LED Lighting Principle-How Phosphors Control Light Wavelength and Color Temperature

I. Core Function: How to "Create" White Light from Blue Light

To understand the role of phosphors, we must first grasp a key point: LED chips themselves can hardly emit high-quality white light directly.

Physical limitation: A single LED chip can only emit light of a single wavelength (monochromatic light), such as blue, red, or green.
Nature of white light: The "white light" we see is a mixture of different colored lights. For example, sunlight contains the entire visible spectrum.

Therefore, scientists devised an ingenious method: using blue LED chips to excite phosphors to obtain white light. This is the famous "Blue LED + Phosphor" technology, for which the inventors were awarded the 2014 Nobel Prize in Physics.

Basic principle: Photoluminescence Phosphors are materials that, when exposed to high-energy light, absorb its energy and emit lower-energy light. This process is called "photoluminescence."

II. Specific Application Methods of Phosphors in White LEDs

There are three main approaches, with the first being the most widely used:

1. "Blue LED Chip + YAG Yellow Phosphor" Method (Most Common)
This approach is adopted by over 95% of white LEDs on the market.

LED chip: Emits blue light with wavelength around 450-455 nm.
Phosphor: Mainly cerium-doped yttrium aluminum garnet (YAG:Ce³⁺) phosphor.
Working principle:
1. The blue LED chip emits high-energy blue light.
2. Part of the blue light passes through directly.
3. Another part of the blue light strikes the YAG phosphor surrounding the chip.
4. The phosphor absorbs this portion of blue light, converting it to lower-energy, broader-spectrum yellow light.
5. The remaining direct blue light mixes with the yellow light from the phosphor, creating white light in the human visual system.
  Advantages: Simple structure, low cost, high efficiency, mature technology.
  Disadvantages: Due to the lack of red components in the spectrum, the Color Rendering Index (CRI) is relatively low, making red objects appear less vivid, and the light color tends to be cool.

2. "Blue LED Chip + Red/Green Phosphors" Method
Developed to improve color rendering.

LED chip: Same blue chip.
Phosphors: Uses multiple phosphors that emit red and green light (such as nitride or fluoride phosphors).
Working principle: Blue light from the chip simultaneously excites red and green phosphors, producing blue, green, and red primary colors that mix to form white light.
Advantages: Very high Color Rendering Index (CRI>90), good light color and color reproduction.
Disadvantages: Higher cost, slightly lower luminous efficacy compared to the YAG solution (due to energy loss in phosphor conversion, known as "Stokes shift").

3. "UV LED Chip + Red/Green/Blue Phosphors" Method
This approach is similar to how traditional fluorescent tubes work.

LED chip: Emits invisible ultraviolet light (e.g., 365-400 nm).
Phosphors: Uses a mixture of phosphors that emit red, green, and blue primary colors.
Working principle: The UV light itself doesn't participate in color mixing; it only serves as an energy source to excite all phosphors. The three primary colors emitted by the phosphors mix to form white light.
Advantages: Excellent color rendering, uniform light color, no risk of "blue light leakage" as in blue chip solutions.
Disadvantages: Greatest energy conversion loss (large energy gap from UV to visible light), lowest efficiency, highest cost. Currently used mainly in professional fields with extremely high color quality requirements, such as museum lighting and photography.

III. Key Impacts on LED Performance

Phosphors directly determine several core performance aspects of white LEDs:

Color temperature: By adjusting the chemical composition, proportion, and particle size of phosphors, the color temperature of white light can be precisely controlled, from warm white (2700K-3000K, yellowish) to cool white (5000K-6500K, bluish).
Color Rendering Index: By adding red phosphors to YAG phosphors, the red portion of the spectrum can be supplemented, significantly improving CRI and making illuminated objects appear more realistic.
Luminous efficacy: The conversion efficiency of phosphors is a key factor in determining the overall luminous efficacy of LEDs. High-efficiency phosphors can convert more blue or UV light into visible light, reducing heat loss.
Lifespan and stability: Phosphors may age and degrade under long-term high temperature and strong light exposure, leading to decreased light output and color shift (e.g., becoming bluer). Therefore, developing heat-resistant, stable phosphors is a research focus.

IV. Common Phosphor Material Types

Silicates: Used in early stages, moderate stability.
Aluminates (such as YAG): Currently the most mainstream yellow phosphor, high efficiency, good stability.
Nitrides/Oxynitrides: Usually red or green phosphors, excellent chemical stability, heat resistance, key materials for enhancing high-CRI LED performance, but higher cost.
Fluorides: Emerging narrow-band emission phosphors (such as KSF red phosphor), can significantly enhance the color gamut of displays, widely used in high-end LCD backlighting.

Conclusion

The application of phosphors in LEDs is a crucial technology that successfully converts efficient blue LEDs into usable white light, launching the solid-state lighting revolution. Phosphors are not just "color creators" but also regulators of LED light quality (color temperature, color rendering). As demands for light quality continue to increase, ongoing innovation in phosphor technology remains one of the key driving forces in LED industry development.

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