Ludwig Genzel would possibly be the last scientist to desire an obituary written on his account, because he would have thought the author’s time could be more wisely spent doing science, listening to Beethoven, or appreciating the fine architecture of Europe. He was one of the cardinals of infrared spectroscopy and perhaps the reigning pope for Fabry–Pérot interferometry in the far-infrared (FIR) during the post-diffraction-grating era. For several decades, his “Genzel interferometer,” with its novel in-focus beam splitter, was an industry standard. Ludwig was a mentor of outstanding pedagogical ability at the Max Planck Institute for Solid State Research. He died of pneumonia on 27 January 2003 in Stuttgart, Germany.

Ludwig was born on 17 February 1922 in Bad Nauheim, about 30 miles northeast of Frankfurt. Following the Great Depression and the early death of his mother, the family moved closer to Frankfurt, and Ludwig received his abitur (high-school diploma) in 1940 before being drafted into the German army.

The horrors of World War II must have been terrible for him because he was a gentle and impeccably sensitive person. He refused offers to be an officer—staying as far as possible from any connection with Nazism.

He managed to join a flak battery and so was involved only in defense operations. Nevertheless, he felt a deep sense of responsibility for the pain that Germany inflicted on other nations, and throughout his life acted to redress those crimes in his position as a director of a major scientific institute. His department at the Max Planck Institute for Solid State Research was a model of cultural and ethnic diversity. During his tenure, a couple of anti-Semitic incidents took place. He firmly and effectively dealt with them, leaving no doubts as to where he stood.

In 1949, Ludwig earned his diplom in physics from the University of Frankfurt am Main under the direction of Marianus Czerny. Two years later, he received his promotion (PhD) from the University of Frankfurt. With the aim of studying condensed matter systems such as dielectric crystals, he and his students built one of the first modern FIR interferometers in 1954. In 1958, he and his coworkers constructed a rapid-scan interferometer for the millimeter and submillimeter range.

Ludwig stayed in Frankfurt until 1960, when he accepted an invitation from the University of Freiburg to join its faculty. But he first spent a year at Ohio State University, which, at that time, had one of the highest-resolution FIR spectrometers. He fondly remembered that stay and the warmth and friendship of his hosts.

At Freiburg, Ludwig made his most original scientific contributions. In 1962, he conceived of and constructed a Fabry–Perot etalon from a metallic mesh for the FIR. For cognoscenti in Brillouin spectroscopy, seeing a “flat” made of ordinary aluminum with a lattice of holes is quite an enlightening experience.

Presumably in the late 1960s, Ludwig had a conversation with Werner Heisenberg in which Heisenberg asked him to help establish the Max Planck Institute for Solid State Research. In April 1970, Ludwig was appointed its first founding director.

In the early 1970s, Ludwig and coworkers measured the IR transmission of microcrystals of magnesium oxide. The crystals contained only about 50 ions. They applied lattice dynamics to obtain the eigenvalues and eigenvectors for the microcrystals. In 1974, he and his colleagues published their highly cited work on long wavelength optical modes in mixed crystals in Physica Status Solidi . Ludwig also published influential work on the dielectric function of extremely small metal particles in Surface Science in 1985. He showed how quantum effects shifted the plasma resonance absorption.

Ludwig published one of his first papers in biological physics, on lysozyme, in Biopolymers in 1979. He later contributed to the observation of low-lying phonons in oligonucleotides. He insisted on constructing a simplified but useful model of DNA in deciding on which experiment to focus.

Shortly after the “Woodstock of Physics” at the 1987 March meeting of the American Physical Society, Ludwig began a detailed study of the optical properties of the first high-Tc superconductor YBaCuO. His spectroscopy group and coworkers made difficult low-temperature reflection measurements on world-class samples of high-Tc superconductor (single crystals, ceramics, and thin films), some of which were made in house. His publications on phonon anomalies and the superconducting gap have been widely read and cited. Later, he started a very fruitful collaboration with the theory group at Los Alamos National Laboratory.

Ludwig loved physics and the fine arts. Franz Schubert’s string quartet in D Minor (Death and the Maiden) was one of his favorite pieces. His great mentorship of a myriad of students, postdoctoral fellows, and resident and visiting scientists lives on as his greatest legacy. Although, on first impression, Ludwig had a reserved and formal presence, he was one of the few prominent German scientists of his generation to insist that his colleagues use the informal pronoun when addressing him. But this sliver of a fault line in a reserved exterior was only the barest indication of a finely cultivated and delightfully engaging sense of humor.

Ludwig’s son, Reinhard Genzel, has continued in his father’s footsteps. A director at the Max Planck Institute for Extraterrestrial Research, Garching, Germany, and a professor of physics at the University of California, Berkeley, Reinhard used the infrared techniques developed by his father to probe the existence of black holes.