Print Legibility from Visual Angle: A Minimum Type Size for Shared Schoolbooks

Shared schoolbooks are often printed in small type on yellowing paper. This paper derives a minimum type size from a stated visual-angle criterion for fluent reading at 35 centimetres, and computes the Michelson contrast of fresh and yellowed paper against typical ink. The result is a printing bound that a textbook specification can enforce at no recurring cost: below the computed size, no amount of pupil effort restores fluent reading. Computed results: xheight_mm_at_35cm = 1.63, min_type_pt = 9.2, contrast_fresh = 0.78, contrast_yellowed = 0.69. Every number in this paper is produced by the accompanying script `compute.py`. SIMULATED, NOT MEASURED.