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UV Irradiance is the radiant power arriving at a surface per unit area. This is the "intensity" pattern to which a point on a surface will be exposed as it passes under a lamp rises to a peak directly under the lamp. Irradiance, in watts (or milliwatts)/cm² , is the measure of UV (or light) photon flux at the surface. (Irradiance is the proper term for radiant power arriving at a point on a surface per unit area).

Irradiance varies with lamp output power, efficiency and focus of its reflector system, and distance to the surface. (It is a characteristic of the lamp and geometry, so does not vary with speed). The intense, peak of focused power directly under a lamp is referred to as "peak irradiance." Irradiance incorporates all of the individual effects of electrical power, efficiency, radiant output, reflectance, focus, bulb size, and lamp geometry.

Irradiance data must always include identification of the wavelength range to which it applies. This is one of the most common omissions in radiometry. When irradiance is measured in any specific range of wavelength, it is called “effective irradiance. When this wavelength range is clearly understood, The term “irradiance” is sufficient.