Digital Prints

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Mastering Digital Printing
Since these types of printers can only list relatively lowly 200 ppi, 300 ppi, or at the most,
400 ppi as their addressable resolutions, the manufacturers have come up with a marketing
term—”apparent resolution”—to put them on equal footing with all the inkjets that
are claiming much higher numbers.
Using the Océ LightJet 430 photo laser printer as an example, here’s how it works. The
LightJet accepts 24-bit, RGB color data. We know that each color is 8-bit, which represents
256 possible values per pixel. The equivalent commercial halftone printing device
would need a 16 × 16 cell to equal that same 256 levels (16 × 16=256). (If you don’t
know what a halftone cell is, don’t worry; you’ll learn about it soon. Just stick with me
for now.) So if you take 300 ppi (one of the LightJet’s two resolution settings) and multiply
that by 16 (16 cell units per pixel), you get 4,800. That’s 4,800 “dots per inch of
apparent resolution.” They’re not really dots in the same way that inkjets have dots, but
that’s what the makers of these devices have come up with as a way to do battle with the
army of inkjet printers covering the land. Unfortunately these “virtual dots” are of no
use in forming sharp-edged vector elements, so dye subs and photo printers are at a disadvantage
in printing fine text.
Some inkjets themselves have used “apparent resolution” to compete in the marketplace.
The now-discontinued-but-still-in-use, drum-based, wide-format inkjet printers IRIS and
ColorSpan’s Giclée PrintMakerFA have addressable resolutions of 300 dpi (the IRIS was
replaced by the IXIA, which is still being sold). However, they both claim 1800–2000 dpi
“apparent resolution,” based on either variable-drop technology, the ability to layer color
dots, or additional ink colors, or all three.
Variable-sized and overlapping dots are
clearly visible on an IRIS print on
Photo Glossy paper (40x blowup).
Courtesy of Martin Juergens