Halftoning: a crucial step in the printing
When discussing the printing process, one of the most critical aspects is halftoning, or screening, the file for print. Halftoning involves converting a digital file into a series of halftone dots or points for printing purposes. This process occurs during lithography and is evaluated during preflight to ensure high-quality output. A halftone screen is generated using software collectively known as RIPs. The number and size of these halftone dots depend on the required print resolution and the specific nature of the final product.
The concept of creating dots to produce a continuous image dates back to analog photography and film development. However, in commercial printing, the conversion of an image into a set of dots is now performed digitally, referred to as digital halftoning. In this process, extremely small dots are placed closely together, and the human eye, due to its structure, perceives these adjacent dots as a continuous tone.
In a general definition of halftoning, the goal is to create a matrix of dots or halftones. This can be achieved by varying the size of the dots or by keeping the size constant and changing the spacing between the centers of the dots. Regardless of the method used to halftone a digital file, the aim is to arrange the dots in such a way that the eye perceives a continuous surface due to the small size of the dots.
Create halftone
As discussed in the print resolution section, the laser in a lithographic machine creates microdots based on the printing requirements. The number of these microdots per inch is expressed in DPI, and the higher the quality of the laser machine, the more accurately the dots are created. After each dot is created, a set of dots are arranged side by side in rows, forming a group of dots known as LPI.
The process of placing more dots closer together, which results in a desirable printed image, can be implemented in various ways. Companies like Heidelberg, Kodak, and others have defined this as a matrix or screen. In this way, the group of dots produced to create an image simultaneously change angle within a grid.
This method helps prevent common errors such as moire, dot gain, and printing banding. Ultimately, it can be said that in halftoning, by creating microdots to form a dot and then arranging all the dots together, an image is created.
Halftoning process
To achieve a desirable printed image, various methods have been defined for creating halftone screens and halftoning. However, each of these methods has its own advantages and disadvantages, and it cannot be said that only one method is suitable for every type of printing job. Therefore, it is sometimes necessary to change the halftoning method according to the production requirements. This change can be effective in reducing printing errors, optimizing ink consumption, and controlling product quality based on the design and production type.
For example, in conventional or AM halftoning, the size of the halftone dots varies, and by placing a number of dots in a row, an image is formed in print. However, the problem with this type of halftoning is that if the printing cylinder pressure is not appropriate, dot gain can occur, and the printed image will be damaged in the shadow areas. On the other hand, the advantage of this method is its easy production and easier control for the machine operator.
In frequency modulated or FM halftoning, the size of the dots is fixed, and their distance varies to form a printed image. This method produces very high quality and dot gain is very low or zero, but this method can cause color shifts in printing when the ink density is high or when the dots are very close together.
The role of designers in production
One of the key parts of quality control and preflight is examining the created halftones. Of course, the production manager first selects the correct halftoning/screening method according to the printing needs, then, by looking at the advantages and disadvantages of each method, they proceed to troubleshoot the file structures, lithography, and printing. This initial level of control relates to the resolution and angle of the screen, and at a more professional level, it can lead to examining the screen structure on the plate, the shape of the screen according to the type of printing press, and controlling potential errors in each type of screen during printing.
It is still necessary to mention that the halftoning process is performed in lithography and according to the type of printing press and plate. The designer has no involvement in creating halftones, but since the final quality in the lean printing process is very important, designers should consider themselves involved in the production and quality control process. Because the first step is defined by designers to create a work, and sometimes printing certain subjects is not easily possible depending on the type of printing press.
If art directors and graphic designers have a little knowledge of production, they will create artistic ideas with a better perspective, and in this process, quality control managers will also achieve better results.
