Étendue

Ok, I had to write a Photon Snacks column about my middle name étendue.  If you have not taken a course from me, you will learn why I say this.  Étendue is a geometric factor that tells you the flux propagation characteristics in a lossless optical system.  Due to the lossless nature of the optical system étendue is conserved, such that as the flux propagates there is a trade between the projected area of the optical beam/cross section of the optical system and the solid angle.  Additionally, there are seemingly some methods to break the conservation limits by using the physical properties of the radiation, which means optical designers can use these properties to push the limits of optical design.