ABSTRACT
Light travels through an optical system, it requires area and angular space. A surface that emits light with this kind of angular distribution is called a Lambertian emitter. The etendue of the light entering the box is reduced because there is loss of light. Conservation of etendue then tells that the product of projected area and solid angle is constant. This means that if the area available for the light is increased, the solid angle decreases. The optical momentum is a vector defined at each point on the path of a ray. It has, as its magnitude, the refractive index of the medium at that point, and the same direction as the light ray at that point. It is tangential to the light ray at each point. The Winston–Welford design method involves placing two mirrors along two flow-lines so that the light is guided between them, while conserving etendue.
