Wilhelm Conrad Röntgen [conventionally Roentgen](1845-1923) awarded the first Nobel Prize in Physics for discovery of X-Rays and inventing radiography. One could develop a collection of stamps just on Roentgen and XRays, but here is just a taste:
Sweden, 1961, Scott 603, 604, 605 celebrating Nobel winners. Roentgen is in front, and has the fiercest in this line up of beards.
India, 1995 They did not do all that well with his eyes so he looks a little crazy.
From his native Germany, 1951, Scott 686:
Finally, a nice one from Afars and Issas, 1973, Scott C84: