The Danish physicist Martin Knudsen (1871–1949) has just passed his 150th anniversary. Growing up, he worked as a shepherd during the summer and finished his career as a professor in physics and president at the University of Copenhagen.

In the period from about 1910 to 1920 he was investigating the behaviour of the gas flow in vacuum systems, in particular, low-pressure systems, in which the mean-free distance and dimensions of the vacuum system were comparable.

He introduced several concepts, the Knudsen Number, Knudsen gas, (Hertz-)Knudsen equation and Knudsen cells, which are still used today. The influence that Knudsen had on recent work will also be outlined.

Jørgen Schou graduated in 1980 at the University of Copenhagen, Denmark, with a study on electron- and ion-induced secondary electrons. In the following years, until 2007, he worked as a staff member of the physics department at Risø National Laboratory with the main emphasis on sputtering of volatile targets by ion and electron beams. The latest 10 years he worked as group leader at the Technical University of Denmark (DTU) mostly on photovoltaic films and retired in 2021 after having obtained the world record efficiency for CZTS-silicon tandem cells of about 7% as a coordinator for a major innovation project.