Traditional printin presses cannot print[print?:printing image graphic cutting solution] continuous tones: smooth graduations from light to dark tones of the same color. The way printers have gotten around this problem is to create an optical illusion called a halftone. Look at any photograph in a newspaper through a magnifying glass and you will see that it is composed of thousands of dots . The halftone converts a continuous tone into a pattern of solid dots. The dots are either very small for highlighted areas or very large (relatively speaking) for shadows. (In stochastic screens, the dots are all the same size, but randomly distributed, producing a similar effect.) Viewed at a distance, the eye cannot distinguish between these tiny printed motes, and fills in the missing information to “reform” the image as a continuous tone .
A color photograph of continuous tone is separated and screened into the four process colors: cyan, magenta, yellow and black (CMYK). By combining various combinations of CMYK dots of differing sizes, many more colors may be simulated. These are called manufactured colors. See an example of how the four process colors combine to form the final image. Most, but not all colors may be reproduced with process inks. The Hexachrome system adds orange and green to the original four, increasing the spectrum of colors that may be manufactured. However, some colors, such as grays, reds and violets do not manufacture perfectly from a combination of the four CMYK inks. In this case, these colors may be matched by a single spot color ink, providing there are enough units on the press.
The number of halftones that are printed in a linear inch is called line screen. It’s value is expressed as Lines per Inch (LPI). Newspapers, which are printed on porous, lower quality paper[paper?:paper sample maker cutting machine] may require as little as a 70 line screen to reproduce a reasonably sharp image, but high quality images printed on packaging[packaging?:DCP-H series packaging sample cutter plotter] will typically be printed between133 and 150 LPI. Glossy, color magazine print may go as high as 300 LPI or more.
