Theodore Lyman

Physicist, born in Boston, Massachusetts, USA. He taught at Harvard (1907–25), then became director of Harvard's Jefferson Physical Laboratory (1910–47). He discovered the fundamental Lyman series of hydrogen wavelengths in the vacuum ultraviolet (1914), and continued to investigate the range of ultraviolet spectra to increasingly shorter wavelengths.

He became an assistant in physics at Harvard, where he remained, becoming full professor in 1917, and where he was also director of the Jefferson Physical Laboratory (1908-17). Dr. Lyman made important studies in phenomena connected with diffraction gratings, on the wave lengths of extreme ultraviolet light discovered by Schumann and also on the properties of light of extremely short wave length, on all of which he contributed valuable papers to the literature of physics in the proceedings of scientific societies. The Lyman Crater, an impact crater that lies in the southern hemisphere on the far side of the Moon, was named after him.

Directivity is also defined for directional couplers and similiar devices for RF and Microwave measurements. It is the properof the device couple power in one direction while rejecting in other direction. For example in a directional coupler if the directivity of a measuring port is 40 dB, it means the power coupled in this port from desired direction will be 40 dB higher than the undesired power coupled from other direction.