Michael Firth
Augmented reality (AR) head-up displays (HUDs) are the next big thing in the automotive industry. The technology has finally advanced to the point where automakers and Tier-1 suppliers are actively developing AR windshield HUDs.
True AR displays require a wide field of view (FOV) of at least 10 degrees coupled with a virtual image distance (VID) of 7.5m or greater. FOV expresses the size of the display in degrees and VID indicates how far out the image is projected. In an automotive HUD, VID indicates how far out over the road the image appears.
The idea behind AR is to overlay digital information on top of the real world, enhancing driver situational awareness and improving the driving experience. The larger the FOV and the longer the virtual image distance, the better.
Two of the biggest challenges in designing an AR display have always been brightness and solar load. An AR display needs to be as wide and bright as possible, which requires a lot of light from the imager. You also need to project images out as far as possible onto the road. Today’s HUDs have an FOV in the range of 7 to 8 degrees or less and can “project” images out 2.0 to 2.5m on the road. These images appear to float over the hood of the car. In an AR HUD, you want to project the image further out, allowing the images to truly augment and interact with the driver’s field of view.
To support longer virtual image distances, it’s not uncommon to design a system with 25 to 30x magnification, but that has the unfortunate side effect of focusing the solar load (i.e. the sun’s energy) onto a very small area of the HUD’s imager panel which can create sever thermal challenges. This high level of magnification effectively moves the imager panel closer to the focal point of the HUD optics, resulting in a higher concentration of solar energy per unit area, as shown in Figure 2.
Note, that’s it’s not the ambient temperature that’s the problem, but the focusing of the sun’s energy coupled with more light entering the system (due to the larger glare trap of an AR HUD) that creates the thermal challenge.
DLP® technology’s unique intermediate diffuser screen architecture enables you to design HUDs that can withstand the thermal loads caused by the magnification of sunlight. As you can see in Figure 3, a DLP technology based HUD projects the image onto a diffuser screen, which the HUD optics then magnify and project onto the windshield for reflection back to the driver. In a TFT HUD, the TFT panel is attached to the HUD optics, replacing both the diffuser screen and supporting electronics.
To better understand why a diffuser screen provides an advantage, let’s look at the physical properties of a diffuser screen vs. a traditional TFT panel (Figure 4). The two primary advantages are a higher operating temperature and, more importantly, that the diffuser screen simply does not absorb much of the sun’s incoming visible spectrum. (The infrared [IR] and ultraviolet [UV] portions of the spectrum are easily filterable.)
The incoming solar load is focused onto the diffuser by the HUD optics, just like it would be for a TFT panel. But in the case of a diffuser, the transmitted light is actually spread out, eliminating the magnification effects of the HUD optics and making for a much simpler thermal problem to manage. With a TFT panel, the sun’s energy is absorbed and can easily increase the panel’s operating temperature above its maximum rating. It’s this solar load advantage – along with superior brightness, contrast and color gamut – that are enabling automakers and Tier-1 suppliers to design and bring to market next-generation AR HUDs.
TI PROVIDES TECHNICAL AND RELIABILITY DATA (INCLUDING DATASHEETS), DESIGN RESOURCES (INCLUDING REFERENCE DESIGNS), APPLICATION OR OTHER DESIGN ADVICE, WEB TOOLS, SAFETY INFORMATION, AND OTHER RESOURCES “AS IS” AND WITH ALL FAULTS, AND DISCLAIMS ALL WARRANTIES, EXPRESS AND IMPLIED, INCLUDING WITHOUT LIMITATION ANY IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE OR NON-INFRINGEMENT OF THIRD PARTY INTELLECTUAL PROPERTY RIGHTS.
These resources are intended for skilled developers designing with TI products. You are solely responsible for (1) selecting the appropriate TI products for your application, (2) designing, validating and testing your application, and (3) ensuring your application meets applicable standards, and any other safety, security, or other requirements. These resources are subject to change without notice. TI grants you permission to use these resources only for development of an application that uses the TI products described in the resource. Other reproduction and display of these resources is prohibited. No license is granted to any other TI intellectual property right or to any third party intellectual property right. TI disclaims responsibility for, and you will fully indemnify TI and its representatives against, any claims, damages, costs, losses, and liabilities arising out of your use of these resources.
TI’s products are provided subject to TI’s Terms of Sale (www.ti.com/legal/termsofsale.html) or other applicable terms available either on ti.com or provided in conjunction with such TI products. TI’s provision of these resources does not expand or otherwise alter TI’s applicable warranties or warranty disclaimers for TI products.
Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265
Copyright © 2023, Texas Instruments Incorporated