Abstract: A spectrophotometer having an optical system for directing a beam of substantially monochromatic excitation light to a liquid sample contained in a well (3) of a well plate for interaction with the sample for absorption or emission measurements to analyse the sample. The optical system includes two apertures (46, 28) for establishing a Kohler illumination region outside the well, that is an excitation beam region between conjugate images (18, 21) of the two apertures. This excitation beam region is then demagnified and imaged (10, 9) into the well (3). The invention provides for the shape of the Kohler illumination region to correspond to the shape of the well space so that all of the liquid sample is uniformly illuminated without the well obstructing any portion of the illuminating excitation beam of light.