Optical system for the photometric measurement of samples, the system comprising - a light source, the light source comprising a plurality of light emitting diodes (LEDs) mounted on the same substrate or chip board in thermal contact with each other such as to enable thermic conduction and heat transfer between the LEDs, said LEDs having different expected lifetimes, comprising a plurality of light emitting diodes (11, 1-3) with longer expected lifetime (L-LEDs) and a plurality of light emitting diodes (11, 4-6) with shorter expected lifetime (S-LEDs), the L-LEDs (11, 1-3) emitting white light or light with a wavelength greater than 450 nm and the S-LEDs (11, 4-6) emitting light with a wavelength equal or smaller than 450 nm, - an optical detector and either a cuvette holder for holding one or more optical cuvettes and arranged such that light is guidable in an optical path from the light source through a cuvette held in the cuvette holder to the detector for the photometric measurement of a sample contained in the cuvette or a multi-well-plate holder for holding at least one multi-well-plate and arranged such that excitation light is guidable in an optical path from the light source to at least one well of the multi-well-plate and emission light is guidable from the at least one well to the detector for the photometric measurement of a sample contained in the well, - a controlling unit for switching the optical system between two light-source modes, characterized in that the two light-source modes are a light-source-lifetime-extending mode and a light-source-lifetime-non-extending mode, wherein in the light-source-lifetime-extending mode all light emitting diodes (11, 4-6) with shorter expected lifetime (S-LEDs) are in a down mode, that is an off mode or a reduced current mode compared to an on mode, and all light emitting diodes (11, 1-3) with longer expected lifetime (L-LEDs) are in the on mode, and wherein in the light-source-lifetime-non-extending mode all light emitting diodes (11, 4-6) with shorter expected lifetime S-LEDs (11, 4-6) and all light emitting diodes (11, 1-3) with longer expected lifetime (L-LEDs) are in the on mode thereby extending the lifetime of the light source while a thermal equilibrium of the light source can be maintained by the heating of the L-LEDs (11, 1-3) which remain in the on mode also in the light-source-lifetime-extending mode. - a light source, the light source comprising a plurality of light emitting diodes (LEDs) mounted on the same substrate or chip board in thermal contact with each other such as to enable thermic conduction and heat transfer between the LEDs, said LEDs having different expected lifetimes, comprising a plurality of light emitting diodes (11, 1-3) with longer expected lifetime (L-LEDs) and a plurality of light emitting diodes (11, 4-6) with shorter expected lifetime (S-LEDs), the L-LEDs (11, 1-3) emitting white light or light with a wavelength greater than 450 nm and the S-LEDs (11, 4-6) emitting light with a wavelength equal or smaller than 450 nm, - an optical detector and either a cuvette holder for holding one or more optical cuvettes and arranged such that light is guidable in an optical path from the light source through a cuvette held in the cuvette holder to the detector for the photometric measurement of a sample contained in the cuvette or a multi-well-plate holder for holding at least one multi-well-plate and arranged such that excitation light is guidable in an optical path from the light source to at least one well of the multi-well-plate and emission light is guidable from the at least one well to the detector for the photometric measurement of a sample contained in the well, - a controlling unit for switching the optical system between two light-source modes,