A multibeam photometric measuring device includes a light assembly (12) comprising a single modulated light source along with a filter assembly (19) for passing only a desired band of wavelengths and an optical system for suitably focusing light from the light source. A fiber rotor arrangement (24) is provided for sequentially connecting the collimated light source to each of the plurality of fibers (34) disposed in a distributor (32). Light passing through the wells of the sample plate (40) is captured by discrete photodetector cells (44) disposed on a one-to-one correspondence opposite the wells of the plate. A reference fiber links (52) light from the light source to a separate photodetector (45) to provide a reference light signal with which the sensed photodetector signals are compared. The sensed signals are suitably multiplexed and processed by circuitry which allows control over the accuracy with which the signals are processed on the basis of their instantaneous strengths as well as reference readings taken during system initialization. Enhancement of colour distribution resulting from chromogenic reactions is provided by automatic agitation by mechanism (54) of the reacting agents within the sample wells before a reading is taken at each of the sampling intervals of a kinetic measurement sequence.