From 1880 to 1970, photographic emulsions were the primary detectors that professional astronomers used to record images. A photographic emulsion consists of silver halide crystals dispersed in a gelatin matrix, coated on a glass plate backing or a flexible plastic film backing. Although silver bromide is the primary silver salt, traces of chloride and iodide may be present. In the manufacture of photographic materials, the size and shape of the crystals is carefully controlled; the most sensitive crystals are approximately one micron across and flattened. In addition, the spectral sensitivity of the finished emulsion depends on dyes that absorb photons and transfer their energy to the silver bromide crystals.
Photographic emulsions differ from the retina and the CCD because exposure to light produces irreversible changes in the detector; in other words, you can use a film or plate once and once only. When a photon of energy greater than about 2 electron-volts impinges on a silver bromide crystal, it creates a defect in the crystal structure. Such defects can migrate through the crystal, and may spontaneously "heal" after several seconds or minutes. When several such defects have been created in a crystal, however, they coalesce into a stable defect called a development center. A stable defect involving only a few dozen atoms is enough to render the entire crystal—containing billions of silver atoms—chemically unstable.
When the emulsion is subsequently placed in a solution of photographic developer, the defect triggers the chemical reduction of the entire crystal to metallic silver. The capture of three to four photons can thus precipitate the formation of a grain of silver containing 10 billion silver atoms—a remarkable feat of chemical amplification. After development, the emulsion is washed in a fixer (a bath that dissolves undeveloped crystals of silver halide), then thoroughly rinsed and dried.
The developed photographic image thus consists of a clear backing coated with a gelatin layer containing microscopic grains of metallic silver where light struck the emulsion. Because the image appears dark or opaque where light fell, it is called a negative. By passing light through the negative to another sheet of photographic emulsion coated on white paper, you can make a negative of the original negative, which is called a positive print.
Photographic images have interesting properties. The process is inherently nonlinear with respect to its exposure to light, because once a grain crosses the development threshold, it is developable; and further exposure cannot make it more
Exposed and Developed Emulsion
Figure 1.6 Photographic detectors are made by coating a thin sheet of plastic or glass with gelatin containing microscopic crystals of silver bromide, iodide, and chloride, protected by a top layer of plain gelatin. After exposure to light and chemical development, the image consists of tiny grains of metallic silver.
developable. Because of this, the number of developed grains is less than proportional to the number of photons. Furthermore, since a developed emulsion is three-dimensional, developed grains shadow other developed grains, resulting in a further undercount of the original number of photons.
Photographic emulsions are also nonlinear with respect to exposure time. Unless three or four defects are created in a sufficiently short interval to produce a development center, individual defects may disappear before coalescing; and the grain will not be developable. Photographic materials, therefore, record a smaller fraction of the incident photons in long exposures to low levels of light than they record when the same number of photons arrive in a short time. At the light levels found in astronomical images, faint parts of an image are recorded less efficiently than are its bright parts.
Finally, because the silver halide grains in the emulsion were scattered randomly, the number of developed grains in small regions that have received the same exposure varies randomly around an average value. The random distribution of developed grains causes the "grainy" appearance of photographs.
During the decades when emulsion photography was king in astronomy, numerous ways were found to enhance the performance of emulsions. Bathing plates in a dilute solution of ammonium hydroxide a day or two before exposure made the plate more sensitive. By exposing plates to faint light just before use in the telescope, astronomers found they could create one or two defects in each crystal, reducing the number of photons necessary to cross the development threshold. Baking plates and films for several hours before exposure likewise created defects, enhancing the performance of some emulsions. However, when researchers discovered that the primary reason that single-photon defects decayed was that
water and oxygen were present in the emulsion, astronomers began baking films and plates in vacuums or gas mixtures containing hydrogen. This process—hydrogen hypersensitizing—drove out the water and reduced the oxygen, and produced a twenty-fold improvement in the fine-grain emulsion of Kodak 2415 Technical Pan. For serious amateur astrophotographers, hydrogen-hypered Tech Pan is the ultimate film.
Modern black-and-white films are made with multiple layers of emulsions with differing crystal size to compensate for the limited dynamic range of simple emulsion coatings. Modern color films are made with multiple layers of emulsions with differing color sensitivity to separate and record the full gamut of color, and sophisticated chemistry to replace developed silver grains with clouds of colored dyes.
Overall, photographic emulsions serve as remarkably good image detectors.
With hydrogen hypering, modern emulsions like Technical Pan perform well in exposures of several hours. Even without special treatment, modern black-and-white and color films are still efficient in exposures ranging from 5 to 20 minutes.
Although their overall quantum efficiency ranges from around 0.5% to about 4%, with spectral sensitivity peaking in the blue and green regions of the spectrum, photographic detectors are readily available in large sizes. Standard 35-mm film has a detector area 24 mm by 36 mm, standard 120-format roll film produces images 60 mm by 70 mm, and standard 4 x 5-inch sheet films have active areas of 100 mm by 125 mm. Although individual grains are only a few microns across, the smallest effective area capable of producing a good signal-to-noise ratio ranges from 5 to 20 microns. Assessed in the terms used for electronic sensors, one frame of fine-grain 35-mm film offers the resolution of a 10 megapixel electronic detector.
Unfortunately, because photographic detectors are inherently nonlinear, and because a piece of film can be used only once, high-precision measurements of light intensity are not possible. With typical quantum efficiencies of 1%, photographic exposure times must be 20 to 60 times longer than comparable exposures with electronic sensors. For capturing physically large images, however, photographic emulsions are competitive with electronic sensors, especially in applications where the detector must be simple, compact, rugged, and inexpensive.
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