Chemistry Of Photography

Type of physical science: Chemistry

Field of study: Chemical methods

The process of photography depends on reactions occurring in two separate steps. First, an image is formed by the ability of certain substances to react to the presence of electromagnetic radiation. Second, further chemical reactions are needed to make this image permanent.

Overview

Photography uses chemicals sensitive to electromagnetic radiation to capture an image, or picture, of an object. Most types of photography use visible light as a radiation source.

Different types of electromagnetic radiation are defined by their wavelengths, measured in angstroms. Visible light ranges between 4,000 angstroms (producing a blue or purple color) and 7,000 angstroms (red). Radiation of shorter wavelengths (ultraviolet light, X rays) and longer wavelengths (infrared radiation) can also be used in specialized types of photography.

Chemicals used to capture an image must be able to respond in some way to electromagnetic radiation. Many such chemicals exist, but most are impractical for one reason or another. For example, melanin, the pigment in skin, changes color but very slowly. The most reliable compounds yet discovered are salts of silver. These salts are naturally pale in color, but many darken rapidly when exposed to light. Many silver salts are known, but the ones used most commonly in photography are silver bromide, silver iodide, and silver chloride. Silver bromide is the most widely used salt, often in combination with a small amount of silver iodide. Used alone, these salts will produce a black and white image. Color photography requires the addition of combinations of dyes. The sensitivity of these silver halides seems to extend the entire length of the electromagnetic spectrum, so they can be used in ultraviolet and X-ray photography as well.

Modern photography involves a two-step process. First, an image is captured on photographic film, a transparent support made of glass or clear plastic. Later, this image can be enlarged and transferred to photographic paper. The following description applies to the production of an image on film.

Photographic film is coated with gelatin, which serves as a matrix within which silver bromide crystals are suspended. Silver bromide crystals are formed as a precipitate when potassium bromide and silver nitrate are mixed in a solution. Photographic film is made by allowing this reaction to occur in the presence of warm, semiliquid gelatin with an excess of potassium bromide. The gelatin prevents the silver bromide crystals from coagulating, and the suspension can be spread over any desired surface while still liquid. This suspension is commonly called a photographic emulsion, although it is not technically an emulsion. When allowed to cool, the suspension will harden rapidly. Other chemicals, such as alum, tannin, and formalin, can be added to the mixture to change the temperature at which the gelatin solution will harden.

Exposure to differing intensities of light will cause an invisible "latent image" to form within the photographic emulsion. The best existing explanation as to how this image is formed was first advanced by R. W. Gurney and Neville Francis Mott, in 1938. According to the Gurney-Mott theory, tiny specks of pure metallic silver can be found within every group of silver halide crystals. When light strikes silver bromide, chloride, or iodide crystals, they release electrons, which are attracted to the metallic silver specks. This causes the silver to become negatively charged. These charged silver specks (called sensitivity specks) attract positively charged silver ions from nearby crystals, causing the silver specks to increase in size. Leftover halogen ions (bromine, chlorine, or iodine) move to the outer surface of the crystal and are absorbed by the gelatin matrix. The sensitivity speck continues to grow as long as exposure to light continues and some silver halide remains. An image will not be made visible until the next step is performed, and the photographic emulsion is exposed to a chemical reducing agent found in photographic developers.

The process of developing causes the latent image to appear on the negative. This step must be performed in the dark to prevent further reaction of the silver halides. Typically, the film or other material is placed into a light-protected tank or box. First, a solution containing a developing agent is added for a specific period of time. The action of the developer is stopped by the addition of a water rinse. The image is then made permanent, or "fixed," by the addition of a solution containing a fixing agent. At this point, the image may safely be exposed to light.

Additional washing and drying steps may follow.

The developer solution has four main ingredients: the activator, the developing agent, a restrainer, and a preservative. The activator is usually an alkali added to the formula to allow the developing agent to work properly. Common activators include sodium carbonate, sodium hydroxide, and borax, as well as proprietary compounds produced by different companies. The developing agent does the actual work, donating ions to the exposed silver halides, reducing them to metallic silver. Unexposed salts remain unaffected. Some examples of developing agents include Metol (sometimes called Elon), hydroquinone, amidol, catechol, and glycin.

The restrainer is added to protect unexposed silver halides from the effects of developing agents. Without restrainers, a smear, or "fog," may appear over the image. Potassium bromide is an effective restrainer, but is also often a by-product of the developing process, so it may often slow the pace of development. Other chemicals such as benzotriazole also have a restraining effect.

Developing agents are often very reactive with oxygen, producing unwanted products that may color or cloud the developing solution. Preservatives keep the developer clear by reacting with the oxidation products and rendering them harmless. Many developers include sodium sulfite as a preservative.

The chemicals used in the developing process must be dissolved in a fluid. Distilled water (water from which most of the salts have been removed) is the solvent of choice.

Occasionally, very concentrated solutions may contain additional organic solvents such as diethylene glycol.

Exposure of the silver halides to developer must be carefully timed. Too long an exposure will result in the reduction of all silver halides present, regardless of whether they have reacted to light. The amount of time required for developing depends on the distribution of silver halide in the emulsion, on the particular chemicals used in the developer, and on other factors, including temperature. With commercial preparation, development usually requires between five and fifteen minutes. The action of the developer can be stopped by rapid rinsing with water.

Once an image has been developed, there will be residual silver halide in the emulsion.

These salts are still reactive to light, and must be removed if the image is to be maintained. The fixation step, which follows, acts to remove the remaining silver halides or to convert them to stable compounds that will not react with light. The typical fixation solution, or "fixing bath," contains either sodium or ammonium thiosulfate. These chemicals dissolve the silver halide and cause complexes to form in which the positively charged silver attaches to the thiosulfate anion and the negatively charged halide attaches to the sodium or ammonium anion. These new complexes are soluble in water and are easily rinsed away. Fixing baths often contain additional ingredients, including weak acids and gelatin-hardening agents. The acids, such as acetic acid or solutions of sodium sulfate or sodium sulfite, serve to neutralize any remaining alkali and ensure that the developing process is stopped. To protect the image, agents can be added that will harden the remaining emulsion in which the metallic silver is suspended. For gelatin, the usual emulsion ingredient, the most popular hardening chemical is alum, usually in the form of potassium alum.

The last step is a simple rinse in clear water, to remove the last remaining chemicals and to dilute the acids from the fixing bath.

Once photographic film has been developed, it can safely be exposed to light. In fact, light is necessary to transfer the image preserved on the film to another surface. This process is called making a print. Multiple prints can be made from a single piece of film. Usually, the image is transferred to photographic paper, but any surface that has been treated with a photographic emulsion will do.

Many types of photographic papers are available. Papers are classified by the amount of contrast they will give, by their weight, by their surface type (glossy or matte), by their speed (time for developing), and by their suitability for color or for black-and-white film. Once paper has been exposed to light, developing the image follows most of the same steps as film development, although different combinations of chemicals may be used.

Color photography is similar in principle to black-and-white photography, except that layers of dyes are needed to filter and select primary colors from white light. Color films have three separate layers, which respond in turn to blue, green, or yellow light. The dyes in each layer respond by dying everything else with a complementary color. For example, the blue layer reacts by staining all images yellow that do not contain blue light. A blue sky (which is blue) or a white cloud (white light contains all colors) would not have a yellow stain. On the other hand, the yellow layer would stain all nonyellow-containing objects blue. In this layer, the blue sky would be blue and the white cloud would still be white. The green layer responds by producing a red dye. Other combinations of colors are produced by the relative responses of the layers of dye.

Applications

The basic principles of photography can be applied to produce many different types of images. Different steps of the process can be altered to produce photographs of very small or very distant images, to enhance resolution, or to record wavelengths of light not visible to the human eye.

In photomicroscopy, a camera replaces the eyepiece of a microscope. The applications of such an apparatus are nearly unlimited. The shapes of microscopic crystal can be studied, computer chips can be examined for flaws, and fingerprints can be compared for identical features. In fact, forensic science, which involves the examination of physical evidence of a crime, has made great use of photomicroscopy to examine ashes from arson investigations or soil types from the tires of cars involved in kidnapping cases.

On the other end of the scale, cameras are also attached to telescopes. By allowing film to be exposed for long periods of time, it is possible to identify objects too dim to be seen by the human eye. To avoid the problems caused by sunlight and distortion from the atmosphere (such as a cloudy night), many satellites are equipped with phototelescopes.

Some objects are best examined using wavelengths of light that are not visible to the human eye. For example, ultraviolet wavelengths are very short, have little penetrating power, and tend to be sharply reflected. The ability of an object to absorb or reflect ultraviolet light may give important clues to its physical properties. Some objects will give off visible light in response to exposure to ultraviolet light. This phenomenon is called fluorescence. Fluorescent light can be captured by color film and normal lens systems, but ultraviolet photography usually uses black-and-white film and special quartz lenses (since normal glass will absorb ultraviolet radiation).

X rays have wavelengths even shorter than those of ultraviolet radiation. Normal photographic emulsions are almost transparent to X rays, so X-ray film will typically have a very dense packing of silver halide crystals. In X-ray photography (or radiography), the specimen is placed between the film and a special X-ray tube; wherever the specimen has absorbed the radiation, nothing will pass through to expose the film. Radiography is used in the health fields and also in industry, to examine metals for cracks and flaws. The study of photographic patterns of diffracted X-rays aimed at crystals of DNA (deoxyribonucleic acid) was used by James D.

Watson and Francis Crick in 1953 to help them determine the structure of the genetic material.

Infrared radiation includes wavelengths that are much longer than those normally visible (yet one can sense some infrared radiation as heat). Infrared photography is often used in the life sciences.

For example, the movement of animals at night can easily be recorded, since animals generate more heat than do plants. In medicine, infrared photography has many applications, including studies of patterns of blood circulation and detection of some tumor types (tumors generate more heat than normal tissues). Infrared photography is also used in hydrological surveys, in which the distribution of water is mapped. Damp areas show up darker (colder) than dry areas, so aerial surveys can be done mapping muddy or swampy areas, or marking the extent of tidal flow.

Another example of the use of cameras and aerial surveys is seen in the science of photogrammetry, in which photographs are used to analyze the dimensions of some object. Most commonly, this object is the earth's surface, and the photographs are used to make topographic maps. Other applications are also possible, including estimating the volume of timber in a given stand of trees or mapping the extent of ruins at a site of archaeological interest.

Context

Modern photography owes its existence to two separate events: namely, the invention of the camera and the discovery of the light-absorbing properties of the silver halides. The modern camera evolved from the camera obscura, the principles of which were first discovered in ancient Greece. The camera obscura (or "dark chamber") consists of a darkened room or chamber with a single, tiny hole in one wall through which light may enter. On the wall opposite the hole, the light forms an upside-down image of whatever lies outside. This device was used by many great artists, including Leonardo da Vinci. The images formed, however, could be preserved only by tracing.

A German physicist, Johann Heinrich Schulze, first established the light sensitivity of silver salts in 1727. He even produced rough images by coating the inside of glass bottles with his silver halide mixture, then covering the bottles with stencils and exposing them to light.

Negative images of the stencils appeared on the surface of the bottles, but only for a short period of time.

One of the first people to attempt to preserve an image with silver compounds was Englishman Thomas Wedgwood, working in the early 1800's. He used paper soaked with a solution of silver nitrate, covered it with a painted piece of glass, and exposed it to the sunlight.

Unfortunately, without fixation, light of any kind soon destroyed his work.

Others who contributed to the early history of photography included two Frenchmen, Nicephore Niepce and Jacques Daguerre. Niepce was an amateur inventor who first used a light-sensitive form of asphalt to preserve images captured by a camera obscura. One of his first photographs, a view of his courtyard, took eight hours to expose and is still in existence today.

Daguerre was a professional scene painter who worked in Paris and who also had an interest in using light to preserve images. Upon learning of Niepce's work, he wrote to him and proposed a partnership, which continued until Niepce's death in 1833. In 1835, Daguerre developed a new technique using plates coated with iodized silver whose light-induced images would appear after exposure to mercury vapor. Two years later, he found that the images could be made permanent after treatment with a sodium chloride solution. This process he named the "daguerreotype," and its popularity spread rapidly around the world.

The title "father of modern photography" might well be claimed by the English scientist William Henry Fox Talbot. Talbot was a contemporary of Daguerre who independently worked out a technique for preserving the images of the camera obscura. Talbot produced light-sensitive paper by soaking it in alternate baths of sodium chloride and silver nitrate. He also had problems with the permanence of his images until another scientist, Sir John Frederick William Herschel, suggested a fixation technique employing sodium thiosulfate. Talbot labeled his process "photogenic drawing."

Talbot contributed to photography in additional ways. By using gallic acid in the development process, Talbot found that he could reduce the necessary time for light exposure to around one minute. Talbot was also one of the first to use photography for scientific purposes. In 1839, he recorded an image from a microscope at low magnification.

Throughout the last half of the nineteenth century, photographic techniques continued to improve. Uses for photography other than portraiture and journalism continued to grow.

Wilhelm Conrad Rontgen discovered X rays in 1895, and the first X-ray photograph (of the bones of a hand) was published in the same year. Photography was also used to study motion, using series of sequential still photographs. Eadweard Muybridge used multiple cameras at different positions to illustrate the movement of humans and many animals. His published photographs of the different gaits of the horse caused quite a stir, since they contradicted "common knowledge" of how those actions occurred.

Photography in the twentieth century continued to improve and to diversify. Color photography; "instant," or Polaroid, photography, and disposable cameras have all made photography more versatile and more accessible. Photography also evolved into the "motion pictures," which employed multiple images, captured over a short period of time and displayed very rapidly. Other applications of the photographic process await only the imagination of scientists and artists.

Principal terms

DEVELOPER: a chemical solution that will allow a latent image, formed within a photographic emulsion; to become visible

FIXING BATH: a solution containing chemicals that will dissolve any silver halides that have not been acted upon by a developer

LATENT IMAGE: an invisible image formed within a photographic emulsion when the latter is exposed to varying intensities of light

NEGATIVE: an image that appears to be reversed in tone compared to the real subject; a negative image on a transparent surface is used as a source for a positive print

PROCESSING: the series of steps by which a latent image is converted to a permanent, visible image

SILVER HALIDES: ionic compounds formed from silver and alkaline salts of halogens (bromine, chlorine, fluorine, or iodine); of these, silver fluoride is not photosensitive

WAVELENGTH: the distance between the crest of one wave and the next; used to define different types of electromagnetic radiation

Bibliography

Balsys, Algis, and Liliane DeCock-Morgan, eds. THE MORGAN AND MORGAN DARKROOM BOOK. Dobbs Ferry, N.Y.: Morgan and Morgan, 1980. This book serves as a practical guide to developing film and prints. More practical than theoretical in nature, it includes many recipes for film and paper developers and toners, as well as descriptions of different types of films and papers, without discussing in any detail how or why they work.

Beaumont, Newhall. THE HISTORY OF PHOTOGRAPHY. 4th ed. New York: The Museum of Modern Art, 1978. Following the trail of both the evolution of the camera and of the photographic process, this is an essential source for anyone interested in the development of the technique. Most of the photographs are devoted to the use of photography in art and in photojournalism, but chapter 12 devotes some space to the history of scientific photography.

Engel, Charles E., ed. PHOTOGRAPHY FOR THE SCIENTIST. New York: Academic Press, 1968. This book is a compilation of articles by different authors and is intended as a resource book for scientists who wish to apply photography to their research. Articles on basic principles, techniques, and equipment are included. Of special interest are articles on underwater photography, photography of living and preserved specimens, and ultraviolet and fluorescence photography.

Langford, Michael. THE DARKROOM HANDBOOK. New York: Alfred A. Knopf, 1981. This text is on a more advanced level than the MORGAN AND MORGAN DARKROOM BOOK, but still concentrates mainly on "how-to" rather than "why." The last chapter, "Rediscovering Old Processes," covers the use of light-sensitive materials used before silver bromide became popular and includes specific instructions for making cyanotypes, milk prints, brown prints, and the like.

Saferstein, Richard. CRIMINALISTICS: AN INTRODUCTION TO FORENSIC SCIENCE. Minneapolis: Burgess, 1987. This text describes many of the important physical and chemical techniques used in criminal investigations, including photography and microscopy. Of special interest is the use of famous cases such as the Lindbergh baby kidnapping to illustrate the importance of such techniques.

Photochemistry, Plasma Chemistry, and Radiation Chemistry

X-Ray Determination of Molecular Structure