Jan 11, 2018: Koito Manufacturing Co Ltd, Tokyo Institute of Technology and Nagoya University have jointly developed a new material—a unique red phosphor for white LEDs, which does not cause colour shift. This new material is called Fluorine Oxygen Ligand Phosphor (FOLP:Eu2+).
The light spectrum of white LEDs is achieved by the conversion of a ultra-violet or violet/bluish light source which passes through a mix of phosphors, that absorb the light and convert it into different wavelengths.
The white light of such phosphor-based LEDs is difficult to adjust bacause the red phosphors that have been used till now can re-absorb some of the shorter wavelengths (blue, green, yellow) that are produced by other phosphors from the mix. As a result, this causes a red shift, altering the white and the colour rendering capability of the white LED.
Usually, white LEDs are short of red components that make it idifficult to obtain enough color rendering property. Therefore, red phosphors are added to improve color rendering property. However, existing red phosphors used for white LEDs also convert a wavelength range from blue to yellow to red light, causing color drift.
The new research has developed al red phosphor, FOLP:Eu2+ (short for K2CaPO4F:Eu2+) which has a crystal structure in which fluorine and oxygen ions are located around a light-emitting element.
The phosphor absorbs light within a wavelength range of 315-420nm, from ultraviolet to violet, to emit in the red with a high conversion efficiency while hardly absorbing visible light in the range from blue to yellow, which means what was once mixed with blue, green and yellow phosphors to obtain white light does not shift colour.
The newly-developed FOLP:Eu2+ absorbs light with a wavelength range of 315-420nm (from ultraviolet to violet) and generates red color with a high conversion efficiency. It does not absorb visible light from blue to yellow. So, when FOLP:Eu2+ is mixed with phosphors of other colors such as blue, green and yellow to obtain white light, it does not reabsorb those colors. As a result, it becomes possible to obtain white light having a stable chromaticity without causing color drift.