Contrary to widely held opinion, optical dot gain is measurable; it is not an illusion. In principle, optical dot gain varies from 0% (for nonscattering paper) to 100% (for relatively high dot density on highly scattering substrates). In practice, optical dot gain is typically between about 10% and 35%.
Optical dot gain was first characterized by Yule and Nielsen (Nielsen's name was misspelled in the printed paper):
Yule, J.A.C. and W.J. Nielsen [sic] (1951), "The penetration of light into paper and its effect on halftone reproduction," in TAGA Proc.: 7-9 and 65-76.
See also Ruckdeschel, Frederick R. and Hauser, O. G. (1978), "Yule-Nielsen effect in printing: a physical analysis," in Appl. Opt. 17 (21): 3376-3383.
See also Physical Dot Gain.