As the phosphorous in LED streetlights degrades, the light the lamps emit adopts a purplish hue. Sometimes incorrectly attributed to faulty bulbs, the underlying technical issue was studied recently by the Raleigh Section of the Illuminating Engineering Society, the results of which were presented in a May 2024 webinar, “Causes of Excess Color Changes in Streetlights.”

Authored by the Department of Energy’s LED Systems Reliability Consortium (LSRC), the presentation explored the scientific basis for the phenomenon. LSRC’s investigation focused on the chip-scale package LED chip, as it’s the type used in the affected streetlights. They are designed with a chip-sized silicone phosphor layer adhered to the top surface of the LED chip, with the phosphor layer converting the blue light from the LED chip into white light for the streetlights.

After evaluating 28 LED modules from just one manufacturer sent from the North Carolina Department of Transportation, LSRC determined that the failure occurs in the silicone phosphor layer, because the silicone loses adhesion to the LED chip over time, resulting in cracks and delamination. This changes the color output.

Modules were first visually inspected for obvious physical damage. They then underwent photometric tests to measure light output and color. Researchers mapped the progression of color from near-standard white light to blue/purple in order to illustrate the degradation over time as the phosphor emission decreased.

The use of thermal imaging determined that overheating was not a significant factor in failure. However, microscopic examination discovered that material failures such as cracks and brittleness may contribute to degradation.

The researchers concluded that the silicone phosphor layer’s integrity was compromised due to material failure, resulting in decreased light quality and a blue/purple hue. Their findings indicate that adhesion between the LED chip and the silicone phosphor layer deteriorates over time. Degradation is indicated by brittleness, cracks, fractures and delamination. If the phosphor layer is completely separated or missing, the LED produces blue light instead of the white light it should emit in streetlights. The impact on each LED can vary.

The LSRC raised safety concerns as a result of this shift of light color from white to blue/purple, citing the decrease in illumination on roads and pathways, causing poor visibility. Furthermore, the unexpected color may be distracting to drivers.

The LSRC recommends all streetlight manufacturers adopt strict guidelines to prevent premature LED degradation. The group also suggests they consult the data to assess LED performance in extreme conditions that may exceed a fixture’s standard operational ratings for current and temperature. Doing this in conjunction with using tools that monitor changes in luminous flux and color stability over time should provide an accurate evaluation of an LED streetlight’s long-term reliability and effectiveness in real-world applications—and ultimately predict their longevity and color consistency.