the set of methods used to obtain permanent images of objects and optical signals on photosensitive layers, referred to as emulsions, by fixing photochemical or photophysical changes that occur in the emulsion upon exposure to radiation emitted by or reflected from the object photographed.
The general sequence of operations in photography is independent of the type of emulsion selected and the process used to obtain a stable image. The sequence includes the following stages: (1) distribution of illumination on the surface of the emulsion, corresponding to the image or signal; (2) appearance in the emulsion of chemical or physical changes induced by radiation, which vary in magnitude in different sections of the emulsion and which are uniquely determined by the exposure in each section; (3) intensification of such changes if they are too insignificant for direct perception by the eye or some device; (4) stabilization of the initial or intensified changes, which allows prolonged storage of the images or signal recordings for subsequent examination or analysis; and (5) extraction of data from the image obtained by examination, reading, measurement, and so forth. This general sequence of operations may be supplemented, for example, by the addition of image multiplication, and individual stages may be divided into smaller ones or combined. In general, however, the sequence is used in all photographic processes.
Photography was initially created as a means of preserving portraits and images of nature over much smaller periods of time than those required by an artist. However, as the range of possibilities for photography broadened, the number of problems the new technique resolved also increased, especially after the emergence of motion-picture and color photography. The significance of photography to mankind grew accordingly. In the 20th century photography became one of the most important means of information distribution and documentation through its recording of people and events, and it provided the technological basis for the most popular form of art—film-making. It serves as one of the fundamental technologies used in printing and is an important research tool in many branches of science and technology. This wide range of problems enables us to regard photography as, simultaneously, a branch of science, a technology, and an art.
Regardless of the field of use, photography may be divided into various, more specific categories on the basis of certain criteria: still and dynamic photography (the most important example of the latter is motion-picture photography), depending on the temporal characteristic of the image; (2) silver (more precisely, silver halide) and nonsilver photography, depending on the chemical composition of the emulsion; (3) black-and-white and color photography, depending on the technique’s ability to render only differences in brightness or differences in color as well; (4) amplitude and phase photography, depending on whether differences in brightness in the subject are registered by means of differences in light absorption in the image or differences in the optical path in the image; and (6) two- and three-dimensional photography, depending on the spatial character of the image.
The last division, however, requires a comment. Any photographic image is in itself two-dimensional. The three-dimensional nature of some images, especially those in stereoscopic photography, is achieved by photographing the subject simultaneously from two points close to each other and subsequently viewing the two images simultaneously, each with one eye. Holography may be regarded as a specific type of three-dimensional photography, but the method used in holography to record optical information from the subject and the subject’s spatial characteristics is fundamentally different from that used in ordinary photography. Holography is similar to photography only in that an emulsion is used to register information.
History. The history of photography begins with experiments in which an image of an object was projected and drawn on paper or canvas with the aid of a camera obscura. This investigation began no later than the end of the 15th century; Leonardo da Vinci knew of it and conducted some experiments himself.
Photography in the proper sense of the word emerged only much later, when the photosensitivity of many substances became known and principles were discovered for the use and preservation of the changes evoked in such substances by exposure to light. In the 18th century, silver salts were among the first photosensitive substances to be discovered and studied. In 1802, T. Wedgwood in Great Britain was successful in obtaining an image on a layer of AgNO3, but he was unable to preserve it. The date of the invention of photography is conventionally given as 1839, when L. J. M. Daguerre reported to the Academy of Sciences in Paris on a method of photography, which he named daguerreotypy. Full credit to Daguerre for the invention is questionable, inasmuch as many of the key features of the method were the achievements of J. N. Niépce—either developed by Niépce alone or in collaboration with Daguerre.
Nearly at the same time, W. H. Fox Talbot in Great Britain announced another method of photography—the calotype process—for which a patent was issued in 1841. The similarity between the two methods was limited to the use of AgI as a photosensitive emulsion. The differences were striking: in daguerreotypy a positive, silver, mirror-like image was directly produced, which simplified the process but made it impossible to obtain duplicates; in the calotype process a negative was produced, from which it was possible to obtain any number of positive prints. In this respect the calotype process was more similar to modern photography than daguerreotypy; as in modern photography, it used a development process not only to render a latent photographic image visible but also to intensify the image.
In the subsequent discoveries of fundamental importance to the development of photography, it is necessary to note the transition from use of a camera obscura with a poor-quality lens chosen at random to use of a camera with a special, well-corrected photographic lens, as first designed by the Hungarian optician J. Petzval in 1840. A second transition was from use of a wet emulsion prepared immediately before photographing to use of a dry emulsion prepared in advance; the dry emulsion was capable of prolonged storage in the dark without undergoing any significant changes. In this respect, an important advance was the use of a gelatin emulsion, as first widely practiced by R. Maddox in Great Britain in 1871, to replace the colloidal emulsion.
Another crucial step forward was the introduction of Ag halides other than Agl, which had the advantage of greater practicality. Today, the most common emulsions are dry gelatin layers containing dispersed microcrystals of a silver halide, in which the halogen may be Cl, Br, Cl + Br, Cl + I, Cl + Br + I, or Br + I and in which the Agl content does not exceed several percent. These emulsions, first commercially mass-produced in the mid-1870’s, were initially applied to a glass backing to produce plates; paper and film backings were subsequently introduced.
The mass production of films began approximately 15 years later than that of plates (after the invention of a flexible nitrocellulose backing by the American inventor H. Goodwin in 1887), and films as a photographic material gradually dominated the field. Their use greatly facilitated the design of small cameras, which in time replaced the bulky, plate cameras, except for special reproduction work. By the 1870’s, films constituted approximately 90 percent of all manufactured silver halide emulsions; at the same time, plates accounted for less than 1 percent. Among currently available photographic materials, films are usually negative emulsions (in addition to motion-picture positives and reversal materials), and papers are usually positive emulsions (except for special duplicating papers); plates are exclusively negative emulsions.
The most important event in the development of silver halide photography was the discovery of optical sensitization by the German scientist H. W. Vogel in 1873. Sensitization refers to the broadening of the spectral region of emulsion sensitivity by the introduction of dyes that absorb light at longer wavelengths than do silver halides; the latter only absorb light in the ultraviolet (UV) region and in a part of the shortwave band of the visible spectrum (down to the blue region). The use of spectral sensitizing dyes overcame the major disadvantage of previous emulsions.
In the 1880’s most of the available emulsions were made orthochromatic—sensitive to yellow—and since the 1920’s the most important mass-produced emulsions have been panchromatic materials, which are sensitive to the orange-red region of the spectrum. Later, silver-halide emulsions were invented that were sensitive to wavelengths of 1.2–1.3 micrometers, corresponding to the portion of the infrared (IR) region adjoining the visible region; however, such emulsions were not for amateur photography but for scientific and technical use. Further advances in the sensitization of emulsions to longer wavelengths is not possible, since the equilibrium thermal radiation of surrounding bodies is concentrated just in the IR region. Such radiation continuously acts on the sensitized emulsion over the entire time between manufacture and use; in the first days or even hours of storage, it fogs the emulsion to an unacceptable degree. It is theoretically impossible for any type of photography using silver halide emulsions to overcome this limitation.
However, the sensitivity of silver halide emulsions is not restricted to shorter wavelengths of the visible and near-UV regions already mentioned; the emulsions are also affected by radiation at still shorter wavelengths, including X rays and gamma radiation, and by nuclear particles and electron beams. As a result, silver halide emulsions have long been used to obtain images in X rays and electron beams; they have also become one of the most widely used means of recording and measuring doses of ionizing radiation. Certain types of such radiation, as well as a number of elementary particles, were actually discovered with the aid of silver halide emulsions.
Manufacture of silver halide materials. Dry silver halide emulsions are obtained by coating a backing with a liquid photosensitive emulsion—a suspension of silver halide particles in gelatin or another protective colloid. The most important properties of such emulsions, apart from physicomechanical characteristics and dimensions, are established before the coating process. Most important among them are the parameters associated with the characteristic curve—photographic sensitivity, fogging, and the contrast coefficient—as well as spectral sensitivity and structural characteristics determined by the size of the silver halide micro-crystals.
There are four primary stages in the manufacture of silver halide emulsions. The first stage consists of emulsification and physical ripening, that is, the formation and growth of the solid phase of the emulsion—silver halide microcrystals. The formation of silver halides proceeds from the reaction between AgNO3 and the corresponding halides (usually, of potassium) in a solution containing gelatin, which prevents the microcrystals formed from adhering to each other. At the same time as the formation and growth of microcrystals are occurring in the solution, recrystallization begins, that is, the predominant growth of larger micro-crystals resulting from the dissolution of smaller ones. The presence of gelatin has a substantial effect on the rate and results of recrystallization. Near the end of silver halide formation, recrystallization becomes the predominant process.
The distinct boundary between emulsification and ripening does not always exist, and the separation of the stage into two processes is sometimes purely formal. As a result of both processes, formation of the solid phase is fully completed and none of the subsequent stages exhibits any practical effect on the size of the microcrystals. Consequently, several properties of the finished emulsion, such as grain size and, sometimes, resolution, are established during the first stage. Another important factor in this process is the mass ratio of gelatin to silver halide, which determines light scattering in the emulsion during exposure and edge definition in the image details obtained. At the same time, the sensitometric characteristics of the future finished emulsion layer depend only indirectly on the conditions and results of the first stage. The primary cause of this is the low photographic sensitivity of the microcrystals. When formed without structural defects, the microcrystals exhibit a negligible effect on the photographic sensitivity of the photographic material even after further processing. The sensitometric characteristics are primarily established during subsequent stages; the photographic sensitivity of the emulsion after the first ripening is always low.
The second stage consists of chemical ripening. The emulsion is maintained at an elevated temperature for a specific time, which facilitates the reaction process on the microcrystal surface between the silver halide and the microcomponents of the gelatin, for example, divalent sulfur compounds and reducing agents. These reactions often involve special additives, primarily sulfur compounds (if their content in the gelatin is low), as well as gold salts. As a result of these reactions and of the second ripening in general, impurity centers form on the surfaces of the microcrystals, principally along surface defects. The impurity centers are small particles of substances other than the silver halide, such as sulfides of Ag or Au, combined gold-silver sulfides, and metallic particles of Ag and Au. Mobile photoelectrons are attached to these particles during exposure of the microcrystals, which gives rise to the formation of a latent image. Thus, it is the very presence of impurity centers that essentially determines the ability of the microcrystals to participate further in the photographic process. On the other hand, the nature and size of the impurity centers determine the efficiency of the process, that is, the final photographic sensitivity of the entire emulsion. Impurity centers are sometimes referred to as digestion nuclei or, more appropriately, sensitivity specks.
That the impurity centers are distributed along the microcrystal surface is extremely important; upon subsequent exposure, the centers of the latent image immediately interact with the developers and accept electrons from the developers’ molecules. However, if the second ripening continues for too long a time or at excessively high temperatures, the interactions of gelatin and microcrystals are carried too far; the impurity centers become excessively large and capable of accepting electrons from the developers without the participation of the latent image. Such an emulsion can be reduced in a developer without exposure; in this case the impurity centers are called fog centers. A moderate second ripening is also accompanied by the formation of fog centers but only to a small extent, on a few microcrystals. The second ripening may be regarded as ideal when maximum photographic sensitivity is attained with minimum fog. This condition becomes more difficult to satisfy with an increase in the difference between individual microcrystals. It is here that the role of the preceding, first ripening becomes evident, since it determines the degree of variety in the microcrystals with respect to size and perfection of the crystal structure. The variety of microcrystals, both before and after the second ripening, is also fundamentally determined by the contrast coefficient of the finished emulsion, which, on the average, decreases with an increase in the variety of microcrystals.
The third stage consists of the preparation of the emulsion for coating, during which the sensitizing properties of the finished emulsion become fixed and the emulsion’s basic physicomechanical characteristics are established. In the preparation for coating, numerous additives are introduced into the emulsion. The most important of these additives are spectral sensitizing dyes, which are absorbed on the microcrystals and which broaden the spectral region of emulsion sensitivity; color exposure components (used only in color photographic materials), which are active in the formation of color images; stabilizers, which prevent fogging and changes in photographic sensitivity during storage of prepared emulsions prior to exposure; tanning agents, which increase mechanical strength, elasticity, and the melting point of the gelatin and of the entire emulsion; plasticizers, which reduce the brittleness of the emulsion after tanning; and wetting agents, which improve contact between the emulsion and the backing during the coating process and which make it possible to obtain more uniform emulsions.
In the fourth and final stage, the emulsion is applied in a thin layer (usually 5–15 micrometers) onto a backing. The resulting material is dried and then cut into the required format. The emulsion dimensions are established during this stage, as well as certain other parameters, such as the maximum attainable optical density (darkening) of the emulsion after developing.
Basic types of processing for silver halide emulsions. Until recently, the most widespread method of black-and-white photography using silver halide emulsions has used the separate negative process and positive process, which were realized for the first time in Talbot’s calotype process. In this method, the exposed emulsion is subjected to development, during which only those microcrystals that have been acted upon by exposing radiation (producing a latent image on the microcrystals) are selectively reduced to metallic Ag. In the fixing stage, which follows developing, the unused microcrystals are dissolved and removed from the emulsion, but the metallic Ag of the developed image remains in the gelatin.
The greatest darkening occurs in the emulsion sections with the largest remaining quantity of Ag, that is, in the sections corresponding to the lightest parts of the subject; thus, the distributions of light and dark in the negative image and the subject are opposite to each other. The same process is then repeated on another emulsion using the negative as the subject; after developing, the resulting image gives a distribution of light and dark opposite to that in the negative but correct in relation to the original subject. This is a positive. Transmission of the actual brightness ratio of the sections of the subject to the image (tone reproduction) is not necessarily quantitatively accurate: accuracy of transmission is limited by the nonlinearity of the characteristic curve of the silver halide emulsion and is possible only along that section of the curve associated with exposure latitude.
Since 1950, increasing use has been made of a method that produces a positive black-and-white image directly on a silver halide emulsion, thus eliminating the need for an intermediate negative. This process is known as reversal. In reversal the emulsion is also developed after exposure, but it is not fixed; instead, the metallic Ag in the image is converted into water-soluble compounds. If the Ag formed during the first developing were to be removed and the emulsion then subjected to a second exposure and again developed, the number of developed microcrystals in each section would be in inverse proportion to the number of microcrystals reduced on first exposure and to the amount of exposure from the subject to the corresponding section of the emulsion; it would also be in inverse proportion to the brightness of the represented detail of the subject. Thus, the resulting image is a positive.
Theoretically, a similar processing method can be applied to any emulsion, but adequate tone reproduction is only achieved on special reversal materials. Reversal is most commonly used in the production of photographs in slide or film form for later projection and viewing on a screen. However, the separate negative-positive technique is much more convenient for the production of prints on paper and for the duplication of images.
Another widely used variant of black-and-white photography with silver halide emulsions uses diffusion transfer processing. In the USSR this process is available to amateur photographers in the Moment system. In other countries similar systems are manufactured in several types according to licenses from the Polaroid Corporation, which first developed such cameras.
The system includes a relatively large-format (for example, 9 × 12 cm) roll film camera; a negative silver halide film; a viscous, multipurpose processing solution, which is evenly applied to the film surface when the film is rewound inside the camera immediately after exposure; and a positive receiving layer, which is rolled onto the developed negative layer during rewinding. The processing solution reduces the exposed microcrystals of the negative emulsion, forming an ordinary negative image; it also dissolves the unexposed microcrystals by converting the Ag in them into salts or complexes. It then reduces the bound Ag from the unexposed microcrystals in the corresponding sections of the positive layer after the Ag compounds diffuse there. Moreover, the positive layer need not be photosensitive; in most cases, it is simply a paper layer with an applied coating containing highly dispersed centers for the deposition of Ag from the reduced compounds. Because of the high viscosity of the solution, the process is practically dry, making it possible to obtain a finished, dry print on the receiving layer within one minute after exposure without removing the negative from the camera.
Color photographic processes constitute a special group of processes using silver halide emulsions. Their initial stages are the same as those in black-and-white photography, including the appearance and development of a latent image. However, the material of the final image is not developed silver but a combination of three dyes. The formation and quantity of the dyes in each section of the emulsion are regulated by the developed silver, and the silver itself is subsequently removed from the image. As in black-and-white photography, there are separate negative and positive processes, wherein the positives are printed either on special colored photographic paper (with enlarging) or on film (in contact), and a direct positive process on color reversal materials. A method similar to the diffusion process that produces color images is also widely used.
Nonsilver photography and scientific and technical applications. Materials and processes based on the use of silver halides have many exceptionally valuable characteristics, such as sensitivity to highly varied radiation, the ability to allow exposure effects to accumulate, thus enabling the material to react to extremely weak radiation flux, and the ability to give a geometrically correct reproduction of an image and its details. However, in many of the new trends of applied science and technology, it has gradually become clear that the characteristics of silver halide emulsions and processes theoretically limit the possible applications of photography. For example, with the emergence of holography, the markedly increasing requirements for emulsion resolution (of the order of several thousand mm–1) and for reduced levels of ambient interference have proved to be at the limit of possibilities of silver halide emulsions because of the characteristic and unavoidable discrete structure of such emulsions. As a result, new emulsions, primarily of the macroscopically structureless type (such as spray-coated layers, polymer films, and glasslike substances), have gained widespread use in holography in addition to silver halide emulsions. In planar process, the demands made on emulsion resolution (no lower than 1,000 mm–1) are only slightly less stringent, for example, in the manufacture of microelectronic circuits and optical memory devices for computers and in microfilming with large-scale reduction.
Yet another major disadvantage of silver halide emulsions and processes is the relatively large time interval between exposure of the emulsion and the production of a visible image, even an un-stabilized one; this interval cannot be reduced to less than several seconds given any type of high-speed developing method, even if most other operations are eliminated. Moreover, in computer-based information systems, closed-circuit television systems, holography, and optical image processing, it is becoming increasingly necessary to compute and process the images or signal sequences recorded on the emulsion on a real time scale, that is, in small fractions of a second. Under such conditions any processes on silver halide emulsions are too slow, and the transition to non-silver emulsions becomes inevitable.
The trend to replace silver halide emulsions with nonsilver emulsions is also connected with the increasing cost and difficulty of obtaining expensive materials derived from the earth’s limited silver reserves. This has spurred two research efforts: the immediate orientation toward nonsilver in all the newly emerging fields of application for photography and the search for possible substitutes for silver halide emulsions in the traditional fields of application. There are, however, considerable obstacles to such transitions, because the levels of sensitivity of nonsilver emulsions have not even approached those of silver halide emulsions (at least for negatives) and will hardly do so in the forseeable future. Therefore, the replacement of silver halide emulsions is still impossible for those applications of photography in which only highly sensitive emulsions are suitable, such as professional and amateur motion-picture photography, aerial photography, and astrophotography.
Until the 1950’s, silver halide emulsions were practically the only type of commercially produced emulsions. Applications for other emulsion types were comparatively insignificant, including diazo process emulsions based on diazonium salts and ferrotype and cyanotype emulsions based on trivalent iron compounds (for copying operations), as well as emulsions that undergo physical changes when exposed to light, such as the pigment paper used in printing, which uses hexavalent chromium compounds. The development, application, and commercial production of nonsilver emulsions only began in the 1950’s. During the same period the applications of photography also expanded significantly, so that the new emulsions were used from the very beginning almost exclusively in the newly emerging fields. On the other hand, the production of silver halide emulsions continued expanding with the continued expansion of the traditional fields of application.
Only in one of the traditional fields did nonsilver emulsions prove more or less adequate substitutes for silver halide emulsions—in the large-scale printing of motion-picture films. The vesicular process found application in black-and-white cinematography. In this process, the image is produced by light-diffusing vesicles of gaseous nitrogen, released in the polymer film upon the photochemical decomposition of a photosensitive diazo compound introduced into the film. Although such emulsions exhibit low photographic sensitivity, they make possible a useful reduction in the use of silver halide emulsions in motion-picture photography. Another nonsilver process—hydrotypy—has been used in the printing of color films; in this process, the differences in the effected exposure are transmitted by the differences in height of the hardened gelatin relief on a special emulsion. The relief is then dyed and used as a matrix for the printing of a color-separation image on a nonphotosensitive receiving layer of blank film.
Among the new applications of photography in which nonsilver emulsions are used, reprography was the earliest to develop independently. Reprography combines the copying and duplication of printed, graphic, and typewritten materials (such as texts, documents, and drawings) with the microfilming and microcopying of such materials for archives. The original materials are thus reproduced and greatly reduced for storage in compact form. Reprography makes the most extensive use of nonsilver emulsions. The most widely used reprographic process is electrophotography, in which the emulsion consists of a layer of amorphous selenium, a ZnO layer with a polymer binder, or, in recent years, a layer of the organic semiconductor polyvinyl carbazole. Electrophotography is used exclusively for copying and duplicating operations, accounting for 80 percent of the total volume of such operations.
Together with electrophotography, other nonsilver processes that occupy a special place in copying and duplicating technology include thermography; diazo copying, which uses emulsions containing diazo compounds; the vesicular process, in which the photosensitivity of diazo compounds is also used; and diffusion processes with dye transfer. High-resolution silver halide emulsions were the mainstay of microfilming and microcopying as long as the scale of microreproduction for archives was relatively modest. During the 1970’s there was a rapid growth of microreproduction and a simultaneous gradual transition from silver halide emulsions to diazotype, vesicular, and photochromic emulsions, which still exhibit the low level of light sensitivity characteristic of the nonsilver emulsions mentioned above.
Another new field of application, based exclusively on nonsilver materials and processes, is associated with the combined use of photography and electron-beam devices, primarily in television systems. Here, the image is recorded not as a whole but as a sequence of signals obtained by breaking the image up element by element. The primary types of materials used in recording such signals are deformed polymer layers, on which the recorded electron or light beam creates or alters the surface charge distribution. When the polymer is subsequently softened by being heated, the electrostatic forces that arose during irradiation deform the polymer surface according to the distribution of potential on the surface, thus producing a relief. This relief, which modulates the layer by thickness, is the recording of the image.
The processes used to obtain this type of recording, as well as the form of recording (grooves, holes, and irregular, filigree-like structures), are extremely varied. Two-layer systems consisting of a deformed layer and a photoconductor are now coming into use; they make it possible to combine phase-relief recording with electrophotographic registration. The recorded image is also read element by element, with the relief thickness of the recording modulating the light beam with respect to phase. Thus, this type of photography is classified as phase photography.
Another new field of photography—photolithography— emerged with the development of microelectronics. In this process, which uses both nonsilver photosensitive layers (photoresists) and high-resolution silver halide emulsions, masks are formed and the photoresists are subsequently exposed through the masks. During the last third of the 20th century, this field has also witnessed the gradual replacement of silver halide emulsions with high-resolution nonsilver emulsions. Emulsions based on palladium salts have been proposed, which would be subjected to physical developing with the deposition of nonprecious metals such as copper and nickel. Emulsions consisting of spray-coated layers of lead and thallium halides, molybdenum oxides, and other substances have already been developed.
The rapid development of infrared technology, including the emergence of various IR-emission lasers, has posed the problem of extending the boundaries of photography into the spectral region of longer wavelengths. Since silver halide emulsions are unsuitable, the applications of photography in this field are based exclusively on nonsilver emulsions and processes. One method of photography in the IR region is evaporography, in which the emulsions are thin layers of volatile substances on IR-absorbing blackened backings. Other emulsions that have been developed include layers of cholesteric materials and ferromagnetic films with a band domain structure.
Great possibilities, still not fully realized, are afforded by semiconductor photography based on IR-sensitive narrow-band semiconductors and materials with electron hole junctions and semiconductor heterojunctions. In order to eliminate the effect of scattered thermal radiation from surrounding bodies, these photographic materials are rendered insensible to radiation prior to and after exposure; no recording outside the exposure interval is possible, because the recording of photographic data on such materials requires a closed electrical or electrochemical circuit. The circuit may be closed with the aid of photogenerated current carriers in the semiconductor emulsion, or it may be completed at the necessary moment by the person conducting the recording, coincident with the beginning of the exposure; the circuit is subsequently opened at the end of exposure.
As a method of recording optical information in a binary code, photography is used in optical memory devices for computers. In such applications, silver halide emulsions are not the optimum formulation for either long-term or, in particular, working storage. Their shortcomings include limited data capacity (recording density per unit area of emulsion), slow processing rate, which delays access to the data, and the impossibility of erasing the recorded data after use and of reusing the emulsion. As a result, computer memory devices have been equipped with photo-chromic emulsions that reversibly change their spectral absorption region upon exposure; that is, they become photochemically colored. The most useful emulsions of this type are organic dyes of the spiropyran class, but inorganic photochromic emulsions made from a number of alkali-halide salts (KCI and others) are also being used. Because of their lack of structure, such emulsions have exceptionally high resolution and, consequently, a large data capacity. The short duration of the photochemical dyeing process ensures the necessary speed of response, and the reversibility of dyeing makes it possible—by thermal or optical action—to erase the recording with adequate speed and reuse the emulsion.
The above list of photographic methods does not exhaust the existing types of nonsilver emulsions, processes, and applications, although it does give a general picture of how far photography has advanced from its original forms. Despite the extremely rapid growth in the number of types and applications of nonsilver photography, scientific and technical photography based on silver halide emulsions has still retained its importance, and its applications are constantly expanding.
New applications of silver halide photography include the study of high-temperature plasmas; the study of the movement of bodies at supersonic speeds in aerodynamics and ballistics; the study of shock waves (in particular, in explosion and detonation); studies of planetary surfaces atmospheres, and radiation with terrestrial devices and spacecraft; the study of nuclear radiation and nuclear reactions; and the study of production processes and the working of mechanisms in chemical and mechanical equipment. In most cases, dynamic photography is used in such research, either to produce a series of successive images of a subject, usually over very short time intervals (down to 10–9 second), or to provide a continuous recording of images. Continuous readings are obtained by means of optical scanning, in which the changes in blackening along the length of the film contain information on the development of the process with time. Static photography has also enjoyed widespread use, particularly in the study of biological and geological objects. Dynamic photography is also used for biological objects, primarily in the form of time-lapse filming for slowly occurring changes.
In the extraterrestrial study of astrophysical processes, photography has become more widely used in the far-UV spectral region, even up to the boundary of soft X radiation. For this purpose it has been necessary to develop special emulsions that contain a silver halide as the sensitive element but that are almost or entirely free of gelatin, beause the gelatin would completely impede radiation in the desired portion of the spectrum. Photography has also retained its importance in such traditional fields as astronomy and astrophotometry. Electronic cameras have been widely used to provide a sharp increase in sensitivity to the light flux from very weak stars; such cameras use silver halide emulsions with some sort of electronic image amplifier, such as an electrooptical transducer. Photographic methods are also used in facsimile communication and in a large number of other processes in highly diverse fields of science and technology.
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photography - the act of taking and printing photographs