irradiance

Irradiance is a radiometric quantity that describes the power of electromagnetic radiation received per unit area on a surface. It is defined as the radiant flux incident on a surface divided by the area of that surface, and its SI unit is watts per square meter (W/m²). Irradiance is an objective, wavelength-dependent measure and does not account for human visual response.

Spectral irradiance, Eλ, specifies the irradiance per unit wavelength interval (for example at a particular nanometer or micrometer). The total irradiance is obtained by integrating the spectral irradiance over all wavelengths: E = ∫ Eλ dλ. In photometry, the corresponding quantity is illuminance, measured in lux, which weights radiometric irradiance by the human eye’s sensitivity.

Irradiance depends on the angular distribution of the incident radiation and the viewing geometry. In general, E = ∫Ω L(Ω) cos θ dΩ, where L is the radiance coming from direction Ω, θ is the angle between the incident ray and the surface normal, and the integration is over the hemisphere of directions above the surface. For a perfectly diffuse (Lambertian) source with constant radiance, E = πL.

Measurement and standards vary by application. Pyranometers and calibrated radiometers measure global irradiance (sum of direct and diffuse components) for solar energy and climate studies, while spectroradiometers measure spectral irradiance across wavelengths. In photography and optics, irradiance is important for exposure and radiant power budgeting in devices and experiments.