  The NA48/2 experiment at CERN collected two data samples with minimum bias
trigger conditions in 2003 and 2004. A measurement of the rate and dynamic
properties of the rare decay $K^\pm\to\pi^\pm\gamma\gamma$ from these data sets
based on 149 decay candidates with an estimated background of $15.5\pm0.7$
events is reported. The model-independent branching ratio in the kinematic
range $z=(m_{\gamma\gamma}/m_K)^2>0.2$ is measured to be ${\cal B}_{\rm
MI}(z>0.2) = (0.877 \pm 0.089) \times 10^{-6}$, and the branching ratio in the
full kinematic range assuming a particular Chiral Perturbation Theory
description to be ${\cal B}(K_{\pi\gamma\gamma}) = (0.910 \pm 0.075) \times
10^{-6}$.
