The emulsions used for printing paper are much slower (less sensitive to light) than most film emulsions.

Figure 2-7. Cross section of variable-contrast black-and-white paper.

A high sensitivity to light is not needed. Prints are often manipulated by providing additional exposure or by holding back exposure to selected areas of the print. If the paper emulsions were fast, exposure times would be extremely short and print manipulation would not be possible.

The color or spectral sensitivity of a paper emulsion indicates the response to specific colors of light or radiation-the same as that for a film emulsion. The terms colorblind, orthochromatic, and panchromatic are also used to describe the spectral sensitivities of paper emulsions. The response of a paper emulsion to wavelengths other than ultraviolet radiation, violet, and blue light is achieved by the addition of sensitizing dyes to the emulsion during manufacturing.

The ranges of sensitivity for all paper emulsions begin in the near-ultraviolet region of the invisible portion of the electromagnetic spectrum. The color sensitivity of undyed silver emulsions extend to blue and blue-green. Panchromatic printing papers are sensitive to ultraviolet radiation, violet, blue, green, and red light. The color sensitivity of a black-and-white paper is NOT a determining factor for selecting a paper to print a specific black-and-white negative. Panchromatic papers, however, are normally used to produce black-and-white prints from color negatives. Color sensitivity DOES determine whether or not a safelight is required an, if so, what color of safelight can be used.

Variable Contrast Papers

Variable contrast papers are used almost exclusively in Navy imaging facilities. The contrast of the print image on variable contrast papers is controlled by the specific color of the exposing light. Variable contrast papers have two emulsion layers. The top layer is a high-contrast emulsion and is sensitive to blue light. The second emulsion layer is a low-contrast emulsion and is sensitive to green light (fig. 2-7). When a normal contrast negative is exposed through a variable contrast filter with blue light, a harsh, contrasty print is produced.

Figure 2-8. Cross section of color printing paper.

When the same negative is exposed on another sheet of paper with green light, a flat or not enough contrast print is produced. When the normal-contrast negative is printed with the proper combination of blue and green light, a print with natural contrast is produced; therefore, to control contrast when using variable contrast papers, you can use a series of yellow and magenta filters to control the amount of blue and green printing light during exposure.