Substrate coverage of CO (left axis), and thickness of the CO ice (right axis) as a function of different VUV fluence. The results for five experiments, starting from different initial thicknesses are shown, and for clarity have been horizontally shifted around the fitted line that is the resulting average of the photodesorption rates of these independent measurements. The photodepletion of CO ice at 20 K can be determined by a linear fit. Surface coverage has been determined with Eqs. (1) and (2), with n = 1.27 and ρ = 0.80 g cm^-3.