## 1 Introduction  
There are three vision states for human beings, photopic vision with luminance values \(>\)5 cd/m\({}^{2}\), mesopic vision with luminance values from 0.005 to 5 cd/m\({}^{2}\), and scotopic vision with luminance values \(<\)0.005 cd/m\({}^{2}\)[1]. The luminance of values of several scenes, like roads, tunnels, and dashboard lightings, likely fall in the mesopic vision range. To measure light source performances in mesopic vision states, the scotopic/photopic ratio (_S/P_ ratio) is used [2]. Effectively, this is the ratio of the luminous efficacy of radiation (LER) in scotopic and photopic vision. There are two photopic spectral sensitivity curves (SSCs): one is the Commission Internationale de l'Eclairage (CIE) 1924 2-degree standard observer SSC [3] and the other is the CIE 1964 10-degree standard observer SSC [4]. Some studies claim that the 10-degree SSC constitutes a better representation [5]. Prior research studies on colour differences also show that 10-degree colour matching functions yield smaller colour differences than 2-degree colour matching functions [6]. Another study [7] suggested that there may be problems in the short-wavelength part of the 2-degree SSC. In road lighting, visual tasks sometimes require large visual fields, while peripheral vision is also important [8; 9; 10]. The rod photoreceptors are mainly distributed in the part of the retina associated with peripheral vision. We may thus conclude that both the 10- and 2-degree systems should be considered for uses in various applications [11]. CIE recommended mesopic vision models for road lighting [2] and published guidelines on how to use the model [12]. However, CIE did not recommend which SSC should be adopted for calculation, implying that either SSC type can be used in applications. In most of the published research studies and practical applications, the 2-degree SSC system has been used. [PERSON] [13] reported S/P ratios of 16 typical light sources in the range from 0.23 to 2.47. [PERSON] et al. [14] reported a light source with an S/P ratio of 2.56 at a correlated colour temperature (CCT) of 5368 K. [PERSON] et al. [15] reported a white-light emitting diode (WLED) with an S/P ratio of 3.04 at the extremely high CCT of 45000 K. [PERSON] et al. [16] proposed light-emitting diodes (LEDs) for CCTs from 2700 K to 45000 K with _S/P_ ratios in the range from 1.83 to 3.84. [PERSON] et al. [17] reported WLEDs that reached S/P ratios from 1.54 to 3.84, which corresponded to CCTs in the range from 2700 K to 7580 K. [PERSON] et al. [18] reported two-hump quantum-dot-based LEDs that reached an _S/P_ ratio value of 2.09 at 4038 K, and a four-hump LED that reached an _S/P_ ratio value of 3.08 at 5015 K. [PERSON] et al. [19] optimised hybrid white LEDs for high _S/P_ ratios at varying CCTs that reached high colour rendition, and compared their differences. Despite the fact that in some studies it has not been mentioned which SSC was used, we rechecked the spectra in the literature and have confirmed that most studies have used 2-degree SSCs. To date, no study has compared the differences of the calculations of _S/P_ ratios using these two SSCs. In practical applications, there are urgent needs for large-scale assessments of urban lighting [20] and nightscape lighting. However, there are some unsolved technical issues to assure photopic adaptation luminance [21; 22], which may cause inaccuracy in assessments. In this study, we examined the differences of the calculated _S/P_ ratios with two SSCs based on tests with more than 300 light sources [23] with CCTs from 2000 K to 8000 K, and with blackbody radiant (BR) light sources with CCTs from 10000 K to 45000 K. We calculated the mesopic luminance levels at photopic adaptation luminance levels from 0.005 to 5 cd/m\({}^{2}\) based on the use of the two tested SSCs and compared the differences. Conversion coefficients between the S/P ratio and mesopic luminance calculated using 2- and 10-degree SSCs were proposed.