  The simulation proposes that the cylindrical cavity around a circular light
source can decreases the divergence angle without changing the emission energy
and source size. This result can provide a way to reduce the etendue of the
light source by scrambling the rays inside the cavity with a lossless
scattering surface. The experiment demonstrates that the metallic cavity around
the surface source reduces the divergence angle. However, the metallic surface
also absorbs quite a large portion of the light energy. The Lambertian nature
of the sidewall surface changes the directions of the rays into horizontal
directions. It increases the number of reflection inside the cavity and
amplifies the small amount of absorption at a single reflection. The effect of
the cavity on the etendue of the light source can contributes to providing more
design flexibility in various lighting applications.
