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 and its spectral characteristics, and these numbers can be grouped into broad categories, but the numbering system is arbitrary within a group and does not encode any information. For example, within the photometric group of color correction filters, the 80A–80D are blue, while the next filters in numerical order, 81A–81EF, are orange. Letters almost always increase with increasing strength (the exception being 2B, 2A, 2C, 2E).