A laser interferometer where intensity is measured as function of incidence angle is described. Number of fringes and/or their location in interferogram defines plastic film thickness. With simplest method of signal processing, thickness accuracy is within 2% for films over 50 micrometers. The fringe interval method improves accuracy to within 0.1% for films over 10 micrometers. Special least-square methods may improve accuracy further or even allow a simultaneous index of refraction and thickness measurement. The method is intended for industrial on-line monitoring of plastic film thickness.