The color gamut known as BT.2020 is one of the Holy Grails of video displays. It encompasses more visible colors than the digital-cinema gamut called P3 and way more than the HDTV gamut called BT.709. However, reproducing it is notoriously difficult because the red, green, and blue color points exhibit extremely narrow spectral peaks.
Most efforts to achieve BT.2020 involve lasers or quantum dots, which emit light in very narrow spectral bands. But what if it could be achieved using conventional LEDs? That’s the idea behind a phosphor called TriGain by GE, which the company demonstrated at DisplayWeek 2017. TriGain produces a much narrower and more intense red peak than most red phosphors used in LEDs. GE calls this spectrum RadiantRed.
To understand how it works, we need to take a close look at the structure of an LED.
The light from an LED originates in a semiconductor that sits within a reflective cavity. The semiconductor emits light when a suitable voltage is applied across the anode and cathode leads. In an LED used in the backlight of an LCD TV, the semiconductor is typically doped gallium nitride (GaN) or indium gallium nitride (InGaN), and the light is blue. Within some such LEDs, the semiconductor die also contains a phosphor material called Ce:YAG (cerium-doped yttrium-aluminum-garnet) that absorbs some of the blue light and emits yellow light. This yellow light combines with the blue light that is not absorbed to create white light.
Ce:YAG is very stable over long periods of time, but the red portion of its emitted spectrum is very broad and low-intensity. This makes it difficult to eke out enough red to paint a full-color video picture.
The blue light from a Ce:YAG LED has a nice, high, narrow peak from the LED itself, but the yellow light from the phosphor exhibits a very broad spectrum, and the red portion is quite low-intensity. (Graphic by Deglr6328 at the English-language Wikipedia)
One solution is to use separate phosphors that emit green and red light when exposed to blue light. These days, silicon-based nitride compounds are commonly used as red phosphors, but their spectrum is still fairly broad and low-intensity.
GE’s TriGain red phosphor is based on potassium fluorosilicate (PFS, sometimes abbreviated KSF after its chemical formula, K2SiF6) that has been doped with Mn4+ manganese ions. The resulting red emission exhibits a much narrower, more intense red peak than other phosphors.
GE’s TriGain RadiantRed phosphor emits a much narrower, more intense red than other common red phosphors. The strongest peak is centered at a wavelength of 631 nm, which is the ideal red point for the BT.2020 color gamut. The green peak is provided by Beta SiAlON:Eu2+ phosphor, which is mixed with the red phosphor in the LED. (Image courtesy of General Electric Company)
In this CIE diagram, you can see that the RadiantRed red point is right where it needs to be for BT.2020. (Image courtesy of General Electric Company)
The term “TriGain” derives from improvements in three key metrics: CRI (color-rendering index), LPW (lumens per watt), and R9 (which represents red color rendering). Normally, increasing CRI lowers LPW and vice versa, but TriGain manages to increase all three.
This is seriously “inside baseball” stuff; GE is at least two degrees of separation from the TV manufacturers we all know. GE sells its phosphors to LED makers, who then sell LEDs to TV companies. But the development of TriGain RadiantRed technology is important to those of us who want TVs to display the widest possible color gamut. Of course, the red color filter needs to be tuned to the same wavelength in order to see that deep, rich red in video images, but GE’s TriGain RadiantRed provides a great starting place.