1. Environmental Conditions of Urban Road Lighting
Since entering the 21st century, especially after 2010, the air quality in our country has severely deteriorated, with smog weather becoming a frequent occurrence. LED street lights, as a type of solid-state cold light source, possess characteristics such as being environmentally friendly, low energy consumption, high luminous efficiency, and long lifespan. Therefore, LED street lights will become the best choice for energy-saving renovations in road lighting. LED street lights are efficient solid-state light sources that emit light using minimal electrical energy based on the formation of semiconductor PN junctions. Under certain forward bias voltage and injection current conditions, holes injected into the P region and electrons injected into the N region diffuse to the active region and emit photons through radiation recombination, directly converting electrical energy into light energy. Smog is caused by air pollution, waste gas emissions from enterprises, vehicle exhaust, and combustion of various materials, leading to excessive levels of various suspended particulate matter in the atmosphere, with PM2.5 being considered the main culprit of smog weather.
In recent years, the air quality in China has rapidly deteriorated, with the number of smoggy days in most cities increasing, along with the associated hazards. In many regions of China, smoke has been incorporated into fog as a warning of severe weather, collectively referred to as “smoggy weather.” For instance, in Beijing, the air quality is below two levels for nearly 200 days of the year, indicating that more than half of the weather throughout the year is hazy.
Other major cities in China are also facing smoggy weather, with many averaging over 100 smoggy days per year. Cities with severe smog are often economically developed. In the promotion of LED street lights, these cities tend to play a leading role. Therefore, studying and discussing the selection of LED street lights under smoggy weather conditions is of great significance.
2. Color Temperatures of Common Light Sources
LED light sources can be categorized into warm white, neutral white, and cool white. Traditional street lights and sodium lamps emit discharge radiation concentrated around the double D lines at 589.0nm and 589.6nm, resulting in a color temperature of approximately 2300K warm white light, which appears distinctly yellow to human vision. LEDs can be manufactured using packaging technology to produce light sources with different color temperatures, providing many options for road lighting.
3. Selection of LED Street Light Color Temperature
Currently, LED packaging technology can adjust the emission of different color temperatures by varying the amount or ratio of phosphors stimulated by different excitation wavelengths.
Different light sources emit light at different color temperatures. Low color temperature light sources are characterized by a relatively higher red radiation in their energy distribution, often referred to as “warm light.” As the color temperature increases, the proportion of blue radiation in the energy distribution rises, commonly termed “cool light.”
For white LEDs, yellow light occupies a relatively high proportion in low temperature light sources, while high color temperature sources have a higher proportion of blue light. From the perspective of human vision, low color temperatures provide a warm and stable feeling, while high color temperatures evoke a cool and refreshing sensation.
4. The Relationship Between Color Temperature and Luminous Efficiency
It is worth noting that in the initial selection of color temperatures for LED street lights, nearly all manufacturers chose 6000K or even higher, which contrasts sharply with high-pressure sodium lamps. Why is this?
This is because, under the early technical conditions of phosphors, cold white light sources had higher luminous efficiency. Compared to color temperature, people prioritize the physical properties of luminous efficiency. The most important method to enhance luminous efficiency is to improve the excitation efficiency of the phosphors. Yellow phosphors have significantly higher excitation efficiency than orange phosphors.
Clearly, higher luminous efficiency can be achieved with higher color temperatures. The luminous efficiency of light sources at color temperatures between 5500K and 6500K is 48.7% higher than that of warm white light below 3500K.
