Why Are Most Street Lights In Cities Yellow?

LED electronic displays are becoming increasingly common, and LED lighting is being tested on some major roads. But what conditions do LED street lights need to meet?

As the day transitions into night and the city lights up, the urban landscape gradually quiets down after a busy day. Streetlights illuminate every city road, from main thoroughfares to tree-lined paths in residential areas, guiding citizens home. Have you ever noticed that most street lights are yellow? Why not white or other colors?

The Science of Light Colors

Firstly, light is composed of seven colors: red, orange, yellow, green, blue, and violet. Different colors of light have different wavelengths, with red light having the longest and violet the shortest. However, the color of the illumination is not necessarily the one with the longest wavelength. Red light, for instance, is not suitable because it is more stimulating to the eyes.

Yellow light, with a wavelength range of 570-600 nanometers, falls within a band that is particularly sensitive to the human eye. Yellow is also a warm color, unlike red or orange, and does not stimulate the eyes as much. Therefore, yellow light is ideal for street lights.

The Era Before LEDs

Before the advent of LEDs, white light was primarily produced by incandescent lights, while street lights used high-pressure sodium lights, which emit yellow light. High-pressure sodium lights are high-efficiency gas discharge bulbs with light wavelengths in the yellow band of visible light. Data shows that high-pressure sodium lights are several times more efficient than incandescent lights, have a lifespan twenty times longer, are cheaper, and have better fog penetration. Additionally, human eyes are sensitive to yellow light, which provides a sense of warmth and helps reduce nighttime traffic accidents.

However, high-pressure sodium lights have significant drawbacks, primarily poor color rendering. Color rendering measures how accurately a light source displays the true colors of objects. Incandescent lights have good color rendering and are suitable for household lighting, but sodium lights display poor color, making everything appear yellow. For road lighting, color accuracy is less critical; the primary need is to see the size and speed of oncoming vehicles, not their color.

Advantages of LED Street Lights

LED street lights for sale offer several advantages over traditional high-pressure sodium street lights:

1. Energy Saving: The current luminous efficiency of white LEDs is about 80 lm/W, which can save 50%-60% of electricity compared to traditional high-pressure sodium lights. With the rapid improvement of LED efficiency, semiconductor street lights have great potential for energy saving.

2. Low Maintenance Cost: High-power LED light sources can last up to 10 years without replacement, whereas traditional high-pressure sodium lights need replacement every one and a half years on average. Using semiconductor light sources can significantly reduce maintenance costs.

3. Good Color Rendering: LEDs have a high color rendering index (75-80), making roads appear brighter and more comfortable, and improving driver safety. High-pressure sodium lights have a narrow spectrum and poor color rendering (20-40), making the environment feel dim.

However, an LED street light typically requires about 100 1W white LEDs, making large-scale replacement costly. This makes it challenging to promote and apply LED street lights extensively in small and medium-sized cities. Nonetheless, LED street lights are being used to replace traditional street lights worldwide. As LED industry standards and technical specifications improve, the replacement and renewal of LED lights in small and medium-sized cities will accelerate.