Effects other than the one under study may set a limit on accelerated testing. The maximum irradiation rate must not damage the sample (or cause non-linear response, Fig. 2). Absorbed visible and infrared radiation in intense solar radiation or simulator beams can quickly char dark fabrics, even though the ultraviolet induced weathering effects is still following the law of reciprocity. We offer simulators that have greatly reduced visible and infrared output. This allows realization of high ultraviolet intensities without complications from visible and infrared heating.
When the action spectrum for the weathering effect is not precisely known, as is often the case, it is important that the test spectrum closely simulates the expected spectrum for deployment. Using a total ultraviolet level for tests can be misleading if the simulator spectrum isn't a reasonable approximation to a time weighted outdoor spectrum. This is particularly true for highly variable action spectra.
Any weathering effect following reciprocity will be proportional to the total effective dose; the wavelength integrated product of the action spectrum and the dose spectrum. The appropriate dose spectrum should take into account the diurnal and annual dependence of the outdoor spectrum and may be expressed as "worst case" annual irradiance, or for materials like those making up patio furniture, the local spectrum for total summertime dose. Reliable information is gradually becoming more available on UV dose spectra for various sites throughout the world.
The integrated effectiveness spectrum indicates how effective each source is in producing weathering. In the example, we are able to compare the sources because we used a known action spectrum. The simulator based on the metal halide lamp produces 10% more weathering than expected from power measurement.
For our example, we use the summer noon UV spectrum and a “weathering action spectrum” based on the ultraviolet absorption of polycarbonate resin. We compute the effect at each wavelength (in arbitrary units) for the solar spectrum by multiplying the value of the solar irradiance by the value for the action spectrum. Integrating the values gives the total weathering effect for this action spectrum and that solar spectrum.
We repeat this process for the simulator with atmospheric attenuation (AA) filter, and a metal halide based simulator. For both of these we scale the output to match the total solar irradiance from 280 - 400 nm. The results are tabulated.
When the action spectrum is not known, the closer the spectral match between the simulator and the average solar irradiance, the easer it is to extrapolate simulator results to outdoor reality. This is particularly important when comparing new product formulations with possible differences in action spectra.
Broadband power or energy meters are sensitive to a broadband of wavelengths. Broadband UV meters are useful when working with a single source or two sources with similar spectral content. Using a meter to measure solar UV and then comparing this value with simulator output can lead to serious errors. In biological applications, Sayre1 showed errors of factors of twenty!
For valid comparison of sources with different spectral outputs using a broadband meter either:
The sources must have similar spectral content
or
The meter spectral response must have the same shape as the action spectrum