Light Exposure Calculator

Lux-hours are just brightness multiplied by time, and the brightness half of that product comes from the season selector rather than from anything you measure. Summer assumes eighty thousand lux outdoors, spring and autumn thirty thousand, winter only ten thousand — an eightfold span that makes the season field the single largest influence on your total, far outweighing any plausible change to the minutes you spend outside. Afternoon minutes are then discounted to seventy percent of the prevailing outdoor figure, on the reasoning that the sun is lower and you are likelier to be in shade, so an afternoon hour is worth less than a morning hour of the same length. Light box minutes are valued at whatever intensity you type, with no discount at all.

The three contributions are summed and compared against a target of thirty thousand lux-hours; the resulting percentage is capped at one hundred, which is why twenty minutes of morning sun reads as sixty-eight percent in spring, twenty-three percent in winter, and maxes out in summer. Roughly thirty-nine morning minutes clears the target outright at spring intensity, after which extra time outdoors stops moving the percentage even though the raw lux-hour figure keeps climbing. Evening screen hours run on a completely separate track: they delay melatonin by fifteen minutes per hour and score impact at two and a half points per hour up to a ceiling of ten, and they never subtract from your daytime total. One rough edge to know about — set the box intensity to zero and the extra-minutes advice reports zero rather than admitting the deficit cannot be filled.