In this work we present an analysis of the microstructures of steels obtained at various heat treatment temperatures. Specific focus has been on quantitative image analysis to determine the plate thickness of the thermomechanically processed steels. The proposed method is based on the observations that the optical intensity of the lamellar structure in a narrow region resembles a periodic pulse train. A Fourier analysis of this information is used to determine the plate thickness of lamellar structures at micron and nanometer scales.