The spectral irradiance responsivity calibrations of InSb radiometers measured on the tunable-laser based Infrared Spectral Irradiance and Radiance Responsivity Calibration with Uniform Sources (IR-SIRCUS) facility are discussed. This work describes the following changes undertaken to reduce the uncertainties of the calibrations: improve the spatial uniformity, reduce the laser-induced speckle from the gold-coated integrating spheres between 1 μm and 5 μm, improve the stability of the optical parametric oscillator (OPO) tunable laser, reduce the noise from the signal-to-monitor ratio, increase the repeatability of measurements, and reduce the stray light and fringe problems of the radiometer under test. Measurements of the spatial uniformity with the use of polytetrafluoroethylene (PTFE) and gold-coated integrating spheres at different wavelengths have been performed. Different approaches for generating a uniform source, removing the speckle, stabilizing the laser, and improving the signal-to-monitor ratio are also described. The spatial non-uniformity after using these approaches has been shown to be reduced to < 1 %. The uncertainty budget of spectral irradiance responsivity calibrations is discussed, and is found to be mainly due to the measurement repeatability uncertainty component of 1 %. Calibrated radiometers are tested against a source-based scale from the calculated spectral irradiances obtained using a precision aperture and a blackbody (BB) with a known temperature.© (2009) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.