Importance of solar load modeling in AR HUD

Augmented reality (AR) head-up displays (HUDs) will revolutionize the driving experience by overlaying critical driving information onto the real world. Today, the best example of AR displays is used in fighter jets, which place a wealth of critical information in the pilot’s direct line of sight.

In an automotive environment, graphics are placed directly in the driver's line of sight, replacing basic warning sounds or symbols, in this way conveying information and identifying threats in the driver's field of view, allowing them to take immediate action. Graphics appear as a natural conformal extension of the real world; they are not just secondary information displays in today's HUDs.

As discussed in my previous blog post on solar loads, solar irradiance presents significant challenges to AR HUD design. Unlike traditional HUDs, AR HUDs have a wide field of view and long virtual image distance, and require real-time integration of the vehicle's sensor data with the HUD display. The long virtual image distance (>7m), and to a lesser extent the wider field of view (at least 10 degrees horizontal x 4 degrees vertical), results in significantly increased solar concentration and corresponding heat rise on the imager panel. To prevent thermal damage from solar irradiance, you must carefully design the AR HUD and run detailed solar load simulations to verify reliable operation.

There are several points to consider when simulating the effects of sunlight load on AR HUD design.

Accuracy of solar load models

While it may seem obvious, I cannot overstate the importance of accuracy in model elements. AR HUD sunlight load simulation requires an accurate sunlight source model (with appropriate angle, spectrum, and irradiance characteristics), as well as accurate spectral transmission curves for the optical elements in the car, including (but not limited to) windshields, glare traps, and hot/cold mirrors.

Effects of off-axis solar irradiance

In everyday driving conditions, sunlight enters the vehicle at various angles as the vehicle turns, drives up and down hills. Therefore, it is important to scan the incoming sunlight over the appropriate range of angles, as shown in Figure 1. TI found that in an AR HUD prototype using TI DLP® technology, the off-axis peak solar irradiance was 2.7 times worse than the chief ray level, resulting in a significant increase in thermal load. The simulated peak solar irradiance is shown in Figure 2. If your system design cannot handle the worst-case off-axis solar irradiance, you will face an unacceptable risk of field failures due to damaged imager panels.

Figure 1: Simulating sunlight over a range of input angles

Figure 2: Peak irradiance on the diffuser screen as a function of input sunlight angle.

Thermal effects of solar irradiance

Simulating peak solar irradiance is only the first step in predicting and avoiding thermal failure. Solar energy is converted to heat based on the spectral absorption of the material it strikes. For example, in our testing, as shown in Figure 3, the temperature of a thin-film transistor (TFT) panel due to sunlight load rises six times faster than the transmissive microlens array diffuser screen used in DLP technology-based systems, making the TFT panel more susceptible to damage from solar irradiance.

At an ambient temperature of 85°C, the Kuraray diffuser screen with DLP technology in HUD systems can withstand solar irradiances up to 82kW/m2 due to its low spectral absorption and high operating temperature. This thermal performance enables DLP technology to support long virtual image distances in AR HUDs.

Figure 3: Temperature rise and solar irradiance

The design challenges for AR HUDs are significantly different than those in today’s HUD designs. The solar loads in AR HUDs are significantly higher, and you must run detailed thermal simulations and account for off-axis solar irradiance in your designs. For a more detailed discussion of solar load modeling, see the white paper “DLP Technology: Solar Loading in Augmented Reality Head-Up Display Systems.”