Wratten number
A system for classifying optical filters that was created by Frederick Wratten and was given his name (1840-1926). There is a number assigned to each distinct color (the orange filters, for instance, have the number 81), and some of the filters have letters to signify the level of filtration they provide (an 81EF is much stronger than an 81A, for example).
Do you have a better definition? Share it in the comments.
