A metric is a well-defined measure that a designer can use to evaluate a design. For many years, the most widely used metrics in general lighting practice have been the average illuminance, the associated illuminance uniformity, the daylight factor, the correlated colour temperature and the CIE General Colour Rendering Index of the light source, the Unified Glare Rating and the power density of the installation. All these metrics have limitations with the result that there are now many proposals for replacing them with metrics more suited to delivering better lighting. This paper, which is based on the presentations given at the LRT symposium held in London on the 15 July 2014, considers the desirable attributes of a metric, summarises the proposed new metrics and considers how likely they are to be adopted and, if they are, whether or not they are likely to lead to better lighting.
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