Matthew Pothen Thekaekara (1914–1976) was an Indian scientist and author of books and papers relating to spectrophotometry and the solar constant besides works on theology. He was instrumental in publishing some of the earliest AM0 spectra, which is a model spectrum of the sun in space. The historic 1973 Thekaekara spectrum was the basis for ASTM E490 from 1974 to 2000, when it was replaced by the most recent AM0 upgrade, in ASTM E490-00.