Abstract: An optical detection and control system is described for controlling the brightness of a lamp, or compensating for variations in the brightness of a lamp, such as is used, for example, in a glossmeter, hazemeter, reflectometer, colourimeter or opacity meter. In one system light is transmitted along a path from the light source (1) to a region of interest (8), means (10) are arranged to detect light scattered from the said path, and a power control (12) is coupled to the detecting means (10) to control the brightness of the light source. In another system light is transmitted along a path from the light source (1) to a region of interest (8), a first detector (10) detects light scattered from the said path, a second detector (9) detects light which has reached the region of interest (8), and a signal is produced which is proportional to the ratio of the outputs of the two detectors (8, 9).