Digital Photography

Laser printers

These printers work on the same principle as photocopiers: electrical charges on the surface of a drum alter according to the light falling on it. The drum picks up toner or not, according to the pattern of the charge, and deposits the toner onto the paper. The toner is then fused onto the paper by heat and pressure. However, where laser printers differ from copiers is in the fact that the charge is built up dot by dot and line by line. A laser beam reflects off a mirror that spins around: as it turns, the beam of light scans across the drum. By rapidly turning the laser on and off, a series of dots is applied to the drum. Color printers use colored toners to build up the image, usually one at a time, so a full print needs four passes. Laser printers are useful for rough proofing and good-quality text output.

Checking printer output

Today's desktop printers can outperform the quality of many digital cameras. But to ensure that you produce the best possible image, color settings have to be correctly made.

Examine closely any areas of very dark tones in your test print: do the dots of color appear to run together into uneven clumps or puddles? If so, the paper is unsuitable. Produce another test print on better-quality paper.

Examine any large areas of even midtones, such as blue sky or evenly painted walls. Do they appear banded (as if painted in strips with tiny gaps between) or uneven in any way? If so, reject the print.

Examine closely any small areas of what should be smooth, even transitions in tone, such as the side of a face falling into shadow or the bodywork of a car. Do they look mottled or banded? If so, reject the print.

Printers can lay more than 2,000 dots of ink per inch (dpi) but since no printer can change the strength of the ink and not all can vary dot size, not all dots are available for defining detail. Thus, printers use groupings of dotshalftone cellsto simulate a grayscale. At least 200 gray levels are needed to represent continuous tone. Therefore, a printer must control a large number of ink dots just to represent gray levels, reducing the detail that can be printed. In practice, detail resolution above 100 dpi meets most requirements. Now, if a printer can lay 1,400 dpi, it has some 14 dots to use for creating the effect of continuous tone. If you multiply together the four or more inks used, a good range of tones and colors are possible.