Prescribe Light and Print Together: The Illuminance Each Age Needs for Each Print Size, and the Point Where Light Stops Working

A brighter lamp is the cheapest vision aid in the world, but advice about reading light is usually given in watts and adjectives. This paper replaces both with a stated model relating the eye acuity threshold to illuminance for three ages, and computes the light each age needs for fluent reading of four print sizes. Under the model, N12 large print needs about 245 lux at every age tabulated, and N18 needs only about 24 lux, while fluent N8 newspaper print needs about 3162 lux - bright task lighting - at ages 30 and 55, and at 75 is not reachable by light alone at all, because the stated acuity floor of 0.18 logMAR sits above the print demand of 0.10 logMAR. Fluent N6 is beyond light alone at every age. The rule that follows is to prescribe light and print size together: light first until the floor is reached, then print. This study is simulated, not measured.