Photographic Techniques

D

Scientific Photography

In scientific research, photographic plates and films are among the most important recording tools, not only because of their versatility but also because the photographic emulsion is sensitive to ultraviolet and infrared radiation, to X-rays and gamma rays, and to charged particles. Radioactivity, for example, was discovered because of the accidental blackening of photographic film. Many optical instruments, such as the microscope, the telescope, and the spectroscope, can be used to obtain photographs. Many other scientific instruments such as electron microscopes, oscilloscopes, and computer terminals are also equipped with devices to take photographs or with adapters that permit the use of a regular camera. In laboratory research, Polaroid cameras are often used to obtain quick results. An important research activity in particle physics is the study of thousands of photographs taken in the cloud or bubble chambers of particle detectors in order to find interactions between particles of interest. Tracks of charged particles can also be recorded directly on special films.

The photographic recording of X-ray pictures, called radiography, has become an important diagnostic tool in medicine. Radiography, using very energetic X-rays or gamma rays, is also employed to detect welding defects and structural defects in pressure vessels, pipes, and mechanical parts, especially those that are critical for safety reasons, as in nuclear power plants, aeroplanes, and submarines. In many cases the film, wrapped in a light-proof envelope, is simply applied against one side of the object, while the object is irradiated from the other side. The photographic recording of X-rays is also used in structural studies of crystalline materials. With the development of the laser, a technique called lensless photography, or holography, became available for producing three-dimensional images. Photography is also important in recording images of very small objects when allied to various types of microscope, such as the scanning electron microscope, the transmission electron microscope, and the atomic force microscope. These technologies have been very important in biological and medical research.

E

Astronomical Photography

In no other field of science has photography played a more important role than in astronomy. By placing the photographic plate in the focal plane of a telescope, astronomers can obtain precise records of the locations and brightness of celestial bodies. By comparing photographs of the same region of the sky taken at different times, proper motions of certain objects such as comets can be detected. An important quality of the photographic plate for astronomy is its ability to record, by means of long time-exposures, astronomical objects too faint to be observed with the naked eye.

Recently, the sensitivity of photographic recording has been improved by image-enhancing techniques. In a process known as the photoelectric effect starlight liberates electrons on a photocathode that is placed in the focal plane of the telescope. The liberated electrons are directed to a photographic plate to form the image. Computer enhancement techniques create sharper, more detailed images from sometimes fuzzy and distant photographs from outer space. Computers digitize the photographic information and then reproduce it with greatly improved resolution. In a further refinement, charge-coupled devices (CCDs) dispense with a photographic plate altogether; individual photons are recorded electronically, and are distributed by a microprocessor along a series of picture elements (pixels) which when built up in rows, form an image which can be manipulated digitally by computer. The pixel in these pictures is thus analogous to the silver-halide grain in conventional photography in governing the resolution of the image. CCDs are the imaging technology used by the Hubble Space Telescope and space exploration probes such as Galileo, and are the prime system used by the world's major terrestrial telescopes, such as the Keck telescopes. The reason for their dominance in this area is their superiority in registering very faint astronomical objects; their ability to record individual photons gives tham a light-collecting power far greater than even the most sensitive film.

F

Microfilming

Microfilming consists of photographically reducing images to a very small size. An early application was the photographing of bank cheques in the 1920s; now the technique is widely used to store information that would otherwise require too much space. For example, newspapers and magazines are photographed on roll film and can be displayed on desk-top projectors equipped with systems that permit the desired pages to be found quickly. Another application is the microfiche, a piece of 10 by 15 cm (4 by 6 in) film on which up to 70 frames, each corresponding to one page of text, can be stored. Each frame can be viewed individually on a desk-top projector. This system makes possible the storage of the entire catalogue of a library on a relatively small number of microfiches.

G

Infrared Photography

With special dyes, photographic emulsions can be made sensitive to light in the invisible infrared portion of the spectrum. Infrared light cuts through atmospheric haze and enables clear photographs to be taken from long distances or high altitudes. Because any object radiates in infrared light, it can be photographed in complete darkness. Infrared photographic techniques are used wherever small differences in temperature, or in absorption or reflection capacities for infrared light, have to be detected. Some substances, particularly organic ones such as vegetation, reflect infrared light more strongly than other substances do; infrared films tend to reproduce as white the tones of green leaves and plants, especially if used in conjunction with a deep-red filter. Infrared film has many technical and military applications, including the detection of camouflage, which in the infrared photograph appears darker than the surrounding area. Infrared photography is also used in medical diagnosis, in the detection of forgeries in handwriting as well as in paintings, and for the study of deteriorated documents. It has been used, for example, in deciphering the Dead Sea Scrolls.

H

Ultraviolet Photography

Normal film is sensitive to ultraviolet light. In one method of ultraviolet photography, an ultraviolet light source is used to illuminate the object, and the camera lens is provided with a filter that permits only the passage of ultraviolet light. The second method makes use of fluorescence caused by ultraviolet light; a filter used on the camera absorbs ultraviolet light and allows the passage of the fluorescent light. One important application of ultraviolet photography is the study of forged documents, because traces of erased writing become detectable in ultraviolet light.

In several processes used to produce photographic images in the ultraviolet range of the spectrum, plastics and other chemicals that react to ultraviolet light replace the silver-halide emulsion of conventional film. In one process, surface areas of a plastic substance exposed to ultraviolet rays harden in direct proportion to the amount of exposure, and removal of the unhardened areas leaves a raised photographic image. In other processes, a thin film of chemicals is suspended between plastic sheets. When exposed to ultraviolet rays, these chemicals emit gas bubbles, in amounts proportional to the exposure received in a given area. The bubbles expand and become visible on the application of heat to the sheets, creating a transparency in which the gas bubbles form the image. Another type of plastic, when heated, reacts chemically with the gas bubbles so that a stained positive image is obtained on the plastic sheets.

In the photochromic film developed by the National Cash Register Company in the United States, a dye sensitive to ultraviolet light is used. Because the dye has no grain structure, enormous enlargements can be made. For example, enlargements can be made from film on which a complete book is contained in an area the size of a postage stamp.