Senior prepress expert's understanding of network attributes
Halftone dots are the basic units that represent the layering and color variations of continuous-tone images. They form the foundation of printed images and play a role in conveying the tonal scale of the layout. The state (size and shape) and behavioral characteristics of halftone dots will affect whether the final print can accurately reproduce the tonal and color variations of the original. In this article, we invite senior prepress experts to share their interpretation of the four attributes of halftone dots.
Dot Size
Dot size is determined by the coverage of the dot, also known as dot area percentage. It is generally expressed in "cheng". A dot with 10% coverage is called "one cheng," 20% is called "two cheng," and so on.
Before color separation, it is first necessary to determine the dot percentage at a certain point on the screen. Dot percentages can be measured using a densitometer: first measure the integral density of the dot, and then convert it into the percentage of the dot area. This method is relatively scientific.
Another method to measure dot percentage is by visual inspection: judging the dot percentage based on the relationship between the dots and the blank areas.
The rules are:
If three dots of the same size can fit between two dots, this size of the dot is called '10%', that is, 10%;
If two dots of the same size can fit, it is called '20%', that is, 20%; if 1.5 dots of the same size can fit, it is called '30%', that is, 30%;
If 1.25 dots of the same size can fit, it is called '40%', that is, 40%;
If black and white are equal, this size of the dot is called '50%', that is, 50%;
When dot percentage starts from 50%, the dots will overlap. For ease of calculation, the black and white dots can be inverted, using the paper color as the blank standard. For example, if three blank dots of the same size can fit between two paper-colored blank points, this is considered '90%', that is, 90%.
Of course, if the sample itself is a printed product, then the dot gain percentage must also be considered. Dot gain percentage is related not only to press pressure but also to dot shape.
Dot Shapes
Dot shapes include circle, ellipse, square, chain (diamond), cross, diamond, line, dispersed, and other special shapes. Currently, the most commonly used dot shapes in China are circle, square, and chain.
First, we need to determine the halftone shape of the sample, and then understand how different halftone shapes expand during printing in order to restore the ideal state of the sample's digital original.
Next, let's discuss the characteristics and properties of several halftone shapes:
Circular halftones, when expressing the layers of an image, have highlights and midtones running parallel, with overlapping only appearing in the shadows. When the circular halftone percentage is about 78%, the perimeter is the longest. At this point, halftones begin to overlap and reach their maximum expansion.
Square halftones have the longest perimeter at 50% coverage. At this point, halftones begin to overlap, reach their maximum expansion, and are prone to banding.
Elliptical (chain-shaped) halftones appear diamond-shaped, especially showing a noticeable chain-like pattern after overlapping. Because elliptical halftones have short and long axes, the overlapping occurs at about 35% and 65% coverage, so these points have the maximum halftone expansion.
It is important to note that the expansion of halftone areas caused by overlapping in printed samples must be taken into account during color separation.
Halftone Angle
Halftone angle refers to the angle between the halftone line arrangement and the horizontal line. Generally, when the angle difference between two halftones is 30° to 60°, the overall moiré pattern appears aesthetically pleasing. When the angle difference is 15° or 75°, the moiré pattern becomes noticeable and can produce interference lines, commonly known as a turtle pattern.
From a visual effect perspective, a 45° dot angle produces the most comfortable and aesthetically pleasing visual effect, and it is the preferred angle for the main color tone in single-color or multi-color printing. A 0° dot angle has the worst visual effect and is generally used for the weakest color plate, such as the yellow plate.
Line screen
The size of the halftone dots is affected by the line screen; the higher the line screen, the smaller the dots and the higher the fineness. Common line screen counts are 80 lpi, 100 lpi, 150 lpi, 175 lpi, and 200 lpi. The line screen can be measured using a line screen gauge.