Emissivity is a measure of a material's ability to emit thermal radiation compared to that of a perfect blackbody, which has an emissivity of 1. In the context of blackbody radiation cavities, emissivity quantifies how effectively the cavity emits infrared radiation at a given temperature. A higher emissivity indicates that the cavity more closely approximates an ideal blackbody, emitting radiation uniformly across all wavelengths and directions.

According to Kirchhoff's law of thermal radiation, for a body in thermodynamic equilibrium, the emissivity equals the absorptivity at each wavelength. This principle ensures that a cavity with high emissivity also has high absorptivity, allowing it to absorb incident radiation efficiently and re-emit it uniformly. citeturn0search8

In practical applications, achieving an emissivity close to 1 in blackbody cavities is essential for accurate calibration of infrared thermometers and sensors. Materials and surface treatments are carefully selected to maximize emissivity, ensuring that the cavity serves as a reliable reference standard for temperature measurements.