Leonard "Lynn" L. Northrup Jr. (March 18, 1918 – March 24, 2016) was an American engineer who was a pioneer of the commercialization of solar thermal energy. Influenced by the work of John Yellott,Maria Telkes, and Harry Tabor, Northrup's company designed, patented, developed and manufactured some of the first commercial solar water heaters, solar concentrators, solar-powered air conditioning systems, solar power towers and photovoltaic thermal hybrid systems in the United States. The company he founded became part of ARCO Solar, which in turn became BP Solar, which became the largest solar energy company in the world. Northrup was a prolific inventor with 14 US patents.