Abstract: An apparatus and method for determining a parameter of a constituent of a sample employ a radiation source, focusing means for focusing emitted radiation at a first position on the sample, detecting means for detecting the radiation reflected from or transmitted through the sample and adapted to generate a signal representative of the detected radiation, processing means for receiving the signal and determining a parameter of a constituent of the sample corresponding to the signal, and translational repositioning means adapted to translate the focused radiation to a second position on the sample.