- White LED Definition: A white LED is defined as a lighting technology that uses various methods to produce white light from LEDs, now widely used in multiple lighting applications.
- Wavelength Conversion: This technique involves converting LED radiation to white light, commonly using blue LED with yellow phosphor or multiple phosphors.
- Color Mixing: Involves combining LEDs of different colors, like red, blue, and green, to produce white light, offering higher efficiency as it avoids energy loss in conversion.
- Homo-epitaxial ZnSe Technology: Developed by Sumitomo and Procomp, this method grows an epitaxial blue LED layer on a ZnSe substrate to produce white light.
- Applications of White LEDs: Used for general lighting, indicators, and back-lighting in displays, with potential for broader uses as technology improves.
White Light Emitting Diodes, or White LEDs, produce a spectrum that the eye perceives as white. They are used in indoor lighting, street lighting, flood lighting, displays, vehicles and indicators. Their efficiency, compact size and controllability have made them the main technology for many lighting applications.
An individual LED normally emits light over a narrow wavelength range rather than a complete white spectrum. White light is commonly made by phosphor conversion, by mixing several coloured LED outputs, or by combining both methods. A ZnSe design discussed later is a separate, historical research approach.
Wavelength Conversion
Wavelength conversion uses luminescent material to absorb some light from a blue, violet or ultraviolet LED and emit it at longer wavelengths. The converted light combines with any unabsorbed pump light to form white. Phosphors are the established converters, while quantum dots are another class of wavelength-selective converter.
Blue LED and Yellow Phosphor
A common design uses a blue LED to excite a yellow-emitting phosphor, often cerium-doped yttrium aluminium garnet (YAG). Part of the blue light passes through, while the phosphor emits a broader band at longer wavelengths. The combined spectrum appears white. This simple design can provide high efficacy, although phosphor composition and package design affect its colour appearance and colour fidelity.
Blue LED and Several Phosphors
A blue LED can also excite several phosphors whose emissions cover different parts of the visible spectrum. Adding red- and green-emitting components can broaden and shape the spectrum, which may improve colour fidelity or produce a chosen CRI and CCT. These benefits involve trade-offs in material cost, optical loss and conversion efficiency, so performance depends on the complete LED package.
Ultraviolet LED with RGB Phosphors
A violet or ultraviolet LED can excite phosphors that emit red, green and blue light. The pump radiation itself contributes little or no visible light, so the phosphor mixture must provide the required visible spectrum. This approach can support broad spectral coverage, but conversion losses, ultraviolet containment and phosphor stability are important design factors.
Blue LED and Quantum Dots
Quantum dots are semiconductor nanocrystals whose emission wavelength depends strongly on their size and composition. In a converted white-light source, blue light can excite quantum dots selected to emit green, red or other longer-wavelength light. Their narrow, tunable emission can help shape the spectrum, but practical designs must also address efficiency, heat, lifetime and material safety. Quantum dots may use cadmium compounds or cadmium-free compositions; they do not have one fixed size or a fixed number of atom pairs.
Colour Mixing
Colour-mixed systems combine the controlled output of several LED channels. Red, green and blue channels can create white light and a range of other colours; additional amber, white or other channels can improve spectral control. A four-channel example uses RED, BLUE, GREEN, and YELLOW LEDs. Direct emitters avoid the wavelength-conversion loss of a phosphor, but the system is not lossless. Individual LED efficiencies, drivers, mixing optics, thermal conditions and the chosen spectrum all affect overall efficacy. Full-colour tuning normally uses at least three independently controlled colours.
Homo-epitaxial ZnSe
Homo-epitaxial zinc selenide (ZnSe) white LEDs were investigated as an alternative to phosphor conversion. Sumitomo Electric and related development work described a device grown on a conductive ZnSe substrate. This section records that research design rather than a main architecture used in present-day general lighting.
In the reported device, a ZnCdSe active layer emitted blue-green light and also excited a deep-level yellow emission in the ZnSe substrate. Mixing these emissions produced light that appeared white. Sumitomo reported operation at 2.75 V, an optical output of 4.25 mW at 20 mA and a room-temperature life longer than 10,000 hours. Those figures describe the reviewed experimental device and should not be treated as ratings for modern commercial white LEDs.
Modern white LED products are primarily phosphor-converted, colour-mixed or hybrid systems. Product suitability depends on the complete lamp or luminaire, including its driver, optics, heat management, colour stability and tested useful life.
