The Heliopan 39mm KB-18 Color Conversion Glass Filter is blue. It is used to balance daylight film under artificial, tungsten, or standard (household) incandescent lighting. We have all taken indoor photos (with traditional film) that turn out yellowish. A portrait under this kind of scenario usually leads to less than desirable results. To bring back a reasonable color balance to the image, a KB-18 is required. The density of the filter requires a 2.6 f-stop increase in exposure. In some instances, that "yellowish" tint may be desired to provide a somewhat "old-style" look to the photo, but in general, it tends to be distracting and unflattering to most subjects.
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 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.