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How Do Electronic Cameras Resemble Photographic Plates and How Do They Differ?

Though it would seem photographic plates and electronic cameras may have as much in common with each other as a penny farthing bicycle and a modern motorcycle do, the similarities are more than just skin deep. Long after they passed out of favor in the rest of the photo world, photographic plates remained the imaging tool of choice for astronomical photography until replaced by CCDs -- a late 20th century technology that shares important characteristics with 19th century glass plates.

Photographic Plate History
- The idea of using a glass plate as a photographic medium dates back to 1851 when F. Scott Archer unveiled his wet collodion process, using a glass plate coated with collodion as a substrate holding the light-sensitive silver halides. The plates were only light-sensitive as long as they stayed wet, and could not be prepared more than 15 minutes in advance. The glass plate took a great leap forward in 1871 when R. L. Maddox developed the dry plate process, using gelatin as the medium for the silver halide. Eventually replaced by the introduction of flexible film bases as pioneered by George Eastman in the early 20th century, photographic plates remained popular for astronomy well into the 1990s for their large size and rigid structure which kept images sharp during long exposures.
Rise of the CCD
- The charge-coupled device first made its way into astronomy in 1979 when Kitt Peak National Observatory installed a 320 x 512 pixel CCD on their 1-meter telescope. By 1983 CCD cameras were steadily replacing glass plates thanks to their higher sensitivity, lower long-term cost, and ease and speed of use. By the late 1990s, the CCD had almost exclusively replaced the photographic plate for astronomical imaging.
Key Similarities
- In both plates and CCDs the basic mechanism of detecting light is similar - a photon excites electrons in a crystal structure to the point where electrons become mobile. Both possess a wide range of color sensitivity and capture images in a two-dimensional manner. Finally, both are rigid detectors unlike flexible film, making them well suited for the physical stability needed to produce sharp images from exposures that can be measured in minutes to hours.
Major Differences
- While both use the same mechanism of detecting light, the process diverges significantly from there. The CCD's electrons remain in an excited state, whereas the plate's electrons return to their original state. This makes photographic plates a non-linear detector, whereas CCDs are linear detectors and far more suited to astrophotography. CCDs also are capable of capturing a far greater dynamic range, the difference between pure black and pure white, and are far more sensitive to light than photographic plates. Image storage also differs, as CCD images can be stored on hard drives, optical media, or other computer-based storage where plates must be physically stored. As such, CCD images are far more easily duplicated and transmitted electronically.

Astronomy