An illuminating device comprising: a plurality of light sources, including a red light source, a green light source and a blue light source, each configured to emit an emission light beam; a plurality of light monitor units configured to respectively detect light monitor values (Qr, Qg, Qb) for a corresponding one of the emission light beams emitted from the plurality of light sources; and a multiplexing unit configured to multiplex the emission light beams emitted from the plurality of light sources and to emit multiplexed light, characterised by: a control unit configured to control each light source on the basis of the light monitor values detected for the emission light beam emitted from the light source and a respective predetermined calibration formula (Fr, Fg, Fb) for the corresponding light source, wherein for each light source, the respective predetermined calibration formula (Fr, Fg, Fb) defines a correlation between the light monitor values (Qr, Qg, Qb) detected for the emission light beam emitted by the respective light source and luminance values (Lr, Lg, Lb) acquired from a light receiving unit external to the illuminating device and configured to detect the luminance values from the multiplexed light emitted from the illuminating device, and wherein the control unit is configured to calculate for each light source, on the basis of the respective predetermined calibration formula (Fr, Fg, Fb), a target light monitor value (Qr_A, Qg_A, Qb_A) to be detected by the respective light monitor unit for the emission light beam emitted by the respective light source, and to control the respective light source to control the light monitor value detected for the respective emission light beam according to the target light monitor value in order to control a color temperature of the multiplexed light. a plurality of light sources, including a red light source, a green light source and a blue light source, each configured to emit an emission light beam; a plurality of light monitor units configured to respectively detect light monitor values (Qr, Qg, Qb) for a corresponding one of the emission light beams emitted from the plurality of light sources; and a multiplexing unit configured to multiplex the emission light beams emitted from the plurality of light sources and to emit multiplexed light, characterised by: a control unit configured to control each light source on the basis of the light monitor values detected for the emission light beam emitted from the light source and a respective predetermined calibration formula (Fr, Fg, Fb) for the corresponding light source, wherein for each light source, the respective predetermined calibration formula (Fr, Fg, Fb) defines a correlation between the light monitor values (Qr, Qg, Qb) detected for the emission light beam emitted by the respective light source and luminance values (Lr, Lg, Lb) acquired from a light receiving unit external to the illuminating device and configured to detect the luminance values from the multiplexed light emitted from the illuminating device, and a control unit configured to control each light source on the basis of the light monitor values detected for the emission light beam emitted from the light source and a respective predetermined calibration formula (Fr, Fg, Fb) for the corresponding light source, wherein for each light source, the respective predetermined calibration formula (Fr, Fg, Fb) defines a correlation between the light monitor values (Qr, Qg, Qb) detected for the emission light beam emitted by the respective light source and luminance values (Lr, Lg, Lb) acquired from a light receiving unit external to the illuminating device and configured to detect the luminance values from the multiplexed light emitted from the illuminating device, and wherein the control unit is configured to calculate for each light source, on the basis of the respective predetermined calibration formula (Fr, Fg, Fb), a target light monitor value (Qr_A, Qg_A, Qb_A) to be detected by the respective light monitor unit for the emission light beam emitted by the respective light source, and to control the respective light source to control the light monitor value detected for the respective emission light beam according to the target light monitor value in order to control a color temperature of the multiplexed light.