The SOLSPEC instrument has been built to carry out absolute solar spectral irradiance measurements from space. It was first flown in December 1983 on mission STS 9 (Spacelab 1), from August 1992 to June 1993 on board the free-flying EURECA platform with mission STS 46 (deployment) and STS 57 (retrieval) then flown three times more with the ATLAS missions STS 45, 56 and 66. Further applications are foreseen in combination with the International Space Station Alpha (ISSA). SOLSPEC covers a wavelength range from 180 nm to 3000 nm split into three separate spectrometer channels (ultraviolet, visible and infrared) and has been calibrated pre- and post-flight using a black body at the Landessternwarte Heidelberg with temperatures up to 3300 K. In this paper the calibration concept is presented, together with a short instrument description and suggestions for improvements in order that higher black-body temperatures can be reached.