Illumination system with four primaries

TTTTTMAXSSSAn illumination system, comprising:-four lamps (A, B, C, D);-four lamp drivers (A, B, C, D) capable of driving their corresponding lamps with respective dim factors (ξ);-a common controller for controlling the dim factors of the respective lamps. The controller is responsive to an input signal indicating a target color point (T) having target chromaticity coordinates (x, y) and target brightness (B). The controller sets the dim factor (ξ) of one lamp to be equal to 1, and calculates an optimum solution for the other three dim factors as a function of the target chromaticity coordinates (x, y), for the maximum allowed value of the luminance (Y) for which 0≦ξ≦1 applies for each of said dim factors (ξ).