In Xenon and UV fluorescent accelerated weathering testing, an irradiance setpoint value is incomplete information without reference to the wavelength or wavelength range it represents. There are two classes of irradiance setpoints.

Narrowband irradiance setpoints include 340 nm and 420 nm, and represent a 1 nm wide bandpass, centered on the single wavelength value indicated (i.e. ½ nm on either side of 340 nm, for example). Narrowband irradiances use units of “Watts per square meter per nanometer.” This can be written as either W/(m2∙nm), W/m2/nm, or W∙m-2∙nm-1.

Wideband irradiance setpoints (usually “TUV” or “total UV”) are an integration of the irradiance from all wavelengths between two endpoints, usually 300-400 nm (accelerated laboratory) or 295-385 nm (outdoor). As a result, wideband irradiance values are generally much larger than narrowband irradiance values. Wideband irradiance is measured in “Watts per square meter,” written as W/m2 or W∙m-2.

The graph below is a spectral power distribution (SPD), which presents irradiance as a function of wavelength. This SPD shows that, for this particular light source, one could describe the irradiance either as having a narrowband irradiance of 0.35 W/m2/nm @ 340 nm, OR as a wideband irradiance of 40 W/m2 from 300-400 nm (TUV).