Optical measurements were carried out using a Perkin Elmer Lambda 950 spectrophotometer. It is a double beam type and is equipped with a 150 mm Spectralon coated integrating sphere: measurements were carried out in the 300–2500 nm wavelength range, with a spectral resolution of 5 nm, covering the whole solar spectrum as defined in the relevant standards [24,25]. The slit aper-ture was set to 2 nm in the visible range and in servo mode (slit size variable according to the optimal energy input) in the near infrared range.