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How PC Luminaires Work

 
Plano-Convex Lens

The Plano-Convex (PC) Lens

Plano-Convex (PC) lenses are flat on the back (plano) and curve outward (convex) on the front.

The glass may be completely clear or the flat side may have a textured surface, the latter sometimes being called pebble-convex lenses. The textured surface softens the beam a little to improve the overall beam quality.

Older PC lenses tended to produce a rainbow around the edge of the light beam and project the filament outline in the centre of the light beam. Their glass was also prone to cracking. Modern PC lenses have largely eliminated these problems.

PC Spotlight Optical System

The PC Luminaire

PC luminaires produce a light beam with a 'crisp' well defined edge with less light scatter than the softer edged fresnel luminaires.

The light beam characteristics of the PC spotlight make it ideal for dramatic highlights when focused to a narrow spot or for more general colour washes when focused as a wider flood.

Adding a light diffuser, such as Rosco 114 Hamburg Frost, changes the light beam to make it soft much like the light beam of a fresnel.

The diagram (left) shows the basic optical system of a PC spotlight.

Plano-Convex (PC) and Fresnel luminaires belong to the same family of "focus spots" with the only difference between the two being the type of lens that is fitted. Both types use a spherical reflector which, in conjunction with the single lens, provides a low cost optical system albeit a little less efficient than the more complex optical systems used in profile luminaires. The beam angle of focus spots is adjustable over a wide range, typically from a narrow spot of 6-10 degrees to a flood of 60-65 degrees. This adjustment is achieved by moving the lamp and reflector relative to the lens which is fixed in position on the body of the luminaire. Some models achieve this movement by means of a locking knob at the bottom of the luminaire that you loosen then slide back or forward while other models have a screw thread system with a knob at the back and/or front of the luminaire that is rotated to move move the lamp tray. Moving the lamp closer to the lens increases beam width towards its widest flood setting while moving it away from the lens reduces the beam width towards its narrowest spot setting.

An accessory called a barndoor is usually fitted to the front of focus spots to provide a means of controlling the edges of the beam.

For safety, most good quality PC luminaires have an electrical safety switch that automatically disconnects the power to the lamp when the lamphouse if not locked fully closed. An additional safety option is a lens safety mesh that prevents pieces of the lens falling out of the luminaire if the lens breaks.

Typical Uses

PC luminaires are ideal for use as tightly focused specials for highlights such as when you want to pick out a single performer for dramatic effect. They are also suitable for use in front of house positions.