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LED color temperature control principle

1.Color temperature

The color temperature of a light source is determined by comparing its color with the theoretical thermal blackbody radiator (referred to as a blackbody, an object whose absorption rate of any wavelength of radiant energy is equal to 1 at any temperature, an ideal model, also called a complete radiator). The spectrum emitted by a thermal radiation source is continuous and smooth, and for black bodies, the color is * different for different temperatures.

There is a unique correspondence between the color of a blackbody's light emission and its temperature. In the expression of the color of a light source, often the color of the light source and the color of the black body luminescence for comparison, if the light source emits the color of light and the black body at a certain temperature of the same color, * the color of the light source as the color of the black body at this temperature, called "temperature color", referred to as " temperature color" for short.

Obviously, "temperature color" refers to the "color", is the color of the black body at a certain temperature. But because of the long-standing convention, the concept is now commonly referred to as "color temperature".

For incandescent lamps and other sources of thermal radiation because their spectral distribution is closer to the black body, so their color coordinate points are basically in the black body trajectory, the concept of color temperature can be seen to properly describe the light color of incandescent lamps. However, for light sources other than incandescent lamps, their spectral distribution is far from that of the blackbody, and their relative spectral power distribution at temperature T

The color coordinates determined by the color product does not necessarily fall accurately on the blackbody temperature trajectory of the color map, so only the color of the light source and the blackbody trajectory * to determine the color temperature of the light source, known as correlated color temperature (correlated color temperature, referred to as CCT).

2. White LED light-emitting principle

White LED is the way to achieve semiconductor lighting. White LED is not a monochromatic light, there is no white light in the visible light spectrum. According to people's research on visible light, the white light that the human eye can see can be generated by a mixture of two or more types of light. Currently there are three methods to obtain white LED lighting source as follows.

(1) Blue LED + phosphor of different colors 

The white LED developed by Nichia is a blue LED excitation coated with yellow YAG phosphor, the phosphor is excited to produce yellow light and the original due to the excitation of the blue light complementary to produce white light, as shown in Figure (a). Can also be obtained through the blue LED chip and phosphor emitted green and red light composite white light, color rendering is better, but this method uses a low phosphor conversion efficiency, especially the red phosphor. At present, the use of blue LED with yellow YAG phosphor white LED packaging technology is more mature, but the problem of uniformity, high color temperature color index is not ideal, the problem is slow to solve.

(2) UV or violet LED + R.G.B phosphor

UV or violet (300 ~ 400nm) LED and R.G.B phosphor to synthesize white light is similar to the principle of fluorescent lamps, but the performance is superior to fluorescent lamps, violet LED conversion factor of up to 0.8, the quantum conversion efficiency of each color phosphor can reach 0.9. (b) shown in this use of violet LED excitation trichromatic or multi-color phosphor to produce multi-color light, and then Mixed into the white light method, better color rendering, but the same problems, red phosphors and green phosphors are mostly sulfide, poor luminous stability, light decay is large.

(3) R.G.B trichromatic LED to form white light 

This method is a combination of green, red and blue LED chips, as shown in Figure (c), while energized, and then the emitted green light, red light, blue light mixed into a certain ratio of white light. The ratio of green, red and blue is usually 6: 3: 1. Using the R.G.B three-color LED directly packaged into a white LED method, white light synthesis performance * good, in the premise of high color rendering index, white light lumen efficiency is also high. Due to the different color temperature and color rendering index required for synthetic white light, the lumen efficiency requirements for each color LED of synthetic white light are also different. However, the main technical challenge of this method is to improve the electro-optical conversion efficiency of green LEDs, as well as to reduce costs.

3. The principle and implementation of white light color temperature adjustable method

3.1 Principle 

Different color temperature white light LEDs mixed into a beam of white light, the luminous flux of mixed white light is the sum of the luminous flux of white light LEDs of different color temperatures. The color temperature spectral power distribution curve of the mixed white light is a superposition of the spectral power distribution curves of different color temperature white light LEDs mixed into a new color temperature spectral power distribution curve, which determines the color temperature value of the mixed white light. By changing the driving current of different color temperatures, thus changing the luminous flux of different color temperatures, and also * changing the spectral power distribution curves of different color temperatures, the new spectral power distribution curves generated by different color temperatures are superimposed and mixed to form a new spectral power distribution curve, and also * obtaining dynamically adjustable white light. 

3.2 Realization method

At present, the methods to achieve color temperature tunable and high color rendering white LED are.

(1) using multiple chips with phosphor

Such as the use of at least two blue chips, a yellow chip, as well as green phosphor and red phosphor module to produce color temperature adjustable high color rendering index (Ra "80) of white LED lights. However, the low excitation efficiency of the blue light to excite the red phosphor, resulting in a low brightness of LED lights, poor practicality.

(2) the use of different color LED combination

Such as the use of white plus red and blue LED combination, by controlling the drive current of white, red and blue LEDs, respectively, to achieve different color temperature range of color rendering index greater than 90 adjustable white light, but the drive circuit is complex, the cost is high.

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