Several newly developed large area photoconductive (PC) mercury cadmium telluride (HgCdTe) radiometers have been tested for spatial and angular responsivity for the purpose of determining what mode of operation (or radiometric quantity) could provide the lowest measurement uncertainty. An infrared (IR) test facility has been developed for the characterization of long wavelength IR (LWIR) detectors and radiometers for spatial response uniformity in power measurement mode and angular responsivity in both power and irradiance measurement modes. We have measured 34% to 53% spatial response non-uniformities and 1.5% to 10.5% changes in angular power reponsivity at different beam positions within the f/4 field-of-view (FOV) of the PC HgCdTe radiometers. The lowest responsivity uncertainty is achieved when these non-uniform radiometers are operated in irradiance measurement mode, where the incident uniform field of radiation averages out the detector's non-uniformity related uncertainties. The angular response deviation from the cosine function within the 16° FOV of the radiometers dominates the uncertainty budget for irradiance responsivity measurements in the 3 µm to 20 µm sensitivity range of these working standard devices.