Guide to UV Measurement
Energy Density
Energy Density or Radiant Energy Density is the energy arriving at a surface per unit area. A square centimeter is the unit area and radiant energy density is expressed in units of joules or millijoules per square centimeter (J/cm² or mJ/cm² ).
The radiant energy density is the time integration of the irradiance. In an exposure where the irradiance value is constant over time (square profile exposure), the radiant energy density could be estimated from the relationship of one watt for one second = one joule.
Most exposures in UV curing and especially in applications where the product is moving are not ‘square profiles'. The product moves into the UV system, into an area of increasing UV, through the peak UV irradiance area and then exits through a decreasing area of UV irradiance. To determine the radiant energy density in this type of profile, the area under the irradiance curve needs to be measured and calculated.
In UV curing, the term ‘dose' has commonly been used to describe radiant energy density. The terms Energy Density or Radiant Energy Density are preferred.
The radiant energy density is important for total and complete UV cure. Historically, the UV radiant energy density (dose) has been the only UV exposure guide number supplied by formulators. Recently, some formulators have also started supplying the irradiance value required for a particular coating.
It is also important to determine the minimum radiant energy density value needed for your process.