Born at Jena, in Germany, Lummer became professor at the University of Breslau in 1905. In 1889 he designed, with Eugen Brodhum, a photometer with an arrangement of prisms, which was an improved version of the grease-spot photometer invented by Robert Bunsen.

Lummer's research was chiefly on radiation energy and temperature. Working with Wilhelm Wien he achieved in practice the black-body radiator, which had been conceived as a theoretical abstraction in the study of radiant heat. In 1899 he did further work with Ernst Pringsheim on the distribution of energy in black-body radiation – work that eventually led to Max Planck's formulation of the quantum theory.

Lummer also built, with Leon Arons, a mercury vapor lamp in 1892.