Ferdinand Kurlbaum (4.10.1857-29.07.1927) was a German experimental physicist. He was born in Magdeburg. In 1887 he received the degree of Doctor of Philosophy in Berlin and worked at the Physico-Technical Institute (Berlin — Charlottenburg) (professor in 1899-1925).
His research concerned optics, spectroscopy and thermal radiation. With O. Lummer he built (1892) a highly sensitive bolometer, with which the accuracy of measurement in the spectrum was brought to 0.01 %. He measured the intensity of thermal radiation. With Lummer he tested (1897) the Stefan–Boltzmann law for temperatures from 290 to 1500°C. Together with H. Rubens (independently of Lummer and E. Pringsheim) he established (1900) by a direct experiment that Wien's law of thermal radiation does not hold for long waves, obtaining a different dependence of the intensity of the radiation of a black body on temperature (as the temperature rose it came closer to being proportional). These measurements by Rubens and Kurlbaum created the experimental preconditions for M. Planck to obtain his formula. Together with Rubens he confirmed experimentally (1900-01) that the Rayleigh law of thermal radiation holds for long waves, and that Planck's formula does too. He made a pyrometric determination of the temperature of the Sun.