Radiance — where it appears
Named by 2 essays across one field — each of them below, with the objects they name alongside it.
A wall does not get darker as it goes away
The inverse square law is about a point source. A surface is not a point source, and the picture of a wall is exactly as bright at twenty metres as at two — the patch one pixel covers grows as the square of the distance and the light per unit area falls as the square of the distance, and a picture records the product. Which is why aerial perspective has to be the air.
A lamp lights less than half a ball
Everyone knows a sphere is half lit. It is half lit by a source at infinity and by nothing else: a lamp two radii away lights a quarter of it, and the boundary it draws is a circle offset toward the light rather than a great circle. The offset is R²/D, which means a photograph of a lit ball carries the distance to whatever lit it.
Named alongside it
The objects these essays reach for when they reach for this one.
DemonstrationDepth cuePoint lightSubtended angleTerminatorcentre of projectionConicDiminutionfield of viewForeshorteningSilhouetteTangent cone