Guide to UV Measurement
Energy, Energy Density
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 (and incorrectly) referred to as “dose,” it is the total accumulated photon quantity arriving at a surface. Because energy density is the product of irradiance and time, it is inversely proportional to speed under any given radiant source, and proportional to the number of exposures (for example, rows of lamps). For an exposure in which irradiance is not constant, such as rising then falling, it is the time-integral of irradiance during the time of exposure (t 0 to t 1 ). This is the UV energy to which a surface is exposed as it travels past a lamp or a sequence of lamps. Effective energy density, E , incorporates irradiance profile, the wavelength range of interest (λ1|λ2) and time:
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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.”
When UV exposure is measured or reported as energy (mJ/cm² ) only , it may be misleading, as the peak of irradiance can be a far more significant factor in the efficiency of cure of many inks and adhesives. Energy and peak should be specified for a UV curable material, as well as spectral response. All of these factors can be significant to the cure performance of the material.