A theoretical approach is developed for distinguishing between the instant nonlinear optical response and the photoinduced accumulation effect on the refractive index. This is performed in terms of superposition of different lens transparencies for the known Z-scan technique. For the case of homeotropic liquid 5CB crystal cell doped with anthraquinone dye, we prove a possibility for decomposing consequent irreversible Z-scan traces into the contribution of instant nonlinear optical response, which takes place before the reorientation threshold, and the accumulated photoinduced refractive index variations occurred due to the exposition.