Which LEDs to select, in order to generate white light?

Diode should be chosen in such a way that the color temperature or Correlated Color Temperature-​CCT of the emitted through the diode radiation would be located in the tolerance area for the light source of 2700 K, 3000 K, 3500 K or 4000 K color temperature. It is important to take a look also on the quality of color rendering by iluminated with white color objects. If in order to determine the quality of color rendering Ra indicator is used, it would be best that it amounted for more than 80. The color temperature of the emitted light radiation or the quality of color rendering of illuminated objects depend on the quality and technology in which LED was made. The basic information on this topic is presented below.

The white light is a mixture of radiations in a range of 380nm to 780nm. For numerical and graphical display depending on the mixing radiations, International Commission on Illumination CIE in 1931 created a triangle of x,y colors shown in Figure 11.1 In this triangle each color has an assigned coordinations of x,y position of its chromaticity. While choosing two colors with different positions of chromaticity points and connecting these points with each other, we can visualize each color of light due to the mixture of suitable proportions of these 2 monochrome radiations. Analogically, 3 colors may be mixtured so the resulting color would lay in a triagle. Skilfully choosen, more than 3 mixtured radiations allows the increase in the quality of obtained white light. In comparison with the other white light sources, LED can present different tones defined by means of the color temperature (similar to fluorescent lamps). That is why, in order to obtain the similar color it is important to ensure that all LED have an identical color temperature.

From the principle of LED operation it is obvious that it emits radiation in a narrow wavelenght range, so in the visible range the human eye picks it as monochrome.

Using principles valid for colors mixing in order to obtain white LED light, 2 groups of methods were developed.

The first group of methods allows to obtain white light by mixing an appropriate proportions radiation of monochromatic LED. Most frequently, few LED chips producing different color light are being used in one housing. Generally, LED housing includes at least two LED chips (producing blue and yellow light), sometimes three (producing red, blue and green light) or four (producing red, blue, green and yellow light). LED produce white color on the principle of colors mixing have potentially the highest luminous efficiency, however the quality of obtained light (the value of the general color rendering indicator Ra) is low.

The second group of methods using luminescence phenomenon taking place in phosphor that is the conversion of wavelengths (radiation falling on luminofor with some wavelength is being conversed by it to the radiation with longer wavelength in comparison with the one which was falling on the luminofor). There are several technological solutions:

  • LED chip emits blue color light. The part of this radiation falls on the luminofor, which converts it to yellow color. This yellow light mixes up with the remaining part of the blue light and as a result we obtain an impression of white light.
  • LED chip emits blue color light, which falls on several luminofors, each of which converts it to the other color light. These different colors mix up with the remaining directly obtaining from the chip blue color light. As a result, we obtain white color light with the higest overall color rendering indicator Ra in comparison with the method of one luminofor.
  • LED chip produces ultraviolet radiation (UV) falling on three-​lane luminofor (producing three-​lane light: red, green and blue). Different colors of light mix up with each other, giving the white color light of the highest quality (the highest overall color rendering index Ra).