Why is the light blue?
White light consists of all the colors of the visible light spectrum, a fact that is known since the days of Isaac Newton. Lightemitting diodes radiate almost monochromatic, i.e. single colored, light. The emitted frequency spectrum of light can be influenced by selecting suitable semiconductor material, but there are only two options for producing white light. You can reverse Newton’s famous prism experiment and “mix” white light by combining the light of three different-colored LEDs.
Unfortunately, this method is rather inefficient. The second option is to coat blue diodes with a yellowish fluorescent layer of doped yttrium aluminum garnet powder (Y3Al5O12). The LED’s blue light and the wideband, yellow-orange-red light of the fluorescent layer complement one another and create a bluish kind of white. Blue diodes are among the most effective LEDs, so it seems only logical to use blue diodes for generating white light. Therefore, our flashlights radiate super-bright light with a bluish white color that is reminiscent of the sun, not of dim candlelight.