Wratten number

Wratten numbers are a labeling system for optical filters, usually for photographic use.

They are named for the man who founded the first company, Frederick Wratten, a British inventor. Wratten and partner C. E. K. Mees sold their company to Eastman Kodak in 1912, and Kodak continued to produce "Wratten Filters" for decades. Even now, as of 2006, Wratten filters are still produced by Kodak, and sold under license through the Tiffen corporation.

Wratten filters are very much an active part of observational astronomy.

Filters made by various manufacturers may be identified by Wratten numbers but not precisely match the spectral definition for that number. This is especially true for filters used for aesthetic (as opposed to technical) reasons; for example, an 81B Warming Filter is a filter used to slightly "warm" the colors in a color photo, making the scene a bit less blue and more red. Many manufacturers make filters labeled as 81B which do similar but not exactly the same filtering of light, according to that manufacturer's idea of how exactly it is best to warm a scene, and depending on their manufacturing techniques. Some manufacturers use their own designations to avoid this confusion, for example Singh-Ray has a warming filter which they designate A-13, which is not a Wratten number. Filters used for printing press color separation or scientific photography tend to have less variation.

Reference Table

The commonly available numbers and some of their uses include:

External links

* [http://www.filmcentre.co.uk/faqs_filter.htm FAQs for DPs: filters]
* [http://www.geocities.com/cokinfiltersystem/color_corection.htm Filters table]