The effects of cavity on the etendue of a light source
  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. 
Ui-Hyung Lee, Young-Gu Ju
physics.optics