Abstract An experiment has been performed for measuring a nonlinear refractive index that is due to the polarizability difference Δ α P between excited ( 4 T 2 ) and ground ( 4 A 2 ) states in a Cr 3+ :LiSAF crystal at λ = 647 nm. The latter one is responsible for a population lensing effect which has been monitored by using the eclipsing Z -scan technique. We have performed a data analysis that allows to distinguish between thermal and population contributions to the lensing effect. We have found Δ α P = 4.6 × 10 −25 cm 3 which is in a good agreement with our previous measurements with a different technique.