The integrated optical absorption I (B) due to the Z3‐triplet exciton in CuCl is measured at 4.2 K as a function of an external magnetic field B (O ≦ B ≦ 0.2 T) in Voigt configuration. I (B) shows the characteristic B‐dependence whose behaviour is ranging between quadratic and linear on B. It is found that this B‐dependence of I is caused by the correlation between the B‐dependent singlet‐triplet mixing and the sample‐dependent damping of the Z3‐triplet exciton. The effects of spatial dispersion due to the finite effective mass of the triplet exciton are taken into account assuming Pekar's ABC. The conceptional transition from a classical Lorentz model to the polariton model is demonstrated experimentally. An application to the characterization of the sample is also discussed. Copyright © 1993 WILEY‐VCH Verlag GmbH &amp
Co. KGaA