Off-axis detection at mesopic light levels in a driving context

Recent research illustrates a robust effect of both light level and spectral power distribution (SPD) on off-axis visual detection at low light levels, a phenomenon that may hold significant energy and safety benefits if extended to actual roadway lighting installations. A laboratory study was conducted to systematically examine the effects of spectral condition, background luminance, target luminance contrast and target location on off-axis detection at mesopic light levels. Four subjects performed a simulated driving task while signaling detection of off-axis targets in a projected roadway scene. Two spectral conditions were used to emulate the scotopic/photopic ratios (s/p) of common roadway lighting sources: high pressure sodium (HPS) and metal halide (MH). The roadway scene was presented at four background luminances between 0.1 and 3.0 cd/m(2). A two-degree disk-shaped luminous target was randomly presented at four eccentricities between 12 and 29 degrees, and at four luminance contrasts between 0.1 and 1.0. Detection rate and reaction time data were collected for over 10,000 target presentations. Experimental results clearly indicate that higher s/p conditions (e.g., MH) produce higher off-axis detection rates at mesopic light levels than lower s/p conditions (e.g., HPS). Similarly, reaction times for the higher s/p condition were significantly shorter than for lower s/p conditions. This difference was most pronounced at the lower target contrasts. In general, performance (i.e., detection rate and reaction time) for the three innermost target locations was the same; at the outermost target location, performance began to decrease.