light gathering/trap

light gathering/trap (In French captage de la lumière) phenomenon in halftone printing in which incident light, in its reflection in the upper layers of the substrate, is prevented from leaving the substrate, because the printing ink layer of the halftone dot blocks the way; this occurs mainly in substrates with low opacity and increases the tonal value increase of the halftone print. The strength of the effect is also dependent on the screen ruling (screen resolution, l/cm or lpi) and the presence of a lacquer coating. For this reason, the light trap correction exponent (Yule/Nielsen) is handled empirically for the screenvalue formula of Murray-Davies; a light trap occurs in the formula as a divisor of the screen density and of the solid density in values between 1 (no light trap detectable) and 1.9.