Screen frequency

The screen frequency (also screen ruling , reciprocal of the screen constant ) is a measure of the resolution of print screens . The screen frequency indicates how many print points occur per unit of length. One differentiates:

• Amplitude-modulated screens that use the same number of dots for all tonal values and produce darker colors with larger dots.
• Frequency-modulated screens that always use screen dots of the same size, but change their number depending on the tone value, i.e. generate darker colors with more dots.

Usual screen frequencies are 30–54 dots per cm in newspaper printing, 54–80 dots per cm in offset printing, and up to 120 dots per cm for high-quality print products. The unit of measurement is often given as lines per centimeter (L / cm) or lines per inch (lpi), with lines per cm being synonymous with points per cm. These names come from the time of optical screening, when the original images were screened by overlaying a distance or magenta screen (see also print screen ).