Irradiance

Irradiance is radiant power incident on a surface per unit area, measured in watts per square metre. It describes incoming electromagnetic radiation at a surface.

Radiant flux: Radiant energy transferred per unit time, measured in watts. Irradiance divides the radiant power arriving at a surface by its area.
Inverse-square law: A law stating that the intensity from a point source decreases with the square of distance. It predicts how irradiance from a compact source falls as a surface moves away.
Solar irradiance: The irradiance produced by the Sun at a location or surface. It is the central radiation input in solar-energy and climate measurements.
Radiance: Radiant power emitted, reflected, transmitted, or received per unit projected area per unit solid angle. Radiance preserves directional information that irradiance sums over incoming directions.
Radiant energy: Energy carried by electromagnetic radiation, measured in joules. Radiant power is the rate at which this energy reaches a surface.
Lambert's cosine law: A relation stating that the radiance of an ideal diffuse surface is independent of viewing angle. It helps connect surface emission and directional radiation measurements.
Solar constant: The average solar irradiance received at the top of Earth’s atmosphere at the mean Earth–Sun distance. It provides a reference value for incoming sunlight before atmospheric effects.
Radiant intensity: Radiant power emitted by a source per unit solid angle, measured in watts per steradian. It characterizes a source’s directional output rather than power incident per area.
Radiometry: The measurement of electromagnetic radiation in terms of physical power and energy. Irradiance is one of radiometry’s basic surface quantities.
Absorptance: The fraction of incident radiant energy absorbed by a material. Incident irradiance may be reflected or transmitted instead of absorbed.

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