Cool white LEDs mix with warm white LEDs to adjust color temperature changes
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Color temperature and color tolerance calculation
1.1 Color temperature calculation
Color temperature is a basic parameter describing the luminescence characteristics of a light source and is an important indicator in color measurement. For light sources other than incandescent lamps, the color of the light does not necessarily fall on the absolute black body trajectory accurately on the chromaticity diagram. 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. The calculation methods of correlated color temperature mainly include triangular interpolation method, black body trajectory method, simulated black body trajectory arc method and McCamy approximation formula method. The McCamy approximation formula is the simplest and easy to operate.
In 1992, McCamy proposed a direct calculation of the correlation color temperature (Tc) from the color coordinates x, y: 
1.2 Calculation of color tolerance
The difference between the spectrum emitted by the source and the standard spectrum is the color tolerance.
According to the color temperature of the light source to be tested, the reference illumination body is selected. When the correlated color temperature of the light source to be tested is lower than 5000K, the reference illumination body should be the spectral power distribution of the Planck radiator; when the correlated color temperature of the light source to be tested is higher than 5000K, the reference illumination The body should be the spectral power distribution of daylight in different phases (standard illuminator D). The difference in chromaticity between the source to be measured (chromaticity coordinates uk, vk) and the reference illuminant (chromaticity coordinates ur, vr):
Both color temperature white LEDs use the same amount of 0.1W low power chip-type phosphor coated LED, and the experimental LED uses the same bracket, InGaN-based blue LED chip, the main wavelength is 440 ~ 460nm, the phosphor is different ratio YAG The yellow phosphor is mixed with the red phosphor. White LEDs are affected by multiple factors, in which chromatic stability and uniformity and heat dissipation conditions have a greater impact on LED performance. It is found that the current change has the greatest influence on the green light spectrum and the least on the white light spectrum. The main wavelength of the blue LED becomes shorter as the driving current increases, that is, the blue color is deepened. As shown in FIG. 1, the driving current changes, and the color coordinate of the white LED changes accordingly, that is, the color temperature shifts toward the high color temperature direction.
As can be seen from Fig. 2 and Fig. 3, regardless of the cool white LED or the warm white LED, the color coordinates are blue-shifted as the driving current increases, that is, the color temperature increases.
In order to continuously adjust the mixed color temperature from 2700K to 6500K, it is necessary to know the relationship between the two LEDs simultaneously adjusting the current and the color temperature. According to Section 2, the following ideas can be drawn: In order to gradually change the color temperature from warm color to cool color, the warm white LED can be driven to the maximum allowable current, and the cool white LED can be driven to the minimum illuminable current, and then gradually reduced. The warm white LED drives the current while gradually increasing the cold white LED drive current.
It can be seen from the data in the table below that the color temperature of the two color LEDs mixed at the maximum current is positive white light, and the warm white LED current is gradually reduced from the maximum to the minimum, while the cool white LED is from the minimum lighting current to the maximum current, The mixed color temperature gradually increased from 2700K to 6500K, thus verifying the original idea. The color temperature adjustment curve is shown in Figure 4.
It can be seen from Fig. 5 that the color tolerance of the color temperature from 2700K to 6500K color coordinates is less than 7 (according to the performance requirements of GB/T29294-2012 LED downlight , the color tolerance should be less than 6).
summary
In this paper, by analyzing the relationship between the driving current and color temperature of the same number of phosphor-coated warm white LEDs and cool white LEDs, the relationship between the driving current ratio of the two LEDs and the mixed color temperature is obtained, with the proportion of warm white light. The larger the color temperature moves toward the warm color temperature direction, and the other the color temperature direction moves. In the adjustment process, the interval adjustment can not realize the stepless adjustment of the color temperature. According to the above test results, the premise of the stepless adjustment is the non-equal interval adjustment of the current, but The practical application value of this point is not great, because the sensitivity of the human eye to color temperature changes is decreasing as the color temperature increases. It can be seen from Fig. 4 that at the approximate 1:1.2 point of the driving current of the cool white light and the warm white light, it is a more obvious inflection point. After this point, the slope of the color temperature change curve is decreased at equal intervals, that is, the change slows down, before this point The slope of the isochronous adjustment color temperature curve can be considered to be approximately the same, but it is also a tendency to slowly decrease (ie, the requirement to achieve stepless regulation is that the adjustment current interval needs to be gradually increased, but further tests are needed to determine the specific value). If the color temperature adjustment section is defined according to the relevant provisions of the initial correlated color temperature requirements in the GB/T29294-2012 standard, a comprehensive conclusion can be drawn from Figure 4: When the adjustment current interval value is set to 20 mA, the standard requirement can be met, that is, the color temperature is realized from 2700K to 6500K eight-speed continuous adjustment.
Detailed analysis of white light LED illumination color temperature adjustment method
The method of adjusting the color temperature of white LED lighting is as follows: (1) The white LED is mixed with the monochromatic R/G/B LED, and the driving current of each channel is adjusted to adjust the color temperature. The advantage of this method is that the spectrum is wider and the color rendering index is higher. However, due to the low efficiency of monochromatic light excitation, the overall light efficiency is not high, and the wavelength of each monochromatic light changes with time, the overall color temperature is not easy to control. Therefore, this method is not widely used; (2) The commonly used method in the industry is to mix the white light LED (2700K) and the cool white LED (6500K) to adjust the color temperature, but the main wavelength of the blue LED chip of the warm white LED and the cool white LED Different from the efficiency of the excited phosphor, to adjust the color temperature from 2700K to 6500K, and the color tolerance is less than 7, the following points must be considered: the relationship between the warm white LED and the driving current and the color temperature; the cold white LED with the driving current and the color temperature. Relationship; and the mixed color coordinate values ​​when the cool white LED and the warm white LED respectively adjust the driving current.