Since the advent of LEDs, people have made great progress in full-color display through continuous exploration of new luminescent materials. People have been able to fabricate LEDs of different wavelengths based on different luminescent material systems, especially in the visible range (380 nm to 780 nm). The realization of LEDs of different wavelengths opens up the prospect of LED application. At present, due to the importance of spectrally tunable light sources, spectral matching studies using LED light sources of different wavelengths have obtained considerable childishness.

Although white LEDs with different color temperatures are widely used, the application prospects are quite good, but for some special fields, such as biology, different kinds of plants need their specific spectral distribution, and LEDs with a certain color temperature cannot satisfy. Plant growth needs. For similar problems, the mainstream solution is to use the principle of color mixing to achieve an ideal spectrally tunable light source, and use different wavelengths of LEDs to achieve some common special spectral distributions by mixing colors, such as D65 daylight spectrum, bromine tungsten lamp spectrum, Solar spectrum and 5500K ideal blackbody spectrum. However, the current spectral matching research of LED light source is mainly focused on the algorithm research of single spectral matching, ignoring its practicability.

The LED simulation of the solar spectrum in this paper is to simulate the solar spectrum at different times of the day through 7 fixed-color LEDs. At different times, the light intensity can be adjusted to meet different color temperature requirements. This overall simulation makes full use of the small size of the LED, and the twist is integrated into an LED module, and realizes the complete simulation of the sun sunrise and sunset from the morning to the evening, providing new non-visual applications. The lighting may be.

LED spectrum matching principle

1.1 Spectral distribution curves of different wavelength LEDs

The light of a single-color LED is not a single wavelength, and its wavelength is generally distributed as shown in Fig. 1. The light intensity of the wavelength A0 is the largest, which is called the peak wavelength. The spectral half-width A-in represents the spectral purity of the LED, that is, the wavelength interval corresponding to one of the peak intensity, and the half-height width reflects the spectral line width, indicating the spectral purity of the LED.

It can be seen from Fig. 1 that the spectral distribution of the monochromatic LED is determined by two key parameters, namely the peak wavelength and the half-height width, so the expression of the spectral distribution of the monochromatic LED necessarily includes the peak wavelength and the half-width. The parameters, but how the monochromatic LED spectral distribution should be expressed, so that the expression is closest to the actual spectral distribution of the monochromatic LED. At present, the Gaussian type is mainly used to express the spectral distribution of a single-color LED. Compared with the actual spectral distribution of monochromatic LEDs, there is still a certain fitting error in the original Gaussian distribution. To this end, the original Gaussian distribution is modified such that the fitting error between the improved Gaussian distribution and the actual spectral distribution is as small as possible. In all of the improved Gaussian distribution models, Equation 1 fits well with the actual spectral distribution of the monochromatic LED. Figure 2 shows the improved Gaussian distribution model and the actual spectral distribution, respectively. It can be seen from Fig. 2 that the fit between the improved Gaussian distribution model and the actual spectral distribution is higher in the case of different colors. Therefore, this research adopts the improved Gaussian distribution model shown in equation (1) as an expression of the spectral distribution of the monochromatic LED.

Considering that to obtain a composite wide-band spectrum, LEDs with different wavelengths are required to work together, so it is necessary to use LED light sources of different wavelengths to match the solar spectrum at several typical times of the day, and each monochromatic LED can be realized. Adjusted separately, according to the principle of spectral superposition, the basic mathematical model of spectral synthesis of a variety of monochromatic LEDs can be obtained as shown in equation (2).

The SMIX (in) in equation (2) is the actual spectral power value after mixing the spectra of various monochromatic LEDs. The color temperature and color rendering index of 7 LEDs under different light intensities can be calculated by equation (2), and then different. The matching of the solar spectrum at the moment also provides the original fitting equation for the subsequent MATLAB fitting.

1.2 Solar spectrum at different times

The color temperature changes of the sun at different times of the day are shown in Table 1.

Considering the practicality of using seven LEDs to simulate the solar spectrum, it is considered that the color temperature of the solar spectrum after sunrise is basically corresponding to that before sunset. Five typical color temperatures were selected from Table 1 as reference spectra, and the color temperatures were 2500K, 3500K, 4000K, 5000K, and 6500K, respectively. The corresponding solar spectrum for each color temperature value needs to be calculated by fitting its corresponding typical solar relative spectral power.

The typical daylight and standard illuminators specified by the CIE are represented by a typical daylight chromaticity trajectory above the Planckian trajectory on the CIE1931U, y) chromaticity diagram. This trajectory is based on the distribution of many measured daylight chromaticity points on the CIE1931 chromaticity diagram, which includes chromaticity points of 4000 to 40,000K typical daylight. Therefore, the typical solar color temperature of 2500K and 3500K is unable to calculate the typical solar relative spectral power distribution, and it is impossible to fit the spectrum with 7 LEDs. The three solar color temperature conditions of 4000K, 5000K and 6500K can be obtained according to the relative power distribution formula (3) of typical sunlight.

In equation (3), S (in) is a correlated color temperature typical daylight wavelength color temperature relative spectral power; S. (A) is the average relative spectral distribution of wavelengths with typical daylight. \(A), S2(A) is the first feature vector of the wavelength A, and the second feature vector 2 is the multiplier of the first feature vector and the second feature vector, respectively.

LED analog solar spectrum

In this model, the selected seven LEDs have center wavelengths of 450 nm, 500 nm, 550 nm, 600 nm, 650 nm, 700 nm, and 750 nm, respectively. The half widths of the LEDs of different center wavelengths obtained by the experimental test are 35 nm, 35 nm, 40 nm, 20 nm, 20 nm, 20 nm, and 20 nm, respectively. On the basis of constructing the Gaussian model mentioned above, the light intensity of each LED in different cases is changed to obtain the corresponding color temperature value, and a higher color rendering index is adjusted. The normalized light intensity values ​​of the seven LEDs corresponding to the solar spectrum at different time periods are shown in Table 2-6.

Among them, the three solar color temperature conditions of 4000K, 5000K and 6500K can be obtained according to the relative power distribution formula of typical daylight, and the standard spectrum is obtained. In these three cases, the fitting curves of the 7 LED simulated sunlight and its corresponding standard spectrum are shown in Fig. 3, Fig. 4 and Fig. 5.

to sum up

It can be seen from the above simulation results that the use of 7 LEDs for fitting can basically satisfy the color temperature change of sunlight within one day and ensure a high color rendering index. However, from the fitting curve of the typical daylight spectrum with 4000K, 5000K, and 6500K, since the number of LEDs is small, it does not fit perfectly with the standard spectral curve, but in terms of trend and spectrum, it is consistent. It is possible to basically simulate the situation of sunlight at different times. In fact, the more the number of LEDs selected to fit the typical daylight spectrum, the better the fit curve can be obtained, but this adds to the complexity of high integration. Therefore, from a comprehensive perspective, it is currently the most feasible method to integrate 7 LEDs into one module to realize integrated luminaires for simulating sunlight.

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