Basic knowledge of printing process: basic elements of printing

Principles, characteristics and applications of outlets. The following content is suitable for offset printing, and some of the content can also be used as a reference for other types of printing. When the specific type of printing is discussed 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 briefly explained and not discussed in depth.

In conventional printing, the dot is the basic printing unit that constitutes the printed image. The contrast between this image unit and the blank on the printed product achieves the effect of reproducing the continuous tone.

The graphic information of the printed matter and the light and shade of the color are composed of dots of different sizes superimposed on each other during the printing process to produce a variety of different hue and different brightness changes. This kind of picture and text formed by dots is called "screen" on printing. The screen was originally a kind of "screen" used in photo-engraving and a traditional screen. When using it, install the screen In front of or above the photosensitive film, it is used to divide the incident light during shooting, and to form dots of different sizes corresponding to the color density of the original on the photosensitive film. The commonly used screens were glass screens in the early days, and later developed into contact screens. Now the new electronic color separation machine or color desktop publishing system no longer needs the screen to screen.

In the amplitude modulation screening method, the photoengraving adopts traditional screen equipment to screen, and the screen separation method used by the electronic color separation machine is generally the IS (Irrational Tangent Screening) dot technology (irrational tangent dot) and RT developed by the German Hell company. (Rational Tangent Screening) dot technology (rational tangent dots). PostScript screening technology is used in the color desktop publishing system.

The dot is the most basic element in the printing process. There are always certain restrictions on the number of screens, and the level change of the image cannot be changed steplessly like a continuous tone image, so the screen image is called a halftone image.

The dot is the basis of the printing and copying process and the most basic unit that constitutes graphics. The functions of the dot are mainly as follows:

1) Responsible for coloring phase, lightness and saturation in printing effect;

2) It is the smallest unit of fat-repellent and water-repellent, and the basic element of image transmission;

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

1. The size of the outlet

Traditional halftone images (generally formed by amplitude modulation dots relative to the FM network) are regularly arranged by dots with different sizes and equal distances (spacing between dot centers) to form a continuous sense visually and achieve the purpose of reproduction . Among them, the regularly arranged dots are all spread out from the center, so the number of net lines arranged by the dots determines the size of the single dot area.

The dot size is determined by the coverage of the dot, also known as the ink rate. It is customary to use "come" as the unit of measurement. For example, a 10% coverage outlet is called "10% coverage", a 50% coverage outlet is called "50% coverage", and a 0% coverage (ie, no outlet) It is called “out of the net”, and the outlets with 100% coverage are called “on the ground”.

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

The following is an example to calculate the size of the actual dot of the dot. Assuming that the area occupied by a single dot is S, specifically the area of ​​the dot is M, the number of screen lines is L, and the dot coverage is n%, then:

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

For example, the pip size of 50% of the 200-line dots 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 the printed matter is generally divided into three levels: bright tone, middle tone, and dark tone. The dot coverage of the bright tone part is about 1 to 30%; the dot coverage of the middle tone part is about 40 to 60%; the dark tone part is 70 to 90%. In addition, there are some nets and field parts.

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