This paper is concerned with designing light source spectra for optimum luminous efficacy and colour rendering. We demonstrate that it is possible to design light sources that can provide both good colour rendering and high luminous efficacy by combining the outputs of a number of narrowband spectral constituents. Also, the achievable results depend on the numbers and wavelengths of the different spectral bands utilized in the mixture. Practical realization of these concepts has been demonstrated in this pilot study which combines a number of simulations with tests using real LEDs (light emitting diodes). Such sources are capable of providing highly efficient lighting systems with good energy conservation potential. Further research is underway to investigate the practicalities of our proposals in relation to large-scale light source production. 1. Introduction It is an aim of lighting source designers to produce sources that deliver effective light output at high efficiency combined with good colour rendering (the ability of a light source to display surfaces in their natural colours). Such aims are usually expressed in codes of practice (see, e.g. [1] for a review of the philosophy underlying lighting codes). With the prevailing emphasis on energy conservation, light sources are now also expected to have the highest practical luminous efficacy, for example, [2]. Since both colour rendering and luminous efficacy depend on the emitted spectrum of the source, there is considerable codependence between these properties. In fact, for the majority of “near-white” sources, these two parameters are generally contravariant. Good examples of this property can be found among the earlier (halophosphate type) families of fluorescent tubes, in which good colour rendering was associated with low luminous efficacy, and vice versa [3]. More recent fluorescent lamp designs have tended to exploit combinations of phosphors with narrow-band spectra at suitable wavelengths—providing enhanced luminous efficacy together with good colour rendering (e.g., [4])—breaking the above-mentioned “rule of thumb”. Our research attempts to emulate this approach, to apply it to the design of mixed LED sources, and in the future to extend it to a more fundamental level—to the generalized design of optimized light sources. This paper reports on a series of exploratory investigations during which the authors have gained an appreciation of the parameters of the questions under investigation, as part of the process of planning for specific future light source designs. It will also be
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