Optical multilayer thin-film filters (OMTFFs) are disclosed. An exemplary filter includes a transparent substrate, a multilayer film (MF) on a surface of the substrate, and a top layer. The MF is of alternatingly laminated layers of a high-refractive-index (HRI) material and a low-refractive-index (LRI) material. The top layer is on an uppermost layer of the MF and is of a material having atoms of lower atomic weight than atoms of either the HRI or LRI materials. The OMTFFs are made in a vacuum environment by alternatingly laminating respective thin films of the HRI and LRI materials on the substrate. The top layer is formed on the MF. Between forming the MF and top layer is a suppression step in which the newly formed MF is exposed to moisture by briefly venting the vacuum to atmosphere. The moisture inhibits migration of the low-molecular-weight atoms into the HRI and LRI materials.