Knowledge What Causes Color Shifts in LED Lights?

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Knowledge What Causes Color Shifts in LED Lights?

Color stability refers to the ability of light sources to maintain their spectral power distribution (SPD) over time. While lumen maintenance is important for LED packages, the change in chromaticity—typically referred to as color shift—must not be overlooked. The term chromaticity refers to the color of light, independent of intensity. The color characteristics of LEDs are derived from their SPD, which describes the relative amount of radiant power at each wavelength of the visible radiation spectrum. The spectral power distribution determines the additive nature of color perception—or, to put it more plainly, how the light appears.

Color shift may be more detrimental when compared with lumen depreciation. In real world applications the chromaticity maintenance life of LEDs is often shorter than their lumen maintenance life. Color shift in LEDs can be triggered by failure mechanisms. Explaining and understanding different color shift mechanisms in LEDs is not possible without having a fundamental knowledge about how LEDs are fabricated.

An LED is a packaged p-n junction device which produces a desired spectrum of light typically through a combination of electroluminescence and photoluminescence. The LED package has one or more semiconductor dies, known as “LED chips”, which comprise a p-n junction sandwiched between two oppositely doped gallium nitride (GaN) layers. When a forward voltage is applied across the doped layers, electrons from the n-doped GaN layer and holes from the p-doped GaN layer are injected into the active region within the p-n junction. As a high energy electron fills a hole in the active region, it drops into a state of lower energy and releases a quantum of energy in the form of a photon. This effect is called electroluminescence.

The photons emitted by an LED chip transport electromagnetic radiation of wavelengths in a very narrow spectral band of wavelengths (about 10-30 nm). The narrow width of the spectral power distribution causes the light emitted by InGaN LED chips to have a single color, e.g. blue or ultraviolet. To achieve broad-spectrum white light the bandwidth of the emitted light is broadened through photoluminescence. The InGaN LED chip pumps light into a phosphor layer which provides full or partial conversion electroluminescence to create a mixture of different wavelengths. Most white LEDs employ this approach to create a spectrum that the human eye perceives as white light. These light sources are also known as phosphor converted LEDs.

Phosphor converted LEDs rely on a packaging structure to complete the electroluminescence and photoluminescence processes. Semiconductor packaging not only allows phosphor down converters to be applied to the LED chip, but also provides the chips with protection from environmental contaminants and mechanically interfaces the LED chip to its operating interface. However, the addition of packaging materials leads to a higher probability of color shift as most chromaticity shifts are caused by chemical changes in the materials contained in the LED package.

There're two types of chromaticity changes in LED packages: short-term, reversible changes and long-term, irreversible changes.

Short-term, reversible chromaticity changes may occur as a result of the changes in LED junction temperature, the changes in the phosphor properties, or dimming with the constant-current reduction (CCR) methods. The wavelength of emitted light will increase one nanometer for every 10°C rise in junction temperature. When the junction temperature of an LED rises from ambient temperature to its typical operating temperature, its dominant emission wavelength may increase by several nanometers and thus there's a color shift over this period. Phosphor thermal quenching can induce color shifts due to the reduction in conversion efficiency at high operating temperatures. The presence of moisture, oxygen or other gas molecules in the ambient environment may induce chemical changes in phosphors. Dimming LEDs to 20% of the rated output can change the correlated color temperature (CCT) by ±200 K.

Irreversible color shifts are consequences of degradation mechanisms that occur in the semiconductor chip, phosphors, encapsulant materials, plastic resins, or lenses. Higher temperatures are the primary accelerator of these degradation mechanisms. Operating LEDs at high drive currents and/or without efficient heat dissipation over time will permanently degrade the quantum efficiency of both the LED chip and phosphor down-converter and also create a color shift. Thermal degradation and photo-oxidation are two culprits behind the discoloration of the plastic resins, encapsulant, and PC/PMMA lenses. Carbonization of the encapsulant due to electrical overstress, hydrothermal ageing of encapsulant due to moisture diffusion, and cracking or delamination of phosphor-binder layer also affect the rate of chromaticity shift.

The direction of color shift varies according to the degradation mechanisms occurring in the LEDs. A shift in the blue direction could possibly be related to loss of phosphor quantum efficiency due to chemical change or temperature effects, operating the phosphor above the saturation flux level, and discoloration of the package resin. A shift toward yellow is observed when there’re delamination and cracking between the phosphor/binder layer and the LED die. An increase in phosphor quantum efficiency, discoloration of the reflector, and oxidation of the secondary lenses can also move the chromaticity point in the yellow direction. Red and green shifts often involve spectral changes in the phosphor.

The maximum acceptable magnitude of color shift depends on the application. Residential lighting allows for a maximum color shift of 0.007 Δu'v' at 6,000 hours. Hospitality, showroom and museum lighting applications may require a much higher chromaticity stability. A color shift of up to 0.003 Δu'v ' over five years is often desired in these applications.
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