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Three Dimensional OLED Displays with Depth-Sensitive Multi-Touch

Gaurav Rajasekar, and Vigneshwaran Shanmugam Madras Institute of Technology Campus, Anna University, Chennai, India

grajasekar@ieee.org , grajasekar@gmail.com

Abstract

OLED displays are thin by nature and can be made transparent. Stacking several layers of the same, one over the other, would make it possible to show a three-dimensional display that carries all the advantages of OLEDs as opposed to traditional LCD and Plasma Displays. These OLEDs are made using the Triplet Triplet Annihilation (TTA) method a.k.a Annihilation-assisted Up-conversion. Such displays are transparent and use less power (20mW per cm2). The creation of such a 3D display would be revolutionary in the field of display technology. This paper aims to compare proposed solution to existing technologies. 1. Introduction 1.1. Present Day Display Technologies

LCDs (Liquid Crystal Displays) have a constant (backlit) image, where the intensity is varied by blocking the light shining through the panel. CRTs (Cathode Ray Tube) use an electron beam, scanning the display, flashing a lit image. In DLP (Digital Light Projection) projectors, the image is created by microscopically small mirrors laid out in a matrix on a semiconductor chip, each mirror represents one or more pixels in the projected image. Plasma displays modulate the "on" time of each sub-pixel, similar to DLP. Every pixel is made up of three separate subpixel cells, each with different colored phosphors.

SED's (Surface Conduction Electron Emitter Display) use nanoscopic-scale electron emitters to energize colored phosphors and produce an image. In a general sense, an SED consists of a matrix of tiny cathode ray tubes, each "tube" forming a single sub-pixel on the screen, grouped in threes to form red-green-blue (RGB) pixels. However they are too expensive for normal use, still unreleased due to patent and production difficulties.

1.2. OLED

OLEDs (Organic Light Emitting Diodes) are any light emitting diodes (LED) whose emissive electroluminescent layer is composed of a film of organic compounds. The layer usually contains a polymer substance (substrate) that allows suitable organic compounds to be deposited. They are deposited in rows and columns onto a flat carrier by a simple "printing" process. The resulting matrix of pixels can emit light of different colors. 2. Three Dimensional View 2.1. Advantages of 3D

Figure 1. The concept of 2-dimensional displays vs three dimensional ones

Many aspects of contemporary work and recreation require the effective visualisation of three-dimensional data:

• studying the structures of biochemicals • designing a new vehicle • obtaining a relationship from multi-variable plots • with non-invasive scans such as CAT scans • planning a new sculpture • playing the latest computer game! Conventional methods for displaying 3D data

exclusively involve flat (2D) displays that give the illusion of depth.

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These range from (albeit sophisticated) rendering techniques that still ultimately generate a flat image, to displays that direct a different 2D image to each eye (through head-mounted displays, coloured filter glasses or optical methods involving lenticular surfaces or parallax barriers).

2.2. Shortcomings of Current 3D Displays

However, these all suffer a number of disadvantages,

related to the fact that the 3D images do not occupy the same space as the observer: • Images typically have a limited angle of view and/or are only visible with special goggles • The observer cannot interact with the images in intuitive ways (e.g. pointing out a part of the image to a colleague with one's hand) • The display can give erroneous impressions of relative size/scale.

These shortcomings can be overcome by a true volumetric display – one where the image of a volume of space actually occupies a volume (composed of “voxels”, volumetric pixels). A number of methods for constructing such displays have been attempted in the past, including the following: •Swept volume method – projecting light onto a moving surface such that the reflected projection appears to originate from the appropriate location within the volume •Dot-matrix LED cubes – individual LEDs in a lattice form the voxels for displaying images •Solid-state fluorescence – invisible laser beams excite specific regions within a crystal to emit visible light [1]

2.3. Types of 3D Displays

2.3.1. Stereoscopic.

Stereoscopic technologies generally involve special spectacles. They are based on stereopsis, i.e. the difference in position of the eyes.

2.3.2. Autostereoscopic.

Autostereoscopy involves integration of the mechanism into the display itself. The change of view is accompanied by the breakdown of the illusion of depth

2.3.3. Computer-generated holography. Display operates on the basis of interference of

coherent light. The effect is similar to looking through a window at the scene being reproduced. Large amounts of calculation required to generate a detailed hologram

largely prevent its application outside of the laboratory and make it extremely expensive

2.3.4. Volumetric displays [2]. Some physical mechanism is used to display points

of light within a volume. These have voxels instead of pixels. Categories under this kind of display include Multiplanar Displays – where multiple display planes are stacked upon one another; and Rotating panel displays – where a rotating panel sweeps out a volume.

3. The Concept Proposed

3.1. Our Idea

We propose the creation of a Volumetric,

Multiplanar Display that makes use of OLEDs as against the conventional LCD type display. Multiple Layers of OLEDs can be used to produce an actual point object in space to represent a voxel. A method of user feedback to the display may also be added.

Comparing this to other solutions, this display eliminates the need for active or passive 3D eyewear, and the need for a mechanical apparatus to rotate the display. Also, we do not require the viewer to remain in one position in front of the display to achieve the perfect viewing experience. The lack of headaches and dizziness or disorientation that may be created due to active-shutter glasses is another advantage. While most other 3D displays enable the user to perceive the image in 3D, they create the illusion of 3D on a 2D screen, whereas this creates a 3D image in actual space. Also, use of OLEDs would eliminate the need for an external light source. 3.2. Working of an OLED

A Positive voltage is applied to the anode. Electrons flow from the anode to cathode of the OLED. Emissive layer is electron-rich, whereas the conductive layer is rich with holes. Recombination occurs closer to the emissive layer, because in organic semiconductors holes are more mobile than electrons. The recombination (Fig. 2) causes a drop in the energy levels of electrons. This loss of energy appears in the form of radiation. Thus, emission of radiation whose frequency is in the visible region occurs. That is why this layer is called emissive. To turn the device off, just give the cathode a positive voltage. [3]

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Figure 2. Recombination in an OLED

3.3. Manufacture The general methods for OLED manufacture

include Vacuum deposition, Organic vapor phase deposition and Inkjet Printing. However, our interest is on a new technique (partial results shown in Fig. 3). The first all-organic, transparent, flexible, versatile colour displays are based upon a new method called triplet–triplet annihilation assisted photon energy up-conversion in viscous polymeric matrix [4].

Figure 3. Screens manufactured by TTA-UC method of OLED manufacture

Why go for this method? By doing this, the necessary excitation intensity is drastically lowered - intensities in order of 20mW cm�2 are suf�cient to create a bright, easily observable image. Moreover an external quantum yield of more than 3% is obtained. These displays are based on metallated-porphyrin sensitizers in combination with emitters dispersed in a transparent polymeric matrix and they are driven by galvo-scanned laser diodes. They require only one excitation source to produce multicolour emission, thus reducing the space needed and the power requirement for a multisource setup. 3.4. Advantages of OLEDs

OLED screens (an example in Fig. 4) are very thin.

They can be just 1mm thick. Though production is expensive as of now, the cost will reduce when these displays become mass producible. The materials used

for the creation of these displays are organic in origin. This makes the screens more eco-friendly as compared to the Mercury lamps used in traditional monitors. The power consumption of these devices is also much less than correspondingly sized LCD and Plasma screens. The colors are also brighter and more vibrant. A major advantage is that OLEDs produce their own light. An LCD works by blocking light from some other source, thus they need an external source of illumination. This source is eliminated when OLEDs are used, and as a byproduct the screens are much lighter and thinner. The field of view of a typical OLED display is also higher – about 170 degrees.

OLEDs are very durable. Some screens can even withstand being hammered repeatedly. Now you get displays which you can bend. This is possible only through the advent of OLED screens. The response time is also enhanced. They switch quicker than LCD displays. The process of making OLED screens seems to have the potential to be less expensive than that for LCD screens.

Figure 4. Typical OLED screen developed for commercial applications

At the current time, manufacturing costs are high

because it’s a new technology but it is believed that future development of this technology could lead to a reduced cost for these screens. The advantages outweigh the cost increase. 3.5. Disadvantages of OLEDs

The lifetime of an OLED is shorter compared to LCDs. It varies with colour of the screen and ranges from 14,000 hours to 230,000 hours. But consider that even these are figures going well into a number of years if run 24x7. In direct sunlight, OLED screens are more difficult to view than LCDs. They are also slightly more prone to water damage. Although there is potential for

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greater savings with OLED screens in the long run, the current cost of manufacturing these is higher than that of standard LCD screens and this could mean that the televisions/displays made using this technology will initially be pricier.

3.6. The Concept

As shown in Fig. 5, our proposed display requires multiple planes. Each plane is composed of an OLED sheet. Remember that the OLED sheets can be made transparent. Hence a number of transparent sheets may be placed one upon the other. The image is then formed by illuminating various sections of the OLED sheets at different levels. This results in the formation of a true 3D object in space.

Figure 5. Schematic Representation of the Concept of 3D displays using several layers

The illusion of depth is maintained because there is indeed an actual 3rd dimension as opposed to 3D using stereoscopic glasses. At the same time, the low thickness of each sheet means that the entire display is not bulky. The greater the number of “layers” in the display, greater is the resolution (in a depth perception sense) of the final image. 4. Basic Model 4.1. Model

To explain what we intend to create we built a basic model [5 , 6] as in Fig 6 and Fig. 7. This model consists of a 3-dimensional matrix of standard LEDs in a 4x4x4 arrangement. These 64 LEDs would have to be controlled individually but we have multiplexed them to allow for just 20 connecting wires to the matrix. All

LEDs in a vertical line share common anodes and all those in a horizontal plane have common cathodes.

The microcontroller used to control the matrix is an Atmel Atmega16. It runs on an external crystal clock of 15Mhz. An RS-232 interface has been included so that future code can be written to the microcontroller and then the pattern being displayed can be transformed in real-time to the monitor of a PC. The supply is given via a 7805 voltage regulator. The device can be programmed using an in-circuit programmer via the supplied ports themselves.

Figure 6. Circuit of the 4x4x4 Cube Model’s Controller Section

Code in C was compiled using AVR-GCC. The

code has been written such that it can be modified to support an 8x8x8 cube too. The circuit diagram is provided. Higher resolution is available on request. The cube displays various 3D patterns to give an idea of the 3D OLED screen in its most basic form.

Building this cube enlightened us to the difficulty in controlling individual voxels in a multilayer display. The model helps to imagine the challenges that would face a multilayer OLED display in that the different layers of LED planes here can represent the OLED layers in a very basic way. Obviously the transparency of an OLED is far greater than that of a standard LED which is more or less opaque. This had us look into solutions for displaying opaque objects on a transparent display. In an OLED display the spacing would not matter, as it does between LEDs in this basic model.

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Figure 7. The 4x4x4 LED Cube constructed to show the concept in its most basic form

5. Special Characteristics of the 3D Display 5.1. Transparency

Substrate, Anode and Cathode are up to 85 percent transparent and they allow light to pass through in both directions. One side transparent OLEDs can also be manufactured for applications such as smart cards 5.2. Power Consumption

Each OLED requires just few billionths of an amp to operate and the sheets are more efficient at extracting light from the power put into it than standard LEDs and LCDs. Consequently they emit less heat. Low power micro-OLEDs are being developed by researchers at Tyndall Institute, Ireland. 5.3. Flexibility

Possibilities include a heads up umbrella displays which broadcast news, t-shirts displaying live TV streams, moving image posters for display advertising, televisions that can be blended to view, newspapers that allow users to upload daily news to an easy to carry contraption etc. They can be rolled, bent and even hammered. Kodak has created a submersible OLED screen too.

5.4. Temperature Range

OLED displays provide a far greater temperature range of operation than conventional LCD based displays, without the need for heaters or loss of contrast. They can operate between 218.15 kelvin and 403.15 kelvin, creating applications in areas where environmental conditions are extreme.

5.5. Present Day Applications

� High resolution, low cost 3D Displays � Medical imaging � Foldable and portable displays � In Mechanical Computer Aided Design � Non-forgeable documents using a thin OLED layer

to store images of 3D facial features � Personal entertainment devices � Military Visualization 5.6. Medical Usage

The 3D image gives a better understanding of the spatial arrangement of various parts of the body organs by looking at it. In CAT scans, each layer of the body is imaged (as in Fig. 8) and then provided to corresponding layers of OLED screens. The resulting image is a 3D composite.

3D viewing is useful for minimally invasive surgery (MIS). Visualization of procedure is accomplished using output from a video system in part placed inside the body of the patient. Overall, the system gives the surgeon a more realistic presentation and can allow more efficient hand-eye coordination. The image is also of better clarity due to improved contrast.

Figure 8. Ordinary 2D images obtained as a result of a CAT Scan 6. Bleeding-edge Implementation

A German firm has realised a Biometric 3D face-recognition tech called VISOTEC Expert 300. Their idea is to combine this with a thin and light portable 3D display for which an OLED setup is ideal. The power source is a heat-based battery system with recharges as the passport is being carried around. Moreover,

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Samsung is building a flexible AMOLED display to be part of a tough, presumably hack-proof document (eg. Fig. 9). This will include movies of 3D biometric info. Also, the screen is heat-resistant and super thin. All these make the idea of a non-forgeable, biometric, 3D passport document closer to reality.

Figure 9. A concept artist’s impression of a 3D OLED passport 7. Touch Capability

Transparent Capacitive Touch Sensors (as shown in Fig. 10) have been developed by Synaptics (under the trademark ClearTouch) and several other companies. These could be placed between each layer as the layers themselves are transparent and don’t have any backlighting setup. The OLED layer itself acts as the dielectric between the capacitors and the entire arrangement will measure ‘depth’ of the applied pressure across the screen. This would make use of the OLED layer’s flexibility and resistance to distortion even under pressure. Thus applied pressure would be transferred from the top layer till the bottom layer of the screen with a gradual decrease in intensity resulting in depth perception with regard to touch sensitivity due to the increased number of touch sensitive layers. An alternative is to use a single layer of capacitive touch sensors that would sense touch as well as measure the amount of pressure. This way, multiple planes of capacitive touch layers would be unnecessary provided that an accurate measurement of quantity of pressure input at each location on the screen is done by each layer.

Thus we would have a display that provides a method for the user to give a touch input (user feedback) at multiple points on the display. Capacitive touch-screens are particularly adept at multitouch and can be used with the human hand directly as is evident in consumer gadgets like the iPhone. The addition of a

method of user feedback such as this is vital in today’s world of limitless interaction.

Figure 10. Transparent Touch Sensors that could be applied to such a screen 8. Challenges

The transparency of each panel is 85% (transmittance of 0.85). The transmittance of the last display pane in stack of panels would be 0.85 raised to a power of the number of panels. Hence realistically, the displayed information on the 21st panel (transmittance less than 4%), for example, would not be very visible. Thus there is a limit to the number of panels that can be stacked in an actual display. However, the depth provided by even 10 panels would be sufficient to create a remarkable 3D display. Mitigation of the problem created by the lower transmittance of light from lower layers can be done by supplying greater light intensity of light from the layers at the back and gradually decreasing light intensity towards the front. This would create a more evenly lit display. The power requirement for the layers at the back of the display would hence be slightly greater than those at the front. Still, requirement is far less than any other display technology, and switching speed is a great advantage over other display technology.

Blending of colours in the viewing direction may occur. Hence a proper tinting of the panel behind must be achieved to prevent distortion of the colour. To display opaque images the pixel on the panel immediately behind is made to emit completely white light forming a base upon which the light from layers behind it will not affect the layer in front unless viewed from an angle. However viewing at an angle would expose the image behind, which is exactly what we

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require in a 3D display that we can look around. This is not a major problem as we do not advocate producing the background and foreground images on separate layers, we instead believe in displaying different “layers”, in a manner of speaking, of the object to be displayed, on the 3D OLED display layer-by-layer from front to back. Such an idea would require that the image signal be in 3D itself.

A standard television signal input would not be 3D compatible. In such an event, the front panel alone would be used to display the standard signal and the back panels would be blanked out to produce a standard 2D display that is not transparent. The algorithms and methods for the creation of 3D signals from a 2D source are beyond the scope of this paper.

How would you display a perfect black if the layers are transparent? Perfect blacks are more easily achieved in an OLED layer display than in LCD based systems. Making multiple layers display a black colour would help in producing a deeper black in any area of the screen. Control of blacks on the different layers of the display enables, as an added benefit, far better contrast ratio than current solutions.

Visibility in direct sunlight is low thanks to this being an emissive technology, and that remains a disadvantage when using OLEDs. This can be solved to a certain extent by anti-reflective screens and with Transparent OLEDs that use the sunlight as it passes through them to increase the illumination that they themselves provide.

9. Conclusion

In the event of getting any kind of funding or support from our university or an organization to proceed with the project, the authors plan to try a hardware implementation of the concept using the

mentioned OLED screens with touch sensors. The experience of seeing a display show images and videos in 3D is surely entrancing to most people. The practical applications are limitless and the new manufacturing process holds great promise with new display technology being quickly adopted by the mass market.

Acknowledgment

The authors thank their department faculty as a whole and Mr. Raman of the E&I Dept, MIT Campus, Anna University, in particular, for extending their help in debugging the model of the cube. The authors also extend thanks to Aravindakshan Parthasarathy (PhD) of Purdue University, USA, for help in acquiring certain hardware components. References [1] Lewis, J.D. Verber, C.M. and McGhee, R.B. , “A true three-dimensional display” IEEE Trans. Electron Devices 1997 [2] Downing E, Hesselink L, Ralston J and Macfarlane R, “A Three-Color, Solid-State, Three-Dimensional Display” Science 1996 [3] Chung-Chih Wu, et al. Dept. of Electr. Eng., Nat. Taiwan Univ., Taipei;, “Electromagnetic Modeling of OLEDs and its Applications to High-cd/A OLEDs” [4] Tzenka Miteva, Vladimir Yakutkin, Gabriele Nelles, and Stanislav Baluschev, “Annihilation assisted upconversion: all-organic, �exible and transparent multicolour display,” New Journal of Physics 10 - 2008. [5] Todd Holoubeck, “LED cube”, http://www.toddholoubek.com/projects/ledpage/ [6] Chris Lomont, “LED Cube”, http://www.lomont.org/Projects/LEDCube/LEDCube.php