We have measured and analyzed the angular scattering patterns in guided-wave depolarized light scattering (GWDLS) from 500 nm thick diblock copolymer thin films of poly(styrene-block-isoprene) using diffraction grating couplers. The measured pattern is decomposed into the sum of contributions from ordered grains and grating birefringence. Measured patterns are well fitted using a grain model consisting of disks lying in the film plane with elliptical cross sections, randomly oriented optic axes, and variable-length major axes.