The B&W 55mm KB20 Cooling Color Conversion Glass Filter (Slim) is the correct filter to use under illumination provided by 40-150 Watt household incandescent bulbs. It is also for correct color conversion with transparencies that display technical interior, architectural photographs or reproductions.
It should not, however, be used for moody interior pictures because unlike the KB15 and KB12 filters, the KB20 corrects the light to a neutral balance, eliminating the warmth that we might intuitively expect from bulbs.
B&W slim filters are for wide-angle lenses and are 3mm thick. They do not have a front thread.
80 Series Color Conversion Chart
80A increases color temperature from 3200-5500 Kelvin (commonly used with 3200K lamps)
80B increases color temperature from 3400-5500 Kelvin (commonly used with photoflood lamps)
80C increases color temperature from 3800-5500 Kelvin (commonly used with flashbulbs)
80D increases color temperature from 4100-5500 Kelvin (commonly used to remove extreme orange or red)
KB20 increases color temperature from 2600-5500 Kelvin