David L. Fried (April 13, 1933 – May 5, 2022) was an American scientist, best known for his contributions to optics. Fried described what has come to be known as the Fried Parameter, or r0 (often pronounced r-naught, but also r-zero). The Fried Parameter is a measure of the strength of the turbulence in the atmosphere of Earth. The turbulence causes what is known as seeing in astronomy and usually limits the optical resolution of ground-based telescopes and the detail in their images of astronomical objects. The Fried Parameter describes the smallest diameter of a telescope aperture at which the image fidelity starts to suffer significantly from turbulent airflows in the atmosphere of Earth. The Fried Parameter may change quickly on the time scale of minutes, or less. Typical values for the Fried Parameter in the visible spectrum may range from less than one centimeter (you can detect turbulence with your eye) to some tens of centimeters at good astronomical sites. Life and professional career David Larry Fried was born in Brooklyn, New York, on April 13, 1933. He received the AB, MS, and PhD degrees in physics from Rutgers University, New Brunswick, New Jersey, in 1957, 1959, and 1962, respectively. From 1957 to 1959 he was with RCA Astro-Electronics Division, Princeton, N. J., where he worked on computer applications analysis. In 1961 he was employed by Rockwell International, where he held the position of Manager in the Electro-Optical Laboratory of the Autonetics Division (Anaheim, Calif.), where, as head of the Laser Techniques Group, he was engaged in the study of devices necessary for laser applications, and in the analysis of system concepts for laser application. He also did extensive work in the study of optical propagation in a randomly inhomogeneous atmosphere and the consequent effects on optical system performance. In 1966 he joined the technical staff of the North American Aviation Science Center, Thousand Oaks, Calif., where he was engaged in a study of the microwave reflectivity and emissivity of rough surfaces. Fried served for 20 years on the U.S. Army Science Board (ASB). For many years, he served on the Army Science Board's standing committee on ballistic missile defense. In the 1960s, Fried published a series of papers on the optical effects of atmospheric turbulence that provided much of the analytic foundations for the development of adaptive optics systems, and that resulted in the definition of the quantity