What is AM screening method
One common halftoning technique employs amplitude-modulated (AM) screens. In this method, the size of the dots varies, but their center-to-center distance remains constant. The versatility of AM screening lies in its ability to create different screen angles, facilitating rapid RIP processing and easier press control.
The fundamental structure of AM screens is analogous to the formation of images in analog photography. Darker areas are represented by larger dots, while highlights are created using smaller dots. Each CMYK color is defined on a separate screen, and the dots within each screen are arranged at a unique angle. Over time, the size, angle, and shape of AM screens have evolved to meet the demands of higher-quality printing.
As previously mentioned, AM screening is characterized by a fixed distance and variable dot size. Individual dots are generated by a laser within a matrix grid, and the overall screen is created by combining and rotating numerous dots.
AM screening is primarily categorized into two types: RT (Rational Screening) and IS (Irrational Screening). Today, IS is more widely used. In addition to these two types, companies like Agfa and Kodak offer alternative algorithms such as Spiral, Crystal raster and Sublima.
RT Algorithm
Rational Screening is a process defined by the continuous repetition of a halftone dot within a matrix grid. The angles in this mode must produce a rational tangent, resulting in a rational number. For example, the tangent angle in a halftone algorithm might be defined at 0, 45, and 18.4 degrees, with halftone dots being repeated identically within a grid. PostScript level 1, 2, and 3 programming languages, as well as Adobe Pixelburst technology found in RIPs, are compatible with RT screen angles.
RT halftoning algorithms are very fast because they create a single halftone cell that is simply repeated at the specified angles. However, this method has its limitations, such as difficulty in printing images with high color variations. While it can still be used for printing monochrome images, with the advancement of technology, the IS mode has become more widely used.
IS Algorithm
Most of the concepts you've encountered in the world of printing and halftoning relate to Irrational Screening (IS). This term signifies that the repetition of dots doesn't follow a simple, regular pattern like in Rational Screening (RT). Instead, various parameters such as dot shape, angle, and print resolution can be adjusted to achieve optimal print quality.
Creating a halftone screen in IS mode takes longer in the RIP (Raster Image Processor) and is more prone to moiré patterns and color shifts. However, it offers the advantage of enabling the creation and printing of a wider variety of images with complex color variations and imagery. Companies like Heidelberg and Kodak have expanded upon the IS method, even modifying the basic shape of the halftone dot within the screening grid to further enhance print quality.
This method is the most common approach to halftoning in conventional printing and is supported by most RIPs and PostScript programming languages across all levels.
Targets
The reason for varying the angles of halftone dots, using different algorithms, or employing diverse shapes for dots in this type of screening is to achieve a texture known as a Rosette. This pattern is obtained through various experiments where the halftone dots of each CMYK color are placed on a separate plate at a different angle. Ultimately, the precise arrangement of halftone dots at specific angles creates a unified, flower-like shape that is both visually appealing and produces high-quality prints.
If, for any reason, the angles of the AM halftone dots are misaligned, even due to a registration error, a wavy texture called a Moiré pattern will be produced instead of a Rosette. One of the primary tasks of quality control personnel during preflight is to prevent the formation of these moiré patterns.
Other Algorithm
As mentioned, the AM method with its IS algorithm is highly practical and controllable for creating printed dots or halftones. However, this approach has certain drawbacks such as dot gain, banding, and moiré patterns.
Therefore, under specific conditions, the FM (Frequency Modulation) screening method can be used. In FM, the dot size remains constant, but the center-to-center distance between the dots varies.
While this method offers various algorithms for achieving optimal quality, it's also possible to combine AM and FM techniques to address the limitations of each individual method and achieve better results.
