Abstract
Photochromism is a phenomenon describing the irradiation-induced reversible transformation of a chemical compound's colour between two states (dubbed as colour switch). This review makes an up-to-date account of inorganic photochromic compounds. In the first part, we summarise the photochromic mechanisms involving transition metal oxides, with an emphasis on the canonical WO 3 large-bandgap semiconductor. In the second part, we discuss the potential contribution of rare earth elements to the widening of the colourimetric contrasts in absorption, as well as the method of modulation in emission.