What is the cycle length of a Metric Ronchi Ruling?

Ronchi Rulings are made up of many pairs of opaque and transparent lines. 1 Line Pair (LP) = 1 cycle for a 100 LP/mm ronchi ruling. 1mm / 100LP = 10μm, or a cycle of 10μm. The width of the line pair is 10μm, so each line thickness will be ½ the line pair thickness, or 5μm.

To calculate how many LP/mm you need based on the cycle length, divide 1mm by the cycle length: 1 mm/ 20μm = 50 LP (½) x 20μm = 10μm line thickness. In other words, you need 50 LP/mm to get a 20μm cycle length with a line thickness of 10μm.