All values are in quantal units
From Table B of Stiles (1978). See also Table 2(7.4.3) of Wyszecki & Stiles (1982).
To measure the field sensitivities of the π mechanisms, Stiles (e.g. 1953, 1959) typically used a 1-deg diameter foveal test flash of 200-msec duration presented in the center of a 10-deg diameter adapting background. Full details of the experimental conditions can be found in Stiles' book (Stiles, 1978, p.19).
The field sensitivities are the reciprocals of the field radiances (in log quanta s-1 deg-2) required to raise the threshold of each "isolated" π-mechanism by one log unit above its absolute threshold. The results were averaged across four subjects: three females aged 20-30 and one male aged 51.
In his earlier work, Stiles (1939) had hoped to find just three mechanisms corresponding to the three types of cone thought to be present in the trichromatic foveola. However, failures of the "Displacement Rules", led him to propose seven π-mechanisms.
The classic `field sensitivities' obtained by Stiles (1953) from normal eyes for the green (π-4) and red (π-5) colour-mechanisms are broader than the protanopic and deuteranopic spectral sensitivities (see Dichromats). His high intensity `test sensitivity' measurements (Stiles, 1964), like those of Wald (1964), have yielded spectral sensitivities in the normal trichromat that are more nearly protanopic or deuteranopic.
On the other hand, one of his field sensitivities for the blue-colour mechanism (π-3) is close to the S-cone sensitivity measured in blue cone monochromats (Stockman, Sharpe & Fach, 1995).
Stiles, W. S. (1939). The directional sensitivity of the retina and the spectral sensitivity of the rods and cones. Proceeding of the Royal Society of London, B127, 64-105.
Stiles, W. S. (1953). Further studies of visual mechanisms by the two-colour threshold technique. Coloquio probl. opt. vision, 1, 65-103.
Stiles, W. S. (1959). Color vision: the approach through increment threshold sensitivity. Proceedings of the National Academy of Science U.S.A., 45, 100-114.
Stiles, W. S. (1964). Foveal threshold sensitivity on fields of different colors. Science, 145, 1016-1018.
Stiles, W. S. (1978). Mechanisms of Colour Vision. London: Academic.
Stockman, A., Sharpe, L. T., & Fach, C. C. (1995). Spectral sensitivity of the short-wavelength-sensitive cones. Investigative Ophthalmology and Visual Science, 36 (suppl.), 189.
Wald, G. (1964). The receptors of human color vision. Science, 145, 1007-1016.
Wyszecki, G., & Stiles, W. S. (1982). Color Science: concepts and methods, quantitative data and formulae. (2nd ed.). New York: Wiley.