. 1835: Macedonio Melloni makes the first thermopile IR detector. 1860: Gustav Kirchhoff formulates the blackbody theorem . 1873: Willoughby Smith discovers the photoconductivity of selenium . 1879: Stefan-Boltzmann law formulated empirically that the power radiated by a blackbody is proportional to . 1880s & 1890s: Lord Rayleigh and Wilhelm Wien