Thermal measurements on a sub-micron scale are non-trivial, but are important for 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 thus 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 100mK temperature resolution are obtained. Experiments performed on semiconductor micro-coolers and micro-heaters are presented.