Panchromatic Imagery

Panchromatic Film

Photography is an analog, chemical method of recording images. Silver halide crystals suspended in gelatin form an emulsion and undergo a chemical reaction on exposure to ultraviolet, violet, and blue light, thus recording the spatial patterns of light that fall on the film. However, because of the limited spectral range of the silver halide crystals, black-and-white photographs produced from this film did not provide an intuitive correspondence to human visual perception. In 1873, Hermann Carl Vogel discovered that by adding small amounts of sensitizing dye, the silver halide crystal sensitivity to light could be extended to include green wavelengths, a type of film known as orthochromatic. Finally, in 1904, Vogel developed a dye that extended the silver halide sensitivity range to include red. The resulting black-and-white photographs were more visually intuitive than those from the earlier films, and the name panchromatic was chosen for the new film. In 1919, infrared films were developed by extending the sensitivity range even further. In contrast to all black-and-white films, including panchromatic film, color films have three emulsion layers, one for each of the red, green, and blue portions of the spectrum.

Panchromatic film is prized for its broad spectral response, from approximately 0.25 to 0.7 micrometer, and was the traditional film used in aerial photography (Figure 1). Because the atmosphere preferentially scatters shorter-wavelength light, and thus especially ultraviolet and blue light, panchromatic aerial film was [Page 2114]normally exposed with a yellow filter to remove the blue and the shorter wavelengths. Thus, despite the name, panchromatic aerial photography typically does not record all visible wavelengths.

A wide range of aerial panchromatic film types were produced in the past, with varying contrast, light sensitivity, and spatial resolution. In recent years, with the increased use of digital aerial sensors, many types of panchromatic aerial film have been discontinued.

Satellite-Based Sensors

During the early part of the space era, traditional photographic film was the primary medium for acquiring images of Earth, both by astronauts and by automated satellite-based sensors. Consequently, aerial film terminology was used in space-based imaging from the beginning, including for Corona, a U.S. intelligence program that was the first satellite-based Earth-imaging program. The Corona program lasted from 1960 to 1972, and the majority of Corona imagery was acquired with the equivalent of panchromatic film.

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