## 4 Time-Varying Environmental Influences
### Downwelling Ambient Light  
The level of radiance reaching the sea surface changes throughout the day, as well as throughout the seasons. The spectrum can be further impacted by cloud cover. The Sun's azimuth and zenith angle at the location of interest changes due to the Earth's rotation, axial tilt, and elliptical orbit around the Sun, causing the penetrating light rays to be refracted at an angle at the air-water boundary. This effect of the incidence angle is evident even in some of the earliest investigations of the underwater light field by [PERSON] (2017) and later, [PERSON] (2018), where at a constant depth, the amount of light received was greatest when the receiver was directed toward the azimuth and zenith angles that corresponded to the refracted rays as per [PERSON]'s law. However, these directional characteristics are lost with increasing depth as the light field transforms progressively from the \"sunny condition\" near the surface to a more diffuse distribution where ultimately, the peak radiance occurs at a receiver zenith angle of zero degrees, where it has reached its \"asymptotic\" form ([PERSON], 2018).  
Furthermore, accurate site-specific downwelling spectral distribution is best obtained through in situ measurements, which can be resource intensive. Alternatively, historical data may be used, but would require rigorous analysis to forecast the spectra accounting for cloud cover and weather conditions for a given date. Instead, the SPCTRL2 model developed by [PERSON] and [PERSON] (2018) may be a good alternative to estimate the spectra based on date, time, location (latitude and longitude), and atmospheric data available in the public domain. Figure 4 shows that the global spectral distribution (including both direct and diffused sunlight incident at Earth's surface) peaks in the blue-green region that also corresponds to the spectrum with the most penetrative ability underwater ([PERSON] et al., 2018), thus raising questions of how significant this could contribute as optical noise.  
\begin{table}
\begin{tabular}{l c c c c c c c} \hline \hline \multirow{3}{*}{**Literature**} & \multirow{3}{*}{**Stratified Ocean**} & \multirow{3}{*}{**Ambient Light**} & \multicolumn{2}{c}{**Sensor Network and Mobile Horizontal**} & \multicolumn{2}{c}{**Vertical and**} & \multicolumn{2}{c}{**Consider**} & \multicolumn{2}{c}{**SNR Insights**} & \multirow{3}{*}{**Realistic Model**} \\  & & & **Mobile** & & **Horizontal** & & **Optical Beam** & & **Transmission** \\  & & & & **UOWC** & & **Entiredly** & & **Wavelength** & \\ \hline [PERSON] et al. (2018) & No & Yes & No & No & Yes & Yes & No \\ [PERSON] et al. (2018) & No & Yes & No & Yes & Yes & Yes & No \\ [PERSON] et al. (2018) & Yes & No & Yes & No & No & No & Yes \\ [PERSON] et al. (2018) & Yes & No & Yes & Yes & Yes & No & No \\ [PERSON] et al. (2018) & Yes & No & Yes & Yes & Yes & No & No \\ [PERSON] and [PERSON] (2018) & Yes & Yes & Yes & No & No & No & Yes \\ [PERSON] and [PERSON] (2018) & Yes & No & Yes & No & Yes & No & Yes \\ [PERSON] et al. (2018) & Yes & No & Yes & Yes & Yes & No & Yes \\ [PERSON] et al. (2018) & Yes & No & Yes & No & Yes & No & Yes \\ [PERSON] et al. (2018) & No & No & Yes & Yes & Yes & No & Yes \\ This work & Yes & Yes & Yes & Yes & Yes & Yes & Yes \\ \hline \hline \end{tabular}
\end{table}
Table 1: Comparison of the findings of this work to existing works.  
The influence of lunar spectral radiance, on the other hand, may be considered less significant due to a smaller irradiation peak at about 5 mW m\({}^{-2}\), shifting toward the more attenuated amber-red spectrum for a full moon [53]. However, despite the lunar spectral variation being a less investigated topic, given that its spectrum also spans a bandwidth like the solar spectrum, it can be assumed that strategies that discriminate solar noise can also apply to lunar noise.