Led display screen

The invention relates to an LED display screen.

Light emitting diodes (LEDs) are widely used in display screens. Lack of brightness and/or contrast is an important concern when using LED display screens. This is often at least partly related to the presence of a considerable amount of ambient light as, for example, emitted from the sun or from luminaires positioned in the neighborhood of the display screen. The reflection of this ambient light towards the viewers of the screen creates glare, which reduces the ability of the viewers to accurately perceive the pictures displayed on the screen. Hence, LED display screens require a minimum amount of reflections of incident ambient light from the screen in order to obtain high external contrast within a viewing cone of the display screen. Moreover, the intensity of the reflected ambient light from the display screen into the direction of the viewers should also be minimized. This will also help to minimize the visibility of seams that are often present between the individual LED tiles from which the entire LED display screen is composed.

To achieve high external contrast, the LEDs with or without their associated lenses (hereafter referred to as LED elements) are typically positioned on a flat screen that is coated with a deep-black layer often consisting of a carbon -black- filled silicone material. The black layer on the screen surface is made sufficiently rough to induce diffuse reflections of the incident light. However, with these practices, the reflected amount of ambient light from the screen is still substantial because the black color and rough surface are unable to sufficiently reduce the intensity of the incident ambient light, thus resulting in an unsatisfactory contrast. The contrast is further deteriorated when the black layer on the screen between the LEDs becomes soiled with deposits such as dust, sand particles and soot-like materials.

A common practice to mechanically protect the LED display screen, e.g. against vandalism, and to prevent soiling of the display screen by airborne particulates is to place a strong and smooth transparent Perspex or Polycarbonate plate in front of the display screen. However, this arrangement induces additional reflections from the front plate and reduces further the external contrast of the screen. The presence of a transparent plate in front

of the screen should therefore be avoided at any time when an image is displayed on the screen.

US patent Application No. 20030231151 discloses an LED display screen in which a plurality of light shades and a light absorber, composed of a plurality of reflective surfaces between the LED elements, are used. The emphasis is on reducing glare caused by downward-cast ambient light that substantially comes from above. The plurality of light shades, however, appears to make the display vulnerable to damage and costly to manufacture. It also significantly complicates the cleaning of the display. Furthermore, glare caused by ambient light coming from the side instead of from above cannot be dealt with; thus, this solution cannot fully reduce glare within the complete viewing cone of the display screen.

Therefore, it would be advantageous to have an LED display screen that provides a high external contrast not only to the viewers situated directly in front of the display screen but also for viewers situated sidewards with respect to the display screen. It would also be advantageous to have an LED display screen that can easily be cleaned. It would be an added advantage to have an LED display that is vandalism-proof.

Accordingly, an LED display screen comprising an LED element and a layer facing away from the display screen is described herein. The layer is arranged relative to the LED element for absorbing at least part of the incident ambient light onto the screen, and has an absorption-enhancing light-trapping topographical structure that is present in each direction along the plane of the display screen. As the light-trapping topographical structure is present in each direction along the screen, the incident light coming from any direction towards the screen can effectively be trapped within at least a restricted range of angles of incidence with respect to the normal of the display screen, resulting in high external contrast. In a preferred embodiment, the absorption-enhancing light-trapping topographical structure in the layer between the LED elements comprises a plurality of cavities. The cavities widen in the direction facing away from the display screen and have light-reflecting surfaces with light-absorbing properties, allowing the cavities to act as light absorbing traps wherein at least part of the incident light undergoes more than one reflection before escaping from the cavities. The cavities can be pyramidal or conical in shape.

In a further embodiment, an LED display screen comprising a protective structure extending along the display screen is described. This structure provides a degree of mechanical protection to the physical integrity of the LED elements on the screen, thus making the screen vandalism-proof. In a still further embodiment, an LED display screen that can easily be cleaned is disclosed. The layer is made from a material having a low surface energy to make it self- cleaning or easily cleanable. The light reflecting surfaces of the cavities are ensured to be hydrophobic, preferably ultra-hydrophobic, so that they are resistant to soiling and can also be easily cleaned.

Various features, aspects and advantages of the present invention will be clearly understood from the following description with reference to the accompanying drawings, wherein: Fig.l shows a top view of an LED display screen according to an embodiment of the invention;

Fig.2 shows a side view of an LED display screen according to the embodiment shown in Fig.l;

Fig.3 shows a side view of an LED display screen according to another embodiment of the invention; and

Fig. 4 shows a three-dimensional view of an LED display screen according to the embodiments shown in Fig. 2 and Fig. 3.

In Fig.1 , a top view of an LED display screen 100 is shown wherein a layer

101 has an absorption-enhancing light-trapping topographical structure 102 arranged relative to an LED element 104. The LED element 104 comprises an LED 106 with an associated collimating lens 105 that protrudes through the opening 103 in the layer 101. The structure

102 comprises pyramidal cavities that widen in the direction facing away from the display screen.

In a side view of an LED display screen 200 shown in Fig.2, a layer 201 is provided with the cavities 202, each cavity widening in the direction facing away from the display screen. The cavities 202 can be pyramidal or conical in shape. They include light reflecting surfaces 203. Opposing light reflecting surfaces 203 within a given cavity are

arranged at an angle φ _c with respect to each other. The layer 201 is positioned between adjacent LED elements 204. Each LED element has a transparent lens 205 that serves to collimate the emitted light from the LED 206 within a collimation angle θ _c . The collimation angles existing in all cross-sections of the display screen along the normal of the display screen, when combined together, form an angular viewing cone of the display screen. Light is emitted within an angular range 2θ _C in the cross-section shown in Fig. 2, the angular range 2θ _C constituting part of the viewing cone.

In Fig.3, a side view of an LED display screen 300 is shown wherein in addition to all the features mentioned in Fig.2 the layer 301 is also provided with a protective structure 307. The protective structure 307 can be placed on top of the layer 301 or partly embedded therein. The layer 301 is provided with the cavities 302. Opposing light reflecting surfaces 303 within a given cavity 302 are arranged at an angle φ _c with respect to each other. The layer 301 is positioned between adjacent LED elements 304. Each LED element comprises a transparent lens 305 and an LED 306. Fig.4 shows a three-dimensional view of possible embodiments of the cavities and the protective structure on the display screen 400. A layer 401 is provided with cavities 402 and LED elements 403. The LED elements 403 protrude through openings 404. A protective structure 405 is placed on the top of the layer 401.

According to an embodiment of the invention, an LED display screen 100 comprises an LED element 104 and a layer 101 facing away from the display screen 100. The layer 101 is arranged relative to the LED element 104 for absorbing at least part of the incident ambient light onto the screen, and has an absorption-enhancing light-trapping topographical structure 102 that is present in each direction along the plane of the display screen. The presence of a light-trapping topographical structure 102 enables a higher degree of light absorption with respect to the incident light than the light absorption with only one reflection from an absorbing surface. The incident light coming from any direction towards the screen can effectively be trapped within at least a restricted range of angles of incidence with respect to the normal of the display screen when the light-trapping topographical structure is present in each direction along the screen. This results in a high external contrast not only for the viewers situated directly in front of the display screen but also for the viewers situated sidewards with respect to the display screen and at various distances therefrom.

The layer can be embodied as a coating layer or as a foil that is present in between the LED elements on the display screen.

According to another embodiment, the absorption-enhancing light-trapping topographical structure in the layer 201 between the LED elements 204 comprises a plurality of cavities 202. The reflection of incident ambient light towards the viewers of the screen is sufficiently suppressed by effectively trapping the incident ambient light inside the cavities 202. The light-reflecting cavity surfaces 203 with the light-absorbing characteristics increase the extent of the incident light absorption when the incident light undergoes more than one reflection inside the cavity 202 before escaping from that cavity 202. Each additional reflection is accompanied by an additional light absorption. The light-trapping cavities 202 are shaped such that substantially all the incident light that is reflected from the screen back towards the viewers within the angular viewing cone of the display has undergone at least two consecutive reflections inside the cavity. This effectively suppresses the emission of glare from the screen within the angular viewing cone of the screen, thus ensuring high external contrast. Substantially all reflecting surfaces associated with the layer between the LED elements 204 that are exposed to incident ambient light should have a non-zero angle with respect to the plane of the display screen in order to preclude that they exist as horizontal surfaces wherefrom incident ambient light can reflect back towards the viewers after only a single reflection event.

It is ensured that the reflecting surfaces 203 of these cavities 202 are black- colored and have a smoothness that substantially supports the occurrence of specular reflections from these surfaces. The black color of the cavity surfaces allows a significant degree of light absorption across the entire visible wavelength region associated with the incident light when this light reflects from the cavity surfaces. A sufficient degree of light absorption generally requires more than one reflection event from a black-colored surface. Therefore, the angle φ _c existing between two adjacent light-reflecting black surfaces within a single cavity should conform to the equation φ _e ≤ (90° -θ _e )

When this condition is met together with the condition of specular reflectivity, it can be derived that stray ambient light that is incident on the screen within the angular viewing width 2θ _C will reflect at least two times against the specularly reflective black surfaces of a cavity before being able to escape back towards the viewer. At least two consecutive reflections will ensure that the intensity of the reflected incident light has reduced to near zero and is thus virtually invisible. This circumstance significantly increases the external contrast of the display screen. A smaller angle φ _c supports the occurrence of

more internal reflections within a cavity, thereby effectively turning each cavity into a light trap. A wide viewing angle 2θ _C necessitates the use of a small angle φ _c .

Preferably, the layer between adjacent LED elements 204 features a plurality of pyramidal or conical light-trapping cavities. The pyramidal cavities in the layer feature square or rectangular openings at their top with preferably sharp edges separating neighboring pyramidal cavities. Another possibility is that the cavities feature elliptical or circular openings at their top (in the latter case, thus forming conical cavities). However, it then becomes important to avoid the existence of flat parts on the top of the layer 201 in between adjacent cavities, because any such flat parts would allow single light reflections to occur therefrom, which would reduce the contrast of the screen. Irrespective of the shape of the cavities, it is important that all exposed surfaces of the layer 201 facing away from the display screen satisfy the design rules given above in any cross section along the normal of the display screen. Because of the light-trapping characteristics of the absorbing layer 201 that is present relative to the LED elements 203, incident light from external light sources that impinges upon the screen cannot effectively become reflected back towards the viewers within the screen's angular viewing cone, thereby only allowing the light emitted from the LEDs to be visible. The result is an enhanced external contrast of the display screen. It provides a much-reduced visibility of the seams between individual LED tiles.

Although only one cross-section of the screen has been shown in Fig. 2, the same situation must hold in any screen cross section sliced along the normal of the screen.

The angle φ _c between reflecting surfaces should be as small as practically possible in order to ensure that the layer 201 functions as an effective light trap. This is because a smaller angle φ _c enables more internal reflections.

According to yet another embodiment, the LED display screen 300 comprises a protective structure 307. A layer 301 between the LED elements 304 is covered with the structure 307 extending along the display area. The protective structure can be placed on the layer or partly embedded therein. Preferably, it is made sufficiently high to extend beyond the top of the individual LED elements on the screen, the height being limited by the collimation angle of the light emitted from the LEDs and the pitch between the neighboring LEDs on the screen. This protective structure with a height that is larger than that of the LED elements avoids direct collision- induced physical contact of LED elements with for instance objects thrown at the screen. The objects will then collide against or contact the protective structure before physically reaching the LED elements. This provides a degree of mechanical protection with respect to the physical integrity of the LED elements. The protective structure

can comprise a metal, a plastic, and/or a rubber material. The height of the protective structure 307 between the LED elements 304 must be chosen such that it does not interfere with the emission of light from the LED elements 304; thus, the height of the structure must be limited and adapted to the collimation of the light emitted from the LED elements 304 and the spacing between adjacent LED elements 304. In Fig.3 it is seen that this condition is satisfied. The emitted light from the LED elements within the angular emission range 2θ _C (dashed lines) does not reflect against the structure, while the height of the protective structure 307 exceeds that of the lenses associated with the LED elements. Such a protective structure obsoletes the presence of a protecting transparent front plate, thereby also eliminating the additional glare from such a front plate. Preferably, it is ensured that the top edges of the protective structure 307 are rounded such that a person colliding with the screen will not be injured. Moreover, the protective structure should be black-colored and should also be optically smooth in order to minimize glare. It is also possible to additionally provide the top surfaces of the structure 307 with light-trapping cavities such as present in the layer 301.

According to yet another embodiment, all exposed light reflecting surfaces in or on the layers 101, 201, 301 and 401 have a (near) optical smoothness to ensure that the reflections from these surfaces are substantially specular. To be specular in the context of this disclosure means that the roughness of the reflecting surfaces should be less than the wavelength of visible light (400 - 800 nm). Thus, the roughness asperities protruding from the surfaces should remain less than a few hundred nanometers. This can be achieved by forming the layers 101, 201, 301 and 401 by casting solvent-based resinous layers onto the screen surface. The resinous layers preferably comprise carbon-black particles in order to provide them with a deep-black color. The carbon -black- filled resinous layers are subsequently subjected to a controlled drying process during which the solvent slowly evaporates. Pyramidal or conical features inside these resinous layers can be created during the drying process with the help of a mold or a stamp. These features can be stamped into the soft solvent/resin/carbon-black layers before they become completely free of solvent.

The layers 101, 201, 301 and 401 are preferably made from a material having a low surface energy in order to render them self-cleaning or render the cleaning easier.

Silicone rubber could be such a material. More preferably, the light reflecting surfaces of the pyramidal or conical cavities are ensured to be hydrophobic, preferably ultra-hydrophobic, so that they effectively become resistant to soiling and can also be easily cleaned. Ultra- hydrophobicity can be obtained by coating the surface with a thin hydrocarbon layer or more

preferably with a thin fluorocarbon layer. Such a layer can be sprayed onto the surface with a standard solvent-based coating process, or by coating it onto the surface from the gas phase with the help of a plasma process. Alternatively, and even more preferably, the surface can be coated with a controlled amount of ultra- fine hydrophobic silica particles or with ultra- fine hydrophobic Teflon-like particles. These ultra- fine particles should preferably be less than 100 nm in diameter. This can be achieved either with the help of a wet coating process (spraying a solvent-based particle dispersion onto the surface followed by drying) or with the help of a dry coating process (spraying an aerosol of hydrophobic particles onto the surface). This results in a particle-coated surface that has a roughness profile with asperities that do not exceed a few hundred nanometers. The particle coating on the surface has preferably a very open (fractal- like) structure. Such a surface is poorly wetted by water, causing raindrops to readily roll away from the surface before they dry (also known as the Lotus effect). This prohibits the deposition of water-borne contaminants on the surface. Furthermore, the adhesion of airborne dust particles to such an ultra-hydrophobic surface is very weak, thus allowing the dust particles to be easily removed from the surface either by soft wiping or by using pressurized air.