## 3 The Integral Role of Lighting in Urban Environment
### Understanding the Role of the Sustainability Coefficient (Sn) in Measuring Environmental Well-Being  
To explore the validity of the sustainability coefficient (Sn), the focus was on basic urban typologies such as the street, square, and city park. The final decision for the study site was influenced by the conviction of the users who argued for the illumination likability of these areas. When selecting sites for the more detailed analysis, we ensured that the average score for illumination likability was L \(\geq\) 3.00. If the lighting attractiveness is lower, the calculation of Sn is not relevant because the site is not perceived as attractive regardless of the perceived illuminance intensity. With a value of 3.00 on a five-level evaluation scale, we can say with at least 60% probability that the illumination is likable. To capture the sustainability coefficient, we add a new constraint to it (Equation (4)). \[\left\{ \begin{array}{ll}\text{L }\geq 3.00,\text{ then Sn is calculated}\\ \text{L }<3.00,\text{ then Sn is not calculated}\end{array} \right. \tag{4}\]  
In the context of a five-level evaluation scale, a rating of 3 functions as a median value and thus as a threshold for acceptability. People often view median values as neutral. Anything above the median value can be considered positive, while anything below it is potentially negative. Illumination likability above the median value means that the lighting is suitable for most users. This means that the lighting meets most needs and is generally acceptable. A value below 3.00 is considered unsatisfactory or inadequate, i.e., too dim, too strong, or annoying in some way. The desire to achieve a high value for illumination likability encourages setting a high standard for lighting quality in public areas. This means that designers and contractors are more likely to focus on providing high-quality lighting with better and innovative lighting solutions. In the main part of the research, we aimed to validate the sustainability coefficient Sn. The main focus was on streets (urban local streets, streets in pedestrian zones, streets in the medieval part of the city) and urban squares. These areas have the highest importance value and missing value in terms of environmental lighting (Table 1). After careful consideration, the category of city park was added as a basic urban category. In the cultural environment studied, it is not common to visit the park frequently at night. Nevertheless, respondents rated the importance value (3.56) and missing value (3.34) of footpath lighting in the city park as relatively high (Table 1). All basic urban settings were therefore covered: streets, squares, and parks. Among all the possibilities, ten centrally located sites in the city of Maribor were selected according to the principle of diversity. The condition for selection was that the average value for illumination likability was \(\geq\) 3.00. The following locations were selected:  
(a) Streets: Gosposka Street (L1), Postna Street (L2), Kocljeva Street (L3), Vojasniska street (L4);  
(b) Squares: Leon Stukelj Square (L5), Castle Square (L6), Main Square (L7), Liberty square (L8);  
(c) Parks: Slomsek Square (L9), City Park Maribor (L10). A pocket park is situated at the Slomsek sSquare location (L9). It bears the name of a square but has all the features of a small park. Further investigation evaluated the sustainability coefficient for each selected location. Before the experiment, all participants were familiarized with the research sites. They had visited these sites previously. All researched sites were evaluated using a questionnaire. The questionnaire was completed in a classroom, with large-scale images of each location projected on the wall to help participants remember and empathize with the environments. The respondents rated the relation between illumination likability and illuminance intensity on a five-level evaluation scale.