Quantum yield is one of the most important properties of photochromic systems. Unfortunately, a lack of data at the solid state exists, because measurements are intrinsically not straightforward. A kinetic model describing the conversion of the photoactive species is reported and both analytic and numeric solutions are provided according to relevant cases. The model is then applied to measure the quantum yield of dithienylethene-based polymers; the ring-opening quantum yield is measured for different laser beam profiles (i.e., Gaussian and uniform) and at different wavelengths, showing an increased value with increasing photon energy.