Xerography

Xerography is a dry electrostatic printing and copying process that forms images by electronic charges. The term combines Greek roots for dry (xeros) and writing. It was invented by Chester F. Carlson in 1938 and commercialized in the 1950s by Haloid Company, which became Xerox. The process is a specific instance of electrophotography, a broader method for producing images on photoconductive surfaces.

Principle and steps: A photoconductive surface (drum or belt) is uniformly charged. When exposed to light from the original, the surface loses charge in illuminated areas, creating a latent electrostatic image. Dry toner particles, triboelectrically charged, are attracted to the latent image. The toner is then transferred to paper and fused with heat and pressure to form a permanent image. The photoconductor is cleaned for the next cycle.

Materials and devices: Early xerography used selenium as a photoconductor; modern systems use organic photoconductors and layered materials. Charging is commonly performed with corona discharge or charge rollers. The process can produce monochrome or color copies; color xerography typically uses multiple passes or a digital color-separation approach with color toners.

Applications and impact: Xerography is the basis for most modern laser printers and photocopy machines, enabling rapid, high-volume document production with low moisture and the ability to print on plain paper. Its non-impact, dry toner approach is distinct from older wet-ink copying methods.

Limitations and considerations: While highly efficient, xerography depends on good-quality originals and proper machine maintenance. Toner powders are fine and require appropriate handling; energy use and waste streams are considerations in high-volume environments.