Memorandum presenting the optical transmission characteristics of turbulent boundary layers in air on a flat plate with negligible heat transfer measured photometrically for ranges of Mach number from 0.4 to 2.5. The results indicated that the scattering from a collimated beam of white light which penetrates a turbulent boundary layer depends mainly on the integral across the layer of the difference between the free-stream density and the local boundary-layer density.