Optimization of solid-state lighting spectra is performed to achieve beneficial and tunable circadian effects. First, the minimum spectral circadian action factor (CAF) of 2700 K white light-emitting diodes (LEDs) is studied for applications where biologically active illumination is undesirable. It is found that white-LEDs based on (i) RGB chips, (ii) blue & red chips plus green phosphor, and (iii) blue chip plus green & red phosphors are the corresponding minimum-CAF solutions at color-rendering index (CRI) requirements of 80, 90, and 95, respectively. Second, maximum CAF tunability of LED clusters is studied for dynamic daylighting applications. A dichromatic phosphor-converted blue-centered LED, a dichromatic phosphor-converted green-centered LED, and a monochromatic red LED are grouped to obtain white spectra between 2700 K and 6500 K. A maximum CAF tunability of 3.25 times is achieved with CRI above 90 and luminous efficacy of radiation of 313 - 373 lm/W. We show that our approaches have advantages over previously reported solutions on system simplicity, minimum achievable CAF value, CAF tunability range, and light source efficacy.

In the study of lighting, as the construction of a physical test room is costly and time-consuming, researchers have been actively looking for alternative media to present physical environments. Virtual reality, photo and video are the most commonly used approaches in the lighting community, and they have all been used by researchers around the world. Most such studies have been conducted without discussing what gives the subjects a better sense of realism, presence, etc., and which type of media is closer to the ideal, the physical lighting environment. In this paper, we aim to select the optimal alternative media that can present physical lighting environments. We compare a human’s subjective feeling towards a physical lighting environment and three alternative reproduction technologies, namely, virtual reality reproduction, video reproduction and photographic reproduction. We also discuss the feasibility of using virtual reality in representing lighting environments. The selection of the most optimal media is based on the perceptual attributes of lighted space, and the findings are only related to these criteria. The main results of this study are the following: (a) The order of the overall presentation-ability of the media is physical space > virtual reality reproductions > video reproductions > photo reproductions. (b) In terms of subjective rating, virtual reality lighting environments are rated closest to the physical lighting environments, and the order of the approximate coefficient of the media is physical space (1) > VR reproductions (0.886) > video reproductions (0.752) > photo reproductions (0.679). (c) Virtual reality can present lighting attributes of open/close, diffuse/glaring, bright/dim and noisy/quiet consistent with the physical environment. (d) Human subjects are most satisfied with VR reproductions.

With the recent advances in photobiology research and light-emitting diode technology, lighting considering circadian effects and the potential health benefits attract much attention. In this study, we demonstrate that the common practice of spectral optimisation of light for high visual efficacy can potentially lead to very inefficient delivery of circadian stimulus, which contributes to the lack of circadian entrainment that is likely to happen in indoor environments with only electric lighting. To optimise spectra of white light-emitting diodes for circadian efficacy, a four-component colour-mixing method with explicit analytical solutions is introduced. Energy-saving up to 29% is achieved at a target circadian stimulus of 0.35, by switching from the traditional maximum-visual-efficacy strategy to a maximum-circadian-efficacy strategy. Moreover, we propose a framework of a novel lighting-design space which allows practitioners to explore the possible combinations of circadian stimulus, visual illuminance and colour temperature. Solutions are provided for scenarios where activation of the circadian system should be avoided while a reasonable visual brightness appearance is maintained.