Feature

The technology behind colour printing

Although printing technology has improved over the years, no single solution offers the ideal combination of low cost and high quality

People regularly brag about the size and speed of their computers, but it's rare to hear them boasting about their printers' DPI and six-colour, photo-quality inks. Too bad, because there's much to marvel at in the ever-advancing technology of colour printing. In fact, printers are in many ways just as complex and interesting as the computers that drive them.

With the advent of digital photography, the printer is finally coming into its own. To meet the rising expectations of consumers, engineers and designers are making constant advances ( the speed, durability and image quality of today's printers are on the rise even as prices fall. Today, photo-quality colour printing is well within reach of the average consumer.

Currently, at least six distinct kinds of colour printers can deliver prints of photographic or near-photographic quality.

Inkjet printers

According to legend, inkjet printing was born in 1977 when a research scientist inadvertently held a hot soldering iron to a syringe filled with ink. As the ink reached a certain temperature, a small drop shot out of the syringe's tip. This gave the researcher an idea ( instead of using traditional printing methods, why not spray tiny, accurately-aimed bits of ink onto paper?

This idea of spraying ink onto paper is now known as the "non-impact" principle and all of today's inkjet printers rely on it. Most inkjet printers employ a heat-based printing method. These printers contain hundreds of tiny nozzles with openings that are finer than a single human hair. Behind each nozzle is a tiny reservoir of liquid ink and behind the reservoir lies a heating element that warms up the ink in the reservoir. Heating the ink forces tiny drops through the nozzle and onto the awaiting paper.

Some inkjet printers also rely on a variation of the thermal method called the Piezo-electric system. Rather than a heating element, each nozzle is connected to a so-called Piezo crystal, a tiny structure that reacts to an electrical charge. When a current passes through the Piezo crystal, the crystal flexes, which in turn forces a tiny drop of ink out the nozzle.

Because different nozzles can contain different colours of ink, inkjet technology has easily adapted itself to colour printing. The first widely available colour printers contained cartridges for cyan (blue), magenta (red) and yellow from which thousands of shades could be mixed. Note the lack of a black cartridge ( inkjet printers mix the cyan, magenta and yellow to yield a brown so dark the eye sees it as black. Today's photo-quality printers add two more colours, light magenta and light cyan. These additions significantly widen a printer's palette, making it possible to create subtler, more realistic images. Colours are mixed either in the reservoir behind the nozzles or on the page itself (the process of mixing colours on the page is called layering, as one colour is printed on top of another to create a new hue).

Inkjet technology has two significant advantages for the average consumer. First of all, it can produce very high-quality images. Second, initial purchase prices are low. On the down side, print times ( while constantly improving ( are still slow relative to laser printers. Nevertheless, inkjet printers produce photographic quality images much faster and at much less cost than traditional photographs, and inkjet technology is the method of choice for most nonprofessional digital photographers.

Laser printers

From the name alone you can tell that laser printing is very different from the inkjet system. For a clear understanding of the principles, let's first consider a black-and-white laser printer.

First, the printer breaks an image down into a "bitmap". That is, it translates the image into a series of tiny squares that form a kind of checkerboard, very much like the pixels that make up a digital photograph. Next, the laser itself comes into play. Row by row, it sweeps across a charged photo-receptor known as a "drum" sending a beam of light to each "bit" that should be black and bypassing areas where the paper should remain white. The light of the laser leaves a positive electric charge wherever it hits, which creates a mirror image of the original document on the drum.

That is how the image is captured. Next it must be transferred onto paper. First, a very fine, negatively charged powder known as toner is applied to the drum, adhering to the positively charged image. Meanwhile, the paper receives a slight negative charge as it passes an electrified wire. When the paper comes into contact with the drum, the paper's negative charge attracts the toner and the image is transferred. Finally, heat and pressure are applied to the paper. This melts the toner, which contains small amounts of wax, and fixes it to the paper.

Colour laser printers operate upon the same principle, except each colour is printed as a separate image. That is, all the magenta bits are recorded and printed, then cyan, then yellow and finally black. Colours are either mixed on the page, or, in less expensive models, they are "dithered" ( which means different colours are produced by alternating tiny dots of different colours. For example, green tones are produced by printing neighbouring dots of cyan (blue) and yellow.

Laser printers have several advantages. First of all, the cost for individual prints is low. That's because toner is inexpensive, plus it adheres well, even to inexpensive paper (not true for inkjet images, which tend to bleed on cheap paper). Second of all, laser printers, even colour laser printers, are remarkably fast. They do have drawbacks, however. Initial costs are high and upkeep can be expensive. Laser printers, especially colour lasers, require regular maintenance, such as replacing drums and ink cartridges. Additionally, the laser-printing process uses large amounts of energy and produces environmentally unfriendly ozone.

Laser printers are popular with businesses where speed and lower per-print costs are important. However high initial costs and less than perfect images make laser printers less attractive to most digital photographers.

Dye-sublimation printers

Designed for professional graphic artists, dye-sublimation printers operate like inkjet printers, except that they reach such high temperatures (upwards of 500 degrees Celsius) that the ink actually turns into a gas. This allows colours to mix extremely well, and instead of laying down individual dots of colour, one colour blends invisibly with the next. Each colour ( usually yellow, cyan, magenta and black ( is printed individually, then mixes seamlessly on the paper.

This technique produces the highest quality images, making it ideal for professional graphic artists and photographers, but it presents a couple of drawbacks for the typical consumer. First of all, the printers themselves are very expensive, costing 10 or 20 times as much as an inkjet printer. In addition, they require paper that is specially coated to receive the ink without bleeding, which poses an additional cost. Finally, their output can be frustratingly slow.

Thermal wax printers

Closely resembling the dye-sublimation method, thermal wax printers are designed especially to produce colour transparencies. The inks are wax-based, enabling them to adhere to the special transparency paper that's required.

Such printers are state-of-the-art for producing transparencies, but they don't necessarily work for all-purpose colour printing. First of all, wax-based ink is less malleable than other inks. That means individual dots are larger, limiting the level of detail they can capture. They're also expensive to buy and materials can cost a pretty penny. Unless you're running an office that regularly requires transparencies, a thermal wax printer is probably not necessary.

Solid ink printers

With the solid ink method, sticks of wax-based ink are heated up until they reach liquid form. Then all three (cyan, magenta and yellow) or four (cyan, magenta, yellow and black) colours are squirted onto a transfer drum, mixing together to form a mirror image of the final image. Finally, paper is rolled across the drum and receives the final image.

Representing something of a compromise between inkjet and laser systems, solid ink printers are fast and require relatively cheap paper and ink (like laser printers), but their initial purchase price is relatively low (like inkjet printers). This combination of speed and low cost makes them a good choice for an office environment. However, solid ink printers lack the level of quality found in either laser or inkjet printers, thus they aren't necessarily the best choice for the digital photographer.

Thermo-autochrome printers

More than any other method, thermo-autochrome printing is most reminiscent of traditional photographic processing. Before printing, thermo-autochrome paper looks white, but in fact it's covered with three layers of light-sensitive ink ( cyan, magenta and yellow ( each of which is activated at a certain temperature. As the special paper passes through the printer, tiny modules heat up tiny areas of the paper, activating the hidden colours one by one. Then an ultraviolet light passes across the paper, "fixing" the colours, that is, preventing the inactivated ink from developing.

While the thermo-autochrome method is designed especially for digital photography and rivals the quality of inkjet printers, it's not necessarily the ideal choice for all consumers. The printer has very limited use since it can only print with highly specialised paper. By contrast, an inkjet printer can accommodate a wide variety of papers, which provides more flexibility and can help keep long-term costs down.

No single printer is perfect under all circumstances. Unless money is no object, you must balance a number of factors: the cost of the printer, the cost of inks and paper, printing speed and image quality. Certain printers function well with inexpensive paper, but they could never deliver a high-quality, glossy image that resembles a traditional photograph. Other printers boast rapid print speed yet can't capture the subtle colours, e.g., flesh tones that make a photograph appear realistic.

Businesses, where sheer volume makes speed and low per-print costs important, tend to prefer laser or solid ink printers. For professional photographers and graphic artists, image quality is vital, so they'll may make the big investment necessary to purchase dye-sublimation or thermal wax printers. But for the individual digital photographer, the thermo-autochrome or the even more flexible inkjet system generally represents the best combination of price and quality.

Compiled by Ajith Ram

(c) Hewlett-Packard 1998


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This was first published in August 1999

 

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