# Design, Deployment and Evolution of Heterogeneous Smart Public Lighting Systems
## 3 Smart Public Lighting: Infrastructure Architecture and Main Tasks
### Light Dimming  
Clearly, the most important task of LCs is to dim light according to some criterion, to generate savings. Over the years, basic control systems have evolved into extremely advanced ones, providing enhanced capabilities to adapt the lighting intensity to the actual operating environment. In contrast to the simplest and old-fashioned LCs, which allow to settle the light intensity only according to a predefined schedule, cutting edge control systems allow to tailor the lumen output to the current vehicular traffic [68], whose amount is monitored by specific sensors, in some cases even taking into account meteorological conditions jointly with measured road surface luminance. Such advanced features are developed in accordance with specific regulations that allow to dynamically reduce the light intensity when specific conditions are met. In particular, according to the CEN/TR 13201-2 lighting standard [9], each road is assigned a given lighting class, which mostly depends on the expected traffic flow. With reference to motorized traffic roads, [9] defines six classes, from M1 to M6 in decreasing importance order, each of which corresponds to a specific average luminance requirement. For instance, an average luminance of 2 cd/m\({}^{2}\) is required for the M1 class, 1.5 cd/m\({}^{2}\) for the M2 class, whereas 0.30 cd/m\({}^{2}\) is sufficient for the M6 class. However, the standard allows to dynamically adapt the lighting class to the actual conditions, assessed by means of real-time measurements. In Italy, for instance, the CEN/TR 13201-2 standard is implemented through the norm UNI 11248, which standardises adaptive lighting strategies, thus allowing the design of systems that regulate lighting conditions according to factors that may vary over time, such as traffic flow, luminance or weather conditions. In particular, two different operating methods can be adoped: (a) TAI (Traffic Adaptive Installation), where only the traffic volume is measured; (b) FAI (Full Adaptive Installation), where even meteorological conditions and road surface luminance are measured. When TAI is deployed, the lighting class can be downgraded by one if the measured traffic is lower than 50% of the nominal value, whereas a downgrade of 2 lighting classes is possible when the traffic is lower than 75% of the nominal value. Within TAI systems, the traffic volume is measured in given time intervals (usually in the order of 10 min), and dimming is allowed when two consecutive samples are below the limit. The EU GPP document [7] provides some examples of operational profiles, which allow to gain an insight on the benefit of light dimming. In particular, a scenario is considered in which the light output is reduced by 50% during hours of low use, from 00.00 to 06.00. However, when traffic sensors indicate that road use is lower than a certain minimum level, the lighting output is automatically decreased to 10%. Although the exact energy savings vary from day to day, the road traffic pattern used in [7] resulted in a 30% reduction of the energy consumption with respect to the simple 100%-to-50% dimmed installation and almost 46% less than the same undimmed installation. In contrast to TAI, which adopts a discrete step dimming, FAI updates the lighting class progressively and continuously between one and the other, to obtain the maximum energy saving. In this case, the dimming level is based on the joint assessment of traffic conditions, road surface luminance and meteorological conditions. Clearly, distinct approaches (preset dimming, TAI or FAI) lead to distinct savings, require different technologies and systems of varying complexity. If the most saving is to be achieved a low-latency communication network is required.