Originally photocopiers were all analogue. They worked by reflecting light onto a photosensitive surface. This surface (plate later to become the drum) reacted to light. Where the surface was charged the resistance of the drum lowered when exposed to light. This produced a latent image where only dark areas remained charged. The plate or drum then had toner projected at it that was oppositely charged and held attraction to the photosensitive surface.
Early photocopiers used liquid toner and were extremely messy but dry toner was introduced to combat these problems. Now the image could be clearly seen on the drum or plate. Transference pulled the toner from the drum onto the paper via transfer charge that was again charged opposite to toner but of a much higher capacity in voltage than the drum or plate. After Transfer the copy is then fused to the paper using heated rollers coated in silicon oil.
Analogue copiers relied on light being reflected onto the photosensitive surface through a lens. Because the light had to be a very high intensity they required fans to keep the machine cool during copying. These fans attracted dust and the copier mirrors often became coated with a film of dust often resulting in poor copy quality. In addition the machine had no memory and required a scan for every copy made.
Digital Copiers were to replace the analogue counterparts. Although they still reflected light it no longer needed to be so intense so cooling was minimal meaning the photocopier attracted less dust into the optics. The light now was reflected into a unit that converted light and dark into a binary signal. The copy then is projected onto the drum via a laser or an LED array and stored onto a hard drive allowing it to recalled. This allowed for scan once print many on modern photocopiers. However storing data on the hard drive caused security risks and manufacturers had to introduce secure data encryption of hard drives to prevent data being stolen from the photocopier.
As copiers seek to be more environmentally friendly charge rollers have largely replaced corona wires leading to reduced or zero ozone emissions on modern copiers. Silicon Oils have been vastly reduced as the modern copier offers superior toner technology.