What does emissivity measure?

Emissivity is a measure of how efficiently a surface radiates energy compared to an ideal black body at the same temperature. A black body has an emissivity of 1, meaning it is a perfect emitter of radiation. A material with lower emissivity emits less energy than a black body, and thus its emissivity value ranges between 0 and 1. In practical applications, emissivity is critical in fields such as thermography and infrared thermometry because it affects the temperature readings taken of surfaces. Understanding the emissivity of materials helps in accurately assessing their thermal properties, which directly relates to their ability to radiate energy.