An Analysis of Knowledge about Halftone Dots

Posted on 04-28-2024 by

In the printing process, the density of halftone dots is adjusted to represent continuous-tone images. By blending the halftone dots of the CMYK four colors, an infinite number of colors can be achieved. Currently, there are two main types of halftone dots used in printing processes: Amplitude Modulated (AM) dots and Frequency Modulated (FM) dots.

AM Dots

AM dots are one type of halftone dot currently in use. Their dot density is fixed, and color depth is achieved by adjusting the size of the dots, thus achieving tone transitions. In printing, the use of AM dots primarily involves considerations of dot size, dot shape, dot angle, and line accuracy.

Dot Size

Dot size is determined by the dot coverage, also known as ink density. It's commonly measured in "percentages"; for instance, dots with a coverage of 10% are termed "one dot," those with 20% coverage are termed "two dots," and dots with 0% coverage are termed "white," while those with 100% coverage are termed "solid."

The tonal range of printed materials is generally divided into three levels: highlights, midtones, and shadows. The dot coverage for highlights ranges from about 10% to 30%; for midtones, it ranges from about 40% to 60%; and for shadows, it ranges from about 70% to 90%. "White" and "solid" are additional categories.

Dot Shape

The dot shape in printing is not limited to a single circular shape. Based on the appearance of dots at a 50% ink coverage, they can be categorized into three shapes: square, round, and diamond.

Square dots, at 50% ink coverage, form a checkerboard pattern. They have sharp edges and are suitable for representing lines, graphics, and some high-contrast images.

Round dots, whether in highlights or midtones, are independent from each other, only connecting in shadow areas. Therefore, they have poor capability in representing gradations and are less commonly used in four-color printing.

Diamond dots combine the sharpness of square dots with the smoothness of round dots, resulting in natural color transitions. They are suitable for general images and photographs.

Dot Angle

In plate making for printing, the selection of dot angles is crucial. Choosing the wrong dot angle can cause moiré patterns.

Common dot angles include 90 degrees, 15 degrees, 45 degrees, and 75 degrees. Dots at a 45-degree angle exhibit good stability without appearing rigid. Angles at 15 degrees and 75 degrees are less stable, but they don't compromise visual effects too much. A 90-degree angle provides the most stability but can make the visuals appear too rigid and lacking aesthetic appeal.

When two or more sets of dots overlap, they interfere with each other. If this interference affects the visual appeal of the image, it results in what's commonly known as "moire patterns."

Generally, when the angle difference between two sets of dots is 30 degrees or 60 degrees, the overall interference patterns are relatively aesthetically pleasing. Next is the angle difference of 45 degrees. When the angle difference is 15 degrees or 75 degrees, the interference patterns detract from the visual appeal of the image.

Line Screen

The size of the line screen determines the image's level of detail, similar to resolution. Commonly used line screens are as follows:

10-120 lines: Low-quality printing for large-scale prints such as posters and billboards, often using newsprint or offset paper and occasionally low-weight art paper and coated paper.

150 lines: Regular four-color printing, suitable for various types of paper.

175-200 lines: High-quality publications like glossy magazines, often printed on coated paper.

250-300 lines: Demanding publications like high-end catalogs, usually printed on coated paper or specialty paper.