Light emitting diode (LED) features high luminous efficiency, adjustable luminance and color temperature, fast response, and long life, and it is widely applied in various fields. During the operation of LED light sources, the temperature change exerts an impact on its emission spectrum and energy efficiency. In previous studies, the designs of heat dissipation structures are mainly adopted to reduce the impact of temperature on LED luminous efficiency and performance. Some researchers have also employed other methods to study the impact. A closed-loop negative feedback system is designed to reduce the influence of temperature on the spectrum of the light source, and the temperature spectrum compensation method of RGBY four-color LED light source is proposed based on visual and non-visual effects. The white LED for lighting is mostly obtained by blue excited yttrium aluminum garnet phosphor. However, its color temperature cannot be adjusted and color rendering is poor. A mixed white LED cluster can achieve dynamic adjustment of color temperature and high color rendering index. Therefore, the optimization method of multi-channel LED white light with tunable color temperature and high color rendering index has been extensively studied. However, the influence of the thermal effect on the optical parameters of a mixed white LED is rarely considered. Thus, this paper studies the impact of temperature on the spectrum of red/green/blue/warm-white (R/G/B/WW) four-color LED and obtains the spectral model with temperature change. Additionally, the spectral optimization model of R/G/B/WW LED mixed white light is built by the adaptive differential evolution (JADE) algorithm based on the temperature and duty ratio, and it is verified through experiments.

The experimental setup mainly consists of microcontroller (MCU), R/G/B/WW LED light source, integrating sphere, HASS-2000 spectral radiometer, and CL-200 temperature control device.Two R/G/B/WW four-color LED clusters are connected in series. The MCU can generate four different pulse width modulation (PWM) signals with specific duty ratios (D_R, D_G, D_B and D_WW) to achieve four-color LED mixed white light. The HASS-2000 spectral analysis system is adopted to measure the spectral power distribution (SPD) of each LED in the R/G/B/WW LED clusters, and the parameters of mixed white light including the luminous flux, color rendering index, and correlated color temperature. The stability of LED temperature is controlled by the CL-200 temperature control device. The temperature range is from 20 ℃ to 90 ℃ with error margin of ±0.1 ℃, and the temperature interval is 10 ℃. In addition, the driving output current of the R/G/B/WW four-color LED clusters is 350 mA and the driving voltage is 6 V. Eight groups of spectral power distribution curves of each monochrome LED light source with temperature between 20 ℃ and 90 ℃ and the interval of 10 ℃ are measured by experiments when the duty ratio is 1. Three modes of Gaussian, Gauss-Lorentz, and Bigaussianare utilized to study the temperature spectral model of LED light source respectively. Additionally, the spectral model changing with temperature can be obtained.

The four-color LED mixed white light with color temperatures of 3000 K, 5000 K, and 6500 K is employed to verify the accuracy of the temperature spectrum model (Fig. 3). Based on the temperature spectral model, the SPDs at temperatures of 20 ℃, 60 ℃, and 90 ℃ are calculated, respectively. The results show that the SPD of the model with temperature change is approximately the same as the measured results (Fig. 4). According to the temperature spectrum model, the parameters of light source containing illuminance (E_V), color rendering index (Ra), correlation color temperature (T_C), blue light hazard factor (η_B), circadian action factor (C_AF), and considering visual/non-visual effects can be calculated at different temperatures. Meanwhile, the calculation results are compared with the measured parameters of the light source at different temperatures (Table 2). The maximum deviation between the model parameters and measured parameters in the three LED light sources is 3.79%, and the chromaticity difference is less than 5.4×10^-3. The light source spectrum and parameter values can change with the rising temperature (Fig. 5). Finally, the JADE algorithm is adopted to acquire the single channel duty ratio to optimize the light source spectrum and related parameters. At the above three LED light sources, the optimization duty ratio obtained by the JADE algorithm shows a linear relationship with the temperature. Based on the optimization model, the SPDs and parameter values of light source at different temperatures after optimization can be obtained. The optimized SPD is approximately the same as the light source spectrum at the initial temperature (Fig. 6), and the maximum relative error of the parameter values is 2.62% (Table 3).

In this paper, according to PWM dimming technology, the JADE algorithm is leveraged to study the spectrum optimization of R/G/B/WW LED mixed white cluster with temperature change. According to the temperature spectral model, the R/G/B/WW LED mixed white spectra changing with temperature are obtained. Based on the obtained mixed white light spectra at different temperatures, the spectral optimization model that can effectively adjust the spectrum of the light source at different temperatures is built in accordance with the compensation results of the JADE algorithm. After compensation, the measured spectra are basically consistent with those of the optimization model, and the maximum relative error of LED light source parameter values is 2.62%. This method can compensate for parameters of light source caused by temperature, and guide the optimal design of health lighting system. In future studies, the method of obtaining real-time junction temperature of LED chips should be further explored, and the feedback control system should be constructed to realize the dynamical compensation of the light source parameters.