Digital Imaging Systems – Part 3

by D.R.Hunt

The Print System.

As the expanded image is in memory, it can be viewed by the display system exactly as a freshly scanned image. And if the compression and expansion part of the system has done its job properly, the resulting image will be a dot for dot replica of the original.

This brings us to the last part of the system. The print system. It’s usual to use a Laser printer for the job and these work in yet another simple fashion. Like scanners, there are two common sizes of printer, A3 and A4. They work in exactly the same fashion, one is bigger, that’s all. The Laser printer is based on the photocopier and uses a Xerographic process, which translated means ‘dry photography’. (Xerographic is the original name coined by the inventor. He then sold his designs to a certain large company who formed a company with a similar name and they did very well – thank you very much – until the patents ran out.)

The Xerographic process in brief, uses a drum coated with a photo-conductor, it used to be the metallic element selenium (selenium isn’t actually chemically a metal), but these days there are many other things which changes their electrical properties when light falls on them, and often the photo-conductive properties of silicon are employed. Anyway, the way it works – the photo-conductive coating is charged with a very high voltage and a static charge builds up on the drum. The drum is then exposed to the light reflected off the brightly illuminated paper original, which moves past as the drum rotates (or a moving mirror reflects the image of the paper onto the drum as it rotates). Where light falls on the drum, it changes its electrical conductance and the charge leaks away. This leaves a pattern of static charges on the drum where the black bits were but no charge in the white areas. The drum continues to rotate past a tray full off very fine soot, well not really soot, it’s finely divided graphite mixed with a dry resin glue. Being a fine dust, the soot is attracted to the static charges on drum, so the drum picks up the soot in the patterns of the black bits of the image. Whilst the top side of the drum is being charged and discharged and picking up the soot, the under-side of the drum is also rolling a piece of blank paper through the works, and the pattern of soot ends up on the paper. The final touch is the paper is passed out through heated rollers which melt the resin glue and stick the soot to the paper. Funny, copier repairmen get upset when I refer to the toner as soot, but I ask you, what else is finely divided graphite !