The response of an emulsion to specific colors of light or radiant energy is termed coIor or spectral sensitivity. You already know, from our earlier study of light in chapter 1, that the visible spectrum is made up of violet, blue, green, and red light. When speaking of film or emulsion sensitivity, you are also referring to its sensitivity to ultraviolet and infrared radiation. Spectral sensitivity of an emulsion simply means that the emulsion is sensitive to some energy of the electromagnetic spectrum. Also, emulsions have color sensitivity, which means they are sensitive to one or more colors of the visible spectrum. In this chapter, the terms color sensitivity and color response are used interchangeably. Color sensitivity or color response refer to ultraviolet and infrared radiation as well as visible light.

All silver halides are sensitive to ultraviolet radiation, violet, and blue light. Color-blind emulsion is the term given to emulsions sensitive only to these radiations. The addition of sensitizing dyes to silver halides can increase the sensitivity of emulsions and extend their sensitivity to green and red light and infrared radiation. Increasing the color sensitivity of an emulsion to other than ultraviolet, violet, and blue is called dye, color, or optical sensitization.

The color sensitivity of a black-and-white film is an important characteristic, since it controls the way colored objects record as tones of gray in the negative

or print. The color sensitivity determines how the film is classed. There are four general classes of black-and-white film emulsions. The four classes are as follows: colorblind (monochromatic), orthochromatic, panchromatic, and infrared. Some of these emulsions respond to a wide range of wavelengths of light. Others respond to only a narrow band of wavelengths. Light-sensitive emulsions are sensitive to all wavelengths of ultraviolet radiation. For all practical purposes, the general classes of emulsions are considered insensitive to the shorter wavelengths of ultraviolet (UV) radiation. This is because glass lenses and the gelatin in most film emulsions completely absorb the shorter wavelengths of ultraviolet radiation. When UV is to be used for photography, a special film with a thin emulsion is required.

Color-blind Emulsions. Black-and-white color-blind emulsions are sensitive only to UV radiation, violet, and blue light. Green and red objects record only as clear areas in the black-and-white negative and reproduce as dark areas in the print. Color-blind films are used primarily for copying and graphic arts photography and may be assigned three or more ISO values; for example, ISO/50 for daylight, ISO/8 for tungsten light, ISO/20 for white-flame arcs, and ISO/12 for pulsed xenon.

Orthochromatic Emulsions. Orthochromatic emulsions are sensitive to ultraviolet radiation, violet, blue, and green light. The sensitivity to green light is gained by the addition of a sensitizing dye to the color-blind silver halides. The emulsions provide an approximate correct reproduction of blue and green objects as corresponding tones of gray in a print; however, red objects record as clear areas in the negative and black areas in the print. since the emulsion is not sensitive to red. Various orthochromatic films with different degrees of contrast, color sensitivity, and emulsion speed are available. Their trade names usually contain the word ortho. Orthochromatic emulsions are used primarily for copying and graphic arts photography.

Orthochromatic emulsions that may be used in either daylight or tungsten light are assigned two separate ISO film speeds. This is because these emulsions are highly sensitive to the predominantly blue colored daylight and less sensitive to the tungsten light that has a higher content of red light.

Panchromatic Emulsions. Panchromatic emulsions are sensitive to UV radiation, violet, blue, green, and red. The emulsion spectral sensitivity to green and red light is gained by adding sensitizing dyes to the color-blind silver halides. Panchromatic film of varying degrees of contrast, color sensitivity, and emulsion speed is available. Panchromatic emulsions are used for copying, portraiture, and general black-and-white photography.

Panchromatic emulsions are assigned only one IS0 film speed. This is because panchromatic emulsions are sensitive to red light and have an almost equal response to predominately blue-colored daylight and predominately red-colored tungsten light.

Infrared Emulsions. Infrared (IR) emulsions are sensitive to UV radiation, violet, and blue light, with little or no sensitivity to yellow-green light but with additional sensitivity to red and IR radiation. The sensitivity to infrared radiation is gained by adding a sensitizing dye to the color-blind silver halides. Infrared emulsions are commonly used for aerial and medical photography as well as forensic photography (photography used for evidence). For best results a black-and-white UV film should be exposed only with IR radiation. To prevent any IR radiation or visible light from affecting the infrared emulsion during exposure, you must use a dark, red filter over the camera lens.

Since infrared radiation does not focus at the same point as visible light, a lens focus adjustment is necessary for critical focusing. Most lenses have a calibrated infrared focusing position on the focusing scale. This position is usually marked by a small, red dot or the letter R in red.

Determining a useful exposure index becomes a problem with infrared film, because exposure meters are calibrated for visible light and similar light sources can emit different amounts of infrared radiation. When using infrared film, you should make trial exposures for each particular film and photographic situation.

Contrast

In the development process, the silver halide grains in a black-and-white film exposed to light remain in the film. These grains form the image of the original scene. The colors of the scene are recorded in the negative as densities of gray instead of appearing as their original colors. These densities of gray can range from very dense to very thin. This depends upon the brightness of the objects in the scene, their color, and the color sensitivity of the film. The ratio of the maximum to the minimum brightness of objects in a scene is referred to as the scene brightness range. Most long scale black-and-white films are capable of recording scene brightness ranges up to 128:1. In a negative, a

light-colored object records as a heavy-density (dark) area, and a dark-colored object records as a low-density (thin) area; therefore, a negative image is reversed compared to the original scene. This reversal is produced by a bright object in the scene reflecting more light than a darker object. The greater amount of reflected light from the brighter object affects more silver halides in the emulsion.

The portions of the negative where the most silver halides have been affected are referred to as HIGHLIGHTS. The portions that are least affected are referred to as SHADOW AREAS. The light reflections from objects other than the brightest and darkest are referred to as MIDTONES.

The amount of metallic silver deposit in any portion of a negative is referred to as density. The amount of light that a negative transmits in a given period of time is controlled by the density of the metallic silver deposits. Therefore, density is used to describe the light-stopping ability of a negative.

The difference in densities between areas in a negative is known as contrast. The total contrast (density range) of a negative is defined as the difference in density between the least-dense shadow area and the most-dense highlight area in a negative.

Emulsion Latitude

The inherent ability of a black-and-white film to record a range of scene brightness differences as differences in density is termed emulsion latitude Normal- and low-contrast emulsions can record a wide range of scene brightness values and are considered to have a wide emulsion latitude or long scale. High-contrast emulsions record a short range of scene brightness values and are considered to have a narrow emulsion latitude or short scale.

Exposure Latitude

The amount the exposure can be varied (increased or decreased) from the ideal exposure and still provide an acceptable negative is termed exposure latitude.

The least amount of exposure that records sufficient shadow detail generally provides the best quality negative and is considered the ideal exposure. The use of a determined exposure index leads to the ideal exposure for a particular film and process combination. Generally, the use of the assigned ISO speed provides acceptable results.