Figure 3.11.23 from Stockman, A., & Brainard, D. H. (2009). Color vision mechanisms. In M. Bass, C. DeCusatis, J. Enoch, V. Lakshminarayanan, G. Li, C. Macdonald, V. Mahajan & E. van Stryland (Eds.), The Optical Society of America Handbook of Optics, 3rd edition, Volume III: Vision and Vision Optics. New York: McGraw Hill.
Stiles’ π-mechanisms. Field spectral sensitivities of the seven π mechanisms: π1 (purple triangles), π2 (dark blue inverted triangles), π3 (light blue circles), π4 (green diamonds), π'4 (green half-filled diamonds), π5 (red squares), π'5 (red half-filled squares). 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. Details of the experimental conditions can be found on p.12 of Stiles' book (Stiles, 1978). Data from Table 2(7.4.3) of Wyszecki & Stiles (1982).
Stiles, W. S. (1978). Mechanisms of Colour Vision. London: Academic Press.
Wyszecki, G., & Stiles, W. S. (1982). Color Science: concepts and methods, quantitative data and formulae (2nd ed.). New York: Wiley.