British Standard BS5489-1: 2003 permits a trade-off between colour rendering and illuminance for lighting in subsidiary streets—if lamps of high colour rendering index such as metal halide are used instead of high- or low-pressure sodium lamps, a lower average illuminance can be used. A series of tests are carried out under mesopic conditions to validate the trade-off, and this paper reports on the new visual performance results. Four tests are carried out: acuity of achromatic and chromatic targets, achromatic contrast detection threshold, and colour identification, these being for on-axis targets. It is found that spectral power distribution (SPD) does not affect the performance of achromatic tasks except for an increase in contrast detection threshold under low-pressure sodium lamps. The performance of an acuity task using coloured targets displayed interaction between target colour and SPD. Colour naming accuracy is found to be significantly higher for metal halide lamps than for sodium lamps. For all tasks there is a reduction in visual performance at lower illuminances, and therefore a reduction in design illuminance leads to a reduction in the performance of some visual tasks which may not be offset by lamp SPD. Implications for the performance of real pedestrian tasks are discussed.
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