The main drawback of discharge lamps is that it is impossible to dim the light with conventional dimmers. This limits the use of discharge lamps to moving lights, follow-spots, or lighting for cinema and television studios.
The output of a discharge
lamp
can be brought down with the aid of a special
ballast
which allows the reduction of the quantity of the light by 30% without changing the color temperature. This however does not provide adequate dimming control for the theatre where continuous control of the light from blackout to full light is essential.
A mechanical dimming mechanism (dowser) can be mounted on the fixture to provide full range dimming control. The dowser works like a venetian blind, the slats of which can be set at any angle ranging from perpendicular to the lens, which gives maximum light output, to parallel to the
lens
surface, which blocks out the light completely. Modern moving luminaires have other types of built-in mechanical dimmers.
