Many substances are affected in some way by light. The light-sensitive substances used in photographic film to record an image are silver salts and are called silver halides. The silver halides react to ultraviolet radiation, violet, and blue light only; however, they can be made sensitive to other colors of light and infrared radiation by the addition of dyes. Depending on the amount of light and the type of silver halide, the light produces a visible or invisible change in the halides of a film or printing paper. An invisible change is made visible by development.

Photographic films and papers are composed of two basic parts: the emulsion and the base, or support. The emulsion is the light-sensitive portion of a film or paper that records the image. The emulsion contains the silver halides and any special sensitizing dyes suspended in a binder of gelatin. The gelatin holds the silver halides evenly dispersed and prevents action by a developer until the silver halides have been made developable either by exposure to light or chemical action. Also, the gelatin acts as a sensitizer for the silver salts.

In photographic films and papers, the primary purpose of the base is to support or hold the emulsion in place. The base, or support, may be transparent or opaque, depending upon how the recorded image is to be used. A transparent base is used for transparencies that are viewed by transmitted light and for negatives that are printed with transmitted light. An opaque base is used for prints that are viewed by reflected light.

The latest state of the art in light-sensitive materials used in photography is the use of the electronic medium. Video disks do not contain an emulsion or a base. When electronic mediums are used, light is converted to electrical impulses and these impulses are stored magnetically on a tape or disk. Since it is the camera itself that converts light to electrical impulses, the recording medium and all stages of the photographic process can be carried out in normal room light.

BLACK-AND-WHITE FILM

The characteristics and use of black-and-white film depend largely on the actual construction of the emulsion. These characteristics include the following: the degree of sensitivity to light, response to various colors of light (color or spectral sensitivity), contrast, exposure latitude, emulsion latitude, and emulsion definition.

There are many types of black-and-white films available. Each type differs somewhat from the others. You should become acquainted with the characteristics of films. This knowledge is helpful in selecting the film most suitable for each photographic assignment.

EMULSION SENSITIVITY TO LIGHT

The silver halides and sensitizing dyes of most film emulsions are very sensitive to small amounts of light. This light causes invisible changes to the emulsion and is called the latent image. The latent image can be physically made visible by the chemical step of development. The extent of the reaction to the light of the emulsion is affected greatly by the size of the silver halide grains and the amount of light reaching the film. The inherent property of a film emulsion to respond to light is termed film speed.

Film Speed

Film speed is important, since it is related to the amount of exposure required to produce an acceptable image. Emulsions are rated as slow, medium, or fast, depending on the amount of light required to produce an image satisfactorily. Fast emulsions require less light to produce an acceptable image than slow emulsions.

To calculate the exposure for a film emulsion accurately and consistently with a light meter, the manufacturer has developed a system of rating emulsion speed. The rating system used is the IS0 film speed system. ISO film speed is a numerical value assigned to an emulsion used for determining exposures.

The International Standards Organization (ISO) is a federation of all the national standard bodies of the world. It has approved a uniform set of film speed standards. The standards call for a universal expression of both arithmetic and logarithmic speed values with the ISO designation. The ISO designation generally looks like the following:

ISO 100/21

Figure 2-1. IS0 of film.

The ISO assigned to a film is labeled on the packaging material and on the film cassette or paper backing (fig. 2-1). Some types of black-and-white films are assigned one ISO number; others are assigned two or more. Whether one or more film speeds are assigned depends on whether the film responds differently to different colors or color temperatures of light. Often, this results in a film having one speed for daylight and another for tungsten light. The ISO for a particular film is valid for calculating correct exposure only when that film is developed as recommended by the manufacturer.

An exposure index is another numerical value assigned to some films for exposure calculation. The exposure index is a more accurate method of film speed, because it is determined by processing a particular film through the actual process in an imaging facility. Exposure indexes are generally assigned to films used for copying or for technical applications.