# New Approaches in Dynamic Metrics for Lighting Control Systems: A Critical Review
## 2 Review
### Dynamic Daylighting Metrics for Health and Well-Being  
Despite their usefulness in addressing energy efficiency through daylight utilization, the metrics previously discussed do not fully describe the impact of daylight on occupant health and well-being--particularly with respect to the spectral composition of daylight. Their focus is placed on the visual system and therefore they do not account for the influence of light on circadian entrainment. To address this deficiency, the Circadian Stimulus Autonomy (CSA) is defined by its authors as \"the percentage of days when a threshold for adequate melatonin suppression is met solely by daylight during a specific time of the day, typically in the morning\" [61; 62; 63; 64]. However, it is important to note that CSA does not account for the timing of light exposure or individual variations in circadian sensitivity, which are critical factors in real-world applications of circadian lighting. These limitations should be considered when interpreting CSA values in design contexts. CSA was developed from the calculation of DA and the illuminance required to achieve an acceptable value of circadian stimulus (CS). To achieve appropriate circadian entrainment, [PERSON] et al. argue that a CS value between 0.3 and 0.4 is adequate [61; 62], based on previous studies conducted on office workers [20]. Circadian stimulus can be expressed as follows (7):  
\[\text{CS}=0.7\cdot\left(1-\frac{1}{1+\left(\frac{\text{CL}_{\text{A}}}{305 \gamma}\right)^{1.1026}}\right) \tag{7}\]  
where \(\text{CL}_{\text{A}}\) is the circadian light. \(\text{CL}_{\text{A}}\) is estimated from the spectral power distribution (SPD) and illuminance perceived by the study subject through [PERSON] et al. 's model of human circadian phototransduction [16; 65], considering an exposure time of 1 h with a fixed 2.3 mm diameter pupil. Thus, CSA must be calculated using a CS value of 0.4--based on the light flux received and its combined SPD, considering the inner reflections of the room analyzed. It can be expressed as follows (8):  
\[\text{CSA}=\frac{\sum_{\text{i}}(\text{w}_{\text{fi}}\text{t}_{\text{i}})}{\sum_{ \text{i}}\text{t}_{\text{i}}}\in[0,1];\text{w}_{\text{fi}}= \begin{cases}1\text { if }\text{E}_{\text{D}}\geq\text{E}_{\text{L,CS}}\\ 0\text{ if }\text{E}_{\text{D}}<\text{E}_{\text{L,CS}}\end{cases}  \tag{8}\]  
where t\({}_{\text{i}}\) is the occupied time in a year, w\({}_{\text{fi}}\) is the weighting factor dependent on the illuminance threshold, E\({}_{\text{D}}\) is the daylight illuminance defined at a given point, and E\({}_{\text{L,CS}}\) is the illuminance threshold required to achieve a CS value of 0.4. Despite its promising potential, the Circadian Stimulus Autonomy (CSA) metric currently faces several challenges. The established threshold fails to account for individual variability, including factors such as age [66; 67], chronotype [68], and exposure history [69], as well as differences in circadian sensitivity among individuals [70]. Furthermore, CSA is based on the circadian stimulus (CS) model developed by [PERSON] et al., which is still undergoing evaluation and refinement by the scientific community, rendering its premature use in regulatory or certification applications. Additionally, a significant challenge in the application of CSA lies in its inherent variability across locations and times. Firstly, natural daylight's spectral power distribution (SPD) changes throughout the day and year due to solar position--angle of incidence and solar elevation--and atmospheric conditions--cloud cover, pollution, and humidity. The spectral composition of daylight, particularly the presence of blue light, crucial for circadian stimulus, varies significantly from morning to afternoon and between seasons. Secondly, geographical location plays a crucial role. Regions closer to the equator experience more consistent day lengths and therefore more stable daylight exposure, while higher latitudes exhibit dramatic seasonal differences in daylight availability and spectral quality.