To improve the effect of augmented reality (AR), the head-up display system (HUD) needs a larger field of view (FOV) and a longer projection distance, requiring the image source to have a larger display size, and the resolution must be kept clear and visible under the expanded FOV. The current mainstream technology is still flat-panel display technology, but its development is constrained by the contradictory relationship between brightness and power consumption, the lighting area and module volume. For liquid crystal display (LCD) backlight modules, studies support independent research and development of large-size LCD image source technology and reduce the volume of image source modules, which is of great significance to future AR projection technology. Therefore, based on the fact that improving the interactive effect of AR-HUD requires a larger projection image source, we study and design relevant methods on how to improve the brightness and uniformity of the backlight module and achieve the development of low-cost and large-size image source backlight modules.

Since the illumination distribution of light emitting diode (LED) point light sources is approximately Gaussian, the illumination distribution of a certain point on the illuminated surface after the superposition of multiple point light sources can be solved by referring to the Gaussian mixture model. We employ a nonlinear constraint algorithm to quickly solve the optimal uniform light distance of the array light source on a certain target surface, which greatly saves time for spacing optimization. According to the principle of total reflection, we propose a turning light stick based on the traditional conical light stick structure. As the reflection number of the light beam in the front conical stick increases, the angle with the optical axis becomes increasingly smaller, which can reduce the numerical aperture of the light beam. To further reduce the volume of the backlight module, we propose an asymmetric turning light stick structure, and the Monte Carlo grid division method is adopted to optimize the energy distribution of the turning light stick on the target surface. During the optimization, a Fresnel lens with a cylindrical front surface and a spherical rear surface microstructure is designed to make the power reach the peak within a given angular aperture and maximize the light intensity.

We utilize the maximum uniform light interval solution method of the array light source to quickly solve the 2×12 rectangular LED array arrangement (Fig. 2). After the design is completed, the backlight intensity toe distribution is first evaluated. The simulation results show that the backlight spatial illumination presents sharp and clear rectangular distribution, with the spatial light efficiency of 0.811 (Fig. 11). Meanwhile, the average brightness of the simulated backlight reaches 1.06×10⁶ cd/m², the brightness of the backlight light-emitting surface is evenly distributed, and there is no brightness or illumination mutation, with the luminous area reaching 123 mm×42 mm (Fig. 12). The backlight module is spliced with the HUD, the simulated virtual image brightness is greater than 1×10⁴ cd/m² (Fig. 13), and the simulated average color temperature of the virtual image is 6504.3 K (Fig. 14), which meets the design requirements. Additionally, a system feasibility verification analysis is conducted, the average brightness of the backlight emitting surface is measured to be greater than 1×10⁶ cd/m², and the overall uniformity is greater than 86% (Fig. 15), with the simulated virtual image brightness and the power values of the two types of light and heat radiation incident on the LCD screen given. As the backlight driving power changes between 1 W and 25 W (Fig. 17), the backlight can be employed normally within this power range, and the backlight brightness can be adjusted later according to usage requirements and with reference to this curve.

We propose a method for quickly calculating the optimal uniform light distance of an array light source on a certain target surface. This method can be utilized to build the uniform light model of most rectangular array LED, which has high universality. To improve the design flexibility, we adopt the turning light stick structure, and the size of the light outlet can be changed according to the usage requirements to adapt to the design and development of liquid crystal image sources of different sizes. Then, the non-microstructure surface of the ordinary Fresnel mirror is designed as a cylindrical surface, which improves the collimation of the light under the extended light source. The light outlet surface of the backlight system reaches 5.1 inch (12.95 cm), the average backlight brightness is 1.06×10⁶ cd/m², and the uniformity reaches 88.7%. The difference between the measured data and the simulation results of the final backlight brightness is within 0.4%. Finally, the backlight and HUD system are spliced to simulate the virtual image brightness. The simulation results show that the virtual image brightness is greater than 1×10⁴ cd/m², the target surface brightness uniformity reaches 85.1%, and the virtual image color temperature simulation average value is 6504.3 K. The research and development of a low-cost large-size image source backlight module has been realized. The design method has certain inspiration for the design of backlight sources of other sizes and has great practical application significance.