A direct derivative measurement technique for determining Brewster's angle has been developed using a lock-in amplifier which synchronously detects the reflected intensity of a directionally modulated or dithered laser beam. In our apparatus the angle of incidence of a polarized laser beam is slowly varied while it is dithered at a high frequency by a galvanometer-mounted mirror. The modulated signal, which is proportional to the derivative of the reflectance versus angle curve, is detected with a lock-in amplifier tuned to the dither frequency. The angle at which the signal crosses zero is Brewster's angle. The angle is more precisely defined using this method as opposed to the more conventional method of determining the angle of minimum reflectance because it is easier to locate a zero crossing than a minimum in a curve. The technique has been tested at visible and infrared HeNe laser wavelengths. The precision of the measurement is typically better than five parts per thousand for materials with refractive index near 1.5 and ten parts per thousand for materials with refractive index near 2.5. Data have been gathered for a number of visible and infrared transmitting materials.© (1980) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.