Lighting Design Glossary

Etendue

(Term of radiometry)

Literally, the french word "etendue" translates to "extent" or "space". In a radiometric context, it is used to specify the geometric capability of an optical system to transmit radiation, its throughput.

The numeric value of the etendue is a constant of the system and gets calculated as the product of the opening size and the solid angle that the system accepts light from. In most practical situations, it is approximated by integrating over the two factors.

G = ∫∫ dA * Ω

Quite often an engineer will be confronted with the need to optimize the arrangement of elements within a system in order to maximize the geometric etendue and thus its throughput.