The LED lamps were spectrally characterised by the Optical Laboratory of the Norwegian Radiation Protection Authority (www.nrpa.no), Østerås, Norway during the years 2005–2011. Except for the years 2007 and 2008 when a CCD spectrometer was used to measure the spectral irradiance (see below), the measurements were performed using a high-resolution spectroradiometer (Bentham DTM 300, Bentham Instruments Ltd, Reading, UK) with a double monochromator and integrating sphere. Emission spectra were obtained in the wavelength band 180–800 nm as close to the surface of the light guide as possible (i.e. excitance measurement). The bandwidth at half maximum and the step size were 0.8 nm and 1 nm, respectively. Input optics was an optical light guide, fitted with a cosine corrected flat Teflon diffuser. Wavelength and irradiance calibrations were made against emission lines from a low-pressure mercury lamp and a 150 W portable halogen lamp, traceable to the National Institute of Standards and Technology, NIST (Gaithersburg, MD, USA) via the Swedish Testing and Research Institute (Borås, Sweden). Irradiance measurement uncertainty was within 9%.
The CCD spectrometer (USB4000-UV-VIS, Ocean Optics, Dunedin, FL, USA) measurements were coupled to the integrating sphere and calibrated with the Bentham instrument described above. The measurement wavelength band was 178–888 nm, corresponding to a step size of about 0.2 nm. The CCD spectrometer measures all wavelengths simultaneously while the scanning Bentham spectroradiometer measures the wavelengths one by one (about 1 nm/s). The spectra obtained with the CCD spectrometer were corrected for stray light and dark current. Measurement integration time was adjusted to avoid radiation flux saturation of the instrument, and the values from these lamps were corrected for differences in integration time. Spectra obtained with the CCD instrument represent one occasion within the first 20 s irradiation time (alternatively the maximum irradiation time). Spectra represent the mean of five values, corresponding to about 1.1 nm, and spectra were subsequently interpolated to integers of nanometres. For pulsed lamps, the values represent the mean of the irradiation time (e.g. 20 s or 5 s). The lamp “FlashMax” was measured using a grey filter (O.D. 0.5) transmitting 37–40% of the light flux, depending on the wavelength.