Plano Convex Spotlight

The Plano Convex Spotlight is the earliest form of theatre spotlight, to use a lens. It consists of a simple 'box' housing containing a lamp and reflector behind a plano-convex lens. Modern units range from 300 to 2000 watts. Lens diameters include 5", 6" and 8" wide. Beam spreads typically are variable from approximately 10 to 65 degrees in a single fixture.
This fixture provides a circular beam pattern with a sharply defined 'hard' outer edge. Unfortunately, the field of light, (distribution of light across the beam) is often quite uneven. Further, optical aberrations in the form of rainbow effects, are often seen at the edges of the beam.
The lamp and reflector are mounted together, and move forward or backwards, in relations to the lens. As they move closer to the lens, the beam expands in size. As they move away from the lens, the beam becomes narrower.