Formatt Hitech 3 x 3" FL-B Fluorescent Filter

A Formatt 3 x 3" FL-B Filter is used to correct the color balance when shooting tungsten film under fluorescent lighting.  

An FL-W filter is used to balance daylight film under warm white or white type fluorescent lamps. An FL-D filter is used when you wish to correct for the greenish tone that appears when shooting daylight film under general purpose fluorescent lighting.