The transient temperature distribution produced by a surface heat source with Gaussian distribution in one dimension and of infinite extent in the other is calculated. The heated medium is a thin metallic film on a thick substrate. The transverse temperature gradient in the thin film is neglected and it is assumed that all the heat is generated in the thin film. The heat source is assumed to be a light beam. A stationary medium and one moving at constant velocity under the beam are considered. Numerical result of maximum temperature and heated area width are given for a glass and copper substrate.