SINCE the invention of the cathode-raytube (CRT), efforts to further develop emissive display technology have been con-siderable and include field-emission displays(FEDs), plasma-display panelss (PDPs), and,recently, organic light-emitting diodes(OLEDs), among others. Compared with abacklit panel such as a liquid-crystal display(LCD) or a reflective display such as amicrodisplay-based projection system, emissive displays may offer significant advantages, e.g., large viewing angle, superiorimage quality, and color richness. With the idea that a combination of emis-

sive and projection technology might be usedfor 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 com-mercial advertising and digital signage; it canturn a glass window of any size or shape intoa fully transparent digital sign with an unlim-ited viewing angle.

Fluorescent Emission and Light-Projective ExcitationThe EPD system consists of a fully transparent

fluorescent screen and a projector source thathas a light output that operates in the blue-to-violet wavelength range.1 The screen technology is based on down-conversion fluorescent nano-materials, with high fluorescent quantum effi-

ciency for brighter emissive images. Unlike conventional phosphor screens in CRT or plasma displays, this type of structureless emissivescreen can be mass-produced economicallythrough a roll-to-roll manufacturing process.

Novel Emissive Projection Display DigitizesGlass 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 signageon the windows of buildings or vehicles. The screen can be applied to any window withoutobstructing the view through the glass.

by Ted X. Sun and Botao Cheng

Ted X. Sun and Botao Cheng are with SunInnovations, Inc., in Fremont, California. TedSun can be reached at ted@sun-innovations.com.

2 Information Display 6/130362-0972/6/2013-0??$1.00 + .00 © SID 2013

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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.

Figure 1 compares EPD with a direct-viewCRT and a conventional projection display.CRT technology, while obsolete due to volume and weight, offers an excellent basisfor comparison due to its superior displayqualities, including high image contrast andlarge viewing angles. In a conventional pro-jection display [Fig. 1(a)], visible light passesthrough a micro-imager device (microdisplay)and is projected onto either a reflective (forfront projection) or a scattering screen (forrear projection), which are also largelyopaque. In a conventional CRT display [Fig. 1(b)], images are formed on an opaquephosphor screen that is excited by raster-scanned electrons in a vacuum tube. In Fig. 1(c),EPD employs the projector as an excitationsource for the fluorescent materials in thescreen. Hence, it combines the superiorimage quality of an emissive display such as aCRT and the image scalability of projectionwith a fully transparent screen.

Novel Color-Rendering Approach inEPDsAs opposed to other emissive display tech-nologies (e.g., CRT or PDP), EPD employs ahomogeneous, structureless fluorescent screento eliminate the need for projector-to-screenalignment. Three layers of vertically stackedtransparent fluorescent films can be addressedseparately by excitation light in multiple discrete wavebands.2 Figure 2 shows theworking principle of the full-color EPD.In order to display full-color images, the

transparent fluorescent screen can be con-structed by stacking films (e.g., red, green,and blue fluorescent films) with distinctiveabsorption and emission characteristics. Theprojector encodes the original color imageinto the projected light at several excitationwavebands (three for full-color displays). Onthe screen, light of each waveband will exciteits corresponding film and generate color

emissions at visible wavebands (RGB). Eachfluorescent layer absorbs its designated exci-tation light with high efficiency, but passesvisible light and the excitation light of otherwavebands. Since each fluorescent film isvery thin, high-resolution and full-colorimages are synthesized in a direction perpen-dicular to the screen. Such a color-renderingmethod is completely different from a conven-tional full-color emissive display (e.g., CRT,PDP, and OLED), which lays out the RGBpixels in the in-plane direction. Recently, Sun Innovations developed mate-

rials that can be effectively excited in threeseparate wavebands in the range of 350–500 nm.Researchers at Sun measured the emissioncolor of the fluorescent film. The blue filmcan be excited by a UV light in the range from

380 to 420 nm; the red film can be excited bylight from 350 to 380 nm and the green filmcan be excited by light from 430 to 470 nm.The color coordinates on the CIE 1931 andCIE 1976 system are listed in Table 1. Thedominant wavelength is 618 nm for the red,438 nm for the blue, and 532 nm for thegreen.The display color gamut of the fluorescent

screen can achieve performance similar to thatof conventional CRT displays. The quantumefficiencies (QEs) of fluorescent conversionamong the RGB emission films vary in therange of 40–60%. Since the emissive materialson the screen have extremely fine particlesizes in nanometer ranges, the image resolu-tion on the screen is principally dependent onthe resolution of the projector. EPD can be

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Table 1: The color coordinates ofEPD film are listed for red, green,

and blue.

x y u’ v’

R 0.678 0.322 0.492 0.526

G 0.378 0.594 0.161 0.57

B 0.148 0.132 0.138 0.277

Fig. 2: The full-color image-formation process takes place in multiple layers of transparentfilm. The excitation light has three wavebands; each excites a specific film/layer and generatesvisible emission at one of the RGB wavebands.

Fig. 3: This possible system architecture for a laser projector uses a galvanometer scanner.

readily implemented onto existing glass win-dows or windshields. It offers an extremelycost-competitive display solution, with scala-ble projection and a flat screen that can beeconomically manufactured roll-to-roll.

Projector in EPDThe EPD system requires the projector to out-put UV/blue light onto the fluorescent screen.There are several technical platforms for the design of the projector. For example, agalvanometer or MEMS scanner-based laserengine with a UV/blue-light laser could beused, or a DLP projector with a gas-dischargelamp, UV/blue-light LED, or laser. Figure 3 shows a possible design architec-

ture for a laser projector based on a galvano-meter scanner. The projector consists of acontroller board, protection board, LD opticsmodule and LD driver, X/Y galvanometer anddrivers, projection lens, power board, temper-ature sensor, IR detection, and I/O interface.A microcontroller unit (MCU) embedded in

the controller board serves as the master con-troller of the projector and provides the linkbetween image display and input data read.There is a temperature sensor built into thesystem for monitoring the LD status. Gal-vanometer drivers will feed back scanner status to the MCU; if the scanners behaveabnormally, the MCU will shut down theentire system to prevent a stationary laserbeam emitted from the projector. An IRdetection module is employed to execute thesafety function, ensuring that the safety level

of the laser display is Class 3a or less. Shouldsomeone 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” barrieron the solid angle of projection to prevent anyaccidental exposure of the laser image tohumans or animals and making the projectionsystem safe for operation in the public.

Results and DemonstrationSun Innovations has developed and demon-strated a laser-vector scanning projector system named Line-Art, in combination witha full-color fluorescent screen (Fig. 4). Threelaser diodes with dominant wavelengths of375, 405, and 445 nm, respectively, werecombined to create the projector’s lightsource. The fully transparent display screenincludes three different layers of fluorescentmaterials. As shown in Fig. 4, the resultingemissive image is easily visible in normalambient room light. The viewing angle of theemissive surface is unlimited. The screen isfully transparent but presents a slight greencast due to the visible-light excitation of thegreen screen. Instead of a laser projector with a galvano-

meter scanner, a DLP-based projector with agas-discharge lamp as a light source can beused.3 In Fig. 5, a commercial DLP projectorwith a UHP lamp has been modified to outputlight in the approximate wavelength range of360–410 nm in order to excite a single blue or

white emission from the screen. The esti-mated UV intensity at the screen surface isabout 0.3 mW/cm2. A water-clear, single,blue fluorescent film was utilized in thisexample. A fully transparent white, red, anddual (red/blue) screen, all without body colorand without haze, are other options.Sun has also recently developed a projector

based on a DLP scanner and LED or lasersources. Solid-state sources such as high-power LEDs and lasers have excellent powerand a spectrum matching the excitation of theemissive materials in the screen. They arealso smaller, consume less energy, and offersignificantly improved optical efficiency andreliability (with a lifetime >10,000 hours).

Other “Transparent” Digital DisplayTechnologies There are a number of other display technolo-gies in the commercial market that claim to betransparent, including a TFT-LCD-basedtransparent “display box,” conventional head-up displays (HUDs), transparent OLEDs, andholographic projection screens (Fig. 6). Thedisplay box places an LCD module with 15–20% transparency in front of a brightlight-box assembly. It is best used in anenclosure such as a vending machine; it doesnot perform as well on an open glass windowwithout a strong and stable backlight. A con-ventional HUD is a virtual image display,using a glass window or windshield as a trans-flective “mirror” for digital projection. AHUD has an extremely limited viewing angle,

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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).

limited image size, and is typically used invehicles only for the driver. Both transparentOLED and holographic screens are only par-tially transparent, with significant haze andpoor image contrast in well-lit environments.

Advantages of EPDCompared with the other commercially avail-able transparent display technologies, the EPDoffers significant advantages for a digital-signage solution on glass windows or clearpanels: 1. EPD offers an unlimited viewing angledue to the isotropic nature of the fluores-cent emission. The image is equallybright on both sides of the transparentprojection screen.

2. EPD presents a water-clear screenwhether an image is being displayed ornot. The screen has virtually no haze;the visible-light transmission is around90% and can reach 95% with anti-reflec-tive coating. It is a true see-through dis-play system for either front or rearprojection.

3. The projected image does not go throughthe emissive screen; there is negligiblephysical penetration for projected lights,unlike a holographic projection screen.

4. Like a projection display, the EPD screenhas no pixel structure. The EPD screencan be manufactured roll-to-roll.

5. As with a projection system, the displayimage size is scalable.

6. Like an emissive display (e.g., PDP orCRT), the image quality of an EPD isexcellent, with a good color gamut andimage properties that are largely inde-pendent of viewing angle.

7. It is versatile. In addition to a fully trans-parent display on a window, it can be

applied to a black substrate4 to create afront-projection display with superiorimage contrast in bright ambient light, atlower projection power.

ApplicationsThe EPD system is able to turn any clear surface into an emissive digital display. It allows audi-ences 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; windowglass in airports, train, subway stations, andother high-traffic public areas; large advertis-ing 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 anima-tions on storefront windows using a scanninglaser projector. The system enables text andpictures to appear to float in air with no appar-ent display boundary. They appear to float inair like an animated neon sign. This systemoffers commercial brands a fresh, exciting

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Fig. 6: Shown above are several transparent display technologies and their applications.

Fig. 5: The display shown uses a transparent and water-clear EPD based on DLP projectionwith a UHP lamp source.

look and is designed to maximize the cus-tomer experience and attract foot traffic toincrease 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 forjewelry or watches. The EPD projector isbeneath the products and projects onto trans-parent fluorescent film that is mounted on theshowcase’s cover glass. In this way, theshowcase has been turned into a novel trans-parent digital display without affecting theviewing of the articles inside. Furthermore, ifcombined with an interactive module such asa transparent touch film, the digital displayshowcase could enable people to search forproduct information on the cover glass of theproduct cabinets in an interactive fashion. EPD systems have also been applied

successfully in various exhibitions. The window glass of a booth or hall can displaycreative, dynamic, and see-though signage,including traffic-stopping commercials. Figure 8 shows an example: the EPD productlaunch at World-Expo 2010 (Shanghai,China).

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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.

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 alsopotentially be used to create large advertisingdisplays on the glass curtain walls of modernbuildings. The unique display screen presentsthe projected image, while being highly trans-parent to visible light. Figure 9 is a computerrendering of such a display on the glass curtain wall of a hotel building. Up to 10,000sq. ft. of the digital display can be producedwith a screen and a high-power laser projectorfor glass-wall laser displays at night. For such an application, a high-powered

laser projector would be placed on either sideof the glass screen for front or rear projection.An additional UV layer would block the laserlight from penetrating the screen and reachingviewers. Since the screen is water clear, thewindows continue to provide natural light andan uninterrupted view. Any advertising displayed by the projector would be viewablefrom miles away at night. Such a displaywould be easier to install and disassemble andmore cost-effective than a large LED sign,without affecting the aesthetics of the buildingor the functioning of the windows. As a final example of a signage application,

the full transparency and large viewing angleof the EPD system make it ideal for display-ing information on the windshields or otherwindows of cars, trucks, trains, buses, aircraft,and other vehicles. The EPD system couldfunction as a novel HUD and could utilize anypart of a windshield as the display screen.A problem facing almost all projection

displays in such an application is the degener-ation of image quality due to intense sunlight.In order to reduce the influence of sunlightand improve the contrast, the display screencan be used in conjunction with a tinted film,which basically eliminates the overlappingexcitation wavelengths from sunlight. Figure10 shows a clear 50-in.-diagonal image onglass facing direct sunlight. This large HUDproduct with unlimited viewing angles hasHDMI and VGA interfaces and is ready to beimplemented in many commercial vehicles(Fig. 11).

A Clear Future for Novel Applications Sun Innovations has spent the past 6 yearsdeveloping a novel transparent emissive pro-jection display that combines the high qualityof an emissive display with the scalability ofdigital projection. It enables the display ofdigital images on windows or windshieldswithout affecting the view or the transmission

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Fig. 9: This artist’s rendering shows how a high-power EPD system could be applied as a verylarge-scale display on the glass-curtain wall of a commercial building at night.

Fig. 10: An EPD screen with a transparent green emissive film and tinted film is applied on awindow facing direct sunlight.

of light through the glass. This innovativenew display system has many commercialapplications where information or advertisingneeds to be presented on glass, without affect-ing the view through the glass. While the technology is still in develop-

ment, multiple products have been developedover the past few years, including a laser animation display based on scanning laserprojectors and a transparent information display (including HUD) on glass. The supe-rior image quality, unlimited viewing angles,best-of-class clarity, economy, and ease ofimplementation make the EPD the potentialtechnology of choice to digitize the glass ofthe future.

References1T. Sun, and J. Q. Liu, U.S. Patent No.6,986,581.2J. Q. Liu, T. Sun, and M. Duan, U.S. PatentNo. 7,537,346.3T. Sun and B. Cheng,”A new emissive pro-jection 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 emissivedisplay,” Proc. SPIE 8254, 82540K-1 (2012).

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Fig. 11: The current EPD HUD product as shown in Fig. 10 can be readily implemented ontovarious vehicle windshields, including (clockwise) cars, tractors, trucks, and ships.