LED street lighting benefits-NEWS

URL:https://www.masonleds.com/news/led-street-lighting-benefits-66874587.html

Street lights have profited greatly from LED technology which produces light by generating radiative electron-hole recombination in solid state semiconductors, rather than by exciting a gaseous medium or heating a thermal radiator in glass envelopes or enclosures. Solid state lighting technology offers compelling advantages over HID systems including high-pressure sodium (HPS), low-pressure sodium (LPS), metal halide (MH) lamps.

The biggest drive that spurred on the shift from HID (HPS, LPS, MH) to LED is the substantial energy savings offered by LED technology. HPS lamps, the ever most popular street light source, can hit a source efficacy as high as 150 lm/W in high wattage products, however in real world applications their source efficacy is around 100 lm/W. When the optical and ballast losses are taken into account, the system efficacy of HPS street lights can drop by 30% - 40%. Whereas phosphor converted LEDs have a potential source efficacy of 255 lm/W, a commercially available source efficacy over 200 lm/W, and financially economical source efficacies between 150 and 190 lm/W. High source efficacy combined with the directional emission pattern of LEDs and high power conversion efficiencies of LED drivers allows LED street lights to achieve a system efficacy of well over 140 lm/W, and a luminaire efficiency approaching 80%. This means LED street lighting brings about 50% - 100% energy savings over conventional technologies.

The maintenance and life cycle cost savings offered by LED street lights also appeal to municipalities and utilities who strive to cut down the operation and relamping cost. LED lighting systems with good thermal management and optimal power regulation can have an operation life of more than 50,000 hours. LEDs are constructed from a block of semiconductor, and do not use glass envelopes or fragile components. The solid state durability of the light source allows LED street lights to survive repeated vibration caused by vehicles traveling at a high rate of speed. Excellent reliability and durability collectively contribute to the long useful life of LED systems and a dramatic reduction in maintenance and relamping.

The spectral power distribution (SPD) of LED street lighting can be optimized for nighttime driving conditions. The visibility provided by a lighting system can be affected substantially by the spectral characteristics of the light source. The human eye contains two visual photoreceptors: rods and cones. Rods are responsible for night vision (scotopic vision) where the luminance level is very low (<0.005 cd/m²). The cones can respond to all colors in the visible spectrum and are most active under photopic conditions where the luminances are generally in excess of 3.4 cd/m². The spectral sensitivity curves for photopic vision and scotopic vision peak at 555 and 507 nm, respectively. The area between photopic vision and scotopic vision is referred to mesopic vision which is responded to by the rod photoreceptors.

By adjusting the ratio of phosphors for the desired colors in down-converters, the light spectrum of LED street lights can be modified to target the most effective spectrum for the roadway vision states, in particular the mesopic vision which applies to the light levels frequently found in street lighting. A good scotopic vision is also important for the eye to detect off-axis objects. Visual acuity has limited importance in visibility for a driver, but a good color rendition allows the cone photoreceptors to be activated and thus makes small objects easier to distinguish from their background. Compared with the poor CRI of HPS lamps, LED street lights typically have a CRI of 80 which is sufficiently good for roadway lighting. In general, a light spectrum with a high scotopic / photopic (S/P) ratio is preferred in order to ensure high visual performance in mesopic vision. HPS lamps has a typical S/P ratio of 0.63, whereas LED street lights can be spectrally tuned to offer an S/P ratio from 1.21 (3000 K LED) to 2.0 (6000 K LED).

A high S/P ratio doesn't always mean good visibility. For conditions with a low meteorological visibility due to the existence of a high density of fog, mist or haze in the atmosphere, higher the S/P ratio is, the more the light is scattered and the less the light is transmitted. High S/P-ratio light contains a high percentage of blue wavelengths in its light spectrum. This raised the concerns on the blue light hazard and physiological impact of high intensity and high CCT street lighting. While the blue-rich cool white light should not be used in indoor lighting during the night to avoid circadian disruption, the light spectrum for roadway lighting may require 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). As such, LED street lights with a color temperature of 4100 K are typically recommended for high way and freeway lighting. In densely populated regions and residential areas, the negative physiological impact of street lighting should be reduced to a minimum and therefore warm white light (e.g. 3000 K) is suggested. Whatever the CCT requirement, LED technology can do the job.

LEDs are semiconductor devices that can work seamlessly with other solid state circuits. Because LEDs react instantaneously to changes in power input, analog dimming based on the constant current reduction (CCR) technique can be implemented by simply controlling the drive current fed to the LEDs. LED street lights can also be digitally dimmed using a pulse width modulation (PWM) technique, which enables full range intensity control while maintaining a consistent color point regardless of the variation in light intensity. In contrast, HPS street lights can only be dimmed to roughly 50% light level and dimming MH lamps is more difficult. The digital nature of solid state lighting opens up opportunities for direct integration of street lights into computer-based systems, and resulting in improved efficiency and automation. This combination of street lighting, sensor technology, and wireless communication opens the door to a wide range of innovative capabilities in the context of IoT.