Panel A): wavelength dependent photodesorption of CO ice at 18 K (Fayolle et al. 2011); panel B): normalised VUV spectrum (integrated area = 1) measured with the absolute VUV spectrometer described in Ligterink et al. (2015); panel C): multiplication of panel A) and B) representing the photodesorption expectance based on the VUV spectrum of the H₂ lamp. Integration of the curve in panel C) provides the estimate of the CO photodesorption rate at 18 K based on the spectral characteristics of the applied VUV-lamp.