Special Uses Of Photography Part II

Spacecraft and satellites are often used to carry cameras that photograph the earth's surface. The Apollo astronauts took many spectacular photographs of the earth. Some satellites-such as the Landsat D, launched in 1982-take very detailed pictures. But Landsat does not use a regular camera. It uses a special device called a scanner.

Infrared photography uses special films that are sensitive to infrared radiation. Infrared rays, which are invisible, have a longer wavelength than visible light. They can penetrate haze that scatters the waves of visible light. For this reason, infrared photographs often are much clearer than ordinary photographs. Pictures can be taken with infrared rays even at might or in complete darkness.

Infrared photography is used in medicine to inspect damage to veins and healing beneath scabs. Faults in the weaving of textiles show up in infrared photography. Even crossed-out words or writings on a charred piece of paper can be read if photographs of them are taken with infrared rays. Astronomers use infrared photography to study stars that are covered with a haze.

Spectrography combines a camera with a spectroscope. This is an instrument that breaks up light into its different wavelengths. Using spectrography, scientists can analyze the light given off by the stars and the sun. Spectrographs of the planets reveal which gases are present in their atmospheres. Spectrographs can be used to analyze materials such as metal alloys. A small piece of the material is heated until it melts, then turns to gas. The pattern of lines in the spectrum identifies each element that is present in the material.

Holography is three-dimensional photography. A laser beam, which consists of only a very narrow range of light waves, is used. Part of the beam is directed at the object. This is reflected from the object to the photographic plate. The other part of the beam is focused directly on the photographic plate. The two parts of the beam interfere with each other. This interference is recorded in the silver compound on the photographic plate. To view the hologram, the plate is exposed to a laser beam. This beam usually has the same frequency as the one that recorded the image.

Holography can be used to test materials without harming them. It can reveal very small changes of size and shape when an object is subjected to pressure, heat, or other forces.

There are even ways to make three-dimensional pictures using sound, rather than light, as the energy source. These are called acoustic holograms. They are used in medicine, underwater studies, and other fields.