When LED street lights are used in rain, fog, and snow conditions, a low color temperature design is needed to improve light penetration, while high luminous efficacy technology is combined to optimize lighting performance and meet road safety requirements.
Requirements for Light Penetration in Rain, Fog, and Snow Environments
Traditional lighting advantages in penetration: Sodium lamps: Low-pressure sodium lamps (color temperature around 2000K) and high-pressure sodium lamps (2200–2500K) produce monochromatic yellow light with longer wavelengths (589.0 nm and 589.6 nm). This gives them strong penetration in rain, fog, and snow, which is why they were long the mainstream choice for road lighting. For example, major roads in Shenzhen once widely used high-pressure sodium lamps. After residents complained that white-light metal halide lamps (6000–6500K) performed poorly in fog, many were switched back to sodium lamps.
Vehicle fog lights: Yellow light (as seen in certain xenon lamp conversions) is preferred for fog lights because of its strong penetration. High-color-temperature xenon lamps (above 6000K) tend to create a white haze in fog, reducing visibility.
Penetration challenges of LED street lights: Conventional white-light LEDs have a high proportion of short-wavelength light (such as 460 nm blue light) in their spectrum. In rain, fog, or snow, this light is easily scattered, producing glare and lowering lighting effectiveness. For instance, the spectrum of a 6000K LED concentrates energy in the blue range, while a 3000K LED is dominated by yellow-red light between 570–630 nm and therefore offers better penetration.
Key Factors Affecting Light Penetration
Wavelength and diffraction capability: Longer wavelengths have stronger diffraction ability and better penetration. Yellow light (570–590 nm) is close to the peak of the human eye's sensitivity curve (555 nm) and suffers relatively little scattering loss, making it the preferred choice for outdoor lighting. Although red light has an even longer wavelength, its luminous efficacy is too low for practical road lighting.
Color temperature and visual comfort: High color temperatures (cool white light) can create a hazy visual impression for drivers and increase psychological tension. China's national standard "Standard for Lighting Design of Urban Roads" (CJJ45-2015) states that the color temperature of LED street lights should not exceed 5000K and that medium-to-low color temperatures (such as 3000K) should be preferred.
Optimization Strategies for LED Street Lights in Rain, Fog, and Snow
Low color temperature design: By adjusting the ratio of red, green, and yellow phosphors, LED packages can be produced with low color temperatures. In a 3000K LED, for example, blue-light energy is almost eliminated and the proportion of yellow-red light rises significantly.
Industry trend: The color temperature of LED street lights has been gradually lowered over the years. Before 2016, practical applications already showed a clear shift from high color temperatures toward medium-to-low color temperatures to better suit foggy regions.
High luminous efficacy technology
Chips and phosphors: High-efficacy LED chips combined with high-conversion-efficiency phosphors raise the luminous output per watt.
Secondary optical lenses: High-transmittance lenses reduce light loss and optimize light distribution.
Thermal materials: High-thermal-conductivity aluminum substrates lower thermal resistance and extend fixture life.
Results: Before 2016, the overall luminous efficacy of LED street lights improved year by year, achieving a practical balance between low color temperature and high efficacy.
Practical Application Cases and Standard Support
Local standard requirements: The Housing and Urban-Rural Development Department of Guangxi Zhuang Autonomous Region explicitly requires that low-color-temperature luminaires (such as 3000K) be used in mountainous and foggy areas. Expressways and arterial roads should give priority to medium-to-low color temperature LED street lights and avoid cool-white sources (above 6000K).
Supporting research data
- Fog environment tests: Red-yellow and red-blue light combinations showed the highest brightness in fog, while blue light had the poorest visibility.
- Rain and snow comparisons: Yellow light produces less backscattering than white light, resulting in lower glare and better overall lighting performance.
Conclusion
Through low color temperature design (typically 3000–5000K) and high luminous efficacy technologies (optimized chips, phosphors, and lenses), LED street lights can significantly improve penetration in rain, fog, and snow while still meeting requirements for energy saving, long life, and good color rendering. As the technology continues to advance, the advantages of LED street lights in outdoor lighting will become even more pronounced, making them an ideal choice for road lighting in foggy regions.
