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Visual displays used in nighttime driving and some other applications require low luminances. Visual performance under mesopic light levels is not well characterized using conventional photopic photometric quantities. Response times to flashed decrement targets against colored backgrounds on a computer monitor display were systematically shorter for blue than for red backgrounds. Responses were consistent with a unified system of photometry that models the relative contributions of rod and cone photoreceptors at mesopic light levels. The empirical data and unified photometric system can have practical application for the design of displays in low-light applications.