Above a bar in Queens, New York City, 75 years ago, Chester Carlson created the first-ever xerographic images. Carlson worked as a patent attorney and was frustrated that the only way to copy documents was to retype or rewrite them. He came up with a process, free of liquid chemicals, that takes advantage of photoconductivity, the tendency of certain materials to become electrically conductive when hit by light.
To make a copy, Carlson laid an image, held by a glass slide, on top of a photoconductive surface and then illuminated it. Where hit by light, the surface became conductive, draining its charge to the plate below; where the image blocked light, the surface stayed charged.
After removing the plate, Carlson sprinkled on a negatively charged powder, which adhered to the areas of positive charge, thereby re-creating the image. It was not until 1949 that the Haloid Company (later Xerox) released the first commercial machine.
Now that photocopying is digital, Xerox has added technology that enables law enforcement to track down counterfeiters. Serial numbers and other data can be encoded in nearly invisible yellow dots on copies.
– Rachel Z Arndt