The development of LED lighting, which generates light by radiative electron-hole recombination in solid state semiconductors rather than by stimulating a gaseous medium or heating a thermal radiator in a glass enclosure or enclosure, has tremendously benefited street lights. When compared to HID systems, such as high-pressure sodium (HPS), low-pressure sodium (LPS), and metal halide (MH) lights, solid state lighting technology provides significant benefits.
The significant energy savings provided by LED technology is what most motivated the transition from HID (HPS, LPS, MH) to LED. Although HPS lamps, the most common street light source, may achieve source efficiencies of up to 150 lm/W in high power products, in practical applications, their efficiencies are closer to 100 lm/W. HPS street lights can lose 30% to 40% of their system effectiveness when optical and ballast losses are taken into consideration. In contrast, phosphor converted LEDs have source efficacies between 150 and 190 lm/W that are both economically viable and have prospective source efficacies of 255 lm/W. LED street lights may attain a system efficacy of well over 140 lm/W and a luminaire efficiency that is close to 80% because to their high source efficacy, directed emission pattern, and high power conversion efficiencies. This indicates that compared to traditional lighting sources, LED street lighting offers energy savings of 50% to 100%.
Municipalities and utilities that seek to reduce operating and relamping costs are drawn to LED street lighting' reduced maintenance and life cycle costs. LED lighting systems may operate for more than 50,000 hours provided they have appropriate thermal management and optimal power control. LEDs are made of a semiconductor block rather than glass envelopes or other delicate parts. LED street lights can withstand continuous vibration from fast-moving automobiles because to the solid state endurance of the light source. Outstanding dependability and durability work together to extend the usable life of LED systems and significantly reduce maintenance and relamping.
For optimal nighttime driving conditions, LED street lighting's spectral power distribution (SPD) may be adjusted. The spectral properties of the light source have a significant impact on the visibility that a lighting system provides. Rods and cones, two types of optical photoreceptors, are present in the human eye. Scotopic vision, which is used at night when the brightness level is very low (less than 0.005 cd/m2), is made possible by rods. All visible colours may be seen by the cones, which are most active under photopic circumstances when luminances are typically more than 3.4 cd/m2. For photopic vision and scotopic vision, the highest spectral sensitivity curves are at 555 and 507 nm, respectively. The rod photoreceptors respond to mesopic vision, which is the region between photopic vision and scotopic vision.
The light spectrum of LED street lights can be modified to target the most efficient spectrum for the roadway vision states, in particular the mesopic vision that applies to the light levels frequently found in street lighting, by adjusting the ratio of phosphors for the desired colours in down-converters. The eye must have strong scotopic vision in order to identify off-axis objects. While visual acuity has a relatively little role in a driver's visibility, a strong colour rendition enables the cone photoreceptors to be engaged, which makes it simpler to differentiate small things from their backdrop. When compared to HPS lamps, which have a low CRI, LED street lights generally have a CRI of 80, which is enough for illuminating roads. To ensure optimal visual performance in mesopic vision, a light spectrum with a high scotopic / photopic (S/P) ratio is often desired. While LED street lights may be spectrally tailored to give a S/P ratio between 1.21 (3000 K LED) and 2.0 (6000 K LED), HPS lamps typically have a S/P ratio of 0.63.
Visibility is not always improved by a high S/P ratio. When there is a high density of fog, mist, or haze in the atmosphere, the meteorological visibility is poor, and the greater the S/P ratio, the more light is scattered and the less light is transmitted. Light with a high S/P ratio has a large portion of blue wavelengths in its spectrum. This sparked worries about the dangers of blue light and the physiological effects of high-intensity, high-CCT street lighting. The light spectrum for roadway lighting may need a minimum content of blue or a moderate S/P ratio to provide good visibility as well as to create alertness and suppress the release of melatonin (which is known as a sleep hormone). However, blue-rich cool white light should not be used in indoor lighting during the night to avoid circadian disruption. So, for high road and motorway lighting, LED street lights with a colour temperature of 4100 K are commonly advised. Warm white light (e.g., 3000 K) is recommended in locations with dense populations and in residential areas to minimise the detrimental physiological effects of street lighting. Any CCT need may be met with LED technology.
Since they are semiconductors, LEDs may easily be integrated into other solid state circuitry. Since LEDs respond immediately to changes in power supply, analogue dimming based on the continuous current reduction (CCR) approach may be used by merely altering the driving current provided to the LEDs. Pulse width modulation (PWM) technology, which permits full range intensity control while keeping a constant colour point despite variations in light intensity, may also be used to digitally dim LED street lights. In comparison, dimming MH lamps is more challenging and HPS street lights can only be reduced to around 50% light intensity. Because solid state lighting is digital, there are prospects for direct integration of street lights with computer-based systems, which would increase automation and efficiency. This integration of wireless connectivity, sensor technologies, and street lighting opens the door to a variety of cutting-edge IoT possibilities.
