Abstract
Experiments in the laboratory to compare the apparent brightness of mercury and sodium light sources of high luminance show that most observers do not judge brightness on the ordinary photometric scale. In some circumstances the mercury luminance must be reduced to one third that of the sodium before observers, on balance, judge them to be equally bright.
Similar, but less marked, results were found in a formal street lighting test, a mercury installation being judged more glaring than a sodium. This, however, did not produce a marked preference for the sodium installation.
The commonly expressed preference for sodium street lighting may be partly explained by the reduced glare from sodium sources as compared with mercury but, it appears, is largely due to the characteristics of lanterns and installations now in use.
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