"80" series filters are color conversion filters for the use of daylight type color films with tungsten light source.
The KB 20 is the darkest blue in the"80" series.It increases color temperature from 2600-5500 degrees Kelvin.
Depending on the percentage of red, conventional household bulbs require extreme color temperature conversion whenever daylight color film is being used indoors.The B+W KB20 provides additional color correction under these circumstances. Filter factor is approx. 2.7.