|
Lens luminaires enable variation of the diameter of the light beam. Their optical systems consist of a spherical
reflector
and a converging lens. The
lamp
is generally located between the focal point of the
lens
and the
lens
itself. Focusing is achieved by moving the
lamp
and
reflector
towards or away from the lens, changing the
beam
angle
Modern PCs employ Pebble Convex or Prism Convex lenses which produce a slightly diffused light beam.
|
|
P.C spot.
|
| The closer the
lamp
is to the lens, the more light it collects, and the light cone is wide and smooth (flood). As the
lamp
moves away from the lens, towards the focal point of the lens, less light is collected and the light becomes more concentrated and has an articulated
light distribution
(spot). The
lens
used in this type of luminaire is either a plano-convex
lens
(PC) or a fresnel lens, each of which has a distinctive type of light beam. |
|
Focusing a PC: A wide and smooth
beam
when the focus is at flood (left). A tight and strong
beam
when the focus is at spot (right). |
Both types of luminaires are available in a large variety of sizes and wattages.
Lens luminaire by CCT. |
|
When choosing a
lens
luminaire, the intended light
throw
must be considered.
 |
| When the
throw
is long (i.e. far away from the stage), the
beam
angle required to light a given area is small. Therefore the
lens
luminaire selected must have a long focal length which, while keeping the
lamp
at a safe distance from the focal point, will produce a smooth narrow angle
beam
without creating a filament image. When the
throw
is short (i.e. close to the stage), the
beam
angle used to cover the same area must be wide. In this case, the
lens
luminaire selected must have a short focal length. In other words, a
lens
luminaire for a long
throw
must be longer than that used for a short throw, in order to accommodate the longer
lamp
|
|
 |
travel. Large diameter lenses, which are easier to attain when the focal length is long, and larger
wattage
lamps, are usually used in order to compensate for the smaller light collecting angle. |
