The number denotes the color of the filter .
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 .
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 .