Abstract: This invention relates to a method for reading, while using optical interference, a barcode that extends into the depth of a substrate. The barcode is represented by an area with marks in the substrate that is partially transparent to electromagnetic radiation. The inventive method comprises the steps of irradiating the substrate with short coherence length light from a broad-band light source, dividing the light up into reference light and measuring light, returning the reference light and the measuring light back-scattered or reflected in the marked area to an analytical unit, determining the reflectance or the reflectivity of the substrate for all layer depths in the marked area from the interference of the reference light and the measuring light and interpreting the result as a barcode.