Diffuse reflectance and transmittance spectra collected in different scattering angle intervals have been analyzed in order to determine the interface roughness cross correlation of thin films. Different angle intervals correspond to different roughness length scales; the cross correlation as a function of length scale can hence be determined. Sputter deposited ZrO2 films were analyzed. The transition from correlated to uncorrelated film interfaces was found to occur at longer length scales, when the film thickness increased.