Light for Study: A Minimum Useful Illuminance Bound at the Page, the Cheapest Optical Design That Meets It, and What Illuminance Does Not Fix

Derives a study illuminance bound: acuity alone gives only 17 lux for 10-pt print, so practical bands are set by reserve - 150 lux minimum for standard print, 300 lux comfortable, 500 lux for small print. A 1 W, 130 lm LED with a simple reflector delivers 596 lux average over 0.12 m2 for $5.90 in components. Glare, flicker, posture, dry eye and refractive error are not fixed by lux.