This paper presents the development of a method to control the light output from a prototype hybrid lighting system. This system transports daylight from a heliostat with a concentrating fresnel lens to a luminaire in a windowless room, via a large core liquid fiber optic. The main artificial lighting system is located outside of the building without the possibility for dimming due to the lamp type chosen. Consequently a control strategy is needed in order to minimize the switching cycles of the lamps (severe reduction in lamp life) while at the same time maintain the lighting levels in the interior. (c) 2004 Published by Elsevier Ltd.