Peak irradiance, also called intensity, is the radiant power arriving at a surface per-unit area. With UV curing, the surface is the cure surface of the substrate or part, and a square centimeter is the unit area. Irradiance is expressed in units of watts or milliwatts per square centimeter (W/cm² or mW/cm²). Peak irradiance is instrumental in penetration and aiding surface cure. Peak irradiance is affected by the output of the engineered light source, the use of reflectors or optics to concentrate or contain the rays in a tighter surface impact area, and the distance of the source from the cure surface. The irradiance for UV LEDs at the cure surface decreases quickly as the distance between the source and the cure surface increases.
Energy density, also called dose or radiant energy density, is the energy arriving at a surface per-unit-area during a defined period of time (dwell or exposure). A square centimeter is again the unit area and radiant energy density is expressed in units of joules or millijoules per square centimeter (J/cm² or mJ/cm² ). Energy density is the integral of irradiance over time. A sufficient amount of energy density is necessary for full cure.
