Laser Printers
In the 1980s, dot-matrix and laser printers were pre-dominant, with inkjet technology not emerging in any significant way until the 1990s. The laser printer was introduced by Hewlett-Packard in 1984, based on technology developed by Canon. It worked in a similar way to a photocopier, the difference being the light source. With a photocopier a page is scanned with a bright light, while with a laser printer the light source is, not surprisingly, a laser. After that the process is much the same, with the light creating an electrostatic image of the page onto a charged photoreceptor, which in turn attracts toner in the shape of an electrostatic charge.
Laser printers quickly became popular due to the high quality of their print and their relatively low running costs. As the market for lasers has developed, competition between manufacturers has become increasingly fierce, especially in the production of budget models. Prices have gone down and down as manufacturers have found new ways of cutting costs. Output quality has improved, with 600dpi resolution becoming more standard, and build has become smaller, making them more suited to home use.
Laser printers have a number of advantages over the rival
Considering what goes into a laser printer, it is amazing they can be produced for so little money. In many ways, the components which make up a laser printer are far more sophisticated than those in a computer. The RIP (raster image processor) might use an advanced RISC processor; the engineering which goes into the bearings for the mirrors is very advanced; and the choice of chemicals for the drum and toner, while often environmentally unsound, is fascinating. Getting the image from the PC's screen to paper requires an interesting mix of coding, electronics, optics, mechanics and chemistry.
A laser printer needs to have all the information about a page in its memory before it can start printing. How an image is communicated from the PC's memory to a laser printer depends on the type of printer being used. The crudest arrangement is the transfer of a bitmap image. In this case there is not much the computer can do to improve on the quality, so sending a dot for a dot is all it can do.
However, if the system knows more about the image than it can display on the screen there are better ways to communicate the data. A standard A4 sheet is 8.5in across and 11in deep. At 300dpi, that is more than eight million dots compared with the eight hundred thousand pixels on a 1024 by 768 screen. There is obviously scope for a much sharper image on paper - even more so at 600dpi, where a page can have 33 million dots.
The major way quality can be improved is by sending a page description consisting of outline/vector information and allowing the printer to make the best possible use of it. If the printer is told to draw a line from one point to another, it can use the basic geometric principle that a line has length but not width, and draw that line one dot wide. The same holds for curves, which can be as fine as the resolution of the printer allows. The idea is that one single page description may be sent to any suitable device, which would subsequently print it to the best of its ability - hence the much-touted term, device independent.
Text characters are made up of lines and curves so can be handled in the same way, but a better solution is to use a pre-described font shape, such as TrueType or Type-1 formats. Along with precise placement, the page description language (PDL) may take a font shape and scale it, rotate it, or generally manipulate it to its heart's content. There's the added advantage of only requiring one file per font as opposed to one file for each point size. Having predefined outlines for fonts allows the computer to send a tiny amount of information - one byte per character - and produce text in any of many different font styles and many different font sizes.
Where the image to be printed is communicated to it via a page description language, the printer's first job is to convert the instructions into a bitmap. This is done by the printer's internal processor, and the result is an image (in memory) of which every dot will be placed on the paper. Models designated "Windows printers" don't have their own processors, so the host PC creates the bitmap, writing it directly to the printer's memory.
At the heart of the laser printer is a small rotating drum - the organic photo-conducting cartridge (OPC) - with a coating that allows it to hold an electrostatic charge. Initially the drum is given a total positive charge. Subsequently, a laser beam scans across the surface of the drum, selectively imparting points of negative charge onto the drum's surface that will ultimately represent the output image. The area of the drum is the same as that of the paper onto which the image will eventually appear, every point on the drum corresponding to a point on the sheet of paper. In the meantime, the paper is passed through an electrically charged wire which deposits a negative charge onto it.