An optical filter used in applications involving spectroscopic measurements is fabricated by depositing layers of optical coatings onto a substrate. The layers are deposited so as to have a substantially constant thickness in a first direction along the surface of the substrate, and a gradually ...http://www.google.de/patents/US5760910?utm_source=gb-gplus-sharePatent US5760910 - Optical filter for spectroscopic measurement and method of producing the optical filter 
