The product of a light source's emitting area and the solid angle into which it emits. It is a strictly conserved quantity in ideal optical systems, meaning you cannot squeeze a diffuse, wide-angle light source into a small, highly collimated beam without losing power.
We can't couple this LED into a single-mode fiber efficiently because the etendue of the source is way too large.
A fundamental rule of optics is that etendue can never decrease in a passive system. This is why you cannot focus sunlight to be hotter than the surface of the sun.
Also heard as
- optical extent
- throughput
Related terms
Beam waist
The narrowest point of a focused or propagating laser beam, where its wavefront is flat.
Divergence
How fast a laser beam spreads with distance far from its waist, given as an angle in milliradians.
Angular aperture
The angle subtended by the edge of an optical element as measured from the focal point.