Laser-comparable inkjet text printing

The HP Deskjet 1200C printer achieves laser quality by means of pigmented black ink and precise, mode dependent control of drop volume. Contributing to laser printing speed are an intelligent print mode forecaster, a large memory capacity, heated drying, improved media handling, a larger printhead, and a high firing rate made possible by careful attention to refill dynamics.

The mission of the HP Deskjet 1200C print cartridge and product development team was to deliver text quality and speed that meets the expectations of the office printer market. The office standard for text printing has been set by the HP LaserJet series of printers. To be a general-purpose office printer, a printer must provide text quality, output speed and connectivity comparable to LaserJet printers, along with LaserJet language compatibility. This article discusses how two of those objectives--text quality and speed--were achieved. The article on page 85 discusses the compatibility and connectivity solutions.
Text Quality Fundamentals

Several attributes define the text quality of a printed page, regardless of whether its source is a serial impact dot matrix printer, a thermal inkjet printer, a laser printer, or a printing press. The fundamental characteristics that define print quality are:

* Character hue and darkness

* Edge smoothness or roughness

* Character edge contrast

* Presence of artifacts

* Uniformity of area fills.

Character darkness, or optical density, is a measure of the blackness (lack of lightness) of the printed image. In general, most surveys indicate that customers prefer darker characters over lighter characters. Hue refers to the tone of the color used to print the character. Even black characters can be slightly cold (bluish) or warm (brownish). The ability of the eye to distinguish small differences in hue diminishes at high optical densities.

In text printing, edge roughness is determined by several factors including printer resolution (often measured in dots per inch), dot placement accuracy, rendering algorithms, and the interactions between the colorant (e.g., the laser toner or the inkjet ink) and the paper. In general, higher resolutions produce smoother edges because they allow smaller changes in dot placement and the individual picture elements (pixels) correspond to smaller areas. Fig. 1 illustrates the improvement obtained by increasing resolution from 150 to 300 dpi with a binary printer. Firmware and hardware-based algorithms can be used to enhance edge smoothness further by judiciously placing dots between the basic grid points or by changing the dot size.

The contrast between the printed character and the background paper is affected by the optical density of the ink and toner, the color and brightness of the paper, and the edge transition sharpness of the printed area. For high-quality, high-contrast printing, the dark printed zones must transition sharply into unprinted zones. If the printed area slowly fades into unprinted paper, the characters appear fuzzy and soft.

Unwanted artifacts, such as inkjet spray and laser background scatter, can also make characters appear fuzzy. Inkjet spray is the presence of small, unwanted dots near the printed zones. In laser printing a similar phenomenon, called scarcer, sometimes occurs, leaving undesirable toner particles near the printed zones.

Solid area fills, used for graphics and large font rendition, should be uniform and dark. Nonuniformity of area fills can occur in a variety of ways, such as the mottle (light and dark areas) caused by the uneven penetration of an inkjet ink, the uneven gloss seen on many laser prints, banding, and density gradients.

Text Speed

Typical inkjet printers have text throughput ratings of one to three pages per minute. Laser print engines deliver four pages per minute in their low-end designs, 8 to lO pages per minute in the midrange designs, and as much as 16 to 20 pages per minute in the relatively expensive shared network devices. Our market target matched the low-end to midrange lasers, so we made our minimum throughput goal for high-quality text a true four pages per minute as measured by industry analysts' latest printer text benchmarks.

Comparing the Technologies

HP LaserJet printers and HP thermal inkjet printers use significantly different printing technologies. Each process has inherent advantages and engineering challenges. Fig. 2 illustrates the electrophotographic printing process and Fig. 3 shows the inkjet drop generation process. The basics of the thermal inkjet printing engine are described in "An Inside View of the Drop Generation Process" on page 11.

Laser printing has several high-value attributes. It provides a high degree of media independence, it is a high-speed page printing process, it is a dry process that doesn't wet the paper so physical distortions of the media are minimized, and it produces durable print unaffected by water and highlighters. Excellent character edge smoothness is achieved by the fusing process and the small toner particle size. Dot size can be adjusted, and customers perceive the process to be highly reliable.