Color rendition describes the influence of light source spectrum on the color appearance of objects. The rendering of object colors in desirable ways has attracted much attention as LEDs have developed as a versatile and efficient lighting technology, with significant research effort to understand human perception and develop new metrics. These have helped manufacturers optimize LED lighting products, increasing both energy efficiency and appeal to drive adoption in workplaces and homes.

Spectrally tunable LED-based lighting is one large and growing segment of emerging products, which has also facilitated new experimental techniques. However, the increased interest and ease of experimentation has not necessarily translated into improved research quality, a more diverse range of experiments, more definitive findings, or increased use of efficient, high-quality LED products. Instead, the collective body of work has sometimes employed questionable methods that produce contradictory and, at times, overgeneralized results.

A new study funded by the Department of Energy and published in Lighting Research and Technology reviews the commonly used psychophysical experimental techniques for investigating color rendition, where human participants are asked to evaluate various subjective aspects of the color appearance of objects, such as color preference, naturalness, or vividness. The work was undertaken to encourage exceptional practices in the conceptualization, design, implementation, analysis, and reporting of such experiments. It is intended to accelerate research progress and the resulting improvements in lighting quality and energy efficiency.

The article synthesizes a large body of evidence on research methods, tailoring the solutions to the specific field using examples. Common pitfalls of existing color rendition research include a lack of clear hypotheses, failing to control for all lighting variables, insufficient adaptation, poor sampling of possible color rendition characteristics, and small sample sizes with insufficient statistical rigor. The study outlines a range of possible work to improve future methods and concludes with a list of recommended practices relevant to performing research on subjective evaluations of color rendition, which may be used as checklist by researchers, reviewers, and readers.

According to Dr. Yoshi Ohno, NIST Fellow at the National Institute of Standards and Technology, who was not involved in this study, there is still much room to improve the color quality of white LED sources for lighting preferences in different applications while ensuring the best use of energy. “Vision experiments are essential to make progress in research for this effort,” he says, “and this article by PNNL covers the whole range of important topics and recommendations for designing and conducting such vision experiments with subjects. It will be very useful for all researchers working in this area toward establishing good recommendations on color quality of LED sources for lighting.”

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