A light source system and method that generates stable optical power over time and temperature for use in laser scanning, turbidity sensing, airborne-particle analysis, fog and visibility monitoring, blood-gas analysis and applications where light source output intensity changes less than one-half percent over a 50° C. range. The system includes a miniature semiconductor light emitter that can be powered by two AAA alkaline batteries for more than 100 hours and is about 1 cm3 in size (TO-5 package). A semiconductor light emitter emits a beam of linearly polarized light through a coated optical element having first and second surfaces that meet at an acute angle, the first surface reflecting a portion of the light to a control system and transmitting the rest through the second surface in a direction normal to it and thereby enabling immunity to light interference in the reflected and transmitted beams and novel, error-canceling properties.