Street lights are essential for our modern cities and towns, playing a vital role in enhancing visibility, reducing accidents, and ensuring safety during nighttime hours. The widespread adoption of LED technology has brought brighter and more energy-efficient white light to our streets. However, since early 2021, a peculiar phenomenon has emerged across the United States and parts of Canada, leaving many puzzled: some street lights are emitting a distinct purple glow.
This unexpected purple hue has sparked curiosity and some concern, raising questions about its potential effects on drivers and pedestrians. The altered perception of nighttime environments due to this purple light has prompted people to wonder about its cause and implications.
The reason behind these purple street lights is now understood to be LED phosphor degradation. While this is the widely accepted explanation, it begs further questions. What exactly is phosphor degradation in LEDs? Is this purple light harmful? Is this issue likely to become more common? And importantly, for those involved in street lighting projects, how can this problem be prevented?
The Science Behind White LED Street Lights
To understand why some street lights are turning purple, it’s crucial to first grasp how LED street lights produce white light. The answer lies in the fascinating technology of Light Emitting Diodes (LEDs).
LEDs are semiconductor devices that emit light when an electric current passes through them. While LEDs can produce various colors, they don’t naturally emit white light. So, how do LED street lights achieve that familiar white glow?
The secret lies in a clever combination of materials. At the heart of an LED street light is a blue LED chip. This chip is then coated with a layer of yellow silicone phosphor. When the blue light emitted by the LED chip strikes the phosphor coating, the phosphor becomes excited and emits yellow light. It’s the combination of this emitted yellow light with the original blue light that results in what we perceive as white light.
By carefully adjusting the composition and thickness of this phosphor layer, manufacturers can fine-tune the shade of white light produced, ranging from warm white to cool white, to meet different lighting requirements.
Phosphor Degradation: The Cause of Purple Lights
The purple hue observed in some street lights is a direct result of the degradation of this crucial phosphor coating over time. When the phosphor layer degrades, the balance of blue and yellow light is disrupted. This degradation can manifest in several ways, including the development of cracks in the coating, adhesion issues between the LED chip and the silicone-phosphor layer, or even complete separation, known as delamination.
Without a fully functional phosphor coating, the LED primarily emits blue light, or in some cases, light that appears purple to the human eye. This shift in color is what is causing the widespread appearance of purple street lights.
But what factors lead to the degradation of this silicone-phosphor layer?
One of the primary culprits is faulty or subpar manufacturing of the phosphor coatings themselves. However, several operational and environmental factors can also contribute to this degradation over time. Heat buildup within the lamp due to continuous operation is a significant factor. The constant vibrations from traffic, especially from heavy vehicles passing by, can also play a role in stressing the phosphor layer. Even the simple, continuous force of gravity acting on the light fixture over extended periods can contribute to the slow degradation process.
Are Purple Street Lights Dangerous?
The emergence of these unusual purple street lights has understandably led to public concern. Some have worried about potential health risks associated with this altered light spectrum, including concerns about eye strain and even impacts on mental well-being. However, currently, there is no scientific evidence to substantiate claims of direct health hazards posed by purple street lights. The human eye is remarkably adaptable to a wide range of light colors.
Nevertheless, purple street lights do raise legitimate safety concerns. The distinct color shift can reduce the overall effectiveness of street lighting. The altered light spectrum may impact visibility for both drivers and pedestrians, potentially reducing the clarity with which objects and road markings are perceived at night. This could lead to a heightened risk of accidents, particularly in areas with high traffic volume or significant pedestrian activity. Furthermore, the unfamiliar and somewhat unsettling purple color might create a feeling of unease or insecurity for some individuals, even if the actual light levels remain technically adequate for illumination.
How Common Are Purple Street Lights?
While the appearance of purple street lights might seem sudden, the timeline aligns with the widespread adoption of LED street lighting. Many cities and towns across the U.S. began transitioning to LED street lights approximately 15-20 years ago. Crucially, a significant portion of these early white LEDs were manufactured using similar phosphor-coating techniques. The current emergence of purple street lights is likely a consequence of these early installations reaching a point where the faulty or poorly manufactured phosphor coatings are now beginning to degrade.
Street lighting projects are typically large-scale endeavors. Governments often award contracts to lighting companies through competitive bidding processes. The winning company then becomes responsible for supplying all the street lights for a specific project. This procurement model explains why clusters of purple street lights are often observed in particular areas or regions, reflecting specific contracts and timeframes.
While some of these purple LED street lights have already been proactively replaced by municipalities, it is possible that more instances may surface in the near future as more early LED installations reach a similar stage of phosphor degradation. However, the good news is that both municipalities and LED manufacturers are now acutely aware of this issue. Steps are being taken to replace the defective lights and, more importantly, to implement stricter quality control measures in the manufacturing process for future LED street lights. As these corrective actions take effect, the frequency of purple street lights should gradually decrease over time.
Ensuring Quality and Preventing Purple Street Lights
The issue of purple street lights ultimately underscores the importance of utilizing high-quality and reliable LEDs in street lighting infrastructure. For LED lighting manufacturers, prioritizing quality is paramount. Adherence to industry standards is crucial in ensuring the longevity and performance of LED products.
One key standard is LM-80, developed by the Illuminating Engineering Society of North America (IESNA). LM-80 is a rigorous lumen maintenance test method designed to measure the long-term performance of LED packages, arrays, or modules. It assesses lumen depreciation, color shift, and failure rates over extended periods, typically a minimum of 6,000 hours with data collection points every 1,000 hours. By rigorously following LM-80 guidelines, manufacturers can more accurately predict the lifespan and color stability of their LEDs, significantly mitigating the risk of premature failures like phosphor degradation.
While LM-80 focuses on LED components, for those evaluating complete LED lighting products like street lights, an LM-79-compliant photometric test report is highly valuable. LM-79 provides standardized methods for testing and reporting the photometric and electrical performance of complete LED luminaires.
As a leading LED lighting manufacturer, AGC recognizes the critical importance of delivering dependable and long-lasting products. We are committed to helping our customers succeed in bids and minimize long-term maintenance costs. We rigorously adhere to relevant industry standards to ensure the quality, reliability, and performance of our LED lighting solutions. Contact us today to explore how we can support your street lighting projects with high-quality, long-lasting LED technology.
