Reproduction of colorful originals in modern printing industrial techniques is possible by using raster, through which a continuous original tone value are converted in raster dots of different sizes and shapes, but equal density of opacity. The impression of halftone is provided by optical additive synthesis of raster dots.
The classical raster consists of fine dark vertical and horizontal lines, which create a raster mesh on a transparent background. The spacing between lines is usually equal to the line thickness and the density of the raster is determined by the number of lines to 1 cm.
Depends on the type of printing process, the surface smoothness of the printing substrate and type of graphic product, the raster density is selected from 20 to 120 lines / cm.
Types of Raster
Glass raster – In the two transparent glass plates with engraving and etching we get lines painted in black. Composed, facing one another at an angle of 90 °, they are giving raster mesh with crossed lines.
Glass raster’s can be rectangular for one color reproduction (with tilt line at an angle of 45 °) and circular for recording of color separations for multicolor printing with mechanism to adjust the angle of lines. Line raster with parallel lines is applied for special purposes.
Contact raster – consists of arranged alternately opaque and transparent boxes that gradually evolve into one another; made on transparent film serves for contact copying colorful tones of negatives and slides. They can be neutral (gray), purple (magenta-raster) or orange. Colored contact raster provide better contrast to reproduction.
Grained raster – for irregular structures of dots is used to reproduce photographs, originals in pastel, charcoal, chalk, etc., as well as to record parts of complementary colors in multicolor reproduction.
Digital raster – Unlike the glass, contact or grained raster, with digital raster all dots (pixels) are the same size.
Here with specific digital programs, each halftone dot is formed with a number of pixels. A group of pixels which forms a half-tone dot is called a halftone cell. The amount of pixels in halftone cell determines the number of gray shades that digitally created halftone raster can simulate.
Digital laser illuminators (photo-plotters) products in the RIP (Raster Imaging Processor) raster structure of an image by offering different types of screen (raster) elements: the most common are round, elliptical, square, line, diamond, sinus, etc.