For various recently purchased, but also for some older NOT filters, transmission curves are not yet available. Until recently we have had the transmission of our filters scanned by the ING. Since the ING scanner is broken, we have investigated obtaining transmission curves with ALFOSC using a Tungsten flatfield lamp as light source.
In general we can get good results, but in the far blue the red lamp does not emit enough photons such that the transmission curves are incorrect due to contamination by scattered red light. Eleven transmission curves, as measured with ALFOSC, have been added to our filter data base on the WWW pages. See http://www.not.iac.es/technical/astronomical/filters/ .