A method and an associated device for determining the transmittance of a flat-glass substrate with a measuring device, with which light of at least one light source is guided from one side of the flat-glass substrate through the flat-glass substrate to the opposite side of the flat-glass substrate, where it is captured by at least one receiving unit and the transmittance of the flat-glass substrate is determined by means of a comparison between the intensity of the light emitted by the light source and the light incident upon the receiving unit. The light source is a surface-like diffuse light source, and the receiving unit comprises at least one spatially resolving receiver. By evaluating brightness values in the measuring image of the spatially resolving receiver, the transmittance is determined in a spatially resolved manner in a partial surface of the flat-glass substrate, which is covered by the measuring image.