Lähtekiirgus

Lähtekiirgus, often translated as emission or exitance, is a term used in physics, particularly in optics and thermal radiation, to describe the total amount of radiant energy emitted from a unit area of a surface per unit time. It is a measure of the power radiated by a surface. The units of lähtekiirgus are typically watts per square meter (W/m²).

This quantity is distinct from radiance, which describes the intensity of radiation emitted in a specific direction. Lähtekiirgus considers all radiation emitted from the surface, regardless of direction. It is an important parameter when analyzing the energy balance of objects, especially in thermal contexts where surfaces emit infrared radiation.

For a blackbody, which is an idealized perfect absorber and emitter of radiation, lähtekiirgus is directly proportional to the fourth power of its absolute temperature, as described by the Stefan-Boltzmann law. Real surfaces, or gray bodies, emit less radiation than a blackbody at the same temperature, with their lähtekiirgus being a fraction of the blackbody emission, determined by their emissivity.

In applications such as remote sensing, understanding the lähtekiirgus of the Earth's surface and atmosphere is crucial for interpreting satellite measurements. Similarly, in lighting design, the lähtekiirgus of light sources influences the overall illumination of a space.