Design Nomenclature / Light & Optics
Directional light
Also called sun light, distant light, infinite light, parallel light
A source at effectively infinite distance, so its rays arrive parallel and its illuminance does not vary with position. Shadows stay the same size at any range and the inverse-square law does not apply — the falloff across a set is real but immeasurable. It models sunlight.
In practice
Sun lights still take an angular-size parameter. The sun subtends about half a degree, which is exactly what gives real sunlight its slightly soft shadow edge.
Not to be confused with
- Point lightAn idealized source of zero extent radiating equally in all directions from a single position. Because it has no size it subtends no solid angle, so it casts geometrically hard shadows with no penumbra whatsoever, and its illuminance obeys the inverse-square law exactly rather than approximately.
- Inverse-square lawIlluminance from a point source falls as one over distance squared, because the same flux spreads across a sphere whose area grows with the square of the radius. It follows from geometry alone, and it holds only for sources small relative to the distance — roughly five times their own width.