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Dots Per Inch? The Fine Art of Print Quality

6 Mar, 2002 By: Terry McManus imageSource

Dots Per Inch? The Fine Art of Print Quality

While manufacturers tout
dpi settings as the key to resolution, it’s not necessarily so. Savvy buyers
look beyond this hype to find the printers and copiers that best suit their


As you walked around your
workplace today chatting with colleagues, chances are nobody was dishing out
"dots per inch". As an opener for small talk, it’s likely to elicit
nothing more than a quizzical look from your co-workers.

Even if you use the
printer every day, the machine’s dots per inch (or dpi) workings remain, for
the most part, a distinctly behind-the-scenes element. Few office employees
waste time worrying about it. Nevertheless, "dots per inch" is a critical
concept for desktop printer manufacturers, almost all of whom trumpet the
machine’s dpi setting much like Hollywood heralds a movie’s box-office take.

However, dpi alone does
not promise quality any more than a big star guarantees a great film. So
understanding what it means, distinguishing sales hype from what comes out of
your printer, and controlling the finished product is truly a fine art, and
you’re the artist.

"Epson basically raised
the whole dpi issue and they’ve done a fantastic job in marketing that,"
says Marco Boer, a consulting partner with IT Strategies, Inc., in Hanover,
Mass., which tracks the imaging technology industry. "A lot of people still
gravitate toward that, because it’s an easy thing to grasp: bigger numbers
means better quality, right?" Boer adds that since dpi is relatively easy to
explain, users have made it the standard measure of print quality. "This is
not quite right," he says, "but sometimes perception is stronger than

To be sure, nearly every
manufacturer’s sales literature talks up dpi with a certain amount of zeal, as
if they’re challenging their rivals to a pole-vault competition. Indeed,
according to Kevin Kern, vice president, digital business, at Konica, most users
now expect to print at 600 dpi, and the norm will likely rise to 1200 in the
near future.

What Is It?

First, some basics: A
printer’s dpi rating refers to the bits of toner the machine delivers within
one square inch of paper (picture a grid with individual dots packed next to
each other). The rating is usually abbreviated, so that 300 dpi, for instance,
really means 300 x 300 resolution, both horizontally and vertically on the page.
Multiply those two numbers and you get 90,000 dots of toner occupying the
square. Notably, a 600 dpi rating does not mean that resolution is double that
of a 300 dpi printer, it’s actually quadruple, since the machine is spreading
360,000 bits of toner into the same grid. Accordingly, a 1200 dpi engine
deposits 1,440,000 bits. With so many droplets of ink filling such a tiny area,
graphical images (as well as text) can look seamless, even though minuscule
grains of paper regularly interrupt the droplets. In fact, under extreme
magnification, color toner particles resemble sprinkles on a scoop of vanilla
ice cream. These color droplets, cyan, yellow, magenta, and black, plus light
magenta and light cyan in some printers, blend together to reproduce complex
images. The printer blends these dots together, creating a palette of millions
of possible colors.

The two most common types
of desktop printers, laser and inkjets, apply the dots in different ways.
Suffice it to say that in the operation of a laser printer, as image is first
painted in memory and then "rasterized" that is, peeled out of memory
bit by bit. In a subsequent step, the paper passes through a heated area of the
printer, which melts the toner’s plastic encapsulate and fuses the carbon-dot
pigments to the paper  (on a
monochrome printer, shades of gray are created by dot dithering. That is, darker
areas have closely spaced dots, lighter areas sparser ones).

Unlike lasers, inkjets in
effect, do not take a picture of the entire page before peeling it from memory.
Rather, they read the page as they go, using nozzles to spray ink droplets onto
the page line by line. This is why an inkjet will take several frustrating
minutes to output a graphics-intensive page, while a laser will spit out six or
eight copies of the same page in rapid succession. For economic reasons, users
often have to choose one type of machine over the other, while weighing their
own quality standards. When doing so, it’s helpful to keep in mind that color
lasers work much faster than inkjets, but the lasers’ initial purchase price
is considerably more than their inkjet rivals. In fact, you can expect to pay
more than $2,500 for newer lasers from HP and Canon, but less than $200 for a
good inkjet model made by a variety of manufacturers 
(the initial price, however, does not take into account the lifetime cost
of ownership).

Adjusting DPI

Regardless of the type of
printer you use, you may eventually have to deal with dpi-related questions,
especially if you purchase printers for your company. The good news is that
manufacturers have been steadily improving resolution through new technologies.

"The quality of the
output devices "laser and inkjet" is phenomenal today," says Daniel
Wilson, associate professor of printing and publishing technology at Vanderbilt
University in Nashville, Tenn. For text output, he says, "Most printers today
have ‘anti-aliasing’ capabilities. That is, the little jagged edges that
would otherwise be there are filled in by the laser printer." Several years
ago, Hewlett-Packard introduced this technique and called it "Resolution
Enhancement Technology" (Ret), which automatically inserts smaller dots at the
edges of lines and characters to smooth out edges and reduce jagged appearances
(similar techniques may be called "edge enhancement" by other

For many office employees,
of course, such considerations vanished at about the same time as the dot-matrix
printer. To be sure, users might notice a problem with text only if their toner
starts running low, and while speed is always a factor, for typical users the
print quality acid test boils down to how well the equipment can reproduce color
images, particularly photographs.

"In the office market,
80 percent of the printing is black and white," says Chris Snapp, marketing
communications manager with Lexmark International. "That said, the Web is one
factor driving color. That’s beginning to eat into mono "The output still has
to be high quality because users will demand that. They’d like to see what
they see on their screen, which is very crisp, very vibrant, very

"Use of the Web in
business has raised quality expectations," agrees Dan Barlett, a document
systems integrator with American Image and Information Systems in Dallas. More
office workers, he notes, are also scanning their own photographs or getting
them from the Web. In such applications, dpi comes into play, and scanning
software allows users to adjust dpi settings before scanning a photo into a
printable file.

The Photographic Standard               

To improve color quality,
manufacturers have added light cyan and light magenta to the standard four-color
scheme. These subtler shades "have led to a huge benefit called ‘high
fidelity color," says Bryan Ballreich, manager of new products with Roland
DGA Corp. in Irvine, Calif. "The [new tones] enable you to represent colors
that would have been darker tones but sparsely sprayed."

For instance, say you had
the two additional colors available to print a photo of a person whose nose had
a highlight caused by a flashbulb. Around the nose, Ballreich says, leading into
the flesh tone, you would see light magenta instead of the sparse droplets of
full magenta that a four-color print engine would have produced. "You won’t
pick up on the fact that the spot grows ever so slightly more pink," Ballreich
explains. "Then the full magenta kicks in on the nose and you get a beautiful,
more natural pink."

Perhaps it’s not
surprising then that manufacturers have begun comparing printer output to
photographic images. In theory, the higher the dpi rating, the more closely its
print product begins to resemble the "continuous tone" (or contone) created
by photography. That’s not to say, though, that some printers can‘t achieve
contone. Thermal transfer and dye sublimation printers, for instance, use
extremely high heat to turn ink into a gas, rendering a photographic appearance
on paper. This technology actually operates at relatively low dpi values "
typically 150 and no more than 300, according to Boer. The problem is that, at
$7,000 or more, these printers have proven too expensive for everyday office
use. Consequently, says Boer, "This technology is fading away."

According to Stephan Ohr,
senior editor of Electrical Engineering Times, "Because of their lower cost
per copy and higher printing speeds (10 pages per minute and up), laser have
pretty much replaced thermal-transfer and dye-sublimation devices as the
preferred color hard-copy devices in networked offices."

By definition, inkjet and
laser printers can’t achieve contone. Rather, print quality is defined by two
underlying factors:  resolution
(actual dots-per-inch) and the number of levels that can be printed per dot.
Since printers have a limited color palette, halftoning methods are used to
create the illusion of continuous tone. The end result is virtually
indistinguishable from true contone and the human eye cannot distinguish between
the different shades of colors.

"Contone is more of a
marketing term than a scientific one," says Kirk Green, president of
CreativeColor.com, a print and graphics company in Salt Lake City. "Office
printers can only simulate contone." The lesson? Take sales hype with a grain
of salt, and beware of your printer’s limitations.

How High Can It Go?

The growing popularity of
color and demand for greater fidelity has spurred manufacturers to make
ever-rising dpi claims. Epson and Canon, for instance, make inkjet printers at
1440 x 720 dpi. That’s 1,440 dots on the vertical axis of the page and half
that on the horizontal plane, a ratio that still yields fewer dots than a
true 1200 x 1200 dpi printer.

However, do such
distinctions really matter to most users? Some say no. "To me, 1440 x 720 is
indistinguishable from photo quality," says David Zoretic, director of
marketing, worldwide digital imaging at Ilford Imaging in Paramus, N.J. "So to
go to higher resolutions, I don’t know what you would gain."

The growing popularity of
color and demand for greater fidelity has spurred manufacturers to make
ever-rising dpi claims.

Boer of IT Strategies is
more emphatic. "We’re at a stage now where it doesn’t make any sense to go
any higher than 600 dpi in laser printers. It makes sense for publishing
applications, but for the general office user, you and I won’t be able to tell
the difference."

Despite the
manufacturer’s ongoing campaign to escalate dpi ratings, even they sometimes
admit that sheer numbers of dots don’t necessarily add up to a superior
output. Hewlett-Packard, among others, makes this argument. "DPI is an easy
statistic to use," says Marie Tadlock, worldwide print product manager for the
company’s LaserJet division in Boise, Idaho. "However, there’s a lot more
to print quality in mono and color than dpi."

Rather than trying to pack
more dots into the print area, HP has made breakthroughs in adding more drops of
toner onto each dot. Its Color LaserJet 4500 is a 600 dpi printer that
incorporates ImageREt 2400 technology. In this process, toner gets fused
together in four layers, forming a solid dot of melted plastic. The system
"gives us the equivalent of 2400 dpi with a 600 dpi engine," says Tadlock.
By employing this layering technology, higher print quality is achieved without
sizes and, consequently, slower printing.

As Tadlock notes, "It
makes our pipeline as efficient as possible when printing, but also gives you
the best print quality possible." She adds that the company backs up those
claims through psychometric testing, in which people view printer output samples at various dpi levels and rank them by preference. "We recommend that [customers] have audiences look at the output and decide what they like."

Other Quality Factors

That’s good advice, says
Green, who believes that dpi figures can be misleading anyway. "The rating is
important, but so is the printer itself. We’ve seen printers that claim to
have a very high dpi, but their math formulas for configuring the dpi are pretty
weak. Even though they claim a higher dpi, the visual impact of the print is
lower than a lower-rated dpi printer, which may have a better ability to
manipulate the data."

Green says printer buyers
can do their own form of psychometric testing. He advises them to create a
computer file of a typical print job and ask the dealer to print it out a low,
medium, and high-resolution settings.  Why?
As Green explains, manufacturers often optimize the sample file for that
particular printer. So for instance, if the printer has a hard time rendering
high contrast values (such as photo mixing bright sunlight and deep shadows),
then their samples may be intentionally low contrast, or, he continues, "If
their printer tends to break down when they’re outputting vibrant colors,
they’ll print out something more subtle, like watercolors."

Green also advises
considering how the dealers print media samples. They may print out on expensive
glossy paper, while your normal choice would be regular office stock. Green’s
advise: Use the lower-quality stock in the demonstration.

What’s Ahead?

It’s not as if the
manufacturers aren’t trying to improve all aspects of their products; it’s
just that dpi gets so much attention that customers may fail to understand the
relevance of other quality factors. Chances are you could distinguish between a
picture produced by a laser or inkjet printer at 1200 dpi from one output at

However, you are not
guaranteed gorgeous documents just because your printer boasts the highest dpi
numbers. As Tadlock observes: "Print quality is in the eye of the beholder;
don’t be hoodwinked by dpi numbers." Other elements have to work in concert
with dpi ratings, or such figures begin to lose meaning. Ink, for instance, is
critical in the printing process; specifically, ink needs to dry quickly once it
hits the page. "If prints don’t dry quickly enough, then in a stack, the ink
could transfer from the top of one page to the bottom of the next," says
Ilford Imaging’s Zoretic. While the printer companies are looking at speed and
image quality, he points out, "the consumables companies are trying to solve
the dry-time problems through ink and media."

Ironically, a lower
concentration of ink may lead to improvements. According to Zoretic, such
"diluted inks" are now coming onto the market. They could enable printers to
drop more ink into highlighted areas, further simulating continuous tone images.
For its part, printer manufacturer Epson claims an advantage in this area
stemming from its Piezo-electric inkjet technology, which differs from other
methods that employ a thermal-based process. "You may wait up to ten minutes before our competitors’ inks dry," says Rajeev Mishra, product manager of consumer inkjet printers at Epson. "Ours dries in a tenth of second, using electronic impulses rather than heat." Increased control over dot size and placement are major advantages to this technology, although printing speed is sacrificed. Also, Epson’s quick-drying inks maintain their integrity when coming in contact with the paper and do not suffer from bleeding effects, like some ink jet devices. This factor plays a major role in improving print quality.

Not to be outdone, Konica,
says Kern, is developing polymer toner, which will allow better control of a
particle’s shape (a smaller, rounder dot is better than a large, jagged one).
The particle sizes of this toner will be in a range of three to seven microns (A
micron is .00004" wide; by comparison, the width of a single human hair is
.0025!"). The toner will fuse with the paper at a lower temperature, Kern
says, and will cut the machine’s electrical usage. "This technology will be
in some of our products by early next year," he adds. When that happens,
don’t expect to see your co-workers brandishing their electron microscopes to
verify manufacturers’ dpi claims. Do expect to see them smiling if they are
able to print out a nice-looking document (or complaining if they can’t).
"The problem is that manufacturers tend to think, and they’ve driven it
into the customers’ mind, that higher is better," says Kern. "And
higher can be better, but in a lot of cases it’s not. What users have to look
for is the overall image quality. Because that’s the real world today of
printing and output.

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