Guide to UV Measurement
Energy
Effective Energy Density is the radiant energy within a wavelength range arriving at a surface per unit area, usually expressed in joules or millijoules per square centimeter (J/cm² or mJ/cm²). Sometimes loosely -- but incorrectly -- referred to as “dose,” it is the accumulated amount of energy delivered by the incident photons at the surface. (”Dose” is the energy absorbed per unit volume or per unit mass -- as defined for absorption of ionizing radiation, such as EB -- UV radiometers measure energy per unit area.) Because energy density is the product of irradiance and time, it is inversely proportional to speed under any given light source and proportional to the number of exposures (for example, rows of lamps). When the surface to be exposed is moving relative to one or more UV sources, then the irradiance will rise and fall. In this case, the energy density is equal to the time-integral of the irradiance profile <Irradiance Profile.doc> over the time of exposure (t 0 to t 1). Effective energy density incorporates irradiance profile, the wavelength range of interest (λ 1 to λ 2) and time:
As with irradiance, when the wavelength range is clearly stated and it is clear that the meaning is “per unit area,” this term can be simply expressed as “energy.”