Top panel: Differential cross section of isolated photons measured in pp collisions at $\sqrt s =13$~TeV. Vertical black lines and grey-filled boxes represent data statistical and systematic uncertainties, respectively. The blue boxes correspond to pQCD calculations with JETPHOX, open boxes for scale uncertainty and filled boxes for PDF uncertainty, respectively. Bottom panel: Ratio between the JETPHOX calculation results and data displayed in the blue boxes, vertical boxes size shows the theory scale and PDF uncertainties. Experimental uncertainties are shown here on the black points centred at unity. The normalisation uncertainty of 1.5\%, described in the text, is included only in the bottom panel and shown as a red box at 200~\GeVc.