What is the 10 degree observer?

Because the 1931 2 degree observer is not really appropriate for large-field visual colour judgements the CIE defined a second set of observer functions in 1964 known as the supplementary observer data based upon colour-matching experiments with a field of 10 degrees. Since the 2 degree data are still in use the 10 degrees data are often differentiated from the original 1931 data by the use of subscripts.