Irradiance (Ee) refers to the radiant power (Φe) per unit area reaching a surface A from one or more sources of optical radiation. It is measured in W/m² or in W/cm². When the incident optical radiation is not parallel to the detector, which in practice is normal, a cosine diffuser must be used to correct the angular response for accurate irradiance measurement.
Ee = dΦe/dA
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