Fig. 4. Visual acuity versus contrast shows rapid improvement in acuity as contrast increases. The upper curve is plotted at a higher background illumination than the lower curve. As contrast increases the two merge and the illumination difference becomes irrelevant. Marked above the curves are the contrast ranges of clinical test material and normal printed materials such as newsprint. From this, one can see that patients see clearer under test conditions using high-contrast typeface than they do at home, where ambient lighting may also be reduced. (From Spalton DJ, Hitchings RA, Hunter PA. Slide Atlas of Ophthalmology. London: Gower Medical Publishing, 1984.)