Abstract
The paper describes subjective appraisals by 20 observers of lanterns with widely different light distributions, both in wet and dry conditions.
The tests were made in a single street at fixed lantern positions for a single light source-the new 400 watt high-pressure sodium lamp. It is emphasised that the results obtained apply strictly to the conditions of test only.
The following principal conclusions may be drawn:
Similar appraisals were obtained on the wet and dry nights for the same installations, suggesting that observers accommodate themselves to the weather conditions; appraisals of brightness, glare and uniformity are strongly correlated; the appraisal of overall impression is associated with a weighted sum of the appraisals of brightness, glare and uniformity; for a given degree of non-axiality there is strong correlation between brightness, glare and uniformity and the elevation, above the peak of 1/2 the maximum intensity (or some similar criterion) in the plane containing the street axis; There is no evidence of correlation between brightness or uniformity and the intensity in the 0–30° zone. There was no significant difference between the assessments of trained and untrained observers.
The present method of specifying the lantern photometric performance in terms of intensity ratio gives results no better than the old system of specifying the vertical beam shape by reference to the fraction of the maximum intensity. To return to the old system would greatly simplify the testing procedure for compliance with the specification, without losing any measure of control.
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