The Tiffen 4 x 4" 81D Light Balancing Filter helps to decrease the color temperature by 500K in order to warm the image slightly in order to match the color quality of illumination. When using an 81D filter, you will be able to attain 3200K balance when the prevailing light source is 3700K. The warming effect of this filter requires a 0.6-stop increase in exposure for normal results.
This filter is made using ColorCore technology, a process that involves laminating the filter substrate between 2 pieces of optical glass and grinding flat to a tolerance of 1/10,000th of an inch. It measures 4 x 4" / 101.6 x 101.6mm.