Illumination control

Controlling a lighting system, which comprises a controller (2, 10), lighting units (6), and a sensing device. Each lighting unit comprises a lighting source (12) and a modulated light source (14). A single light source may be used to function as both the lighting source and the modulated light source. Each modulated light source emits uniquely modulated light. A radiation pattern of each modulated light source coincides substantially with a radiation pattern of a lighting source of the same lighting unit. The sensing device is suitable to sense modulated light in a viewing area. Lighting units from which the sensing device senses modulated light are identified from the modulation of that modulated light. The sensing device measures the intensity of the modulated light from the identified lighting unit. The lighting sources are controlled dependent on control data which comprises measuring values of measured light intensities