Output

Output

THE DIGITAL CORNER

In the last two articles I wrote about acquiring a digital image and "processing" it with the computer. The final step is to output (print) the finished image.

The technology for image output lags considerably behind acquisition and manipulation technology. A "printer" capable of maintaining the quality level of a conventional photographic print is generally well beyond the financial means of the amateur photographer or even a professional with a smaller business. Fortunately, there are lots of alternatives available, including both lower quality printers for home use and excellent quality devices through service bureaus. Color ink jet printers are very popular and cost as little as $150 [many are much cheaper now, but $100 is probably as low as you can go for "photo quality" printing]. They are often advertised as being capable of "photo-realistic" images, but this is really not true [still arguable, but it is a lot closer today]. Even the best ink jet devices, such as the Iris printer, produce images that, while quite attractive, look little like a photograph. Ink jet technology, like most "printer" technologies, has quite high resolution, but can only make each dot "on" or "off". To produce continuous tone they must "dither" the image, trading spatial resolution for greyscale. To get a reasonable number of tonal levels (say 64), the spatial resolution is reduced by a factor of about 8. Some inkjet printers designed specifically for printing photos alleviate the problem to some degree by using two different densities of ink for each primary (referred to as six or seven ink colors). Thus in highlight areas there will be a more even coverage of lightly colored dots, rather than a very sparse set of dark dots. [some inkjet printers also have variable dot size]

Dye sublimation printers produce continuous tone pixels. Most of them are moderately expensive, but recently several manufacturers have introduced printers for around $500. They are limited to small prints (typically 4 by 6 inches), and are not as good quality as the more expensive units, suffering from slight banding and lack of detail in shadow and highlight areas, but the image is still better than that produced by ink jet and the average person would accept a print from such a printer as a photograph. Another small problem with these prints is that they do not age well, particularly when handled like a "snapshot" print. A problem with many inexpensive printers, and especially dye sublimation printers, is that the print heads are under a great deal of stress, which causes them to wear out quickly. You will find that the cost of the print head is essentially the cost of the printer.

Professional output can be done in several forms: Film recorders put an image back on film (of just about any type or format). The quality level varies tremendously, and through service bureaus the price often does not reflect the level of quality. Quality is usually expressed in "lines", which measures spatial resolution and, unlike most other imaging devices, is the number of pixels across the whole image rather than per inch. A less frequently quoted specification is the number of bits per pixel, which is important to the "smoothness" of the image tonal range. A good quality film recorder will have a 16,000 line resolution and 36 bits per pixel.

Several technologies produce prints by imaging onto photographic paper. The image might be generated by a CRT (cathode ray tube) or a scanning laser beam. Some such printers limit the print size to 8 x 12 inches or so, while others can produce very large prints. The cost of such prints is dominated by the cost of the conventional print processing and materials, but is always higher due to the expensive equipment.

As I mentioned earlier, another alternative is an Iris print, which is a state-of-the-art ink jet device. By modulating very small ink nozzles and using special inks this device produces very clean, colorful images. It is generally used for more "digital art" than photographs because the appearance is unlike a photographic print, the process is more expensive than a photographic print, and (at least until recently) the dyes are less permanent.

There are also large format ink jet devices (usually called "plotters" instead of printers), which can make poster size prints quite economically, but with limited quality.

In all the above cases a big concern is calibrating your computer to the output device. It is often problematic to get two machines working together well, especially with "analog" functions. In this case the problem is compounded by the fact that many of the output devices work in the subtractive color space of CMY dyes while your computer's CRT display works with additive (RGB) color. Furthermore, CRTs and various other imaging devices have quite non-linear responses. Lasers and light-emitting diodes are available in rather limited colors, requiring yet another color space transformation. The science of coordinating all these factors is improving every day, but like setting up a color enlarger, there is still a lot of trial and error involved.

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