Method of manufacturing a linearly variable optical filter

A method for producing a spectral gradient filter on a substrate comprising the steps of:

• providing the substrate with a first surface to be coated
• providing a shadow mask that comprises at least one bordered coating area with an edge, wherein the geometry of the shadow mask is adjusted to a desired gradient profile of the gradient filter
• creating a masked substrate by fixing the shadow mask on the first substrate surface to be coated in such a way that parts of the first substrate surface are covered, but the first substrate surface is exposed in the coating area,
• inserting the masked substrate into a coating system which is based on physical deposition from the gas phase (PVD), and
• carrying out the PVD coating,
• wherein at least parts of the shadow mask lie directly on the surface of the substrate so that no vapour migration occurs in an area of these parts during the coating and wherein the shadow mask is fixed to the substrate in a mechanically detachable manner so that the shadow mask can be used for several coatings, and
• wherein the coating system is a drum system, in which the substrates to be coated are guided past a coating source with a target surface, wherein the axis of rotation of the drum is arranged parallel to a straight line lying in the target surface and the substrates are mounted on the drum shell in such a way that the plane of the substrates spanned by the substrate surface does not intersect the axis of rotation of the drum.