Wratten is a brand name from Kodak used for their series of coloured gelatin correction and conversion filters.
There’s a wide range – each one identified by its Wratten number.
The filters are very thin so are optically very good, but can be easily damaged
Wratten filters can be held or taped in front of the lens (or flash). Alternatively you can buy a gelatine filter holder for most filter systems for a neater way to attach the filter to the lens. Example include the Hoyarex Gelatine Filter holder and the Cokin P-Series Gelatine Filter Holder
They are also used in the darkroom for colour printing using an enlarger with a filter drawer. Cyan, Magenta and Yellow are available in increments of 5CC. Owners of the Bowens Illumitran and other similar professional slide copiers can use the filters to correct colour casts when copying from one film to another.
Photographers can tape a wratten filter over their flash and have a cancelling one on the lens so anything illuminated by flash will be the correct colour while the background will be the colour introduced by the filter over the lens.