Wratten number

Wratten numbers are a labeling system for optical filters, usually for photographic use comprising a number sometimes followed by a letter. The number denotes the color of the filter, but is arbitrary and does not encode any information (the 80A–80D are blue, the next filters in numerical order, 81A–81EF, are orange); letters increase with increasing strength.

They are named for the man who founded the first photography company, Frederick Wratten, a British inventor. Wratten and partner C. E. K. Mees sold their company to Eastman Kodak in 1912, and Kodak started manufacturing Wratten filters, which remain in production, and are sold under license through the Tiffen corporation.

Wratten filters are much used in observational astronomy by amateur astronomers. Color filters for visual observing made by GSO, Baader, Lumicon or other companies are actually Wratten filters mounted in standard 1¼″ or 2″ filter threads. For imaging interference filters are used. Wratten filters are also used in photomicrography.