Thermal measurements on a sub-micron scale are non-trivial, but are important of the characterization of modern, semiconductor and opto-electronic devices. In this paper we will discuss the application of the thermoreflectance method for real time sub-micron thermal imaging. By using light in the visible spectrum, the diffraction limit, and this spatial resolution is improved over a traditional infrared camera based on blackbody emission. With active excitation of the sample and frequency domain filtering, thermal images with 100 mK temperature resolution are obtained. Experiments performed on semiconductor micro-coolers and micro-heaters are presented.