Abstract: A circuit for controlling the power supplied to a lamp includes a sensor, located at the lamp, which measures the portion of the lamp's output which is reflected from an illuminated area. This sensor is connected to a sensing circuit which measures the reflectance of the illuminated area. Using this reflectance measurement, the total illumination incident on a work space, both from the light source and from other illumination sources, is determined and this total incident illumination is adjusted by adjusting the power supplied to the light source to provide a controlled illumination. The ability to measure reflectance of the illuminated area using a sensor at the light source itself permits the manufacture of a light source which will self-adjust to illuminate a work area at a desired incident illumination level, without adjustment of the light after installation, and without a requirement that the light source be adjusted if the reflectance of the work area changes.

Abstract: An illumination control device for regulating the illumination in a controlled lighting area responds to natural illumination to control artificial lighting. The illumination control device turns the artificial light on and off at independently variable natural illumination levels to provide a hysteresis between the turn-on and turn-off points of the artificial lights. In addition, a variable time delay is included in the illumination control device to delay the turn-off of the lights for a variable period of time after the natural light level has risen above the level required for artificial light turn-off. Furthermore, the illumination control device includes a circuit for turning on all of the lights, regardless of the natural illumination level, when the circuit is initially energized so that the lights can be checked by maintenance personnel.