Topic Filters & coatings

An uncoated glass surface reflects about four per cent. A broadband antireflection coating brings that below half a per cent, but only inside its design band. Outside it, the coating can be worse than no coating at all, which is why the wavelength belongs at the top of every optics enquiry.

A colour glass filter absorbs unwanted light in the bulk of the glass; an interference filter reflects it from a coating. Absorption is insensitive to angle and cheap, but heats up and has gentle edges. Interference gives steep edges and narrow bands, but the band shifts as you tilt it.