Parabolic reflectors and ellipsoidal reflectors are used to maximize illumination or projection system output by increasing light source efficiency. These devices resemble small satellite dishes in their parabolic geometry, with a small hole either in the center, or offset a few degrees, to focus and project a light beam.
Types of Reflectors
There are three specific types of parabolic reflectors and ellipsoidal reflectors, differentiated by the location of their foci in relation to the axis, or in the case of ellipsoidal reflectors, their distinctive shape. On-axis parabolic reflectors collect radiation from a source at its focal point, and reflect it as a collimated beam, parallel to the axis. Off-axis paraboloid reflectors are a circular segment from one side of a full paraboloid. The focal point is off the mechanical axis, giving full access to the reflector focus area. Unlike standard parabolic mirrors, these mirrors direct and focus incident collimated light at a specific angle, allowing unrestricted access to the focal point.
Ellipsoidal reflectors have two conjugate foci. Light from one focus passes through the other, after reflection. Ellipsoids collect a much higher fraction of total emitted light than a spherical mirror. Because ellipsoidal reflectors can enhance light collection without dramatically increasing heat in the illumination plane, they are an ideal selection for many projection and general illumination applications.
Parabolic reflectors and ellipsoidal reflectors are placed at a specified distance from the point to which an incident bundle of parallel light rays will converge. Primary length is determined as the length from the center of the reflector to the point where reflected collimated light converges for parabolic reflectors. When working with elliptical reflectors this measurement is determined as the distance of the light source from the reflector. A secondary focal length determination is also needed for proper focusing of ellipsoidal reflectors. This range is the point at which the reflected light converges outside the reflector.