Blue filters are used for colour conversion when exposing daylight-balanced film in artificial light (Hoya 80A, B and C), and to increase colour temperature when slightly cooler tones are required (Hoya 82A, B and C).

• An 80A filter increases colour temperature from 3200oK to 5500^oK for use with 3200^oK lamps;
• an 80B filter increases colour temperature from 3400oK to 5500^oK for use with Photoflood lamps;
• an 80C filter increases colour temperature from 3800oK to 5500^oK for use with clear flash bulbs;
• an 82A filter provides a small increase in colour temperature for slightly cooler tones;
• an 82B filter should be used when exposing tungsten type B film (3200^oK) under tungsten domestic lamps (2900^oK). It may also be used in portraiture to eliminate reddish skin tones in early morning and late afternoon; and
• an 82C filter provides a larger increase in colour temperature for more noticeable cooler tones.

82-series filters should not be mixed with the 81-series warm-up filters as each cancels the effect of the other.

The filter factor for an 80A filter is generally 4 x. For an 80B filter it is generally 2.75 x, or 1.7 stops.