" humans. As for myself, I have no doubts whatever that computers will, first, never replace humans and, second, that the "chemical" man has far from being at the end of his tether. With the addition of yet another "computer" hand be can think up many new things.
And one also gets the impression that the aforesaid also applies to "chemical" photography. Its potential is far from being exhausted yet. Electronic devices have but only one carrier of "life"-the electrons. In the chemical processes there are whole numbers of such carriers, including, in addition to electrons, also ions and molecules. And that means a whole range of development prospects.
And there is one more thing. Photographic processes are being constantly developed and improved. So far chemical photography has not yet made full use of the basic principles created by nature over the centuries of evolution and consisting of the "cellular" organization, multistage amplification and chemical programming of processes. There can be no doubt that the use of principles selected through natural evolution will give new impetus to the development of photography in a not too distant future.
Like books and paintings, photographic images will turn increasingly into works of art-the highest concentrations of the human thought and emotions per square centimeter of data carrier.
Looking at some photo we often say-what a fine picture! This admiration, however, is somewhat misplaced because what we really admire is not the picture, but the people and/or nature it portrays. And we are very lucky indeed to have the medium of photography capable of capturing this beauty!
Permanent link to this publication:
LRussia LWorld Y G