etendue UK /ˌeɪ.tɒnˈd(j)uː/US /ˌeɪ.tɑnˈdu/ noun1 •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.
