An optical sensing unit is provided which comprises at least one sample measurement cell, at least one excitation light source acting upon the or each measurement cell to provide one or more sensor fields defining an array of measurement fields, a photoelectric detector array for detecting the intensity of light emitted from the or each measurement cell in response to excitation light, and means for directing light emitted from each measurement field to a respective portion of the photoelectric detector array. The sensing unit may be used in combination with multiplexed waveguide arrays or multiplexed planar cappillary chromatography or electrophoresis chips.