Measuring light

This paper is a review of the existing primary standards for the measurement of the optical radiation; blackbody, synchrotron radiation and electrical substitution radiometer. A summary of the recent and significant advances in this field with the carbon nanotube absorber in the electrical substitution radiometer and the development of the predictable quantum efficiency detector is described. Photon counting principles for the realization of the radiometric and photometric quantities are introduced and finally the route for the realization of the candela, the measurement of the luminous intensity, is depicted.