How Everything Works
How Everything Works How Everything Works

Question 220

In the photocopying or xerographic process, what is the intensity, wavelength, and normal exposure time of the light that is emitted from lamps of these office machines? How does this light differ from sunlight?
The light sensing surface in a xerographic copier is a semiconductor or "photoconductor" film on a metal drum or belt. Light causes this film to convert from an insulator to a conductor of electricity, a change that is ultimately responsible for the formation of the copy image. However, the light particles ("photons") must each carry a certain amount of energy in order to cause that conversion. Since the photons of blue light carry more energy than those of red light, blue light tends to be more effective in the xerographic process than red light. In fact, far red and infrared light have no effect at all on the photoconductor film. However, considerable effort has been made over the years to make the photoconductor films used in xerographic copying very sensitive to all wavelengths of visible light. As a result, it doesn't take much light from even a normal lamp to produce a xerographic copy. Sophisticated copiers expose the original document to visible light from an incandescent lamp, a fluorescent lamp, or a xenon/krypton flashlamp and measure the light reflected by that document. They use this measurement to set the exposure time and/or the aperture of the lens that forms the image of the document on the photoconductor film. The light used in a copier doesn't contain as much ultraviolet light as sunlight, but otherwise the differences aren't very important to the xerographic process. As for the intensity and exposure times, you can see these for yourself when the machine operates. Just open the cover and watch the lamps or flashlamps in action.

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