Led color temperature is the ratio of changing different light. Increase red light, warmer color temperature, increase blue light, and cool color temperature. Adjust the brightness, change the current flowing through the LED, the current is larger, it will be brighter. On the contrary, it is darker. The regulation of the current is achieved by changing the PWM. The so-called PWM is the pulse width adjustment. The method of pulse width adjustment, the most fundamental is to change the value of the resistance and capacitance value that determines its width. If the product of RC is large, the width will be larger. The specifics should be discussed in conjunction with the circuit diagram.
1 color temperature
The color temperature of the light source is an ideal model, also called a complete radiator, by comparing its color and theoretical thermal blackbody radiator (abbreviated as blackbody, the absorption rate of radiant energy at any temperature is equal to 1 at any temperature). ) to determine. The spectrum emitted by the heat radiation source is continuous and smooth. For the black body, the temperature is different and the color is different. There is a unique correspondence between the color of the black body and the temperature. When expressing the color of a light source, the color of the light source is often compared with the color of the black body. If the color of the light source is the same as the color of the black body at a certain temperature, the color of the light source is regarded as a black body. The color at this temperature is called "temperature color", referred to as "warm color". Obviously, "warm color" refers to "color", which is the color of a black body at a certain temperature. However, due to long-standing conventions, this concept is now commonly referred to as "color temperature."
For incandescent lamps and other thermal radiation sources, because their spectral distribution is close to that of blackbody, their chromaticity coordinate points are basically on the black body trajectory, and the concept of visible color temperature can properly describe the color of incandescent lamps. However, for light other than incandescent lamps, the spectral distribution is far from the blackbody, and their relative spectral power distribution at temperature T
The determined chromaticity coordinates do not necessarily fall on the black body temperature trajectory of the chromaticity diagram accurately. Therefore, the color temperature of the light source can only be determined by the color closest to the black body trajectory, which is called the correlated color temperature (CCT). ).
White LEDs are the only way to achieve semiconductor lighting. A white LED is not a monochromatic light, and there is no white light in the spectrum of visible light. According to people's research on visible light, white light that can be seen by human eyes can be produced by mixing two or more kinds of light. There are currently three methods for obtaining white LED illumination sources.
(1) Blue LED+ different color phosphors: The white LED developed by Nichia Corporation is a yellow YAG phosphor coated by blue LEDs. The yellow light generated by the phosphor is excited and the blue light originally excited. Complementary and produce white light, as shown in Figure (a). It is also possible to obtain white light by combining the green light and the red light emitted by the blue LED chip with the phosphor, and the color rendering property is good, but the phosphor conversion efficiency used in this method is low, especially the red phosphor. At present, the white LED packaging technology using blue LEDs combined with yellow YAG phosphors is relatively mature, but the problem of uniformity, high color temperature and poor color rendering index cannot be solved.
(2) Ultraviolet or violet LED+RGB phosphor: The principle of synthesizing white light with ultraviolet or violet (300-400nm) LED and RGB phosphor is similar to that of fluorescent lamp, but superior to fluorescent lamp performance, purple LED conversion The coefficient can reach 0.8, and the quantum conversion efficiency of each color phosphor can reach 0.9. (b) The method of using a violet LED to excite three primary or multicolor phosphors to produce polychromatic light and then mixing it into white light has better color rendering, but there are also problems. Red phosphors and green phosphors are mostly Sulfide, poor luminescence stability and large light decay.
(3) RGB three primary color LEDs form white light: This method combines three kinds of green, red and blue LED chips, as shown in Figure (c), simultaneously energizes, and then emits green, red and blue light. The ratio is mixed into white light. The ratio of green, red and blue is usually 6:3:1. The method of directly packaging into white LEDs by using RGB three primary color LEDs has the best comprehensive performance of white light, and the white light lumen efficiency is also high under the premise of high color rendering index. Since the color temperature and color rendering index required for the synthesis of white light are different, the lumen efficiency requirements for the LEDs of the synthetic white light are also different. However, the main technical difficulty of this method is to improve the electro-optical conversion efficiency of green LEDs and reduce costs.
3 White LED color temperature dynamic adjustable principle and implementation method3.1 Principle:
Different color temperature white LEDs are mixed into a bundle of white light, and the luminous flux of the mixed white light is the sum of the luminous fluxes of the white LEDs of different color temperatures. The color temperature spectral power distribution curve of the mixed white light is a spectral power distribution curve in which a spectral power distribution curve of different color temperature white LEDs is superimposed and mixed to form a new color temperature, thereby determining the color temperature value of the mixed white light. By changing the driving currents of different color temperatures, thereby changing the luminous flux of different color temperatures, and 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. Get dynamically adjustable white light.
3.2 Implementation method:
At present, methods for realizing white LEDs with adjustable color temperature and high color rendering are:
(1) Using a multi-chip plus phosphor, such as a module consisting of at least two blue chips, a yellow chip, and a green phosphor and a red phosphor, to produce a white light with a color temperature adjustable high color rendering index (Ra 80) LED lights. However, the excitation efficiency of the red phosphor excited by blue light is low, resulting in low brightness of the LED lamp and poor practicability.
(2) Using different color LED combinations, such as white light plus red and blue LED combination, by controlling the driving currents of white, red and blue LEDs respectively, to realize adjustable white light with color rendering index greater than 90 in different color temperature ranges, but the driving circuit Complex and costly.
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