Int J Thermophys (2011) 32:278–284 DOI 10.1007/s10765-011-0911-4 Calibration of the Irradiance Responsivity of a Filter Radiometer for T Measurement at NIM X. Lu · Z. Yuan · X. Hao · Y. Lin · J. Yang Received: 23 March 2010 / Accepted: 3 January 2011 / Published online: 19 January 2011 © Springer Science+Business Media, LLC 2011 Abstract At the National Institute of Metrology of China (NIM), silicon photodiode-based narrow-band interference filter radiometers (FRs) have been designed for the radiometric determination of the thermodynamic temperature. The FR calibrations were performed on a new spectral comparator with a trap detector which was calibrated against the cryogenic radiometer at several discrete laser lines. The new spectral comparator is constructed from two grating monochromators assembled to give lower stray light and higher transmitted flux. Applying a transmittance mea- surement of the filter in the out-of-band region and careful control of the temperature, the irradiance responsivity of a 633 nm centered FR has been obtained over a dynamic range of nearly eight decades in the wavelength range from 450 nm to 1200 nm. The relative standard uncertainty of the responsivity is also analyzed and estimated to be less than 7 × 10 -4 at the 1 σ level. Keywords Filter radiometer · Monochromator · Spectral responsivity · Thermodynamic temperature 1 Introduction The new MeP-K at high temperature will allow direct thermodynamic temperature measurement as an alternative to the International Temperature Scale-90 (ITS-90) [1]. This could be through the direct measurement of temperature, or mediated by X. Lu (B ) · Z. Yuan · X. Hao · Y. Lin National Institute of Metrology, Beijing 100013, China e-mail: luxf@nim.ac.cn J. Yang Beijing University of Chemical Technology, Beijing 100029, China 123