Knowledge How Stokes Shift Impacts LED Efficiency and Color Quality?

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Knowledge How Stokes Shift Impacts LED Efficiency and Color Quality?

Phosphor conversion (PC) is the most widely used approach for generating white light with LEDs. The technique involves using a blue LED (InGaN chip) to pump light into a phosphor down-converter. A portion of short-wavelength photons is absorbed by phosphors and converted into longer wavelength photons. This wavelength change between the absorbed and emitted photons is commonly referred to as the Stokes shift of the phosphor material. White light is then generated by mixing the re-emitted broad-band yellow light from phosphors with blue emission from the LED.

In the wavelength conversion process, high energy blue photons are absorbed by atoms in the phosphor, which become excited and emits yellow fluorescence. The emitted photons that appear as longer wavelength light have less energy than the absorbed ones. The excess energy is dissipated as heat, which is known as Stokes heat. The energy lost during wavelength down-conversion is referred to as Stokes loss. Wavelength conversion in the phosphor material results in a significant amount of Stokes loss. Typically, about 15–25% of the radiant energy absorbed by the phosphor is converted to heat in the phosphor layer. This translates to a substantial efficiency loss for LED packages.

The Stokes loss in phosphor-converted LEDs creates a trade-off between LED efficiency and color quality. Because a higher CRI puts a heavy demand on the amount of longer wavelength light, particularly in the red wavelength range at the edge of the visible spectrum, a large amount of blue photons have to be down-converted. This inevitably accompanies a huge Stokes loss. Thus the color rendering ability of LED products is often compromised unless there is a specified color rendition requirement from end-users.

The market is flooded with cool white lighting products. Repeated, prolonged periods of exposure to blue-rich white light at night can disrupt human circadian rhythms and result in a substantial increase in heath risks. However, lighting manufacturers seldom care. They're mostly concerned about the efficacy loss in warm white products. To create warm white light from phosphor-converted LEDs, the blue content in the spectrum must be minimized and the red content must be increased. Likewise, a huge Stokes loss occurs in creating warm white light from blue-pump LEDs.
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