The principle, characteristics and application of the network. The following contents are applicable to offset printing, and some contents can also be used as reference for other types of printing. When the printing type is specifically mentioned later, it will be explained one by one. Because this series of articles is a tutorial for design and color separation output personnel, it is not a professional discussion of printing. Some principles and algorithms are only explained slightly and are not discussed in depth.

In conventional printing, the dots are the basic printing units constituting the printed image, and the contrast between the image units and the blanks on the printed matter achieves the effect of reproducing the continuous tone.

The graphic information and color shading of printed matter are composed of different color hue and different brightness changes by superimposing different sizes of dots in the printing process. The graphic formed by the dot is called "screen" in printing. The screen is originally a kind of "screen" in the traditional sense used in photo-engraving. When used, the screen is installed. In front of or above the photosensitive sheet, the incident light is split at the time of shooting, and dots of different sizes corresponding to the original color shade are formed on the photosensitive sheet. The commonly used screen is a glass screen in the early stage, and later developed into a contact screen. Nowadays, the new electronic color separation machine or color desktop publishing system has no need for screen screening.

In the amplitude modulation screening method, the photoengraving method uses the traditional screen equipment to add the net. The hanging network method used by the electronic color separation machine is generally the IS (Irrational Tangent Screening) dot technology developed by Hell Company of Germany (irrational tangent outlet) and RT. (Rational Tangent Screening) dot technology (rational tangent outlets). PostScript screening technology is used in the color desktop publishing system.

The dot is the most basic element in the printing process. The number of stages in the screening is always limited, and the level change of the image cannot be changed steplessly like the continuous tone image. Therefore, the screen image is called a halftone image.

The dot is the basis of the printing and copying process, and is the most basic unit that constitutes the graphic. The functions of the dot are:

1) The task of coloring phase, brightness and saturation on the printing effect;

2) is the smallest unit of sensory water and water, is the basic element of image transmission;

3) In the color synthesis is the organizer of the image color, hierarchy and outline.

First, the size of the outlet

Traditional halftone images (generally formed by amplitude-modulated dots relative to the FM network) are arranged by dot rules of different sizes and distances (center spacing of dots) to form a continuous sense of vision for replication purposes. . Among them, the regularly arranged dots are outwardly diffused from the center, so the number of network lines arranged by the dots determines the size of a single dot.

The dot size is determined by the coverage of the dot, also known as the ink rate. It is customary to use "cheng" as a unit of measure. For example, a network with 10% coverage is called "one network point", a network with 50% coverage is called "five network points", and 0% coverage (ie no network). For “unlimited”, the network with 100% coverage is called “field”.

The number of screen lines is initially defined according to the number of lines above the screen. The more the number of screen lines, the higher the fineness. Common screen lines are 80lpi, 100lpi, 120lpi, 133lpi, 150lpi, 175lpi, 200lpi. That is, the number of network cables per square inch. It is thus also possible to calculate the number of dots per square inch, which is equal to the square of the number of wires.

The following is an example to calculate the size of the actual point of the network. It is assumed that the area occupied by a single network point is S. Specifically, the area of ​​the network point is M, the number of network lines is L, and the coverage rate of the network is n%.

M = S × n% = 2.54 cm × 2.54 cm / (L × L) × n% The area unit obtained by the formula is square centimeter.

For example, the 50-point dot size of a 200-line dot is:

2.54×2.54/(200×200)×0.5=0.000080645cm2=0.0080645mm2

That is, the dot diameter is about 0.09mm

The tone of a printed matter is generally divided into three levels: bright, middle, and dark. The coverage of the bright spot portion is about 1 to 30%; the coverage of the halftone portion is about 4 to 60%; and the darkness is 7 to 90%. There are also net and field parts.

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