Read an Excerpt
Photographic Digital Printing
The Expanded Guide
By David Taylor
AE Publications LtdCopyright © 2013 AE Publications Ltd
All rights reserved.
To convert the pixels on the screen of your PC's monitor to a physical form you need a printer. Printers come in all sorts of shapes and sizes. Some types are better at certain tasks than others.
The rise of the printer
It used to be so simple. Once, there was very little choice if you wanted a printer. Generally, the same manufacturer made the only printer available for a particular computer model. Now, however, we are spoilt for choice. Printers are ubiquitous. This has partially been driven by improvements in technology. However, the biggest influence on the rise of the printer has been digital photography. Interest in photography is booming like never before. It's only natural that photographers want to be able to make a permanent, physical copy of their favorite images. Printers allow a photographer to control every aspect of image making, from initial exposure through to the final print. Not so very long ago this level of control was only available to those who were willing to spend hours in a darkroom.
This book is an introductory guide to printers and printing. It is biased more toward image-making with inkjet printers, but there is more general information about the concept of printing to be found too. As much as I like seeing images onscreen, there is nothing more satisfying than looking at a well-crafted print. Creating a print is an art in itself, but it is an art that can be learnt and, for the satisfaction it brings, ¡swell worth learning.
This is the simplest and most basic of the printer types currently on sale today. Dot matrix printers are only really suitable for printing text — though simple graphics can also be printed. The print is formed by a print head comprised of a vertical array of rods. These rods can be raised or lowered to create different patterns. The print head scans back and forth across the printer paper. As it does so the rods strike an ink-soaked ribbon to transfer ink to the paper. It's the rapid repositioning of the rods that forms the shape of the text or graphics.
Dot matrix printers, though simple, are generally very rugged devices. They are also usually cheap to run. Although the print quality isn't usually high, they are ideal for situations where a large amount of text or simple graphical data needs to be printed regularly.
Laser printers use light in the form of a laser to create a printed output. The laser beam projects an image of the page being printed onto a photosensitive rotating drum. The drum is initially given a positive electrostatic charge. As the laser projects the page onto the drum, the areas where there is type or an image switch to being negatively charged. Once the drum is ready it is coated in toner, which is also positively charged. The toner clings to those areas of the drum that have a negative charge, but is repelled by those areas with a positive charge.
Paper is then drawn into the laser printer and given a negative charge in the process. The paper is fed around the drum, and toner is transferred from the drum to the paper. As the paper passes over the drum it is given a positive charge to stop it sticking. Once past the drum, the paper passes through a pair of heated rollers that fuse the toner permanently to the paper (and make the paper warm to the touch as it comes out of the printer). Although the process sounds complicated — which it is! — laser printers are capable of printing documents extremely quickly. They are also generally inexpensive to run, and a typical toner cartridge is often capable of printing thousands of pages before it needs replacing.
Initially, laser printers only used black toner. Now, color laser printers are available at relatively affordable prices. Color laser printers work on the same principle as their monochrome cousins. However, they also include three other toner cartridges: cyan, yellow, and magenta. It's the toner from the four cartridges combined that allows the laser printer to print in full color.
A big drawback with laser printers is that they can only use a limited range of paper types. Since the printing process involves heat, paper with a plastic or resin coating cannot be used without risking damage to the printer. Although the print from modern color laser printers is very high quality it is not considered to be photo-quality. Laser printers are excellent for creating proof documents for checking, but less suitable for making exhibition-ready photographic prints.
Dye-sublimation printers use heating elements in a print head to transfer dye onto the surface of the medium being printed to. The dye is stored as a solid on a long cellophane ribbon comprising three colored panels — cyan, magenta, and yellow — followed by a protective overcoat. This type of ribbon is often referred to as CMYO. The panels are the same size as the material being printed on, so to create a 10 x 12in. (25 x 30cm) print the ribbon would need to be at least 10in. (25cm) wide by 48in. (1.2m) long (four panels, 12in. in length). However, most ribbons are longer to allow a number of prints to be made before a replacement ribbon is required.
When an image is printed, the first of the colored panels and the medium being printed to are aligned and pass through the printer. During this process the heating elements inside the print head rapidly change temperature to vary the amount of dye being transferred. As the dye is heated up it turns to gas, which solidifies on the medium as it cools.
The medium is then drawn back into the printer and aligned with the next colored panel, and the process repeats. The three colors combined produce the full color image. The final panel — the overcoat — bonds to the medium to form a protective layer. This is partially to protect the print from light scratching, but also to stop the dye from running if exposed to warmth later on.
The main advantage of the dye-sublimation process is that the print is made of a continuous tone (unlike inkjet prints — see page 14). This means that prints have the look of a traditional darkroom print.
Another advantage of dye-sublimation printers is that they are very clean. Unlike inkjet printers the dye is never in a liquid form, so there is never any risk of ink spills. A dye-sublimation print is also dry as soon as it is finished — unlike an inkjet print, it can be handled immediately. The final advantage is that it is very easy to calculate the cost of a print, as a ribbon is designed to produce an exact number of prints before it needs to be replaced.
For the creative printer, the big disadvantage is that the media that can be used to print to is limited, particularly in comparison to an inkjet printer. The paper used must have a special coating able to take the dye successfully. This makes dye-sublimation printers unsuitable for printing text documents on standard office paper.
The range of paper sizes that can be used is also limited by the size of ribbons available for a particular dye-sublimation printer model. Dye-sublimation printers are also weak when neutral black and white images are required. This is mainly due to the lack of a black dye panel on a standard CMYO ribbon. However, some dye-sublimation printers — such as the Canon Selphy ES range — do accept black and white (or BWO) ribbons.
Dye-sublimation printers aren't for everyone. Commercially, they are ideal for event photographers who want to sell prints of an event immediately — indeed some dye-sublimation printers can be powered by batteries, making them highly portable. However, if you want the greatest degree of choice when it comes to paper size and quality, inkjets take some beating.
Think of a printer for creating photo-quality prints and you'll probably picture an inkjet printer. Inkjet printers for home and small-office use have revolutionized photography, allowing the printing of photo-quality imagery at a relatively low cost.
The basic principle of an inkjet printer is fairly straightforward. Paper (or another suitable medium) is fed into the printer using a rotating roller. As the paper passes through the printer, the document being printed is gradually created by a moving print head. The print head squirts tiny amounts of colored ink as it scans backward and forward over the paper. Like color laser printers, inkjet printers use four colors to produce a full color image: cyan, yellow, magenta, and black (some inkjet printers refine this further by adding lighter versions of these four colors for improved color rendition). Each of these colors is held in individual reservoirs, either altogether in one cartridge block or as a series of separate cartridges.
The clever and less straightforward part of an inkjet is the mechanism used to squirt the right amount of ink out onto the paper. There are two technologies commonly used to achieve this. The first method, known as thermal, uses heat to create bubbles within an ink-filled chamber in the print head. These bubbles cause a change in pressure that forces droplets of ink out of the print head and onto the paper. Canon, HP, and Lexmark commonly use this method in their range of inkjet printers (Canon using the name Bubblejet for this reason).
The second method, known as piezoelectric, uses piezoelectric material in a chamber inside the print head. When a current is passed through the material it changes shape. This causes a change in pressure within the printer that forces ink droplets out. Epson commonly uses this technology.
Both systems have advantages and disadvantages. Thermal inkjet printers are usually cheaper to produce. However, the ink in a thermal inkjet needs the addition of a volatile compound for the process to work. This compound can cause clogging in print heads.
A slightly underrated aspect of inkjet printers is the precision of their roller mechanisms. The paper must be pulled through the printer extremely precisely to avoid noticeable gaps in the output. And this happens over and over again without fuss. Inkjet printers are astonishing pieces of technology.
The company that produces a particular inkjet printer will also produce the ink cartridges for that printer. This means that the ink will have exactly the right formulation for the print head to work effectively. Some manufacturers, most notably Epson, fit their ink cartridges with microchips. These microchips communicate information about the level of ink in the cartridge to the printer. Once the cartridge is empty the printer will stop working. However, to prevent the user from simply refilling the cartridge, the microchip cannot be reset.
You won't be surprised to learn that your inkjet printer uses ink to create a print. However, there's more to the story than that. There are two types of inks currently used by inkjet manufacturers: Dye and pigment. Both have advantages and disadvantages, so there's no clear winner between the two.
A dye is a colored substance that is either a liquid or is soluble in liquid (water is most often used as a base in which to dilute a dye). When applied to a material (such as cloth or paper) the dye alters the color of the material either temporarily (dyes can be washed away) or permanently (when fixed using a mordant). Dyes were originally derived from natural substances such as plants. Think of the way that the juice from berries stains clothing — a natural dye in action. However, today most commercial dyes are created synthetically using industrial processes.
Using dyes to create the ink for an inkjet printer has a number of advantages. Dyes are inexpensive, so reducing the cost of creating inks. Dyes are also vibrant in color, which means that highly saturated prints can be made reasonably easily. However, dyes aren't particularly lightfast. A print made using dye-based ink is prone to fading even over relatively short periods of time. That said, technology moves on and modern dye-based inks are much improved compared to those from ten or even five years ago.
A pigment is a colored particulate substance that isn't soluble in liquid. Instead, the particles of pigment are held in suspension by the liquid (again, this is most often water). Pigments aren't absorbed by material in the way that a dye is. Instead the pigments sit on the surface of the material.
Pigments are far more lightfast than dyes. They are an archival product, retaining their color over long periods of time. A print created using pigment ink should potentially last one to two-hundred years without fading (when using archive-grade paper and when displayed under reasonable light levels).
However, dye-based printers are still being manufactured, so if pigment inks are so good, why don't all printers use them? Pigment inks have their drawbacks. For a start, they are more expensive to produce than dye-based inks. Nor are pigments as vibrant as dyes — though again technology continually develops and modern pigment inks are far superior to those from even a few years ago. Pigments can also create problems with print heads — the particles in the ink can bond together and clog the print head nozzles if a printer isn't used regularly.
The prints from early pigment-based printers also suffered from a visual effect known as metameric failure (more often, and inaccurately, referred to as metamerism). A print that appears to shift color under different light sources suffers from metameric failure. For example, a cool gray under daylight-balanced lighting may appear to be a warm gray under tungsten lighting. Again, ink technology has improved and metameric failure is not the problem it once was.
Matte and photo black
All inkjet printers use a black ink cartridge. However, some inkjet printers require the fitting of either a photo black or matte black cartridge depending on the type of paper you use. This is because the ink in these cartridges is optimized for a particular finish on the paper. Photo black is required when printing on gloss or semigloss papers. Matte black is required when printing to matte, watercolor, or cotton rag papers. If the wrong cartridge is used with the wrong paper, the blacks in the print will look flat and have a slightly different reflectivity to the other colors in the print.
Older printers do not generally have space for both cartridges to be fitted simultaneously, so if you swap paper types you need to swap the cartridges over as well. Each time you do this the printer will have to perform a cleaning cycle to flush out the old ink from the print head. This wastes a lot of ink, and so discourages swapping paper types too often.
More recent printers tend to have space for both types of black cartridge, which is far more efficient in terms of saving ink. The cartridge used is set automatically after the media type is selected when setting the printing options for an image.
Droplets of ink
The concept that inkjet printers squirt droplets of ink onto paper is a very simplified explanation of what happens when a print is made. The droplets of ink are extremely small, typically between 50 and 60 microns in diameter (as a comparison, a human hair is approximately 70 microns in diameter). Because droplets of ink are used, inkjet printers can't print continuous tones in the way that dye-sublimation printers can. Instead, it is the extremely small size of the individual dots bunched together that conveys the illusion of continuous tone.
As previously mentioned, printers use four basic ink colors: cyan, magenta, yellow, and black. These four inks can be used to create a full color print by a process known as dithering. Dithering is a technique where the printer adjusts the sizing and spacing of the individual droplets of ink. By dithering the colored inks in the right proportions, the eye is fooled into seeing a full range of colors. It is only when the print is looked at microscopically that it's possible to see how the trick is done.
One way to assess the capabilities of a printer is to look at its maximum dpi value. Dpi stands for dots per inch and is a measure of how many individual droplets of ink a printer can print across one inch of paper.
The higher the dpi value, the finer the droplets of ink and the sharper the final print. The dpi value is therefore the resolution of the printer (dpi is subtly different to ppi, which is discussed in chapter 4. Dpi should only ever be applied to printers).
Usually printers can print an equal number of dots vertically as well as horizontally. If a printer is shown as having a maximum dpi of 600, it's usually fair to assume that this means for every square inch of paper 600 x 600 dots will be printed. If a printer has a higher resolution in one axis compared to the other, two figures are usually shown (4800 x 2400 dpi for example).
However, just because a printer has a high maximum dpi doesn't mean that it has to use its capabilities to the full every time a document or image is printed. Printers will typically lower their dpi when printing out in draft or normal mode in order to conserve ink.
The maximum dpi of a printer also only tells part of the story. The type of paper you use will affect the sharpness of the final print. Gloss paper suffers less from dot gain (see below) than matte paper, and so prints will look far sharper on this type of paper.
Excerpted from Photographic Digital Printing by David Taylor. Copyright © 2013 AE Publications Ltd. Excerpted by permission of AE Publications Ltd.
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