Hybrid vs. Stochastic Screening
As the old saying goes, there is more than one way to screen a cat…the image of a cat that is–a little printing humor there. All lame puns and disturbing metaphors aside, I am referring to the different types of screening in lithographic printing. Within any manufacturing process there can often be multiple production method options with pros and cons in each. In addition to a general explanation of the process, I will discuss the benefits and challenges of two of the standard screening methods used in offset lithographic printing.
What is a screen?
In printing, a screen can refer to a tint of a single color and/or a resolution of sorts. A lighter tint of black would produce gray, and how smooth that gray is could be considered the resolution, or line screen (lines per inch)–which I’ll discuss later. The type of screening refers to the tint, particularly, how the tint is produced. Imagine a screen door material that has holes in it to allow air to pass through. Now imagine putting that material on top of a piece of paper and pressing ink through the screen. The size of the holes and the number of holes would determine how much ink gets to the paper. Less holes or smaller holes would allow less ink, yielding a lighter tint, whereas more holes or larger holes would allow more ink, yielding a darker tint. This is basically how screen-printing works. Screen-printing involves using an emulsion process to block out areas of a fine mesh to control the ink applied to a substrate. The ink is applied directly through the mesh to the substrate. Each color has its own screen and is applied one at a time. It is primarily used for simplistic designs in textile printing and high-volume signage.
The process in lithographic printing is similar, except the “screen” that determines the tint is imaged onto a metal plate which transfers the ink to a rubber blanket which then transfers it to paper. This process is also called offset printing. Most printers use CMYK process to produce full-color images. With CMYK process all colors are reproduced through the combination of four base colors–cyan, magenta, yellow, and black. Each of these colors is printed in various tints and combinations to produce all the colors we see in images and graphics. How the tint is produced for each color is referred to as the screening method.
There are two types of screening methods used in lithographic printing. The first, and most common, is amplitude-modulated screening (AM screening). This method uses a fixed dot pattern. As the tint gets darker the size of each dot is increased, as the tint gets lighter the size of each dot is decreased, but the number of dots in a given area stays the same.
AM screening is commonly used among printers due to its controllable nature. Most printing plates and blankets available on the market today can easily support a standard AM screen and there is less sensitivity in how well the ink is carried and transferred to the paper. Although, the quality and balance of consumables and proper calibration will always greatly affect the level of quality and consistency that can be achieved.
Because AM screening uses a fixed dot pattern, each of the four CMYK process colors must be at different angles so that the dots do not line up on top of each other. The four colors are typically at (Y) 0˚, (C) 15˚, (K) 45˚, and (M) 75˚. However, this combination of the same pattern at different angles creates a rosette pattern and is one of the well-known drawbacks of AM screening (figure 5). If you look at any full-color printed piece with a magnifying glass or loop you will likely be able to see this pattern of interlocking circles.
The second type is frequency-modulated screening (FM screening). FM screening is also referred to as stochastic screening. This method uses a very small fixed dot size. As the tint gets darker the number of dots is increased, as the tint gets lighter the number of dots is decreased, but the size of the dots stays the same.
FM screening has many advantages when reproducing fine images. Because FM screening utilizes a small random dot pattern, the level of detail and smoothness that can be achieved is far greater than with AM screening. FM screening does particularly well in reproducing high-resolution images of fabric, leather, skin tones or any image where there are very fine details in the textures. Another drawback with AM screening is the potential to produce a moiré effect. Though, a moiré is most often seen when reproducing a scan of a printed piece resulting in a multiplication of the rosette pattern, it can also occur when reproducing an image with a fine geometric pattern like fabric. This is because the rosette pattern of the screen can interact with the pattern of the fabric. This is not an issue with FM screening because there is no consistent pattern to the dots. FM screening also tends to use less ink because there is more dot gain (as there are generally more dots) – meaning less ink is required to produce the same visual tint.
AM screening also involves a line screen. A line screen is the number of halftones per inch, expressed as lines per inch (LPI). This determines how fine the dot pattern is, or how close together the dots are. The closer together the dots are the higher the resolution that can be achieved. However, with higher resolution and finer dots come more challenges in maintaining a consistent result or a smooth image. Most sheetfed lithographic printers will run a 133 to 175 line screen. This is high enough to produce a very smooth image and still be somewhat forgiving. There is also a type of screening that utilizes both methods called hybrid screening. Hybrid screening is primarily AM screening, but at tint values below 5% and above 95% it uses FM screening. This allows for smoother transitions and imaging in highlights and shadows.
FM screening does not use a line screen because the size of the dot is fixed and the pattern is considered random. The equivalent aspect for FM screening would be the size of the dot, which is measured in microns. Most printers running stochastic will use a 20 to 30 micron dot size. Not all printing plates can handle maintaining a clean dot that size. Even if the platesetter (CtP) can image the dot, the plate surface must be able to maintain the dot during printing. If the dot erodes during printing the image will start to disappear or become lighter. This erosion of the plate image is called blinding. The fountain solution, which is the water/chemical solution that keeps the plate clean where there should not be ink (the water versus oil principle), can also play a part in whether blinding occurs.
You may be wondering why, if FM screening looks so much better and uses less ink, more printers don’t use it. In addition to some of the potential challenges mentioned above, there is usually an additional license purchase and it does take a lot of effort in creating and maintaining proper calibration as well as discovering the right balance of consumables to yield a consistently good result. However, with diligent color management, equipment maintenance, and the right consumables, stochastic screening can be a huge benefit to the printer and produce a much higher quality product for the client.
Phillips utilizes both hybrid and stochastic screening methods and is G7 color-space certified in both. If you would like to see examples of printed pieces using these different methods or have any questions, feel free to contact us or call us at 888-ask-phil.