Field angle refers to the angle between the two planes of light where the intensity is 10% (or less) of the maximum intensity at center beam. This is sometimes referred to a ghosting or spill, it is not considered usable light. The type of lens used in the fixture will usually determine the amount of field angle or spill. For example, a lens that has a textured surface or frosted coating will create a wider field angle. This is because the textured surface on the lens (or frosting) will improve color mixing, but also diffuses the light and creates softer edges along the beam creating a wider field angle, and more spill.