The lamp that is used as the source in the measurement of transmittance must emit a continuous spectrum of radiation with no peaks over the measurement range of 290-400nm, and its irradiance should be sufficiently low so that photostability of the product is not unduly challenged (e.g., a xenon flash lamp is a convenient solution). The dose of UV delivered during one measurement cycle should, therefore, not ex-ceed 0.2J/cm². For all wavelengths, irradiance must be at least 100 times higher than the so-called “dark irradiance”, measured when the lamp is switched off.