etendue

Meaning

Noun

• A conserved property of the light in an optical system which characterizes how "spread out" the light is in terms of angle and area: it is the product of its cross-sectional area (normal to the direction of propagation) and the solid angle it subtends.

Origin

• Borrowed from French étendue ("area, expanse; size; extent, range, scale"), short for étendue géométrique.