What is FM screening method
One of the most commonly used methods in halftoning to create continuous-tone printed images is frequency modulation (FM) screening, also known as stochastic screening. In this method, the dot size remains constant, while the distance between the dots varies. The distribution of dots is calculated based on specific mathematical formulas to accurately create grayscale steps for the appearance of continuous-tone images.
Unlike conventional AM (amplitude modulation) screening, FM halftone structure does not involve the concept of angle. Instead, the focus is on creating the smallest possible dot size and adjusting the spacing between dots. The absence of a dot angle eliminates the possibility of moiré patterns.
Additionally, the definition of print resolution for FM screening is different because, unlike AM, the dot size is fixed, and quality is assessed based on the repeatability and size of the dots. In FM screening, the dot size is typically defined between 10 and 20 microns, resulting in higher print quality.
FM screening is used when high print quality is required and the design file is well-defined. For example, if a simple illustration with flat colors is used or the final product does not have complex visual elements in terms of texture and color, there is no need to use FM screening. Controlling this type of screening, in addition to defining the appropriate material, a correct design file, and rigorous quality control during printing, also requires knowledge of halftoning principles.
Bayer Algorithm
In the early stages of FM screening development, each dot was evaluated and created individually. This algorithm was similar to conventional screening but operated with new distribution rules. While it produced good quality, it consumed a significant amount of time in the RIP (Raster Image Processor). The halftone structure was divided into 8x8 matrices, and microdots were defined within these matrices.
A distinctive feature of the Bayer system was that it was defined using numbers. If the darkness level exceeded a certain value, the laser would consider that point positive and create a microdot in the halftone.
However, this is a general definition, and the halftone creation structure became so complex that the described model could not meet the desired quality. Over time, the analysis of each point individually was redefined using a different method called the neighborhood algorithm. A characteristic of halftones created using the Bayer method is the visible texture in the details of the halftone structure.
Error Diffusion Algorithm
Years later, a newer algorithm for frequency modulation or stochastic screening emerged, known as the neighborhood or adjacency technique, often referred to as Floyd-Steinberg. The term "error" in this method doesn't refer to production errors or quality degradation but is used to achieve greater accuracy in halftone creation through a specific formula.
For example, in the Bayer method, we examined how each point was created individually, and a point was only created if the darkness exceeded a certain value in the formula. The Error method follows a similar process up to this point.
However, the key difference is that if the darkness falls below a specific value, instead of simply removing the microdot, the Error method involves a recalculation cycle to further examine the reason for its absence, ensuring optimal print quality. In this method, the structure of microdots and points in the neighborhood of the current halftone also play a significant role in the creation of laser points. A characteristic of this algorithm is the appearance of lines and dots in the details of the halftone structure.
Advantages and Disadvantages
While FM screening produces exceptional results in terms of print quality and final color, it also comes with certain drawbacks. For instance, controlling halftones in this method is quite challenging. Due to the appearance and scattered structure of microdots in creating the screen, controlling midtones (darkness levels between 35% and 70%) is not straightforward, and there is a risk of excessive darkness.
Moreover, unlike conventional screening, you cannot freely adjust the nature of the ink and ink coverage in printing. The amount of ink absorbed by the FM screen is highly precise and defined, and it can rarely be increased during printing. Furthermore, the color profile for FM is significantly more complex, requiring more sophisticated color measurement devices and a deeper understanding of color management for printing.
Despite these limitations, the final print quality, especially in areas with textures or very fine elements, is high due to FM algorithms. Issues like moiré patterns and rosette breakup are less prevalent.
Additionally, the color gamut in this type of halftoning is wider than in AM. Register errors in printing are also less problematic, and the quality of colors in lighter areas (below 30%) and darker areas (above 70%) is exceptionally high.
