We present a large-area perfect blackbody sheet, which would offer a planar standard radiator for high-precision thermal imager calibration. Polydimethylsiloxane (PDMS) sheets with nano-precision surface micro-cavity structures achieve both ultra-low reflectance (ultra-high emissivity close to unity) over the thermal infrared wavelengths and high durability to mechanical contact. The investigation on the geometrical parameters of the conical micro-cavities, that is, radii and aspect ratios (ratio of height to radius), confirmed that the PDMS blackbody sheet with a micro-cavity radius of ∼6 µm and an aspect ratio of ∼4 exhibits the optimum hemispherical reflectance of less than 0.002 (emissivity of higher than 0.998) at the thermal infrared wavelengths (6-15 µm). Furthermore, the large-area PDMS blackbody sheet of 100 mm × 80 mm maintained an excellent in-plane uniformity of the emissivity. This unprecedented large-area perfect blackbody conforms to the International Electrotechnical Commission (IEC) standard recommendation regarding thermal imager calibration for fever screening in terms of the emissivity performance.