Abstract: The opening sphericity is widey used in the metrology of radiation, the scaling of radiation and the measurement of laser energy. As taking the radiative heat exchange into consideration, the true emissivity is an important parameter due to multiple reflections at the surface of the sphere, so the energy absorbed by the opening spherical cavity is more than a flat plate.The article introduced the visible emissivity and ‘opening phenomena’——cavity effect which makes the emissivity and the absorptivity increased correspondingly. With the visible emissivity, the thermal loss of radiation was calculated.