This nearly medium blue Heliopan 40.5mm KB 6 Cooling Color Conversion Glass Filter is used to balance daylight film under artificial, tungsten or standard (household) incandescent lighting.
An 80D filter is the least "powerful" of the 80 series. When using daylight film and you'd like to leave even more yellow than if you were using and 80C filter, the 80D is the way to go. An 80D doesn't bring the temperature up to daylight as much as an 80A, B or C.
This filter allows you to somewhat "balance", i.e. bring back, correct color tone to your images. There is some loss of film speed that may require the use of a tripod but, at least, your photos will demonstrate the proper tonality they so richly deserve.
Heliopan Lichtfilter-Technik of Munich, Germany, has changed their black anodized brass filter rings on most Heliopan non-rotating filters to their "slim" mounts. These slim mounts are made of black anodized brass stock and have so little rim in front of the lens (5.4mm) that they will not vignette with lenses as short as 21mm.
Heliopan offers a wide selection of sizes, screw-in, series, bayonet, square and rectangular filters including digital, video, center weighted, graduated, wide angle, Kaesmann polarizers, drop-in polarizers, etc.