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Overview
By overlaying key driving information onto the real world, augmented reality (AR) head-up displays (HUDs) change how drivers receive situational data. A common example of AR displays is in fighter aircraft, where large amounts of critical information are placed directly in the pilot's line of sight.
In automotive applications, graphics placed in the driver's view can replace basic warning tones or symbols. These graphics communicate information and identify threats within the driver's field of view, enabling immediate responses. The graphics are intended as natural extensions of the real world rather than secondary information displays found in many current HUDs.
Why sunlight load matters for AR HUDs
Sunlight irradiance presents significant design challenges for AR HUDs. Compared with conventional HUDs, AR HUDs typically require a wide field of view and a long virtual image distance while also integrating vehicle sensors and HUD graphics in real time. Long virtual image distances (greater than 7 m) and relatively wide fields of view (for example, at least 10° horizontal by 4° vertical) increase sunlight concentration and produce corresponding thermal rise on the imager panel. To prevent thermal damage from sunlight irradiance, AR HUDs require careful design and detailed sunlight load simulation to verify reliable operation.
Key considerations when simulating sunlight load
Accuracy of the sunlight model
Model accuracy is critical. AR HUD sunlight-load simulations require precise sun-source models with appropriate angle, spectral, and irradiance characteristics, together with accurate spectral transmission curves for optical elements in the vehicle, including but not limited to the windshield, glare traps, and hot/cold mirrors.
Impact of off-axis solar irradiance
During normal driving, sunlight enters the vehicle at a range of angles as the vehicle turns or traverses grades. It is therefore important to scan incident sunlight across the appropriate angular range. TI found that in an AR HUD prototype using TI DLP technology, the off-axis peak solar irradiance was 2.7 times higher than the on-axis horizontal peak, producing a significantly increased thermal load. Simulated peak solar irradiance is shown as a function of input sun angle in the referenced figure. If a system cannot handle the worst-case off-axis solar irradiance, it risks unacceptable field failures from damaged imager panels.
Figure 1: Simulating sunlight across a range of input angles
Figure 2: Peak irradiance on the diffuser as a function of input sun angle
Thermal effects of solar irradiance
Simulating peak solar irradiance is only the first step in predicting and preventing thermal failure. Sunlight is converted to heat according to the spectral absorption of the materials it strikes. In testing, thin-film transistor (TFT) panels exhibited a temperature rise due to sunlight load up to six times faster than transmissive microlens-array diffusers used in DLP-based systems, making TFT panels more susceptible to solar irradiance damage.
At an ambient temperature of 85°C, a Kuraray diffuser used in DLP-based HUD systems can tolerate solar irradiance up to about 82 kW/m^2 due to its low spectral absorption and high operational temperature limit. This thermal performance enables DLP-based approaches to support long virtual image distances in AR HUDs.
Design implications
AR HUD design challenges differ substantially from those in conventional HUDs. Sunlight loads in AR HUDs can be significantly higher, so designers should run detailed thermal simulations and include off-axis solar irradiance in their analyses. Consideration of materials, spectral transmission, and angular solar exposure is necessary to mitigate thermal risk to imager panels and other optical components.
