# The Circadian Effect Versus Mesopic Vision Effect in Road Lighting Applications
## 1 Introduction
In almost all species, including human beings, melatonin levels are high at night and low during the day [23; 24]. Human physiology and behavior are influenced by retinal illumination [25]. To better apply the circadian design mechanism, the circadian parameters should be better expressed, similar to illuminance. Therefore, researchers have proposed different metrics for this application. [PERSON] proposed equivalent melanopic lux (EML) [5] based on melatonin spectral sensitivity. In the nocturnal application, 50 EML is taken as the upper limit [26]. However, the circadian rhythm may be influenced not only by ipRGCs but also by rods and cones. Thus, [PERSON] proposed a comprehensive model expressed as circadian light (CL\({}_{\text{A}}\)) and circadian stimulus (CS) to describe the circadian impact considering influences from all neuro paths [27; 28]. The CS ranges from 0 to 0.7, namely, from 0 to 70% melatonin suppression levels. In the daytime, the CS \(\geq\) 0.3 (equivalent to 30% melatonin suppression) is required for at least 1 h to maintain circadian health [29]. [PERSON] adopted a CS equal to 0.1 as the threshold [30], and [PERSON] et al. used a CS value of 0.05 as the target threshold [28]. A series of indoor circadian lighting research [31; 32] and optimization works [33; 34] show that circadian-guided lighting design helps improve light quality for human health. This influence is significant in a brightly lit environment. However, some research also shows that there is likewise significant melatonin suppression under dim light conditions [35]. [PERSON] et al. showed that illuminance as low as 1.5 lx affects circadian rhythms [36]. Under certain conditions, as little as 1 lx or less can suppress melatonin in humans [30]. [PERSON] et al. [37] showed that a 50% melatonin suppression level corresponds to 13.5 lx for Hour 1, 19.4 lx for Hour 2, and 38.9 lx for Hour 3. These illuminances are mainly in the mesopic vision range. Exposure time under light is also an important factor that decides the melanopic effect. Most studies adopt 'hour' as their measurement scale [27; 28; 31; 37], and a one-hour duration is used in most studies. If the exposure time is much shorter than one hour, the melanopic effect might be minimized. Recent studies found that ipRGCs, as well as S cones, might enhance scene brightness in the photopic range covering low, moderate, and high illuminance levels (i.e., 2 to 20 lx [38], 6.3 lx, 108 lx [39], and 1000 lx [40]). This might be a good measure, but the relevant mechanism study is currently still in progress. Moreover, mesopic models might already include scene brightness enhancement in the mesopic luminance range, although former researchers might not think that this phenomenon is caused by ipRGC. We thus do not include this part in the research. The spectral sensitivity of mesopic vision peaks at 507 nm and melanopsin at 484 nm for crystalline lens transmittance. These two action spectra are close to each other and sensitive to blue light. At nighttime, rich blue light sources may improve mesopic efficiency and suppress melatonin secretion. High correlated color temperature (CCT) white LEDs suppress melatonin secretion about 4 to 5 times [41] more strongly than high-pressure sodium (HPS) light sources in humans using a fitted action spectrum of melatonin suppression [42]. Meanwhile, white LEDs may provide mesopic efficiency 2 times higher than HPS [15]. The relation or tradeoff between mesopic efficiency and the circadian (or melatonin suppression) effect requires clarity.