The invention relates to an optical standard (10) for the calibration or characterization of optical measuring devices and as a reference system for intensities and intensity measurements. The standard (10) according to the invention, constructed sandwich-like, comprises a combination of at least two layer-like optical standard modules (12) having defined optical properties, joinable or joined together plane-parallel, wherein the standard modules (12) in each instance differ from each other by at least one optical property, namely, by their absorption, emission, scatter and/or reflection properties, and the standard modules (12) are made so that they enter into physical interaction with electromagnetic radiation striking one of their two principal surfaces (12.1).