GaN-based mini-LED matrix applied to multi-functional front light

Reflectors are typically used to project low beam or high beam rather than project adaptive headlight, as the reflector is used for non-imaging projection. A design with light field technology used a single reflector to perform low beam and high beam with two LED modules where three LED arrays are composed in each module. Due to the constraint of the LED module used, the lighting area/range is not adjustable. This property cannot provide achievable illumination based on different pavement conditions. Therefore, the design in this article should be divided into two parts, namely the light source and the reflector.

As for the light source, to ensure the maximum feasibility of the illumination range and save the cost, we select the LED matrix with dimensions of 500μm × 500μm, which is the maximum size classified as mini-LED.27. To reduce cost, all emitters are blue GaN arrays so that array bonding, phosphor coating, and electrical drive are consistent. To make the light source as narrow as possible and the adjustment range as wide as possible, we design a light source with a 5×6 matrix as shown in Fig. 1. To simplify the technical instruction, we choose K-mark regulation as the reference of the headlamp28. In the regulations, there are several checkpoints for low beam and high beam as shown in Fig. 2, where point A is the main light point for dipped headlights, and the ratio between the illuminance at point A (EA) and at the HV point in the dark area (must be less than 2 lx) is defined as the contrast, which must be greater than 20. In high beam, HV is the main light point and the illuminance at this point must exceed 50 lux.

Figure 1

The detailed layout of the mini-LED matrix.

Figure 2
Figure 2

The K-mark settlement for (a) dipped headlights and (b) the high beams.

In the previous study, the LED matrix with an area of ​​1 mm2 is powerful enough to support low beam or high beam29. Thus, at least four LED matrices in the mini LED matrix must be used to support low beam or high beam. In the design, the LED dies in the lower two rows are for low beam and the upper three rows are for high beam.

The optical design of the reflector should support low beam and high beam with the designed mini LED array. We define the matrices numbered C4/D4/C5/D5 as the main light source for the low beams (called group L) and B2/C2/D2/E2 for the high beams (called group H). The reflector has been divided into 68 segments shown in Fig. 3. The optical design follows the rule of light field technology30. Since the extent of the light source received by each segment is different, the specific segments (called HL segments) with a smaller extent are used to project a dipped beam with a high-contrast cut-off line, and the others segments are projected downwards for floor illumination. The LED arrays responsible for the high beam are spaced away from the reflector, so that the rays reflected from the HL segments propagate upwards to form the high beam. So the HL segments are for both low beam and high beam, and the difference is made by the placement of the LED arrays. Also, the mini LED array contains more LED arrays along the side extension other than L-group and H-group. The result is that the light pattern will be wider when these LED arrays are on, and this property could be used to extend the lighting area and provide wider vision.

figure 3
figure 3

The geometry of the 68-segment reflector.

In the design, four arrays of the mini-LED array as a unit light source were used to provide sufficient flux for the K-mark low beam light pattern. The dimensions of each LED chip were 500 μm × 500 μm. The output flux of each miniLED was set at 50 lm, which was the average flux of each matrix obtained by measurement. The simulation was performed using ASAP for Monte Carlo ray tracing31. The upper surface of each LED chip has been set to the Lambertian surface due to the strong diffusion of the phosphor layer32.33. The reflector is shown in Figure 3, where the height, width, and depth dimensions were 20 mm, 44 mm, and 19.76 mm, respectively, and the reflectivity was set at 80%. Figure 4 shows the simulation of the patterns lit with different LED matrices at 10 m from the reflector. The observation plane covers an area of ​​4 m × 2.5 m. The results of the simulation are shown in Fig. 4. Figure 4a shows the passing beam by 2 × 2 grating. The illuminance at point A of the regulation was 84.7 lx and the illuminance at point HV is less than 2 lx. Figure 4b shows the 1×4 grating driving beam, which was used to extend the light pattern laterally. The illuminance at the HV point in the regulations was 90.4 lx, which was higher than 50 lx as requested in the regulations. All light patterns have been reviewed and met the requirements of K-mark regulations.

Figure 4
number 4

The lit LED dies (yellow) in different places and the corresponding light patterns, where (a) corresponds to dipped headlights, and (b) is for high beam.

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