e all know, a combination of red, green and blue lights can produce the impression of white light. So does a combination of blue and yellow lights. Most white LEDs (WLEDs) in production today are based on the technology of blue LEDs.
1. Mixing the light produced by red, green and blue LEDs to achieve the white light.
Blue LEDs are based on the wide band gap semiconductors GaN (gallium nitride) and InGaN (indium gallium nitride). They can be added to existing red and green LEDs to produce the impression of white light, though white LEDs today rarely use this principle. Generally speaking, the proportion of brightness of red, green and blue light should be 3:6:1.
White light can also be achieved by mixing the light produced by red and green LEDs or by mixing the light produced by blue and yellow LEDs. However, this kind of approach to achieve the white light is rarely adopted today for the high cost of manufacturing white LEDs.
2. Utilizing blue InGaN as a semiconductor material to excite yellow phosphors to achieve the white light.
Most white LEDs in production today are modified blue LEDs: GaN-based, InGaN-active-layer LEDs emit blue light of wavelengths between 450 nm and 470 nm. This InGaN-GaN structure is covered with a yellowish phosphor coating usually made of cerium-doped yttrium aluminum garnet (Ce3+:YAG) crystals which have been powdered and bound in a type of viscous adhesive. The LED chip emits blue light, part of which is efficiently converted to a broad spectrum centered at about 580 nm (yellow) by the Ce3+:YAG. The single crystal form of Ce3+:YAG is actually considered a scintillator rather than a phosphor. Since yellow light stimulates the red and green receptors of the eye, the resulting mix of blue and yellow light gives the appearance of white, the resulting shade often called "lunar white". This approach was developed by Nichia and has been used since 1996 for the manufacture of white LEDs. Nichia essentially holds a monopoly on patents of blue LEDs.
The pale yellow emission of the Ce3+:YAG can be tuned by substituting the cerium with other rare earth elements such as terbium and gadolinium and can even be further adjusted by substituting some or all of the aluminum in the YAG with gallium.
3. Utilizing an InGaN chip that emits ultraviolet (UV) light to excite RGB (red, green and blue) phosphors to produce the impression of white light.
White LEDs can also be made by coating near ultraviolet (NUV) emitting LEDs with a mixture of high efficiency europium-based red and blue emitting phosphors plus green emitting copper and aluminum doped zinc sulfide (ZnS:Cu, Al).
According to the three principles of generating white light shown above, white LEDs may be classified into bi-wavelength white LEDs and tri-wavelength white LEDs.
Bi-wavelength white LEDs may be classified into two subcategories, Single Chip (Blue InGaN+YAG) (Blue ZnSe+ Substrate) and Multi Chip (Blue InGaN+ GaP Amber/ Yellow-Green).
Tri-wavelength white LEDs may also be classified into two subcategories, Single Chip（RGB1 Chip LED）（UV Chip+ RGB Phosphors）and Multi Chip（RGB LED 3 Chip）.
Most white LEDs in production today are modified blue LEDs: GaN-based, InGaN-active-layer LEDs emit blue light of wavelengths between 450 nm and 470 nm. This InGaN-GaN structure is covered with a yellowish phosphor coating usually made of cerium-doped yttrium aluminum garnet (Ce3+:YAG) crystals which have been powdered and bound in a type of viscous adhesive. The LED chip emits blue light, part of which is efficiently converted to a broad spectrum centered at about 580 nm (yellow) by the Ce3+:YAG. The single crystal form of Ce3+:YAG is actually considered a scintillator rather than a phosphor. Since yellow light stimulates the red and green receptors of the eye, the resulting mix of blue and yellow light gives the appearance of white, the resulting shade often called "lunar white". This approach was developed by Nichia and has been used since 1996 for the manufacture of white LEDs. Nichia essentially holds a monopoly on patents of blue LEDs.
The pale yellow emission of the Ce3+:YAG can be tuned by substituting the cerium with other rare earth elements such as terbium and gadolinium and can even be further adjusted by substituting some or all of the aluminum in the YAG with gallium.
3. Utilizing an InGaN chip that emits ultraviolet (UV) light to excite RGB (red, green and blue) phosphors to produce the impression of white light.
White LEDs can also be made by coating near ultraviolet (NUV) emitting LEDs with a mixture of high efficiency europium-based red and blue emitting phosphors plus green emitting copper and aluminum doped zinc sulfide (ZnS:Cu, Al).
According to the three principles of generating white light shown above, white LEDs may be classified into bi-wavelength white LEDs and tri-wavelength white LEDs.
Bi-wavelength white LEDs may be classified into two subcategories, Single Chip (Blue InGaN+YAG) (Blue ZnSe+ Substrate) and Multi Chip (Blue InGaN+ GaP Amber/ Yellow-Green).
Tri-wavelength white LEDs may also be classified into two subcategories, Single Chip（RGB1 Chip LED）（UV Chip+ RGB Phosphors）and Multi Chip（RGB LED 3 Chip）.
