Coudray discusses the art of managing dot gain and the science of manipulating dots in the midtone region to improve image clarity and quality.
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.
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