Characterising mesopic spectral sensitivity from reaction times

The spectral sensitivity of the eye was investigated using reaction times to broadband chromatic stimuli over a range of background luminances. Relative sensitivity was determined from the nonlinear reaction time curve by converting reaction times to a linear measure that was independent of spectral sensitivity. Two models for mesopic spectral sensitivity were compared. The first was a linear combination of V(lambda) and V'(lambda), and the second included input from the L-M colour-opponent mechanism and the S-cones. The second model produced a significantly better fit to the data. The chromatic mechanisms appear to contribute to reaction time when there is an appreciable chromatic signal but luminance contrast is low.