etendue: a standalone construct

Etendue is the geometrical measure of how spread out a bundle of light is in both area and angle at once, the product of the cross-sectional area it passes through and the solid angle it fills, and its defining feature is that it can never be decreased by any passive optical system: a lens or mirror can trade area for angle but not shrink the product. Chaves's Introduction to nonimaging optics treats etendue and its conservation as the organising principle of light collection and concentration. The referent is the conserved throughput of an optical system, which caps how tightly a source of given size and angular spread can be concentrated and sets the brightness limit of illumination and imaging systems.[1]

Unit

The product of area and solid angle (with index squared), typically in mm^2·sr

Recovery means

Not applicable

Measured by

Computed from the beam's area and angular spread at any plane, and checked against its conservation through the system