Thomas Eugene Everhart FREng (born February 15, 1932, in Kansas City, Missouri) is an American university president, educator, and physicist. His area of expertise is the physics of electron beams. Together with Richard F. M. Thornley he designed the Everhart–Thornley detector. These detectors are still in use in scanning electron microscopes, even though the first such detector was made available as early as 1956.

Everhart was elected a member of the National Academy of Engineering in 1978 for contributions to the electron optics of the scanning electron microscope and to its use in electronics and biology. He was appointed an International Fellow of the Royal Academy of Engineering in 1990. He served as chancellor of the University of Illinois at Urbana-Champaign from 1984 to 1987 and as the president of the California Institute of Technology from 1987 to 1997.

Everhart's parents were William E. Everhart and Elizabeth A. West. Everhart received his A.B. in physics from Harvard University in 1953, and his M.S. in applied physics from the University of California, Los Angeles, in 1955. He held a Marshall Scholarship at Clare College, Cambridge, where he completed a PhD in physics under Professor Charles Oatley in 1958.

Everhart began working on electron detection and the design of scanning electron microscopes (SEMs) as a student with Charles Oatley at Cambridge in 1955. An initial prototype, the SEM1, had been developed by Dennis McMullen, who published his dissertation Investigations relating to the design of electron microscopes in 1952. It was further modified by Ken C. A. Smith, who developed a way to efficiently detect low-energy secondary electrons. Oatley and his students used SEM to develop a variety of new techniques for studying surface topography.

Everhart developed techniques to detect low-energy secondaries. His Ph.D. thesis, in 1958, was Contrast formation in the scanning electron microscope. Analyzing the electrons detected by the SEM, he reported that about 67% of the signal measured could be attributed to low energy secondaries from the specimen. About 3% was due to higher-energy reflected electrons. He also presented equations to model the noise introduced.

Use of the term "voltage contrast" to describe the relationship between the voltage applied to a specimen and the resulting image contrast, is attributed to Everhart. As of 1959, Everhart produced the first voltage-contrast images of p-n junctions of biased silicon diodes. Voltage contrast, the ability to detect variations in surface electrical potentials on a specimen, is now one of several imaging modes used for the characterization, diagnosis and failure analysis of semiconductors. As many as half of the SEMs sold are believed to be used in semiconductor applications.

Everhart studied contrast mechanisms in detail and developed a new theory of reflection of electrons from solids. He also made some of the first quantitative studies of the effects of beam penetration on image formation in the SEM.

In 1960 Everhart and Richard F. M. Thornley published a description for the improved design of a secondary electron detector, since known as the Everhart–Thornley detector. Everhart and Thornley increased the efficiency of existing detectors by adding a light pipe to carry the photon signal from the scintillator inside the evacuated specimen chamber of the scanning electron microscopes to the photomultiplier outside the chamber. This strengthened the signal collected and improved the signal-to-noise ratio. In 1963, Pease and Nixon incorporated the Everhart-Thornley detector into their prototype for the first commercial SEM, later developed as the Cambridge Scientific Instruments Mark I Stereoscan. This type of secondary electron and back-scattered electron detector is still used in modern scanning electron microscopes (SEMs).