Optical Design for Vehicle-Mounted Head-Up Display for Micro-LED
Objective An augmented reality vehicle-mounted head-up display(AR-HUD)projects driving information directly onto the windshield within the driver's view,which facilitates quick interaction between the car and driver and reduces accident risk.However,challenges for AR-HUDs include sunlight backfilling,large size,and low brightness.In this paper,we propose a new picture generation unit(PGU)approach to address sunlight backfilling and size issues while simultaneously increasing brightness.The PGU,based on a micro-LED display on silicon,achieves high brightness due to its large current.However,the display size is relatively small compared to that of the TFT-LCD displays widely used in commercial AR-HUDs.We propose an AR-HUD optical design with a 125-times magnification for image size.A micro-LED intermediate image system,consisting of a lens group and a diffusion plane,is demonstrated.This configuration has the potential to address the challenges of achieving a large virtual image size and a large distance in AR-HUDs.Methods We develop an optical design for a large magnification AR-HUD,which is composed of an off-axis three-mirror system with free-form mirrors,a magnifying lens group,and a diffusion mirror.The initial configuration is determined based on factors such as virtual image size,virtual image distance,and field of view.An inverse optimization model,starting from the virtual image plane,is constructed and subsequently optimized to achieve the best design.The image quality is evaluated using the inverse model,and the ray direction is flipped to simulate imaging quality when rays originate from the micro-LED display.Additionally,a lens group for imaging the micro-LED display onto the diffusion plane is designed and optimized.Finally,sunlight backfilling,which can cause glare and excess heat,is mitigated through the design of an optical filter film based on an F-P resonant cavity.Results and Discussions Initial parameters are determined based on the 0.6-inch micro-LED display and the requirement for virtual image size(Table 1).As shown in Fig.2,an AR-HUD optical system with free-form mirrors is designed,with the mirrors represented by Zernike polynomials.The parameters of the HUD system are listed in Tables 2 and 3.In the reverse design,the cutoff frequency is calculated using the 5-times magnified PGU.The modulation transfer function(MTF)for each field is greater than 0.3 at 10.75 lp/mm,as shown in Fig.3(a).In the forward design,the spatial frequency is calculated using the amplified virtual image pixel spacing.For cutoff frequencies up to 0.37 lp/mm,the MTF at the center of each focal surface is greater than 0.5,as shown in Fig.4(a).The system's mesh distortion is less than 2.1％,as illustrated in Fig.4(b).Figure 4(c)shows that the root mean square(RMS)radius of the spot diagram is within the Airy disk diameter of 879.4 μm,which indicates good imaging quality.Finally,the virtual image size is 26 times larger than the image size on the diffusion plane.An image lens group is placed between the diffusion plane and the micro-LED display,as shown in Fig.6 and Table 4.A plane mirror is used to fold the optical path,which makes the system compact.The grid distortion of the designed lens group is less than 5.2％,and the RMS radius of the spot pattern is smaller than that of the Airy spot,as shown in Fig.7.Additionally,a narrow-band pass filter film is designed for the monochromatic PGU,as shown in Table 6.The optimized filter film achieves high transmittance in the central wavelength band of the PGU light,as shown in Fig.10.Tolerance analysis confirms that the system can be manufactured.The proposed lens group and micro-LED display have been integrated into a homemade HUD system,and the image quality has been evaluated,which shows consistency with the theoretical design.Conclusions In this paper,we design an in-vehicle HUD with a micro-LED display.The system features a magnification lens group and an off-axis mirror configuration.The lens group provides a magnification of 5 times,while the off-axis triple inverse structure achieves a magnification of 25 times.As a result,the virtual image is 125 times larger than the image of micro-LED,meeting the design requirements.A narrow band-pass filter film,with a thickness of 1.24 μm,is optimized to suppress sunlight backfilling while ensuring high transmission for the PGU light.The comparisons of simulation results of film systems with and without the metal layer and the adhesion layer confirm the role of the metal layer in preventing peak cracking and reducing transmission peaks.The lower transmittance due to the metal layer can be offset by using an appropriate adhesion layer.The designed lens group has been integrated into a custom HUD system along with the micro-LED display.Experimental results validate the feasibility of the diffusion screen scheme and offer a practical approach for designing and implementing an in-vehicle HUD with a micro-LED display.
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