The standard observer has been defined in 1931 for the CIE
1931 color model. The standard observer resulted from experiments where
observers were asked to match monochromatic wavelengths of light with
mixtures of three primary colors (red, green and blue). The standard observer
is in fact a table showing how much of each primary color would be used
(by an average observer) to match each wavelength of light.
In the CIE 1931 color model, the 2 degree standard observer has been
defined from color matching experiments with an arrangement that meant
that the stimuli activated an area of the retina of 2 degrees. The distribution
of rods and cones is not uniform over the surface of the retina and this
implies that the tristimulus values
obtained from the 1931 data are strictly only valid for observations made
under 2 degrees viewing conditions. This is equivalent to viewing a small
coin held at arm-length and does not correspond particularly well with
the viewing conditions often used in the coloration industry.