Purity of the isolated-photon sample as a function of $p_{\rm T}^{\gamma}$ calculated using Eq. (5) and with the statistical and systematic uncertainty discussed in Sect. 4 for $R=$ 0.2 and 0.4. The curves (plain for low $p_{\rm T}^{\gamma}$, dashed for high $p_{\rm T}^{\gamma}$) are the two sigmoid functions as defined in Eq. (6) obtained by fitting the points as explained in the text.