Fig. 4. Rat increment-threshold spectral sensitivity functions for four different achromatic backgrounds [from top to bottom: log(luminance=1.12, 1.43, 1.76 and 2.09, where luminance is measured in cdm^-2]. The top curve is a replot of the mean results from Fig.3 and below that are spectral sensitivity functions obtained from an animal tested on three backgrounds that were progressively elevated in luminance. Each scale division on the sensitivity axis is equal to 0.5 log unit. The four spectral sensitivity functions have been best fitted with linear summations of the cone templates. The percentage of the 509nm component required in these best fits is (from top to bottom): 58%, 64%, 58%, 49%.