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Vanadium dioxide (VO2) has been extensively investigated due to its thermal-induced metal-insulator transition (MIT) at about 67 °C; which can be lowered by doping. During the transition from semiconductor to metallic phase electrical conductivity can increase by up to 3-4 orders of magnitude, while optical reflectance can drop by almost 50 %. Possible applications include thermally controlled electrical and optical switches. As the thin film is conductive optical changes can be induced electrically by Joule-heating. In this paper we present electro-optical measurements carried out on 100 nm thick VO2 thin films deposited on sapphire substrates. Phase transitions were electrically induced in contacted films and observed using an optical microscope. Strong spatial inhomogeneity has been detected: a few micron wide dark path connected the two contacts. The visible channel is a metallic area within the semiconducting film as the domain undergoes a MIT because of the Joule-heat of the flowing current. The optically observable fine structure found suggests that VO2 thin films could be used to visualize isotherms with a resolution of a few microns, similar to that of liquid crystals. Possible benefits include the simple and reliable use of the once deposited solid phase precision films and a higher resolution compared to liquid crystal thermography.