Irradiance is a measure of the power of solar radiation or other forms of electromagnetic radiation received by a surface per unit area. It is typically expressed in watts per square meter (W/m²). Irradiance quantifies how much solar energy is available at a given location and time, making it an important parameter in fields such as solar energy, meteorology, and climatology.
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Irradiance measures incident radiant flux per actual receiving area. Its units match radiant exitance, but it counts the incoming side of the radiation exchange. Measure incoming angles from the unit normal pointing toward the incident hemisphere. ThenFor distributed uniformly over , . If the receiving plane is tilted in the same broad uniform parallel beam, the incident power per actual area is halved to . This receiving-surface cosine factor is geometric and does not assume Lambertian reflectance. Together, irradiance and radiant exitance describe how radiation supplies and removes energy at a surface, while radiance preserves the directional information.