The Laser Printer

Carlson's xerography could already turn a pattern of light on a charged drum into fused toner on paper. What it could not do was invent the pattern: it needed an original, lit and imaged through a lens. A copier copies. Gary Starkweather's argument at Xerox from 1969 was that the lens and the original could both be thrown away and the latent image DRAWN, one dot at a time, by a single laser beam swept across the drum by a spinning polygon mirror. He had a working scanner on a Xerox 7000 engine by 1971. The page no longer had to exist before it was printed. The design is then pure arithmetic. Eight and a half inches at 300 dots per inch is 2550 dots a line; eleven inches of them, eight times a minute, is 440 lines a second; multiply and the laser must switch at over a megahertz while the mirror holds a facet on the page. Push to 1200 dpi and 20 pages a minute and it is 200 megahertz and 22,000 rpm on air bearings - which is why the idea had to wait for the semiconductor laser. Two subtleties decide whether it works. The spot must be about one dot pitch across, and diffraction ties that to how wide the beam is on the mirror, so higher resolution forces a bigger facet on a faster-spinning mirror. And a mirror turning at a constant rate sweeps at a constant ANGULAR rate, so an ordinary lens would place the spot at f-tan-theta and stretch the dots apart towards the margins. The scan lens is deliberately barrel-distorted to give f-theta instead. Its aberration is the feature.