Chapter II, Digital Color Theory: Lesson IV Digital Printing http://www.kodak.com/country/US/en/digital/dlc/book3/chapter2/digColorM4_1.shtml

                             All methods of printing images on paper have this in common: they all render color through subtractive color mixing.

                             Many digital color printers use only three colors: cyan, magenta and yellow. A three-color process produces very good quality images, depending on the nature of the colorants.

                             Inks used on printing presses are by nature somewhat impure. They are not perfect filters. They absorb some light that should be transmitted.

                             Deficiencies of printing inks have led to a four-color process that includes black ink. It makes shadows appear denser, and improves overall image contrast. Substituting black for overprints of 3 colors reduces ink usage. It reduces ink costs, and it makes the process easier to control.

                             Even when the four-color process is used, printed images have a color gamut smaller than transparency film or monitors. The gamut must be compressed to some degree during reproduction.

                             Color gamut varies, depending on the quality of paper and colorants. Those shown on the right will reproduce a wider range of colors.

Digital color printers use a variety of colorants.                              Digital color printers use a variety of colorants.

                             Ink jet printers form dots by spraying color inks from a matrix of tiny jets, controlled by digital signals.

                             Color laser printers are similar to color copiers. They use color toners. A photosensitive drum receives the laser-exposed image and transfers toner to paper.

                             Thermal printers use a heating element to transfer color from donor ribbons to a receiving material.

                             In thermal wax printers, the donor ribbon is coated with colored wax. A print head containing thousands of heating elements melts the wax, transferring it to the receiver.

                             After each color is transferred in a thermal printer, the receiver is automatically repositioned for the next pass. Proper registration of each pass is critical.

                             Thermal dye transfer printers use donor ribbons coated with special dyes. They do not use a screen process to print images. They produce continuous-tone output that rivals photographic prints for quality.

                             The four-color process used in high-volume printing requires screening to break up continuous tone images into dots. Dots for cyan, magenta, yellow and black inks are recorded on color separation films. These films are tightly registered for precise alignment.

                             The screened halftone separations are combined to produce a composite image. Type and line are not screened, but are included on one of the films usually the black film.

                             Halftone dots produce full-color images when printed with inks called "process inks." Dots vary in size with small dots in highlight areas and large dots in the shadows.

                             After color images are edited, an image setter processes the image information and records dot values on film to create the separations.

                             Halftone screens use a grid measured in lines per inch. Smaller dots can be formed within this grid. 150 line screens are common in commercial printing on good-quality paper.

                             Image setters produce page-size or larger films, with all page elements in the proper position. This eliminates hand-assembly of films called "stripping."

                             Color proofs are needed to check the image after separation and screening. Overlay proofs carry four colors on clear sheets, combining them for the full-color image.

                             Single-sheet, laminated proofs are more common. They provide a close representation of what the final print will look like.

                             Some proofing systems allow transfer of images to the paper that will be used to print the job. Since paper affects color reproduction, this is the most accurate type of off-press proof.

                             Digital color presses make color printing viable for new applications, in addition to mass reproduction. Today, color pages can be printed cost-effectively in quantities of hundreds, as well as thousands.