How Color Images are Printed on Photo Printers

Color printers create images by dividing a page into thousands, or even millions, of tiny dots, each of which can be addressed by the computer. As the printer moves across and down the page, it can print a dot of color, print two or three colors on top of each other, or leave the spot blank (white). To understand digital printing, you need to know a little about the colors that are used and the patterns in which they are printed.

As you've seen, color displays use three colors, red, green, and blue (RGB) to create color images on the screen. This process is referred to as additive color because adding all three colors together forms white. Color printers use a different process, called subtractive color. This process uses three subtractive primaries—cyan, magenta and yellow. When two of these are overprinted, they form red, green, or blue. When all three are overprinted, they form black. Most printers include a separate black color to provide a deeper black than that formed by combining the primaries. This is useful, not only for richer blacks in photographs, but also when printing text. These four colors give the color system its name—CMYK (C for cyan, M for magenta, Y for yellow, and K for black).

These low-cost printers do an amazing job of printing photo-realistic images on a wide variety of papers
These printers use ink-jet cartridges. Although you can print photos on plain paper, you'll find that liquid inks tend to soak into the paper taking the color along with them. You'll get richer colors using coated papers that are less absorbent and designed specifically for photographs. The ink dries partly by absorption and partly by evaporation. If the paper is too absorbent, the image looks washed out.
A cartridge of ink is attached to a print head with up to hundreds of nozzles, each thinner than a human hair. The number of nozzles and the size of each determines the printer’s resolution. As the print head moves across the paper, a digital signal from the computer tells each nozzle when to propel a drop of ink onto the paper. On some printers, this is done with mechanical vibrations. Piezoelectric crystals change shape when a voltage is applied to them. As they do so, they force ink through the nozzles onto the paper. Each pixel in the image can be made up of a number of tiny drops of ink. The smaller the droplets, and the more of them, the richer and deeper the colors should be.

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