When screen printers get together to talk about halftone printing or printing process color, the discussion falls into three primary areas: controlling moiré, dot gain, and halftone choices (angles and dot shape). The conversation usually evolves into two main subjects: the effect of halftone selection on dot gain, and moiré generation. Many misunderstood and downright in-correct assumptions are made regarding common halftone practices.
When screen printers get together to talk about halftone printing or printing process color, the discussion falls into three primary areas: controlling moiré, dot gain, and halftone choices (angles and dot shape). The conversation usually evolves into two main subjects: the effect of halftone selection on dot gain, and moiré generation. Many misunderstood and downright in-correct assumptions are made regarding common halftone practices. This month, I would like to look at some specific aspects of dot gain, particularly how we can use dot-shape selection to dramatically minimize the negative effects of dot gain.
We can begin our discussion with dot-shape options. Conventional halftones, as opposed to FM or stochastic, are known collectively as dot-centered, ordered-array-cluster patterns. This means the dots are arranged in regular rows and that each halftone cell grows from the center outward. The most common dot shapes in use today are round, square, diamond, and elliptical. For screen printers, the best choices are diamond and elliptical, for reasons soon to be revealed.
Our main focus is on the midtone region. This is the range of tones between 40-60%. This is the region where dot gain is at its maximum. Dot gain is not uniform. The least amount of dot gain occurs in the highlights and shadows. It becomes progressively greater until it reaches the 50% point. This is the point at which the dot has the largest geometric area. It also has the greatest circumference. Since dot gain acts like a stroke, the greater the circumference, the more dot gain (as a percentage) exists.
Dot gain is always measured as an absolute percentage, and it is always referenced against the 50% dot, because that’s where the maximum dot gain occurs. When we say we have 30% dot gain, it means a 50% dot will grow during printing to yield an 80% dot on the print substrate. Dot gain in the range of 35% or higher is extremely common in textile printing. For graphics printers, dot gain in the area of 20% is more common. You’ll have to measure a tone scale with a reflection densitometer to determine your exact dot gain.
The shape of the halftone dot has a major bearing on how much dot gain you will experience in the midtones. If you’re not aware of the differences in the dot-shape options, you may be in for an unpleasant surprise. Programs like Adobe Photoshop often default to lithographic preferences and deliver halftone separations at the wrong angle and with the incorrect dot shape for screen printers. Even experienced prepress technicians fall victim to inappropriate default settings.
When referring to the shape of the dot, we concentrate our attention on the transition area of the midtones. This is where the dot area grows to the point where the corners of adjacent dots join. This is a huge problem for our printing process. When the corners of the joining dots connect, they immediately bleed together. We know this as dot gain, and the associated, simultaneous corner growth is referred to as midtone jump. The combination of the geometry of the joining corners and the high film thickness of screen-printing inks contributes to how much midtone jump you experience. The midtone jump can easily exceed 20%. Even the very best printers find it impossible to compensate for this kind of behavior.
A major point that I want to reinforce is the fact that the midtone value still has the maximum amount of dot gain. The midtone jump is in addition to the expected dot gain in the midtones, which is why the midtone jump is so incredibly damaging to an image. Where normal, expected dot gain might be 20% (50% becomes 70% value), the midtone jump may add up to 20% more gain. Therefore, 50% can suddenly yield a 90% tone value. This huge jump in tone value shows as dark banding and exaggeration of shadow transition. It can be totally disruptive and destructive to the tone values and reproduction of the image.
Round dots transition through the tone range as round dots. All four corners of the adjacent dots connect at a simultaneous tangent point in the midtone area. The angle formed at this tangent point is acute (less than 90°). Because it is so tight, it is very easy to fill with ink. The round dot is the absolute worst choice for our process. Figure 1 clearly illustrates the acute angle and the resulting filling that occurs.
Printers select round dots for two reasons. The first is that this dot is more tolerant to moiré formation. The uniformity of the dot helps screen printers to minimize the effect of thread eclipsing. The second reason has to do more with the printing process—not necessarily the screen-printing process. On high-speed web and sheeted offset, the dot distorts as the paper passes between the blanket and impression roller. The round dot tends toward an ellipse. As we shall soon see, this helps to minimize the midtone jump. In the case of offset litho, the distortion is controlled by shape compensation. A round dot was developed for that purpose. Screen printing sees no benefit from this compensation.
The square dot was developed to address the problems of the round dot. In this example, the halftone dot starts out round. As tone darkens toward the midtones, the halftone shape transitions from a round dot to a square with round corners, finally becoming a perfect square at the 50% tone (Figure 2). The four corners of the square now connect simultaneously at an improved 90° angle. The possibility still exists for the corners to fill and midtone jump to occur. Square dots will spontaneously increase tonal value by 10-12% under ideal conditions. The jump is greater when printing conditions are poor. Past the 50% value, the dot shape reverses itself, eventually becoming a negative round dot before finally becoming a solid tone.
Square dots are noted for their ability to carry extremely sharp detail in the midtone region. The downside of the square dot is that it tends to be less tolerant of moiré. The mesh threads easily eclipse the narrow corners of the dot. Because all four corners connect simultaneously, any disruption of the connecting pattern is readily apparent to the human eye, giving the final printed image a grainy or rough look.
Diamond and elliptical dots were designed to further support even tone transition. They seek to address the geometry of the midtone join and shift the values at which the corners connect. In these two designs, the halftone dot starts off round, transitions to diamond (Figure 3) or ellipse, and then transitions back to round, before becoming a solid value. Changing the shape of the dot allows us to connect two ends of the dot at a lower tonal value—usually in the 40% range. The dot continues to grow until the opposing axis corners join in the 60% region. The corners can join at almost any percentage value when we control the aspect ratio of the dot.
Besides the huge value of connecting at different tonal values, the geometry of the joining corners is now obtuse, meaning the angle formed by the connecting points is greater than 90°. This greater open angle is less prone to filling as the ink spreads. The result is a reduction of the midtone jump to near zero.
The chance of thread eclipsing increases as the dot flattens to a diamond or ellipse. These two shapes are particularly susceptible to moiré. The flatter dot also creates a more visually objectionable delineation of the regular halftone pattern. This effect can be particularly distressing in areas of large, uniform value or in areas of slow tonal change. Examples include a sky gradient in the first case and a flesh tone in the second. Carefully selecting the halftone-angle set helps minimize these visual disturbances.
Halftone-dot design is a highly technical undertaking that requires extensive knowledge of mathematics. The ability to engineer a specific dot shape or transition sequence involves equally extensive knowledge of the imaging process and the method by which the halftone is printed. It’s entirely possible that the ideal halftone dot (or dots) for screen printing has yet to be invented. Until that happens, we’re pretty much limited to working with diamond or elliptical dots in an attempt to minimize tonal jump, an issue that has been with us since we first used halftones in our printing process.
Mark A. Coudray is president of Coudray Graphic Technologies, San Luis Obispo, CA. He has served as a director of the Specialty Graphic Imaging Association Int’l (SGIA) and as chairman of the Academy of Screen Printing Technology. Coudray has authored more than 250 papers and articles over the last 20 years, and he received the SGIA’s Swormstedt Award in 1992 and 1994. He covers electronic prepress issues monthly in Screen Printing magazine. He can be reached via e-mail at [email protected]
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