Radiation

The emissivity of a substance refers to its ability to emit energy as radiation. In certain cases, particularly in materials that are good emitters of radiation, the emissivity can be greater than the absorptivity. This is because emissivity refers to the efficiency with which a body emits thermal radiation, while absorptivity is the fraction of incident radiation a body absorbs. For most practical materials, emissivity is greater than absorptivity due to varying efficiency in emission and absorption, especially for non-perfect black bodies.