How Printers Work

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Print Quality

The two main determinants of color print quality are resolution, measured in dots per inch (dpi), and the number of levels or graduations that can be printed per dot. Generally the higher the resolution and the more levels per dot, the better the overall print quality.

In practice, most printers make a trade-off between opting for higher resolution and providing more levels per dot. This is often determined by the printer's intended use. Graphic arts professionals, for example, are interested in maximizing the number of levels per dot to deliver higher 'photographic' image quality, while general business users will require reasonably high resolution so as to achieve good text quality and reasonable image quality.

The simplest type of color printer is a binary device in which the cyan, magenta, yellow and black dots are either "on" (printed) or "off" (not printed), with no intermediate levels possible. If ink dots can be mixed together to make intermediate colors, then a binary CMYK printer can print only eight 'solid' colors (cyan, magenta, yellow, red, green and blue, plus black and white). Clearly this isn't a sufficient palette to deliver quality color printing, which is why there are half tones.

Half toning algorithms divide a printer's native dot resolution into a grid of halftone cells and then turn on varying numbers of dots within these cells in order to mimic a variable dot size. By carefully combining cells containing different proportions of CMYK dots, a half toning printer can 'fool' the human eye into seeing a palette of millions of colors rather than just a few colors.

In continuous tone printing, there's an unlimited palette of solid colors. In practice, 'unlimited' means 16.7 million colors, which is more than the human eye can distinguish. To achieve this, the printer must be able to create and overlay 256 shades per dot per color, which obviously requires precise control over dot creation and placement. Continuous tone printing is largely the province of dye sublimation printers. However, all of the mainstream printing technologies can produce multiple shades (usually between 4 and 16) per dot, allowing them to deliver a richer palette of solid colors and smoother halftones. Such devices are referred to as "contone" printers.

Six-color inkjet printers have now appeared on the market, specifically geared for delivering photographic-quality output. These devices add two further inks - light cyan and light magenta - to make up for current inkjet technology's inability to create very tiny (and therefore light) dots. Six-color inkjets produce more subtle flesh tones and finer color graduations than standard CMYK devices, but are likely to become unnecessary in the future when ink drop volumes are expected to shrink to around 2 to 4 picoletres. Smaller drop sizes will also reduce the amount of half toning required, as a wider range of tiny drops can be combined to create a broader palette of solid colors.

Market-leader Hewlett Packard has consistently advocated the advantages of improving color print quality by increasing the number of colors that can be printed on an individual dot rather than simply increasing dpi, arguing that the latter approach sacrifices both speed and causes problems arising from excess ink - especially on plain paper. In 1996 HP manufactured the first inkjet printer to print more than eight colors (or two drops of ink) on a dot. Its DeskJet 850C being capable of printing up to four drops of ink on a dot. Over the years it has progressively refined its PhotoREt color layering technology to the point where, by late 1999, it was capable of producing an extremely small 5 pl drop size and up to 29 ink drops per dot representing over 3,500 printable colors per dot.