Correspondence: In support of the IES method of evaluating light source colour rendition

It is well known that the colour of illuminated surfaces can look different under different light sources. In the mid-20th century, as the diversity of light sources began increasing, the Commission Internationale de l’Eclairage (CIE) developed a metric for assessing colour fidelity called the colour rendering index, or CRI. Since 1965, CIE CRI has estimated the colour fidelity of a light source for 14 selected sample surfaces, and also an average accuracy, Ra, for an eight-sample subset.1

CRI has been a helpful aid in light source selection, but CIE has never claimed that a lamp with a higher Ra value is necessarily the more appropriate source for illumination. That is, in part, because it is inefficient to provide perfect colour fidelity; there is generally a tradeoff between the Ra value for a source and other important factors such as luminous efficacy, cost, and lifetime.2 More generally, colour fidelity is only one aspect of colour rendering—colour fidelity does not always correlate with application-specific considerations, such as colour preference or colour discrimination, and does not take into account other aspects of colour appearance such as the influence of illuminance. Lighting design is both an incomplete, complex science, and an art, and the selection of optimum light sources is a key component of that work. Ultimately, designers must make their best judgments, and understandably they seek practical tools to help them do so. Ideally, such metrics would provide more information about the nature of colour shifts of various surfaces and would present the information in ways that would help guide designers.