The complex optical rays method application to the gaussian beam propagation in the region with so-called thermal waves is presented in this work. The case of light beam perpendicular incident relatively to the plane thermal waves of (Delta) z width direction of propagation is considered. Here was assumed that the light refraction index changes caused by thermal waves influence only on the light wave phase. The typical experimental setup was considered in which the light probing beam intensity changes are registered by quadrant photodiode. Such parameters as size and position of the gaussian beam waist, the laser-screen distance, the thermal waves beam position, as well so 'traditional' parameters as thermal waves frequency and the probing beam - thermal waves source distance influence on so-called normal signal from quadrant photodiode is analyzed. The final analytical relations are illustrated by normal signal graphs quantity caused by thermal waves in air.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.