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Conventional Screening
Conventional halftone screening technology (AM screening) was developed long before the advent of digital imaging technologies and was achieved with the tools available to the prepress trade at the time - cameras, glass & contact screens, rulers, and colour filters. Automation began with the introduction of electronic scanners that generated screened halftones direct to film. With the development of page description languages such as Postcript®, conventional screening methods were transitioned from proprietary electronic systems to more open digital processes. However, while making the computerization of the prepress process more palatable by applying a proven screenoing tecnique, problems inherent in the traditional method remained and were joinedby new problems resulting from limitations and characteristics of digital output devices.
The conventional halftone screening process breaks a continuous tone black and white image into a series of dots of varying sizes and places these dots in a rigid grid pattern. For a continuous tone color image it is necessary to break the colors down into the four process color inks - CMYK (cyan, magenta, yellow, black. Aprocess known as seperation). Each color is screened to a different piece of film or plate to create a color seperation. To avoid image distortion, the rows and columns of dots for each seperation are rotated to print at different angles. Gradiations of color and intensity are reproduced by increasing or decreasing the actual size of the dots in a particular area of th image while the distance between dots remains constant. Sharpness is controlled by changing the screen ruling, or the constant distance between the dot centers, measured in lines per inch. The closer the dots in a particular screen (the higher the screen ruling), the better the representation of image detail, but the more difficult it may to print or repoduce.