The present invention is an optical measurement system for measuring a liquid sample within a well. The system comprises a light source configured to transmit light though the well, a detector configured to measure optical signals derived from the transmitted light, and a tunable optical element. The tunable optical element is positioned between the light source and the well. The tunable optical element is operable to shape the light to compensate for distortions induced by a surface of the liquid sample. The detector is preferably located below the well for receiving a forward scatter signal indicative of at least one characteristic of the particles within the liquid sample.