Photography revealed: the principles of development

Photography revealed: the principles of development Jacqueline Belloni-Cofler, Jean Amblard, Jean-Louis Marignier, and Mehran Mostafavi

Although modern photography depends on a host of technical advances, two are of outstanding importance. One was the transparent negative, making it possible to make an unlimited number of copies from a single original. The other was the process of development, which in effect enormously magnifies the initial effect of incident light, making possible very short exposure times. Despite its crucial importance, however, it is only comparatively recently that the physico-chemical basis has been fully understood.

Photography is 150 years old, and ever since the first black-and-white pictures were obtained, with fairly rudimentary equipment, the technique has been con- tinuously advancing. Manufacturers vie with one another in developing more and more sophisticated equipment, and yet more sensitive films. Who would imagine, therefore, that the theory of photography still presents problems?

Jacqueline Belloni-Cofler, D&S

Was born in 1935 and after doina research in the Laboratoire Curie, Radium Institute, Uni- versity of Paris, joined the CNRS in 1961. In 1971 she joined the Laboratoire de Physico- Chimie des Rayonnements, University of Paris- Sud, Orsay, of which she is now Director. She has a particular interest in radiation and aggregates chemistry.

J. Amblard, D&S

Was born in 1942 and in 1966 ioined the Ecole Superieure de Physique et Chimie Industrielle, before moving to CNRS. In 1984 he joined Dr Belloni-Cofler’s team at Orsay to study the early stages of metal growth under radiolytic conditions, and their applications for electrode mod- ification

Jean-Louis Marignier, D&S

Was born in 1953 and was awarded his doctorate in 1979 by Paris-Sud University, Orsay, working first on the radiolysis of ligand sul- phide compounds and then on the dynamics of anion solvation in the Nuclear Research Centre, Saclay. He joined CNRS in 1981, latterly with a special interest in photochemical aspects of the origin of photography; in particular, reconstitu- tion of Niepce’s original photographic process in 1824.

Mehran Mostafavi, D&S

Was born in Iran in 1962 and awarded his doctorate by Paris-Sud University in 1989 for research on the principles of photographic development. He subsequently joined CNRS, working on the dynamics of aggregation of metal clusters in solution at the microscopic level.

Euro-Article (see page ii) This article is Dublished in association with La Lechekhe, France. Translated by Storm Dunlop.

Endaavour, New Series, Volume 15, No. 1, 1991. 01~9327/91$3.00 + 0.00. @ 1991. Pergamon Press pk. Printed in Great Britain.

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Development, the chemical process that produces such an enormous amplification of the effects produced by light, was one such problem. We can now say ‘was’, because recent work has finally lifted the veil of mystery that long en- shrouded the process.

For thousands of years people were obsessed by the same dream: how to record permanently the fleeting image of scenes that had inspired them. Evi- dence for this is provided by the works of artists on the walls of caves and tombs, on pottery and, more recently, on canvas. This is why photography, invented by N. Niepce, aroused an en- thusiastic response that has never waned: it put the power of capturing ephemeral scenes within everyone’s grasp. With a total of some 40000 million photographs taken every year (not counting tine film), the photographic industry is one of the most flourishing in the world. It makes every effort to achieve two goals: a constantly increasing image quality, and greater and greater simplicity of operation that will nevertheless ensure successful photographs under any conditions. This simplicity is only apparent: it is in reality the result of extremely sophisticated technology, which is improving all the time.

What is the actual basis of photography? Fundamentally, it utilizes the transformation caused in a substance - known as a photosensitive material - by the influence of light. The image obtained reflects the different levels of illumination in the original scene. This technique would probably have remained without much future if the Frenchman, Louis Daguerre, had not discovered, 150 years ago, that the change could be amplified to a very considerable degree by the process of development. Paradoxically, despite being the key process in photography, development has remained an empirical technique, and has not been satisfac- torily explained until now. It was nevertheless the technological key, because

thanks to the development process and to the exceptional progress that has subsequently been made by the various photographic companies, very little light is required to activate a modern emulsion. This amplification of the image, which completes the process begun by the photons of light, also allowed cinematography to be introduced. It was equally used to control the deposi- tion of dyes that became the basis for colour photography.

A decisive historical step: development The first photographic discovery dates back to the Frenchman, Nicephore NiCpce [l], who succeeded, in 1816, in obtaining an image by combining a ‘camera obscura’ - known since antiqui- ty - and a chemical process capable of retaining the impression left by light. Widely used by landscape painters from the time of the Renaissance onwards, the camera obscura used a small aper- ture to cast the image of an object onto a flat surface at the rear of a darkened chamber. All that had to be discovered was the best chemical process. NiCpce first tried photosensitive silver salts, but was unable to prevent the image from darkening completely. After having searched for a process where some irreversible change took place, he found, around 1826, that bitumen of Judaea - a heavy hydrocarbon - har- dened under the action of light. We are now able to understand the process that took place. The light caused new links to be formed between the hydrocarbon chains, creating a crosslinked network, so that the solubility of the exposed material was much reduced. NiCpce had only to dissolve the unexposed bitumen in a mixture of lavender oil and light petroleum, and the image was fixed. This process, called ‘heliography’ by Niepce, suffered from a particular dis- advantage, however: the exposure- times required [2] were very long, often a few days.

Research towards a faster process

however. He introduced true photographic development, using mercury vapour which acted on iodized silver plates that had been exposed to very little light. The immense significance of this discovery lay in the reduction of exposure times in light to just a few minutes. This advance indicated the real potential for photography, and we have lately celebrated the 150th anniversary of Francois Arago’s announcement of the combined work by NiCpce and Daguerre. At the same time, and completely independently, William Henry Fox Talbot, in Britain, and HippoIyte Bayard, in France, per- fected the development with gallic acid of negative images on paper that had been impregnated with silver salts. They thus opened the way to the repro- duction of multiple positive images from a single negative one. Develop- ment consists of an enormous amplification of the direct effects of light by a subsequent chemical reaction.

Before going into the details of the development process, we need to summarize, very broadly, the principles behind the modern photographic process. The first stage is to take the picture with a camera, and the mechanism by which this occurs is explained very well by the model by R. W. Gurney and N. F. Mott [3] (figure la). Photons of light reflected from the object hit the film. The latter consists of an inert plastic material covered with a layer of gelatine containing a suspension of crystals of silver bromide, which project through the surface. As has recently been emphasized [4], silver bromide possesses a range of properties (spectral sensitivity range, photoelectron life- time, etc.), which cause it to be a unique material for this application. The dimensions of the silver bromide crystals has to meet two specific require- ments. The desire for high definition in the image means that small crystals should be used; on the other hand, a large surface area means that larger crystals are better at intercepting light, giving greater sensitivity. Many modern emulsions contain crystals that are very flat (tabular crystals), which represents a good compromise.’ The action of the photons causes the bromine ions (Br-) to yield electrons, which neutralize neighbouring silver ions (Ag+). The gelatine does not act just as a cement between the mosaic of crystals. It also indirectly helps to maintain the efficien- cy of the incident light by reacting with the bromine atoms and preventing them from recapturing the electrons trapped by the silver ions. The atoms of silver thus produced cluster together. De- pending on the intensity of the illumina- ‘tion, the crystals receive greater or lesser numbers of photons. After the exposure, each crystal therefore contains several silver atoms (in the brightly

Figure 1 Silver-based photography consists of several distinct steps. During the exposure (a) the photons of visible light excite an isolated crystal of silver bromide held in the gelatine on the film. The electrons of the bromide ions Br- are photodetached and neutralize the silver Ag+ ions. The atoms of silver thus formed cluster together. Depending on the intensity of the exposure lb), the crystals receive a greater or lesser number of photons. After the exposure, they therefore contain several silver atoms in each crystal (in bright areas); no atoms (in the darkest areas); or varying numbers in areas of the image that contain a lot of fine detail. The overall distribution forms the latent image, which is far too weak to be visible to the eye. Development (c) consists of completing the conversion of all the Ag+ ions into metallic silver within any crystal where a sufficiently large cluster of atoms has been created by the light. This is carried out by a chemical agent, the developer, which is an electron donor. The number of atoms inside each of the exposed crystals, which was in single figures in the latent image, increases to several billion. The whole mass of minute particles of silver appears black to the eye. The fixer (d), dissolves and washes away the unexposed crystals that are otherwise still photosensitive. The image thus obtained is a negative of the original scene: the black silver crystals correspond to bright areas. (Photo: Roger Viollet)

took a decisive step forward thanks to another Frenchman, Louis Daguerre, who was an artist and also painted scenery for theatres. In 1829 he joined forces with Niepce, and in 1839 succeeded in obtaining images using silver iodide. The surface of a plate of silver was subjected to the action of iodine

vapour and thus became covered in silver iodide, which served as the sensitive layer. The unexposed areas were subsequently dissolved by a solution of sodium chloride, thus fixing the image.

Daguerre eventually made a far greater contribution to photography,

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number of atoms in the cluster

Figure 2 Development essentially consists of a transfer of electrons from the developer to crystals of silver bromide that already contain clusters of sufficient size. This transfer is possible only when the (variable) potential of the cluster exceeds the (fixed) potential of the developer.

The diagram shows how the electrochemical (or redox) potential of silver clusters varies. This potential -which is proportional to the difference in the energy required to detach an electron, relative to that needed to release one from the standard hydrogen electrode that is chosen as a reference - increases with the number of atoms in the cluster. In a vacuum, on the other hand, the potential decreases as the number of atoms increases. It should be noted that in the latter case the potential measured is an ionization potential, an expression of the energy required to extract an electron from the cluster and raise it to the vacuum energy level. The ionization potential and the electrochemical potential are linked by a simple function and both vary in the same sense. It is observed that the stability of an aggregate with respect to the loss of an electron changes with an opposite trend in the gas and in the solution. This difference in behaviour explains the failure of the first attempts to understand the mechanism of development from the behaviour of clusters in vacua.

illuminated areas), no silver atoms (in the very darkest areas), or varying numbers of atoms (in the intermediate areas of the image). The overall distribution constitutes the latent image, which is far too weak to be visible to the eye (figure lb).

Before the discovery of the development process, the silver image was obtained by direct blackening produced by the action of light alone. This de- manded a prolonged exposure, which had to last until a sufficient number of photons had accumulated to completely convert the silver halogen crystals in the illuminated areas of the photosensitive layer into black crystals of silver. This explains the prolonged exposures originally required. The success of modern photography lies in the replacement of this single long process by two distinct stages: the exposure, which is often very short (a fraction of a second), and the development, which, in a separate step, changes the invisible latent image into a visible one. In fact, development consists of converting all the Ag+ ions into metal in those crystals where clusters of sufficient size have been formed by the action of light during the exposure. Crystals that are too little exposed, or

4

unexposed, remain intact (figure lc). This conversion is effected by a chemical agent that is an electron donor, the developer, which has the task of multi- plying the few atoms that are the result of the exposure into several billion - which is the number of Ag+ ions in a small crystal in an emulsion. The gain in sensitivity offered by development was crucial in making possible very short exposures and the introduction of cinematography.

Development is, therefore, the first process that the film undergoes in the laboratory, in the dark. The second process is fixation, which eliminates the undeveloped crystals that are still photosensitive by dissolving and washing them away. The image thus obtained is a negative version of the imprint left by the light: the minute crystals of silver, which appear black to the eye, correspond to areas that were brightly illuminated (figure 1). A positive image results when a second photograph is made either by direct contact or by projection enlargement of the negative on to sensitive paper, which is then treated as just described.

The chemical basis for the considerable gain obtained by development lies

in the way in which the developer acts as an electron donor, and the way in which the Ag+ cations act as receptors. But not all the silver bromide crystals in the emulsion are equally blackened: a fortunate distinction protects the crystals that have not been exposed to light, or have been only slightly exposed, from any alteration, thus preserving the information content of the image. How can this be explained? Why is it that only crystals containing clusters that are larger than a certain critical number of atoms are able to accept electrons from the developer and thus darken? This is the secret of development that has defied our understanding for such a long time.

Attempts to understand the nature of development Over more than a century, numerous theoretical models [5] have been proposed to explain how the clusters of silver atoms created by the light act as nuclei. In general, the transfer of electrons effected by the light, and that of electrons donated by the developer, have been assumed to obey the same laws. The oldest models suggested that the first silver atoms remained dispersed within the silver bromide. The next phase could grow only beyond a certain threshold value of supersaturation. Above this threshold, the nucleus that acted as catalyst to development was able to form. In this type of model the cluster of silver atoms was assumed to have the same properties as the metal in massive form. This positively contra- dicted chemical knowledge: massive silver is a noble metal, and resists attack by weak oxidizing agents. Yet in an exposed, but undeveloped film, the clusters of a few silver atoms are very sensitive to the action of various compounds, especially weak oxidizing agents. We must therefore assume that when silver is in an ultrafine form - that is, when it consists of clusters of just a few tens of atoms - it possesses properties different from those of the massive metal.

The subsequent group of models [6] is based on a concept that first appeared some 15 years ago: the thermodynamic properties of a metallic cluster vary with the number (n) of atoms that it contains. The theory is that an isolated atom, or a few atoms linked together as in a molecule, possess distinct electron levels. This is opposite to what occurs in a macroscopic metallic crystal, where the quasi free electron levels are structurated in bands. This is known as a quantum-size effect, which is one of the significant concepts in modern research into materials in an ultrafine state. It enables us to explain, for example, how the transition from a non- metallic to a metallic state occurs at a specific number of atoms in clusters of

mercury atoms in a gaseous state. Under these conditions, the number of atoms contained within a cluster has a strong influence on the ionization potential of clusters of silver [7] (figure 2) or of copper [8]. The Coulomb attraction between the electron and the positively charged cluster undergoes discontinuities caused by the layered electron structures as well as fluctua- tions corresponding to the changes in the numerical parity of the electrons: odd-numbered clusters are more stable against the loss of an electron than even-numbered ones. The properties of this ultrafine material - somewhere ‘between atoms and crystals’ - and the quantum effects that depend on size [9], have aroused increasing theoretical and experimental interest, particularly as it concerns the gaseous phase, over the last 15 years.

The mechanism of photographic development thus essentially rests on the transfer of electrons to a nucleus that contains 12 atoms of silver, and which is positively charged by the surrounding silver cations. It is, therefore, important to know the tendency that this nucleus has to capture electrons as a function of n. As the American theoretician J. F. Hamilton emphasized in 1977 [lO], the transfer of electrons from the developer should become more and more difficult as n increases, judging by the properties measured in the gaseous phase. We may add that this knowledge of clusters in the gaseous phase cannot account for the existence of a critical size, despite this being well known to every practis- ing photographer. In addition an isolated atom is more stable in a vacuum than any cluster. The transfer of electrons from the developer to an Ag+ ion to produce an atom of silver ought to be spontaneous, but this certainly does not occur in photographic development. These ‘atomistic’ theories were, therefore, unable to explain the growth of the nucleus and hence the development mechanism. All these theories were based on data concerning free clusters in the gaseous phase. This ignores the fact that photographic development brings an emulsion containing solid crystals into contact with an aqueous solution of an electron-donor - the developing agent. The transfer of electrons from the developer does not occur under the same conditions as those under which electrons are produced by the action of light. It was, therefore, essential to understand the role of the aqueous solvent in this exchange.

For a long time, our group has spe- cialized in studying the interaction of ionizing radiation with condensed chemical media. There is a similarity between the action of such radiation on silver salts and that of light. We may recall that Henri Becquerel discovered radioactivity in 1895 because his photo-

graphic plates were fogged by radiation emitted from a piece of uranium that was placed in the same drawer. In trying to understand the growth of clusters created by radiation, it was therefore natural that we should be interested also in the models suggested by scien- tists working on photographic theory.

On the other hand, ever since 1973 we have known that copper clusters in a quasi-atomic state are capable of being oxidized in solution [ 111, whereas massive copper is not oxidized under the same conditions. Because of the analogous behaviour of clusters in photographic development and in our experiments, we slowly came to suspect that in both cases the mechanism was governed by the variation in properties with the number of atoms involved. This is why our team decided to try to simulate the stage of photographic development that occurs in solution. Thanks to ultra-rapid techniques, we have been able to observe the growth, through the action of a developer, of silver clusters surrounded by Ag+ ions. The method consists of following the course of reactions by recording the very rapid changes of colour that occur (figure 3). There are changes in the spectra of both the silver clusters, and of the developer, which is also of a chosen colour [8].

As this method relies on the absorption of light, we avoided employing a system that was itself photosensitive. (Photographers take the same sort of precautions in the darkroom when they examine paper prints under faint red light, to which papers have a very low sensitivity). We therefore chose silver sulphate, which is sensitive only to high- energy radiation, such as X-rays, gam- ma-rays, or beams of particles. In this process the atoms of silver are created in the same way as in the exposure of radiographic film. This technique, used in conjunction with high-speed, time- resolved spectrophotometry, is known as pulse radiolysis. It allows the indi- vidual steps in a wide range of chemical mechanisms to be determined [12]. For example, a pulse of radiation causes the formation of isolated atoms of silver in a very short time, from Ag+ cations (figure 3) and this step does not inter- fere with subsequent reactions. It is analogous to the exposure stage in photography, except for the fact that the atoms and electrons are free to diffuse through the solution, rather than being confined to the silver bromide crystals suspended in the gelatine.

This method reveals the very rapid steps occurring in the process: formation of the first atom of silver, association reactions of the atoms with excess Ag+ ions, and repeated dimerization of the small clusters until they coalesce into stable aggregates (figure 3). The nature of the reactions, together with their absolute rate constants, may be

determined with this method from ex- amination of the kinetics in the evolu- tion of the optical absorption spectrum. Depending on the time that has elapsed since the pulse, the solution contains isolated atoms, atoms combined with silver ions, or larger and larger clusters also combined with ions, causing them to have a positive charge. It is possible to study the reactivity of these different species, despite their ephemeral nature, by adding the desired reactant before- hand to the solution to be irradiated. The method also allows one to choose the conditions for intervening at the desired stage of clustering. The different sizes of clusters that coexist in the latent photographic image are spread over time, and are, in fact, identified by the elapsed time and by their spectra.

The team led by A. Henglein [15], at the Hahn-Meitner Institute in Berlin, showed in 1978 that silver in atomic form is a powerful electron donor, whereas silver in the massive form cannot be oxidized. These results con- firmed our own findings about copper [ll], namely that the thermodynamic properties of a chemical element may depend on its clustering state. Con- versely, in the simulation of the photographic process that we were hoping to achieve [8], this role of electron donor would have to be played by the reactant analogous to the developer. The silver ions combined with silver clusters would be now electron acceptors, as in development. Donor in certain circumst- ances and acceptor in others: the exchange of electrons depends on the respective tendency to capture electrons of the various systems present. This tendency is measured by the electrochemical potential, which may be negative or positive (zero for the standard hydrogen electrode). For our experiments we chose the molecule of sulphonatop- ropylviologene (SPV) as the developing reactant. When the pulse occurs, trans- forming the Ag+ ion into an atom of silver, this molecule is transformed into the SPV-, electron-donor ion, which is blue in colour. Let us now compare the potentials of the various systems that are present. The potential of the isolated Ag+ ion, which is very reluctant to become a silver atom, is strongly negative, - 1.8 V [15]. That of an Ag+ ion on a massive silver electrode, on the other hand, is + 0.8 V and has a strong tendency to capture an electron. The potential of the SPVISPV- pair is intermediate in value at -0.4 V. The transfer of electrons from SPV- to an Ag+ ion on a silver cluster is therefore only possible if the potential of the cluster is greater than -0.4 V.

The developer (whose potential is fixed) thus differentially develops clusters of different dimensions (whose potential varies with size). The experiment showed us, in effect, that the

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Figure 4 The model provided by pulse radiolysis has led to a new interpretation of the mechanism of development. The black points within the bromide crystals represent atoms of silver created by the action of light. The electrochemical potential of these clusters varies as a function of the number of atoms n that they contain. On the other hand, the potential of the developer, an electron donor, has a constant value. It is only thermodynamically possible to transfer electrons from the developer to clusters whose electrochemical potential is greater than that of the developer. Transfer therefore occurs only above a certain critical size and it subsequently becomes more and more favoured. Conversely, if the developer were to be replaced by an electron acceptor, the smallest clusters would behave as electron donors. The transfer of electrons from the clusters to the oxidant would cause the progressive corrosion of the silver nucleus until the latter was completely dissolved, as happens when the latent image is destroyed.

SPV- species, originally formed at the same time as the isolated atoms of silver, does not undergo an immediate reaction. A certain lapse of time occurs during which the charged clusters are still very small and thus have a lower tendency to capture electrons than the developer itself. After this critical time (the induction time), which corresponds to the growth of the clusters above a minimum size, the difference between the potential of the clusters and that of the developer is reversed and transfer can begin. Detailed kinetic study of this induction time shows that SPV- becomes an electron donor as soon as a cluster contains five atoms of silver sharing one positive charge. From this we may con- clude that the potential of the cluster is about -0.4 V. Subsequently, capture of electrons on to this nucleus, alternating with the accretion of new Ag+ ions, causes the cluster to grow provided the species Ag+ or SPV- are not ex- hausted. The system behaves exactly like a collection of nuclei that grows with a velocity proportional to the

CKD 15-1-R

abundance of the SPV- donor. This is known as autocatalyzed growth.

By changing the nature of the electron-donor developer, and thus its electrochemical potential, we found that the critical size of the nucleus also altered. The potential of the cluster of this size was determined as previously described. For the Cu+ ion (Cu2+/Cu+ potential = 0.16 V), transfer begins when eleven silver atoms shared one positive charge [16]. For the Ni+ ion (Ni’+/Ni+ = -1.5 V), transfer begins when two silver atoms carried a positive charge [S]. Empirically, photographers increase the number of darkened crystals (and ultimately the contrast) by modifying the strength of the developer.

A new interpretation of the photographic development process The various aspects of development revealed by these kinetic studies of solu- tions [8] correspond with a number of characteristics known empirically to photographers. These include growth

by an auto-catalytic chain reaction; critical size required for a nucleus to grow (recently observed directly thanks to a method of grafting clusters of known size on to an emulsion [17]); the suscep- tibility of sub-critical clusters to oxida- tion; and the variation in the critical size in direct proportion to the electrochemical potential of the developer.

In fact, the conditions are very similar in both situations. In the pulsed experiment, the creation of developable silver nuclei and their development occur suc- cessively in the same system, so that the very rapid initiation of development can be better observed. In photography, on the other hand, these two stages are separated in time. The other difference is that, in our experiment, the silver atoms created by the radiation are able to diffuse freely throughout the solution. Development takes place on a free silver nucleus, which is positively charged by Ag+ cations and is accompa- nied by sulphate anions. In photography, however, the silver atoms remain trapped within a crystal of silver bromide that is surrounded by gelatine. Development therefore takes place at the interface between the silver- bromide crystal and the developing solution. But it should be noted that such a developer generally contains a crystal solvent designed to facilitate the release of Ag+ ions - this stage being known as ‘physical development’ - which means that overall the two situations are very similar. Apart from these small differences, the resemblance is therefore sufficiently close for one to be able to assume that the conclusions about the growth of nuclei in solution can be applied to photographic development (figure 4).

Our team’s explanation of the development mechanism therefore runs as follows. The latent image, produced by the action of the light, consists of clusters of silver atoms, each within a crystal of silver bromide. The numbers of atoms in these clusters vary (being zero, sub-critical, or super-critical). When in contact with a solution, the electrochemical potential of these clusters together with the neighbouring Ag+ ions - this potential is a measure of their tendency to attract an electron - directly increases with the number of atoms. If the solution contains a developer (an electron donor), transfer can occur only towards clusters that contain more atoms than a certain threshold number. This threshold occurs when the potentials of the developer and of the cluster are equal (figure 4). In effect, therefore, the whole emulsion consists of a myriad of microscopic electrical cells, of which one pole (the solution of developer) is fixed, and the other (the population of crystals of silver bromide that have been more or less exposed to light) is variable. The passage of electrical current

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Figure 5 The three-layer emulsions for colour photography contain silver bromide associated with a coupler which is a precursor of the dye (a). At the development stage (b), the developer discriminates the supercritical clusters as for black-and- white photography. Its oxidized form reacts with the associated coupler to generate the appropriate dye, as in this example of a negative picture. During fixing, the undeveloped crystals are eliminated, together with the metallic silver that would otherwise form a grey veil.

requires a positive difference in potential. The discrimination shown by the developer is therefore the consequence of a quantum effect caused by size, whereby the electrochemical potential of the silver nucleus (or its ionization potential) increases with the cluster number. With each new electron transfer, the cluster, which is surrounded by Ag+ ions, increases by one unit and the potential becomes more and more favourable for transfer. The developer being present in excess, all the silver ions in a crystal are therefore suc- cessively neutralized to exhaustion, provided that the crystal originally contained a nucleus that was of super-critical size. It is important to note that there are no grounds for be- lieving that from this point onwards the speed of development increases with the size of the nucleus: according to our results the speed is zero below the critical size, and constant above it.

It will also be seen that for a given emulsion, it is possible to vary the development threshold and thus the contrast by altering the choice of developer and its electrochemical potential. The greater the (negative) value of this potential, the smaller will be the critical size of the clusters. This will accentuate

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the bright areas at the expense of the grey tones. When the object is only weakly illuminated, it is possible to resort to a two-stage process: the actual exposure, which may be preceded or followed by a second, weak, even (i.e., neutral grey) exposure of the whole negative. This second exposure creates a few atoms of silver in all the crystals, but these are insufficient in themselves to initiate development. When these atoms are added to the atoms created by the actual exposure, however, the size of certain clusters crosses the critical threshold.

If an oxidant is added to the solution instead of a developer, the inverse transfer process can take place: clusters whose electrochemical potential is less than the potential of the oxidant lose an electron. Such a loss lowers the potential of the cluster, causing it to become even more susceptible to reaction. No- thing can stop this process, which con- tinues until the clusters with strongly negative potentials are completely dissolved. This explains why the latent image is so prone to degradation by even mild oxidants. Such fragility (re- gression of the latent image) is known to occur also in nuclear emulsions designed to detect high-energy radiation

[18]. Unlike the latent image, a true image consists of a dense clump of silver crystals, which are naturally minute but which contain sufficient atoms for them to behave as a metal that cannot be oxidized, just like massive silver. This is why a silver photograph resists aging so well.

Ever since the first results were achieved by Daguerre, this key stage in the photographic process, like all the others, has benefited from innumerable empirical improvements. Various additives that have been combined with the emulsion or the developer have achieved better control of the process. It is quite possible that some of these additives influence the interrelationship between the potentials that we have discussed above. Using the pulse technique, it has thus become possible to observe directly their role in the transfer of electrons.

With its extraordinary amplification factor, development has enabled considerable gain in sensitivity to be achieved. It opened the way for the great technological advances in photography that have been achieved. Expo- sure times have certainly been reduced by several orders of magnitude, an important advance in itself, but also one that gave rise to the cinema. After all, tine-photography is merely a flood of images, each of which is static, but which were all obtained by a succession of extremely short exposures at very short intervals. This enormous gain in sensitivity has led, without any great sacrifice in performance, to the manu- facture of emulsions with finer and finer grain. The result has been a definition which still amazes our eyes, which have perhaps become somewhat jaded by the positive bombardment by images that they undergo. How many people real- ize, for example, that an ordinary photographic emulsion in everyday use contains such a large number of microscopic, juxtaposed crystals of silver bromide (three million per square cen- timetre) that the entire information content of a television image can be stored, without loss, in just five square milhmetres of silver emulsion? Such a high definition enables large images to be projected on to enormous screens, and also the enlargement of scientific images so that infinitesimal details become visible. Finally, thanks to the oxidized state that arises in the developer after it has reacted with exposed silver crystals, coloured molecules can be synthesized, thus producing the basis of still or tine photography in colour.

Photography in colour Colour photography is founded on the trichrome theory proposed in 1802 by Thomas Young. According to this theory, three fundamental colours - red, green, and blue -suffice for the eye

to reproduce all other colours. The action of light in these three colours is combined in the retina, and the process is therefore known as ‘additive’. In 18.59, the physicist James Clerk Max- well demonstrated the physical princi- ple of trichromatic vision by succeeding in obtaining the first additive colour synthesis. The method of applying trichrome theory to photography was proposed in 1869, simultaneously, but independently, by Charles Cros, who merely established the principles, and by Louis Ducos du Hauron. In addition, the latter was the first to obtain actual colour photographs.

Initially, the visible radiation coming from the object to be reproduced is separated into the three bands: red, green, and blue. In practice, this separation is achieved nowadays by the use of three superimposed photosensitive layers coated on to a transparent base, and separated by filters (top left of figure 5a).

These three layers are capable of being excited selectively by photons of the three basic colours. In intimate association with each crystal of silver bromide inside each capsule of gelatine in this multi-layer emulsion, is a coupler. This serves as a reservoir of specific molecules that are destined to form a given colour. For example, an acetyl- acetone derivative is used as a coupler in order to produce azomethine, which gives a yellow image.

Figure 5b explains the various phenomena that occur in the colour negative-positive development process: the chromogenic developer (i.e., the developer that creates the colour, here p-dialkyl-amino-aniline) transfers its electrons to the exposed crystals of silver bromide containing an aggregate that exceed the critical level (as for the black-and-white photographic process that is shown in figure 4), and it becomes oxidized. Unlike the black-and-white case, this oxidized form, quinone diimine, reacts with the couplers, creating the cyan (blue-green), magenta (purple) and yellow dyes, which are complementary colours to red, green, and blue. The negative colour image is produced in this manner and the pro-

cess naturally has to be repeated to obtain a positive. Finally, fixing eliminates non-reduced, silver bromide crystals as well as the crystals of metallic silver that would otherwise form a grey veil.

Our new understanding of the development mechanism will undoubtedly be applied to practical photographic processes. In addition, it will cast light on the phenomena of nucleation and growth of crystals, as well as on the mechanisms underlying the transfer of electrons that are catalyzed by metals in the ultrafine state.

In the course of 150 years, taking photographs has become an extremely commonplace process, but it has still not lost its magic. We may appropriate- ly end by quoting the poet Charles Cros, the inspired inventor of the paleophone or phonograph and, in 1869, with Louis Ducos du Hauron, of colour photography:

‘J’ai voulu que les tons, la grbce, Tout ce que reflkte une glace, L’ivresse d’un bal d’opCra Les soirs de rubis, I’ombre verte Se tixent sur la plaque inerte Je l’ai voulu, cela sera’.

(I wanted the tones and the graceful effect,

Everything that a mirror can reflect, The headiness of an opera ball, The ruby red colour of evening, and the

green shade, To be fixed by a lifeless plate. I wanted it, and yes, shall see it all.)

References [II

PI

[31

[41

Jay, P. ‘NiCpce, ge&e d’une inven- tion’, Ed. Amis du Mu&e Nitpce, Cha- Ion-sur-Sabne, 1988. Marignier, J. L. Nature, Lond., 376, 115, 1990. Gurney, R. W. and Mott, N. F. Proc. ROY. Sot. London. Sect. A. 164. 485. 19j8.

I I

Materials Research Society Bulletin, 14, No. 5, 1989.

[S] Moisar, E. and Granzer, F. Photogr. Sci. Enp.. 26. 1. 1982: Moisar. E. in ‘Contri&ion of clusters physics to material science and technology’, Dave- nas, J. and Rabette, P.M. (eds), NATO

ASI Series E, Applied Sciences no. 104, p. 311. Nijohff, 1986.

(61 Baetzold, R. C., ibid, p. 195. 171 Morse, M. D. Chem. Rev., 86, 1046, . _

1986. [8] Mostafavi, M. etal. Z. Phys. D-Atoms,

Molecules and Clusters. 12. 31. 1989: Mostafavi, M. et al., R&at. Phys: Chem., 34, 605, 1989.

I!?] Cyrot-Lackmann, F. (ed.), ‘Entre l’atome et le crystal: les agrtgats’, Edi- tions de Physique, 1981.

[lo] Hamilton, J. F. in ‘Growth and properties of metal clusters’, Bourdon, J. (ed.), p. 289, Elsevier, Amsterdam 1980.

Ill] Delcourt, M. 0. and Belloni, J. Radio&em. Radioanal. Left., 13, 329, 1973.

[12] Jonah, C. D. in ‘Chemical reactivity in liquids. Fundamental aspects’, Moreau, M. and Turq, P. (eds), Plenum Press, New York, 1988.

1131 Baxendale. J. H. ef al. in ‘Pulse radiolv- , sis’, p. 207, Academic Press, London, 1965.

1141

iI51

WI

(171

1181

Von Pukies, J. et al. Ber. Bunsenges. Phys. Chem., 72, 842, 1968. Tausch-Treml, R. et al. Ber. Bunsenges. Phys. Chem., 82, 1335, 1978. Henglein, A. and Tausch-Treml, R. J. Coil. Interf. Sci.. 80, 84, 1981. Fayet, P. et al. Phys. Rev. Left., 55, 3002, 1985. Haissinsky, M. ‘La chimie nuckaire et ses applications’, Masson, Paris, 1957.

Bibliography 1.

2.

7.

Daguerre, L. ‘Histoire et description des pro&d& du daguerrotype’, ed. A. Giroux, 1839 or ed. Rumeur des Ages, 1982.

Glafkidks, P. ‘Chimie et physique photo- graphiques’, 5th edn, Editions de I’Usine Nouvelle, 1987.

Henglein, A. Current Chem., 143, 113, 1987.

Hamilton, J. F. Adv. in Physics, 37, 359, 1988.

Schumacher, E. Chimia, 42, 357, 1988. Physics Today, 42, No. 9, September 1989, with several, very comprehensive articles about the unique properties of silver halides. Belloni, J. et al. Magie de I’image, ‘La photographie r&&e’, a video film by Universitt Paris-Sud - CNRS, 1989. The film exists in the languages: French, English, German, Spanish, Portuguese, Japanese, Russian.

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Photography revealed: the principles of development