Emissive Projection Technology Enables a Full-Windshield Head-Up Display
The authors have developed a full-windshield head-up display (FW-HUD) for automotive applications. This new display is based on emissive projection display technology, and forms photo-quality images on a fully transparent RGB emissive screen, after selective excitation of the screen by images in multiple ultra-violet wavebands from a projector. With this FW-HUD, information can be graphically displayed anywhere on a windshield without limitation on viewing angles.
by Ted Sun
Automotive manufacturers have been experimenting with display technologies that allow graphical information to be displayed through or onto the windshield so as to
convey critical information to drivers while still allowing them to keep their attention on the road. In this regard, head-up displays (HUDs), invented during WWII for military aviation,1 which project reflective virtual imagery at a precise angle to the driver, have been the technology of choice. Since their invention, HUDs have undergone a series of evolutionary changes in their design. However, state-of-the art HUDs still suffer due to limited field of view and viewing angle, which compromise their display capabilities.
Sun Innovations recently developed a novel emissive projection display (EPD) that uses the principles of fluorescent conversion of luminescent materials that can be created in transparent forms. This display allows visible light to be generated on a fully transparent glass surface, displaying superior quality images. Figure 1(a) shows a schematic of the system: Highly efficient fluorescent material is applied onto a glass panel with less than 2% in haze level and under 10% reduction in visible-light transmission. An invisible image from a custom-built UV image light engine is projected onto the coated glass. The projected UV image is absorbed and converted by the fluorescent materials on glass to re-emit images in the visible-light spectrum.
We utilized a novel “wavelength selective excitation” (WSE) method2 to render the first full-color emissive display without complex screen pixilation [Fig. 1(b)], taking advantage of the transparent nature of the display phosphor films. A full-color screen can be constructed by stacking three layers of transparent RGB emissive films with distinctive absorption and emission characteristics. The projector encodes the original color image into the projected light at three excitation wavebands. Each waveband of the projection light excites its corresponding phosphors in the films and generates a primary color (e.g., R, G, and B) image without interfering with the excitation or emission from the other two layers. Since each fluorescent film is very thin (<50 µm), high-resolution and full-color images are emitted directly toward the intended observer position rather than being scattered or absorbed. The patented WSE process eliminates the need to register pixels in an emissive projection display; the flat transparent screen can be economically produced roll-to-roll, without pixel structures and the corresponding haze.
Fig. 1: The schematic (a) shows an emissive projection display while (b) outlines full color rendering by wavelength-selective excitation (WSE) on a transparent emissive screen.
The Key Transparent Emissive Screen for Full-Windshield HUDs
In order to achieve FW-HUD, we developed a series of visibly clear emissive materials, with the optic haze level under 2% and a reflectance of ~5% on normal incident visible light. Figure 2 shows the RGB emissive materials in coatings on (a) glass and (b) polymer films. RGB layers are stacked together and present a full-color image with gray scale using the WSE approach (c).
Fig. 2: RGB emissive materials appear in (a) as fully transparent coatings on glass and in (b) as optically clear polymer films. In (c), three transparent RGB layers are stacked together for a single full-color emissive screen without pixilation, using the WSE process.
Our team also laminated the transparent emissive materials directly inside the existing polyvinyl butyral (PVB) resin that is commonly used in windshields to keep the glass layers from shattering. It is a much easier solution to produce HUD-integrated windshields or window glass than the conventional HUD approach, which requires precisely mounted “wedge” reflectors inside the windshield. There is no extra coating step and no change to the display-windshield manufacturing process; hence, it is also a better route to mass producing display windshields than applying display films to windshield surfaces.
Figure 3(a) shows a display glass with our emissive material inside the PVB, which was
sandwiched inside two glass panels. Figure 3(b) shows the optical clarity of a large-panel display glass with a transparent display demo on it.
Fig. 3: At the top of (a) a cross section of a display glass with emissive material inside the glass PVB (shown at bottom) is shown. In (b), a demo image on full-sized glass with a phosphor-loaded PVB interlayer is shown. The image is generated by a blue-ray laser projector.
The material lifetime was subjected to several accelerated tests. Figure 4 shows the variation of the fluorescent emission intensity of the transparent display screen after being continuously exposed to harsh conditions for over 2000 hours. Figure 4(a) shows a damp heat test, in which the screen was subjected to a combination of 85ºC and 85% RH (relative humidity). Figure 4(b) shows the temperature cycling of 85-0ºC at ~10 minutes per cycle. The result is no noticeable variation of fluorescent intensity under identical UV excitation. The display films used the same polymer base as window film (PET) or glass lamination film (PVB), which can withstand extremely cold conditions; in fact, the phosphor emission efficacy increases at lower temperature. Sunlight has a great deal of UV emission, which affects material reliability and also contributes to emissive background noise; hence, a clear UV blocking film or layer is typically used before the phosphor films to shield the materials from solar UV.
Fig. 4: The reliability of the display-screen materials was tested under various conditions, including (a) high humidity and (b) high and low temperatures. Multiple screen samples were tested simultaneously for statistical studies and are shown in different colors. The screen luminance is ~4400 cd/m2 on the tests. The screens remain visually clear after the tests.
Miniature Full-Windshield HUD (FW-HUD) Projector and a Complete FW-HUD Kit
For HUD applications, the projector must be small in order to fit into automobiles with volume restraints. In that regard, we are developing a miniature FW-HUD projector based on blue-ray laser technology and x-y laser image scanners. Figure 5 shows the layout of such a custom-designed miniature HUD projector. It consists of a set of x-y galvanometer scanners, the mechanical base of the scanner, and a light-emitting module. The driver boards, controller boards, and input interfaces are integrated in another housing that is connected to the projector. This separated design allows for flexible and convenient installation. A miniature palm-size FW-HUD projector was built [(Fig. 5(b)], which is provided along with an optically clear phosphor-coated display windshield [FW-HUD Standard Development Kit (SDK)] for custom-full-windshield display applications.
Fig. 5: In (a), a drawing of the overall display engine design is shown. A photo of the miniature laser projector for FW-HUD is shown in (b).
An FW-HUD SDK system has been developed for use in cars and other vehicles. Speed, GPS information, and warnings are projected onto the windshield. The controller has an open software interface; users can build their apps to control the projector display. Figure 6 shows an augmented-reality application from GM using the FW-HUD SDK, outlining the road, and pin-pointing the destination in a poorly lit environment, through integration with various sensors.
Fig. 6: Examples of the FW-HUD projector (shown in Fig. 5) are shown on a transparent phosphor-coated windshield.
Sun Innovations recently demonstrated an FW-HUD in HD color, using a custom-built LED/laser hybrid DLP projector that output three wavebands – 405, 423, and 450 nm – to excite the emission of red, blue, and green, respectively. Figure 7(a) shows the projector prototype, along with a demo of an HD-image in color on the transparent phosphor-coated windshield (b).
Fig. 7: (a) An HD-HUD projector prototype. (b) An FW-HUD demo shows an HD image in color.
The FW-HUD technology turns any vehicle glass into a real-image transparent display, with great flexibility to display anywhere desired on that glass with viewability from any angle. Unlike existing HUDs that present a reflective virtual image outside, it does not present an image beyond the glass. While the focal plane is shorter than a “virtual image” HUD, it is longer than any other vehicle display on the dash, and it stays head up. The projected light is completely blocked by a windshield coated with the phosphor film and converted to visible imagery. Energy efficient laser or LED projectors can be built to display adequate
contrast for daylight applications. This technology will complement other vehicle displays, including HUDs, and enable some unique display solutions, including FW
This novel EPD technology has been applied to the first-ever demonstration of a FW-HUD with unlimited viewing angles. This display technology can be readily applied to any glass windows in any vehicle or building structures.3 It can also be applied to convert any surface to a high-quality emissive display, without hiding or affecting the surface appearance. For example, it can enable the first projection display on a pitch-black screen, with high image contrast in bright ambient light that rivals that of flat-panel displays. As a new tool for human– machine interfaces in future vehicles, FW-HUDs will enable advanced augmented-reality solutions over the entire windshield after integration with various sensors.
1R. L. Newman, “Head-Up Displays: Designing the Way Ahead,” ISBN 0-291-39811-1 (1995).
2J-Q. Liu and X. Sun, “System and Method for a Transparent Color Image Display Using Fluorescent Conversion of Nano-Particles and Molecules,” U.S. Patent 7090355.
3T. Sun and B. Cheng, “Novel Emissive Projection Display Digitizes Glass Windows,” Information Display 29, No. 6, 2-8 (Nov./Dec.) 2013. •