A rate nephelometer of the type useful in automated chemical analyzers is provided. The nephelometer includes a laser for generating a polarized laser beam having an S-wave component and a P-wave component. The beam is split by a beam splitter specially constructed so that a known proportion of one of the two polarized portions of the beam is directed to a reaction container. In the reaction container, a first polarized component of the laser beam is used in a nephelometric chemical analysis. The remainder of the laser beam passes through the beam splitter to a laser control light detector. Before the remainder of the laser beam reaches the laser control light detector, however, the polarized component which is not used in nephelometric chemical analysis is filtered out. The laser control detector uses the non-filtered portion of the laser beam to control the output of the laser.