## 1 Introduction  
In the past century and a half, many research projects have investigated the effect of lighting on the perception of the environment, contributing to a comprehensive understanding of the mechanism of visual perception. Studies aimed at finding a correlation between the spectral power distribution (SPD) of light and human visual perception, including colour fidelity [1, 2], colour naturalness [3, 4], colour discrimination [5, 6], colour preference [7, 8, 9], lighting comfort [10, 11], and the whiteness of lighting [12, 13], have obtained positive results. More than fifty colour quality metrics involvingobjective and subjective aspects of perception have been proposed, including the Colour Rendering Index (CRI: _Ra_) developed by Commission Internationale de l'Eclairage (CIE) [1], Colour Quality Scale (CQS: Qa, Qf, Qp, Qg) [2] developed by National Institute of Standards and Technology (NIST), IES-TM-30 metrics (_Rf_ and _Rg_) [14] developed by American National Standards Institute (ANSI) & Illuminating Engineering Society of North America, Memory Colour Rendering Index (MCRI) [15], Colour Discrimination Index (CDI) [16], and Gamut Volume Index (GVI) [17]. These metrics are essential and effective tools to inform good professional practice, for example in applied lighting design and product development in museums and art galleries [11; 18], surgical applications [19; 20], industrial inspection [20], and the retail environment [5; 21]. In 2001, a third type of photoreceptor, the intrinsic photosensitive retinal ganglion cell (ipRGC), was discovered [22; 23]. These novel cells are located in the retina of mammals and they have been shown to have special nerve connections to the suprachiasmatic nucleus (SCN), which is the biological clock in the brain. The SCN, in turn, has a nerve connection with the pineal gland, responsible for the regulation of some types of hormones. The discovery of additional nerve connections from the ipRGC in the eye to the brain has prominently contributed to the understanding of many non-visual biological effects of lighting, for example, circadian rhythms [24; 25], cortisol production [26], melatonin production [27], concentration [28; 29], and alertness [27; 30]. Over recent decades, researchers, lighting designers, and architects have realized that lighting design should be required to consider both visual and non-visual aspects of lighting [31; 32; 33; 34]. Based on such a scientific consensus, researchers have recently focused on the concept of human-centric lighting (HCL) [35; 36; 37; 38; 39], which aims to maximize the benefits of proper lighting and covers a broad set of technologies. Specifically, HCL provides evidence-based lighting solutions optimized for vision, performance, alertness, and general human health, in the way of balancing the visual, emotional, and biological benefits of lighting. In a recent work by [PERSON] et al. [35], taking selected spectral aspects of HCL into consideration, a preliminary usefulness metric was proposed to characterize the energy efficiency of light sources. This HCL metric comprised widespread used measures of colour quality (e.g., CRI _Ra_), descriptors of brightness (e.g., CIE _L\({}_{mes}\)_[40]), as well as an indicator of the circadian effect (the so-called DIN melanopic factor [41]). Additionally, in the latest study by [PERSON] et al. [38] that concentrated on the up-lighting method and freeform optics, a design of freeform lenses for HCL was developed. It was proved to be beneficial in obtaining the maximum luminous uniformity over an indoor space, which might result in less chance of myopia and improvement in health and mental wellness. It is necessary, however, to recognize the limitations of what we know at the current stage [37]. For the visual effects of lighting, we know much about how light affects our visual capabilities and perceptions but, due to the variations in psychophysical studies, a consensus has not been reached in those topics.