Hybrid vs. Stochastic Screening in Lithographic Printing: What You Need to Know
As the saying goes, there’s more than one way to skin a cat—or, in this case, screen one! All joking aside, today’s post delves into the two main types of screening used in lithographic printing: hybrid screening and stochastic screening. Each method comes with its own set of benefits and challenges. In this article, I’ll explain both methods and explore their respective pros and cons in offset lithographic printing.
What Is a Screen in Printing?
In the context of printing, a “screen” refers to both a color tint and a resolution setting. For instance, a lighter tint of black creates gray, and the smoothness of that gray can be considered the resolution or “line screen” (measured in lines per inch, or LPI).
To illustrate, think of a screen door: the holes in the mesh allow air to pass through. Now, imagine pressing ink through this mesh onto paper. The size and number of holes determine how much ink gets transferred to the paper. Fewer or smaller holes yield a lighter tint, while more or larger holes allow more ink, resulting in a darker tint.
This principle is similar to how screen printing works, where ink is forced through a mesh screen to create designs on various materials. However, in lithographic printing, a screen is imaged onto a metal plate that transfers ink onto a rubber blanket, which then applies it to paper. This process is known as offset printing.
Two Types of Screen Methods: AM vs. FM
There are two primary types of screening methods used in lithographic printing: Amplitude-Modulated (AM) screeningand Frequency-Modulated (FM) screening (also called stochastic screening). Let’s explore both.
AM Screening: Traditional and Controllable
AM screening is the most common method in lithographic printing. This method uses a fixed dot pattern where the size of each dot changes based on the tint. When the tint gets darker, the dot size increases; when the tint gets lighter, the dot size decreases. However, the number of dots in a given area remains constant.
AM screening is widely used because of its controllable nature. Most printing plates and blankets support standard AM screens, making it easier to use. The quality of the print, however, depends heavily on proper calibration and balancing of consumables.
One limitation of AM screening is the rosette pattern (see Figure 5). This pattern occurs because each of the four CMYK colors is printed at different angles to prevent the dots from aligning. Despite this, the rosette pattern can still be visible in the final print, especially when viewed under magnification.
FM Screening: Fine Detail with Stochastic Dots
FM screening, or stochastic screening, uses a fixed, very small dot size. As the tint darkens, more dots are added, and as the tint lightens, fewer dots are used. The key difference here is that the size of the dots remains constant throughout the process.
FM screening excels at reproducing fine, detailed images, such as textures in fabric, leather, or skin tones. It delivers higher resolution and smoother transitions, making it ideal for high-quality reproductions of intricate images.
One of the major advantages of FM screening is that it avoids the moiré effect—a common issue in AM screening. The moiré effect happens when a scanned image interacts with the rosette pattern of AM screening, creating unwanted patterns in the print. Since FM uses a random dot pattern, this problem doesn’t occur.
Additionally, FM screening uses less ink than AM because it has more dots in the same area, resulting in less ink required to achieve the same visual effect.
The Line Screen: Key Differences in Resolution
In AM screening, line screen refers to the number of halftone dots per inch (LPI). This controls the fineness of the dot pattern. The closer together the dots are, the higher the resolution. However, higher resolution can be more challenging to maintain consistently.
Most lithographic printers use a line screen between 133 and 175 LPI, providing a balance between smoothness and consistency.
FM screening doesn’t use a line screen because its dot pattern is random. Instead, the size of the dots is measured in microns, typically between 20 to 30 microns for stochastic printing.
Hybrid Screening: A Blend of AM and FM
Hybrid screening combines elements of both AM and FM screening. It uses AM screening for most of the image but switches to FM screening for tints below 5% and above 95%. This hybrid method creates smoother transitions in the highlights and shadows, improving the overall image quality.
Challenges with FM Screening
Despite its many advantages, FM screening does have challenges. The primary one is that it requires high precision in both equipment and calibration. Not all printing plates can handle the small, random dots of FM screening, and plate erosion (called blinding) can occur if the dot degrades during printing. This results in a lighter or disappearing image.
Additionally, FM screening often requires an additional license and considerable effort to maintain proper calibration. The right balance of consumables is essential to achieving consistent, high-quality results.
Conclusion: Which Screening Method to Choose?
So, if FM screening offers better quality and uses less ink, why don’t more printers use it? The answer lies in the complexity and additional cost of implementation. However, when properly managed, stochastic screening can lead to much higher-quality prints, making it an excellent choice for certain high-end applications.
At Phillips Prepress, we use both hybrid and stochastic screening methods and are proud to be G7 color-space certified in both. If you’d like to see samples or learn more about how these methods work in print, don’t hesitate to reach out to us at 888-ASK-PHIL.