# Designing Safe General LED Lighting that Provides the UVB Benefits of Sunlight
## 2 Methods: UVB Characteristics of Natural Light  
UVA and UVB are components of UV rays that have different levels of effects on the human body [30]. The local solar UV irradiance was measured and analyzed to provide a typical solar irradiance used to calculate UV exposure. Measurements were taken at a latitude of 36.85 and a longitude of 127.15 on the roof of a 10-story building to minimize shadow interference. The spectral irradiance was measured at noon on a clear day in summer since it would be advantageous to identify the characteristics of UV light under strong sunlight. The measurement was conducted on 21 July 2018, and the concentrations of the fine dust were 76 \(\mathrm{\mu g/m^{3}}\) for pm10 and 50 \(\mathrm{\mu g/m^{3}}\) for pm25, which are \"normal\" levels (according to the Korea Meteorological Administration). A spectrometer (CAS 140 CT 152) capable of measuring spectral irradiance in the range of 200-800 nm was used as the measuring instrument. Its specifications are shown in Table 1. In addition, the results of the measured natural light are shown in Figure 1.  
The sunlight characteristics based on the measured results are shown in Figure 1. We calculated the total irradiance of sunlight, UVA, and UVB irradiance using Equation (1). Erythermal weighted UV radiation (EUV), which is UV radiation that produces erythema at a given wavelength, was calculated using Equation (2). EUV is a weighted measure of the likelihood of erythema, a type of skin burn occurring in the UV wavelength band, which is needed to calculate safe levels of exposure to ultraviolet light and to calculate the amount of vitamin D synthesis [14]. At this time, the erythema weighting function (\(\mathrm{S_{er}}\)) of Equation (3) was applied according to the definition of Commission Internationale de I'Eclairage (CIE) [31; 32].  
\[\mathrm{UVA}=\ \int_{315}^{400}E\lambda d\lambda\ \left(\mathrm{W/m^{2}} \right)\mathrm{,\ UVB}=\ \int_{280}^{315}E\lambda d\lambda\ \left(\mathrm{W/m^{2}} \right) \tag{1}\]  
\[\mathrm{EUV}=\ \int_{280}^{400}E\lambda S_{er}d\lambda\ \left(\mathrm{W/m^{2}} \right)\mathrm{,\ EUVA}=\int_{315}^{400}E\lambda S_{er}d\lambda\ \left(\mathrm{W/m^{2}} \right)\mathrm{,\ EUVB}=\int_{280}^{315}E\lambda S_{er}d\lambda\ \left( \mathrm{W/m^{2}}\right) \tag{2}\]  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Model (Company)** & **Image** & **Spectral Range [nm]** & **Number of Pixels** & **Resolution [nm]** \\ \hline CAS140 CT 152 (Instrument Systems) & & 200–800 & 1024 \(\times\) 768 & 2.7 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Specifications of the spectrometer (CAS 140 CT 152).  
Figure 1: Characteristics of sunlight by wavelength.  
\[S_{\sigma}(\lambda)=\ \left\{\begin{array}{cc}1&(250\leq\lambda\leq 298)\\ 10^{0.094(298-\lambda)}&(298\leq\lambda\leq 328)\\ 10^{0.015(140-\lambda)}&(328\leq\lambda\leq 400)\end{array}\right.,\ (\lambda:\text{Wavelength}[\text{nm}]). \tag{3}\]  
Thus, the total irradiance was 547.3 W/m\({}^{2}\), of which UVA and UVB were 47.7 and 1.4 W/m\({}^{2}\), respectively, accounting for 8.71% and 0.26% of the total irradiance; thus, the spectral irradiance of UVA was 35 times higher than UVB. However, the results of applying the erythema weighting function were 0.05 and 0.15 W/m\({}^{2}\) for EUVA and EUVB, respectively. Thus, EUVB was three times higher than EUVA, and UVB had a considerable effect on the human body compared with UVA. EUV was 0.2 W/m\({}^{2}\), which can cause erythema upon exposure for longer than 25 min if the person has a skin color of beige to light brown [33]. A previous study revealed that Asians (Koreans) could produce 200 IU of vitamin D when exposed to an EUVB intensity of 0.15 W/m\({}^{2}\) for ~12 min (720 s) [34]. Therefore, in order to support more than 200 IU of vitamin D synthesis through lighting, EUVB doses greater than 0.15 W/m\({}^{2}\)\(\times\) 720 s = 108 J/m\({}^{2}\) (Joule = Watt \(\times\) Time [35]) could be provided to skin types commonly found in the Asian region.