Abstract: A method and an associated device for verifying the transmittance of a flat-glass substrate. Light of a light source is guided through the flat-glass substrate and is captured by a receiving unit. 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. In a first measurement, a light source generates polychromatic light, wherein the transmittance is determined at a point of the flat-glass substrate. In at least one further measurement, a light source generates monochromatic light of a defined wavelength, the transmittance also being determined at the same point of the flat-glass substrate. By comparing the at least two measurements, it is determined whether the transmittance is approximately the same or not the same in all measurements, in order to narrow down the cause for a reduced transmittance.