A Cokin Z-PRO 139 FLD Graduated Resin Filter is used when you wish to correct for the greenish tone that appears when shooting daylight film under general purpose fluorescent lighting. An FL-W filter is used to balance daylight film under warm white or white type fluorescent lamps. An FL-B filter is used to correct the color balance when shooting tungsten film under fluorescent lighting.
Cokin color graduated filters are color-toned in one half of the filter and have a smoothly graduated transition into the clear portion. Colored and clear image portions can be brought into the desired position by adjusting the filter in the holder. This unusual filter is for correcting fluorescent lights on part of the image only, i.e. a ceiling with fluorescent lights.
NOTE: The FL series of filters alone does not always guarantee 100% correct color balance when shooting under fluorescent lighting. They are general purpose filters intended for moderate correction and will not, by themselves, necessarily provide perfect color rendition. Fluorescent lamps burn at many different temperatures and often require more demanding filtration for exact results.
The Z-PRO Series is recommended for focal lengths of 20mm onwards (in the 35mm format). This filter is made of CR39 Resin which is a reputable lightweight and shock-resistant optical material also used for ophthalmic glasses.