# The Circadian Effect Versus Mesopic Vision Effect in Road Lighting Applications
## 1 Introduction  
There are two kinds of visual imaging photoreceptors [1], cone and rod cells, and one kind of non-visual imaging photoreceptor, iPRGCs [2; 3]. The cones, rods, and ipRGCs relate to the spectral sensitivity of photopic vision, scotopic vision, and melatonin suppression [4; 5], respectively. For the visual imaging system, the activation levels of the cones and rods differ at different luminance (or illuminance) ranges. Road lighting is mainly in the mesopic vision stage, where both cones and rods activate. For non-visual imaging systems, melatonin suppression occurs under a large range of illuminance levels, even as low as 1 lux [6]. Most studies focus on indoor circadian lighting applications with illuminance values of about 100 to 1000 lux in photopic vision. Only a few works [7] focused on dim outdoor lighting applications (<30 lux), which might also be in the mesopic vision stage. It is not clear how the circadian effect and mesopic vision effect work for dim road lighting applications. For road lighting applications, the lighting should meet several needs, including the detection of small objects and safety [8; 9]. Generally, luminance for road lighting is about 0.5 to 2 cd/m\({}^{2}\) forenergy-saving concerns in a mesopic vision state. Mesopic vision is a vision state in which both cone and rod cells activate at a luminance level from 0.005 to 5 cd/m\({}^{2}\)[10]. In road lighting, the empirical rule of E = L\(\pi\)/\(\rho\) governs transforming luminance to illuminance, where E is the illuminance, L is the luminance, and \(\rho\) is the reflectance rate of the road. Roads are mainly paved with two materials, concretes and asphalts. There are several kinds of common concrete road surface materials. The average reflectance rate of concrete's surface is about 0.18 to 0.32 [11], according to data from the National Aeronautics and Space Administration (NASA) Jet Propulsion Laboratory's Advanced Spaceborne Thermal Emission Reflection Radiometer (ASTER) library. The average reflectance rate of the asphalt surface varies greatly from several percent to about 70 percent due to the aging process [12]. We adopt a reflectance rate \(\rho\) as 0.3 for the road surface. For road lighting luminance measurements, one method is to measure from 83 m ahead of a vehicle at a 1.5-meter height with a 1-degree angle representing the driver's view. Here, we adopt the above-mentioned empirical formula to calculate road luminance more simply. Therefore, the illuminance levels for mesopic vision can be roughly taken as 0.05 to 50 lx. As is well known, human sensitivity to light shifts to shorter wavelengths from photopic to scotopic vision [13]. The peak wavelengths of photopic and scotopic spectra sensitivity are 555 and 507 nm, respectively. A mesopic vision state is a combination of these two vision states, and the lower the luminance level is, the greater the sensitivity to blue light. Light sources with rich short wavelengths may have high mesopic efficiency. This was a claimed advantage during the transition process from high-pressure sodium to white light-emitting diode (LED) light sources a decade ago. Many works have been dedicated to optimizing high mesopic efficiency light sources [14; 15; 16]. Although mesopic vision applications in road lighting have been widely accepted, [PERSON] et al. 's work showed that there are no significant differences in the detection distances for high and low correlated color temperature (CCT) sources when the speed is over 56 km/h [17]. This might change the optimization strategy under certain conditions. The circadian rhythm is mainly influenced by distinct photoreceptors in the eye, melanopsin-containing intrinsically photosensitive retinal ganglion cells (ipRGCs) [2; 3]. The spectral sensitivity of melanopsin is similarly invariant across species, with peak intensity at approximately 480 nm [18; 19; 20; 21; 22]. Melatonin secretion is decided by both intensity and spectrum.