An absolute cryogenic radiometer (ACR) with a detachable optical window was designed and built for high accuracy optical radiant power measurement and photodetector spectral responsivity calibration. The ACR receiver is an electroplated pure copper cavity with a 50-μm-thick wall and inner surface coated with a specular black polymer material mixed with highly dispersible carbon nanotubes. The absorptivity of the cavity receivers was evaluated to be ≥0.9999 in the 250 nm–16 μm wavelength range and ≥0.99995 in 500 nm–16 μm. The cavity receiver works at the temperature of ∼5.2 K with nanowatt-level noise-equivalent power. The relative standard uncertainty is 0.041% for the measurement of ∼100 μW optical radiant power (250 nm–16 μm) and 0.015% for ∼1 mW (500 nm–16 μm).