BS EN ISO 11664-1:2011

1   SCOPE This International Standard specifies colour-matching functions for use in colorimetry. Two sets of colour-matching functions are specified.

a)
Colour-matching functions for the CIE 1931 standard colorimetric observer

This set of colour-matching functions is representative of the colour-matching properties
  of observers with normal colour vision for visual field sizes of angular subtense
  from about 1° to about 4°, for vision at photopic levels of adaptation. 

   
      
         

b)
Colour-matching functions for the CIE 1964 standard colorimetric observer

This set of colour-matching functions is representative of the colour-matching properties
  of observers with normal colour vision for visual field sizes of angular subtense
  greater than about 4°, for vision at sufficiently high photopic levels and with spectral
  power distributions such that no participation of the rod receptors of the retina
  is to be expected.