The invention relates to an illumination device (1) with a number of light emitters, for example LEDs (L1, L2, L3, L4) of individual emission spectra. Sensor units (D1, D2, D3, D4) can produce a vector of measurement signals (S1, S2, S3, S4) that represent the light output of a single active light emitter. Based on a linear relation obtained during a calibration procedure, a characteristic value of the light output of that light emitter (L1, L2, L3, L4) is then calculated from the measurement vector, wherein said characteristic value is based on the coefficients of a decomposition of the individual emission spectrum into basis functions.