The two main determinants of colour print quality are resolution, measured in dots per inch (dpi), and the number of levels or graduations that can be printed per dot. Generally speaking, the higher the resolution and the more levels per dot, the better the overall print quality.
In practice, most printers make a trade-off, some opting for higher resolution and others settling for more levels per dot, the best solution depending on the printer’s intended use. Graphic arts professionals, for example, are interested in maximising the number of levels per dot to deliver photographic image quality, while general business users will require reasonably high resolution so as to achieve good text quality as well as good image quality.
The simplest type of colour 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 (or toner) dots can be mixed together to make intermediate colours, then a binary CMYK printer can only print eight solid colours (cyan, magenta, yellow, red, green and blue, plus black and white). Clearly this isn’t a big enough palette to deliver good colour print quality, which is where halftoning comes in.
Halftoning 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 halftoning printer can fool the human eye into seeing a palette of millions of colours rather than just a few.
In continuous tone printing there’s an unlimited palette of solid colours. In practice, unlimited means 16.7 million colours, 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 colour, 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 colours and smoother halftones. Such devices are referred to as contone printers.
In the late 1990s, six-colour inkjet printers appeared on the market, specifically targeted at delivering photographic-quality output. These devices added two further inks – light cyan and light magenta – to make up for inkjet technology’s inability to create very small (and therefore light) dots. These six-colour inkjets produced more subtle flesh tones and finer colour graduations than standard CMYK devices, but some doubted that they’d be needed in the future, when ink drop volumes were expected to have shrunk to around 2 to 4 picolitres. The smaller drop sizes will reduce the amount of halftoning required, allowing a wider range of tiny drops to be combined to create a bigger palette of solid colours.
Long-time market leader Hewlett-Packard has consistently espoused the advantages of improving colour print quality by increasing the number of colours that can be printed on an individual dot rather than simply increasing dpi, arguing that the latter approach both sacrifices speed and causes problems arising from excess ink – especially on plain paper. HP manufactured the first inkjet printer to print more than eight colours (or two drops of ink) on a dot in 1996, 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 colour layering technology to the point where, by late 1999, it was capable of producing an extremely small 5pl drop size and up to 29 ink drops per dot – sufficient to represent over 3,500 printable colours per dot.