A method to determine the power of a continuous wave laser from photothermal heating of a poly(dimethyl siloxane) film is reported. The base was doped with oil red O and cured on a surface relief grating. The grating period was shown to increase proportional to the temperature by monitoring the decrease in diffraction angle of a probe beam. A 10s illumination period of a continuous wave (CW) pump beam absorbed by the film increased the film’s temperature, which resulted in a local strain that could be modeled using the two-dimensional heat kernel. The amplitude of the transient response was found to be linearly correlated with the pump laser power.