Novel Emissive Projection Display Digitizes Glass Windows
An innovative emissive-projection-display (EPD) system consisting of a fully transparent fluorescent screen and a blue-light-emitting digital projector can be used for digital signage on the windows of buildings or vehicles. The screen can be applied to any window without obstructing the view through the glass.
by Ted X. Sun and Botao Cheng
SINCE the invention of the cathode-ray tube (CRT), efforts to further develop emissive display technology have been considerable and include field-emission displays (FEDs), plasma-display panelss (PDPs), and, recently, organic light-emitting diodes (OLEDs), among others. Compared with a backlit panel such as a liquid-crystal display (LCD) or a reflective display such as a microdisplay-based projection system, emissive displays may offer significant advantages, e.g., large viewing angle, superior image quality, and color richness.
With the idea that a combination of emissive and projection technology might be used for a new type of digital-signage application, Sun Innovations developed an emissive-projection-display (EPD) system that uses fluorescent emission and projective excitation. This system can be readily applied to commercial advertising and digital signage; it can turn a glass window of any size or shape into a fully transparent digital sign with an unlimited viewing angle.
Fluorescent Emission and Light-Projective Excitation
The EPD system consists of a fully transparent fluorescent screen and a projector source that has a light output that operates in the blue-to-violet wavelength range.1 The screen technology is based on down-conversion fluorescent nanomaterials, with high fluorescent quantum efficiency for brighter emissive images. Unlike conventional phosphor screens in CRT or plasma displays, this type of structureless emissive screen can be mass-produced economically through a roll-to-roll manufacturing process.
Figure 1 compares EPD with a direct-view CRT and a conventional projection display. CRT technology, while obsolete due to volume and weight, offers an excellent basis for comparison due to its superior display qualities, including high image contrast and large viewing angles. In a conventional projection display [Fig. 1(a)], visible light passes through a micro-imager device (microdisplay) and is projected onto either a reflective (for front projection) or a scattering screen (for rear projection), which are also largely opaque. In a conventional CRT display [Fig. 1(b)], images are formed on an opaque phosphor screen that is excited by raster-scanned electrons in a vacuum tube. In Fig. 1(c), an EPD employs the projector as an excitation source for the fluorescent materials in the screen. Hence, it combines the superior image quality of an emissive display such as a CRT and the image scalability of projection with a fully transparent screen.
Fig. 1: (a) The conventional projection display, (b) the CRT, and (c) the projection-based fluorescent display are shown in simple schematic form as a basis for comparison.
Novel Color-Rendering Approach in EPDs
As opposed to other emissive display technologies (e.g., CRT or PDP), EPD employs a homogeneous, structureless fluorescent screen to eliminate the need for projector-to-screen alignment. Three layers of vertically stacked transparent fluorescent films can be addressed separately by excitation light in multiple discrete wavebands.2 Figure 2 shows the working principle of the full-color EPD.
Fig. 2: The full-color image-formation process takes place in multiple layers of transparent film. The excitation light has three wavebands; each excites a specific film/layer and generates visible emission at one of the RGB wavebands.
In order to display full-color images, the transparent fluorescent screen can be constructed by stacking films (e.g., red, green, and blue fluorescent films) with distinctive absorption and emission characteristics. The projector encodes the original color image into the projected light at several excitation wavebands (three for full-color displays). On the screen, light of each waveband will excite its corresponding film and generate color emissions at visible wavebands (RGB). Each fluorescent layer absorbs its designated excitation light with high efficiency, but passes visible light and the excitation light of other wavebands. Since each fluorescent film is very thin, high-resolution and full-color images are synthesized in a direction perpendicular to the screen. Such a color-rendering method is completely different from a conventional full-color emissive display (e.g., CRT, PDP, and OLED), which lays out the RGB pixels in the in-plane direction.
Recently, Sun Innovations developed materials that can be effectively excited in three separate wavebands in the range of 350–500 nm. Researchers at Sun measured the emission color of the fluorescent film. The blue film can be excited by a UV light in the range from 380 to 420 nm; the red film can be excited by light from 350 to 380 nm and the green film can be excited by light from 430 to 470 nm. The color coordinates on the CIE 1931 and CIE 1976 system are listed in Table 1. The dominant wavelength is 618 nm for the red, 438 nm for the blue, and 532 nm for the green.
The display color gamut of the fluorescent screen can achieve performance similar to that of conventional CRT displays. The quantum efficiencies (QEs) of fluorescent conversion among the RGB emission films vary in the range of 40–60%. Since the emissive materials on the screen have extremely fine particle sizes in nanometer ranges, the image resolution on the screen is principally dependent on the resolution of the projector. An EPD can be readily implemented onto existing glass windows or windshields. It offers an extremely cost-competitive display solution, with scalable projection and a flat screen that can be economically manufactured roll-to-roll.
The Projector in an EPD
The EPD system requires the projector to output UV/blue light onto the fluorescent screen. There are several technical platforms for the design of the projector. For example, a galvanometer or MEMS scanner-based laser engine with a UV/blue-light laser could be used, or a DLP projector with a gas-discharge lamp, UV/blue-light LED, or laser.
Figure 3 shows a possible design architecture for a laser projector based on a galvano-meter scanner. The projector consists of a controller board, protection board, LD optics module and LD driver, X/Y galvanometer and drivers, projection lens, power board, temperature sensor, IR detection, and I/O interface.
Fig. 3: This possible system architecture for a laser projector uses a galvanometer scanner.
A microcontroller unit (MCU) embedded in the controller board serves as the master controller of the projector and provides the link between image display and input data read. There is a temperature sensor built into the system for monitoring the LD status. Galvanometer drivers will feed back scanner status to the MCU; if the scanners behave abnormally, the MCU will shut down the entire system to prevent a stationary laser beam emitted from the projector. An IR detection module is employed to execute the safety function, ensuring that the safety level of the laser display is Class 3a or less. Should someone or something accidentally enter the laser-projection space, the MCU will shut down the system and will not restart it until the person or obstacle leaves the projection area. Such IR sensors can define a “virtual” barrier on the solid angle of projection to prevent any accidental exposure of the laser image to humans or animals and making the projection system safe for operation in the public.
Results and Demonstration
Sun Innovations has developed and demonstrated a laser-vector scanning projector system named Line-Art, in combination with a full-color fluorescent screen (Fig. 4). Three laser diodes with dominant wavelengths of 375, 405, and 445 nm, respectively, were combined to create the projector’s light source. The fully transparent display screen includes three different layers of fluorescent materials. As shown in Fig. 4, the resulting emissive image is easily visible in normal ambient room light. The viewing angle of the emissive surface is unlimited. The screen is fully transparent but presents a slight green cast due to the visible-light excitation of the green screen.
Fig. 4: The Line-Art projector based on a galvanometer scanner (left) is used in a full-color (RGB) EPD display system that uses a clear fluorescent multi-color screen (right).
Instead of a laser projector with a galvano-meter scanner, a DLP-based projector with a gas-discharge lamp as a light source can be used.3 In Fig. 5, a commercial DLP projector with a UHP lamp has been modified to output light in the approximate wavelength range of 360–410 nm in order to excite a single blue or white emission from the screen. The estimated UV intensity at the screen surface is about 0.3 mW/cm2. A water-clear, single, blue fluorescent film was utilized in this example. A fully transparent white, red, and dual (red/blue) screen, all without body color and haze, are other options.
Fig. 5: The display shown uses a transparent and water-clear EPD based on DLP projection with a UHP lamp source.
Sun has also recently developed a projector based on a DLP scanner and LED or laser sources. Solid-state sources such as high-power LEDs and lasers have excellent power and a spectrum matching the excitation of the emissive materials of the screen. They are also smaller, consume less energy, and offer significantly improved optical efficiency and reliability (with a lifetime >10,000 hours).
Other “Transparent” Digital Display Technologies
There are a number of other display technologies in the commercial market that claim to be transparent, including a TFT-LCD-based transparent “display box,” conventional head-up displays (HUDs), transparent OLEDs, and holographic projection screens (Fig. 6). The display box places an LCD module with 15–20% transparency in front of a bright light-box assembly. It is best used in an enclosure such as a vending machine; it does not perform as well on an open glass window without a strong and stable backlight. A conventional HUD is a virtual image display, using a glass window or windshield as a transflective “mirror” for digital projection. A HUD has an extremely limited viewing angle, limited image size, and is typically used in vehicles only for the driver. Both transparent OLED and holographic screens are only partially transparent, with significant haze and poor image contrast in well-lit environments.
Fig. 6: Shown above are several transparent display technologies and their applications.
Advantages of EPD
Compared with the other commercially available transparent display technologies, the EPD offers significant advantages for a digital-signage solution on glass windows or clear panels:
1. EPD offers an unlimited viewing angle due to the isotropic nature of the fluorescent emission. The image is equally bright on both sides of the transparent projection screen.
2. EPD presents a water-clear screen whether an image is being displayed or not. The screen has virtually no haze; the visible-light transmission is around 90% and can reach 95% with anti-reflective coating. It is a true see-through display system for either front or rear projection.
3. The projected image does not go through the emissive screen; there is negligible physical penetration for projected lights, unlike a holographic projection screen.
4. Like a projection display, the EPD screen has no pixel structure. The EPD screen can be manufactured roll-to-roll.
5. As with a projection system, the display image size is scalable.
6. Like an emissive display (e.g., PDP or CRT), the image quality of an EPD is excellent, with a good color gamut and image properties that are largely independent of viewing angle.
7. It is versatile. In addition to a fully transparent display on a window, it can be applied to a black substrate4 to create a front-projection display with superior image contrast in bright ambient light, at lower projection power.
The EPD system is able to turn any clear surface into an emissive digital display. It allows audiences to experience a vivid, high-definition image while clearly seeing through the transparent screen. This enables a wide variety of potential applications for displaying digital information.
Applications in the digital-signage market include storefront windows and other in-store advertising displays; shopping malls; window glass in airports, train, subway stations, and other high-traffic public areas; large advertising displays on the glass walls of buildings; HUDs for various vehicles; and much more.
An example of EPD technology in use is the Line-Art projection system (shown in Fig. 4), which displays messages and animations on storefront windows using a scanning laser projector. The system enables text and pictures to appear to float in air with no apparent display boundary. They appear to float in air like an animated neon sign. This system offers commercial brands a fresh, exciting look and is designed to maximize the customer experience and attract foot traffic to increase sales. Figure 7 shows one frame of a continuous animation displayed on the window glass of a furniture store in the San Francisco Bay Area.
As an example of an in-store application, Fig. 7 demonstrates a digital showcase for jewelry or watches. The EPD projector is beneath the products and projects onto transparent fluorescent film that is mounted on the showcase’s cover glass. In this way, the showcase has been turned into a novel transparent digital display without affecting the viewing of the articles inside. Furthermore, if combined with an interactive module such as a transparent touch film, the digital display showcase could enable people to search for product information on the cover glass of the product cabinets in an interactive fashion.
Fig. 7: EPD technology (left) on the window of a home-furnishings store in the San Francisco Bay Area enhances a product launch; at right, EPD enables a dynamic transparent display showcase for jewelry.
EPD systems have also been applied successfully in various exhibitions. The window glass of a booth or hall can display creative, dynamic, and see-though signage, including traffic-stopping commercials. Figure 8 shows an example: the EPD product launch at World-Expo 2010 (Shanghai, China).
Fig. 8: The EPD product launch at World-Expo 2010 (Shanghai) featured six red Chinese words on the glass.
The Line-Art-based EPD system can also potentially be used to create large advertising displays on the glass curtain walls of modern buildings. The unique display screen presents the projected image, while being highly transparent to visible light. Figure 9 is a computer rendering of such a display on the glass curtain wall of a hotel building. Up to 10,000 sq. ft. of the digital display can be produced with a screen and a high-powered laser projector for glass-wall laser displays at night.
Fig. 9: This artist’s rendering shows how a high-powered EPD system could be applied as a very large-scale display on the glass-curtain wall of a commercial building at night.
For such an application, a high-powered laser projector would be placed on either side of the glass screen for front or rear projection. An additional UV layer would block the laser light from penetrating the screen and reaching viewers. Since the screen is water clear, the windows continue to provide natural light
and an uninterrupted view. Any advertising displayed by the projector would be viewable from miles away at night. Such a display would be easier to install and disassemble and more cost-effective than a large LED sign, without affecting the aesthetics of the building or the functioning of the windows.
As a final example of a signage application, the full transparency and large viewing angle of the EPD system make it ideal for displaying information on the
windshields or other windows of cars, trucks, trains, buses, aircraft, and other vehicles. The EPD system could function as a novel HUD and could utilize any part of a windshield as the display screen.
A problem facing almost all projection displays in such an application is the degeneration of image quality due to intense sunlight. In order to reduce the influence of sunlight and improve the contrast, the display screen can be used in conjunction with a tinted film, which basically eliminates the overlapping excitation wavelengths from sunlight. Figure 10 shows a clear 50-in.-diagonal image on glass facing direct sunlight. This large HUD product with unlimited viewing angles has HDMI and VGA interfaces and is ready to be implemented in many commercial vehicles (Fig. 11).
Fig. 10: An EPD screen with a transparent green emissive film and tinted film is applied on a window facing direct sunlight.
Fig. 11: The current EPD HUD product as shown in Fig. 10 can be readily implemented onto various vehicle windshields, including (clockwise) cars, tractors, trucks, and ships.
A Clear Future for Novel Applications
Sun Innovations has spent the past 6 years developing a novel transparent emissive projection display that combines the high quality of an emissive display with the scalability of digital projection. It enables the display of digital images on windows or windshields without affecting the view or the transmission of light through the glass. This innovative new display system has many commercial applications where information or advertising needs to be presented on glass, without affecting
the view through the glass.
While the technology is still in development, multiple products have been developed over the past few years, including a laser animation display based on scanning laser projectors and a transparent information display (including HUD) on glass. The superior image quality, unlimited viewing angles, best-of-class clarity, economy, and ease of implementation make the EPD the potential technology of choice to digitize the glass of the future.
1T. Sun, and J. Q. Liu, U.S. Patent No. 6,986,581.
2J. Q. Liu, T. Sun, and M. Duan, U.S. Patent No. 7,537,346.
3T. Sun and B. Cheng,”A new emissive projection display technology and a high contrast DLP projection
display on black screen,” Proc. SPIE 7932, 793209 (2011).
4T. Sun, G. Pettitt, etc, “Full color high contrast front projection on black emissive display,” Proc. SPIE 8254, 82540K-1 (2012). •