Measurements of chromatic sensitivity in the mesopic range

We measured chromatic threshold sensitivity in the mesopic range using a combination of techniques that mask the detection of photopic and scotopic luminance contrast signals. The measurements were carried out at a number of light levels in the range 45-0.004 cd/m(2), both foveally and with the stimulus centered 3.5 degrees in the periphery. In order to investigate the effect of rod signals on chromatic detection thresholds in the near periphery of the visualJield, we measured chromatic threshold ellipses when fully dark-adapted and during the cone plateau region of the dark-adaptation curve. The results confirm a number of previous observations and reveal new findings: A reduction in background adaptation causes a loss of chromatic sensitivity that becomes more rapid as one enters the mesopic range. This loss is observed both foveally and in the near periphery and cannot therefore, be attributed entirely to rod intrusion. A small increase in chromatic thresholds is observed in the near periphery when compared with foveal measurements. Comparison of foveal and peripheral measurements also reveals a tilt in the orientation of the major axis of the chromatic threshold ellipse away from the tritanopic towards the deuteranopic colour confusion line: The loss of chromatic sensitivity is not uniform, with the tritan axis being most affected. The ellipticity (i.e,, the ratio of major to minor axis of the ellipse plotted on the CIE 1976, UCS diagram) can increase by as much as a factor of two, as the light level decreases from 10-0.056 cd/m(2) At lower light levels, some subjects show an asymmetry in chromatic thresholds along the tritan axis. This asymmetry is consistent with greater sensitivity to increases than decreases in S-cone excitation. Measurements of chromatic sensitivity following either complete dark-adaptation or during the cone plat eau phase of the dark-adaptation curve yield essentially the same results. These findings, therefore, suggest that rod signals have little or no influence on chromatic sensitivity at this eccentricity (C) 2000 John Wiley & Sons, Inc. Col Res Appl, 26, S36-S42, 2001.