Stage Lighting: PC Luminaires Overview

Plano-Convex Luminaires Overview

Plano-Convex (PC) spotlights use a plano-convex lens and are often called PC spots. They are also known as focus spots. The lens is flat on one side and curved outward on the other side. The light beam is "crisp" with even light distribution across the beam angle and less spill than a fresnel.

The beam angle adjustment, typically from a narrow spot of 6-10 degrees to a flood of 50-60 degrees, is achieved by moving the lamp and reflector relative to the lens. An accessory called a barndoor is often fitted to the front of a PC luminaire to provide a means of controlling the edges of the beam.

The diagram above shows the basic optical system of a PC spot. The flat (plano) side of newer PC lenses is often slightly frosted to provide a slight diffusion and improve the beam quality. The pebble texture of this frosting gives rise to the term 'pebble-convex' as a means of differentiating the frosted lenses from their unfrosted counterparts.

Early PC luminaires became unpopular because of their tendency to project a beam with a rainbow around the edges and the lenses were prone to cracking due to thermal stresses in the glass. Technology has now overcome these problems and the PC is regaining popularity because their beam quality offers many of the advantages of a soft focused profile spot without the high cost.

Typical Uses

The PC spot is a versatile luminaire that is ideal for creating dramatic highlights when focused to a narrow spot, while being equally effective for stage colour washes and front of house applications at wider beam angles.

Revised: 23-Dec-2008