Coudray explains how following time-honored recommendations for pairing halftone line coutns can lead to undesirable printed results.
The basic line count is really not a very good indicator of what is happening in the image. When we speak of a halftone at 65 lines/in., we are really referring to the frequency ruling. In the early days of glass halftone screens, glass was scribed with parallel lines. These lines were filled with a black or gray dye. Two sheets of glass were glued together with the lines perpendicular to each other. This created a grid of lines and square spaces that was used to generate the halftone dot on the film during exposure. The frequency of the grid was only partially responsible for determining how much area the dot actually covered. With a short exposure, a very small dot was formed in the center of each grid opening. As the exposure increased, the dot grew in size until it finally connected to adjoining dots, forming a checkerboard at the 50% tonal size. This is typical of a square-dot halftone structure.
Moving to today's digital prepress model, halftones no longer are formed using glass or contact screens and varied exposure levels, but rather by depositing or exposing a matrix of pixels using a digital imagesetting device. Each halftone dot in the digital model is called a cell and is typically made of 255 pixels that can be turned on or off. Figure 1 shows a halftone cell that represents one halftone dot of a 6% tone. It's clear that the area covered by the dot is much smaller than the cell, which is where the problems begin for screen printers. The actual area covered by halftone dots varies by percentage, with 50% representing the largest area of coverage. The actual area of the halftone is mirrored on either side of this 50% value. For instance, a 40% dot has the same area of coverage as a 60% dot. The former represents a positive dot area and the latter a negative area.
The problem with dot area becomes more complicated when we rotate the halftone (change its angle). Rotating the angle dramatically changes the actual area covered by each dot. Figure 2 illustrates two different types of dots: square and elliptical. While both represent the same dot area, the orientation of the dots will clearly influence the potential for mesh interference.
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