## 2 The Sustainability Coefficient of Outdoor Lighting Ambiences
The same participants also were used later in following phases (Phase 2 and Phase 3) the presentation of which is the main purpose of this article. Initially, 12 locations (1-12) and 10 objects (13-22) were selected according to the principle of diversity. All respondents were familiar with all cases and rated on a 5-point rating scale how much the lighting arrangement met their expectations regarding illumination likability and illuminance intensity. \begin{table}
\begin{tabular}{l c c c c c c c c c} \hline \hline \multirow{3}{*}{**Case of Consideration**} & \multicolumn{2}{c}{**Illumination**} & \multicolumn{2}{c}{**Illuminance**} & \multicolumn{2}{c}{**Illuminance**} & \multirow{3}{*}{**Case of Consideration**} & \multirow{3}{*}{**L**/I} & \multirow{3}{*}{\(\mathbf{x_{\text{L}}}-\mathbf{x_{\text{I}}}\)} & \multirow{3}{*}{**t**} & \multirow{3}{*}{**2p**} & \multirow{3}{*}{**r**} \\  & \multicolumn{1}{c}{**L**} & & & & & & & & \\ \cline{1-1} \cline{6-10}  1 & \(\mathbf{x_{\text{L}}}\) & \(\mathbf{\sigma}\) & \(\mathbf{x_{\text{I}}}\) & \(\mathbf{\sigma}\) & & & & \\ \hline
1 Gosposka Street & 3.12 & 0.727 & 3.30 & 0.677 & 0.95 & 0.180 & -2.656 & 0.009 & 0.198 \\
2 Postna Street & 3.16 & 0.073 & 2.87 & 0.780 & 1.10 & 0.290 & 4.441 & 0.000 & 0.512 \\
3 Vojasińska Street & 3.24 & 0.947 & 2.90 & 0.824 & 1.12 & 0.335 & 4.964 & 0.000 & 0.448 \\
4 Stukelj Square & 4.09 & 0.911 & 4.10 & 0.822 & 0.98 & 0.011 & -0.141 & 0.888 & 0.283 \\
5 Castle Square & 3.34 & 0.803 & 3.28 & 0.727 & 1.02 & 0.067 & 1.214 & 0.226 & 0.534 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Assessments of the sustainability coefficient. The results of Phase 1 (Table 1 [48]) show that a higher sustainability coefficient corresponds to more sustainable lighting. The sustainability coefficient, which is calculated in the sixth column of Table 1, is usually around 1.00. Values above 1.00 indicate that likability surpasses illuminance. The correlation coefficient (r = 0.198 to 0.598) indicates a weak to moderate correlation between illuminance and likability in all cases. The results of the \(t\)-test show significant differences in both directions for most cases. Where 2p \(>\) 0.05, there are no statistically significant differences. Cases such as Castle Square and Slomsek Square, the university, the theatre, the market, the ski slope and the plague monument have S \(\geq\) 1.00 at the same time. The ski slope and the plague monument are included in this group despite a negative \(t\)-test result because rounding to two decimal places gives an S value of 1.00. Cases such as the city hall, Stukelj Square, and the football stadium also show 2p \(>\) 0.05 with S \(<\) 1.00, indicating no statistically significant differences between the averages. In all cases where no significant differences were found, the sustainability coefficient (S) is very close to 1.00, specifically between 0.98 and 1.03. Since this range overlaps with S \(>\) 1.00, it can be considered as an interval in which the lighting of urban open spaces remains compliant, defined as S \(\geq\) 0.98. The threshold S = 0.98 represents the lowest rounded value at which no significant difference between illumination likability and illuminance intensity is observed. In this study, all but five cases where S \(<\) 0.98 were classified as compliant. The lighting compliance value was determined on the basis of the statistical parameters of 22 cases. In all cases where S \(\geq\) 0.98 (Sn \(\geq\) 1.00), the lighting is adequate and lighting compliance is achieved. To simplify the expression, a normalized value Sn was introduced, which sets the boundary of the closed interval to 1.00 (as shown in Equation (2)). \[ \begin{array}{l}\text{Sn}=\text{S}+\Delta\,\text{S}\\ \Delta\,\text{S}=0.02\end{array}  \tag{2}\]  
In cases where Sn \(<\) 1.00, the lighting is not adequate and lighting compliance is not achieved. The illumination intensity exceeds the likability, resulting in higher energy