Radiant

Radiant is an adjective used to describe something that emits light or heat, or more generally something that radiates energy. In everyday language it can describe people, objects, or phenomena that appear bright, glowing, or cheerfully lively.

In physics and engineering, radiant energy is energy carried by electromagnetic radiation or by particles. Radiometric quantities quantify this energy: radiant flux (phi) is the total power carried by radiation; radiant intensity is power per unit solid angle; irradiance is the power incident on a surface per area; radiance describes the power per unit area per unit solid angle and relates to how bright a surface appears. Radiant heat transfer refers to energy exchange between bodies through radiation, which can occur across a vacuum and does not require a medium.

Mean radiant temperature (MRT) is a concept used in building science and HVAC to characterize the thermal environment. It represents the uniform temperature of an imaginary surrounding surface that would produce the same radiant heat exchange as the actual environment, influencing human thermal comfort.

In astronomy, the radiant of a meteor shower is the apparent point in the sky from which the meteors seem to originate. The radiant’s position helps identify the meteor stream and its activity.

Etymology traces radiant to Latin radians, radiantis, from radius meaning ray or spoke. Across disciplines, the term emphasizes emission, brightness, and the exchange of energy through radiation.