An improved particle monitoring sensor is described which uses a variety of techniques to prolong the effective life of the optical surfaces within the particle monitoring sensor. Substantially inert purging gas is directed over the particle monitoring sensor windows, which are normally exposed to a harsh operating environment. The surfaces of these windows are heated by heating elements in direct thermal contact with the windows. In addition, a restrictive slit is placed over the detector window to reduce the exposed area and to increase the velocity of gas flowing over the window surface. While this slit reduces the detector's field of view, the signal loss is reduced by using a linearly polarized light source and aligning the elongated slit's major axis with the direction of polarization.