Patent Application
20090179870
This invention relates to light guides for luminous touch screens and, in particular, to an interstitial layer between a light guide and a touch screen.
In a personal electronic device, such as a hand-held computer or a laptop computer, a touch screen is often used as an input device. The touch screen is a separate device or is built into the personal electronic device. A touch screen uses a capacitive element or a piezo-resistive element to indicate a point of contact between a user's finger and the surface of the touch screen. The location is calculated by a microcontroller in the touch screen or in the personal electronic device. The location, or a series of locations, is then used for control or as data entry.
Literally, “touch screen” can include membrane switches but this is not the meaning of the words in the art. As used herein, a touch screen is a device that does not have predefined contact areas but has a single area in which touch is detected and, typically, the location of a touch is measured. Sophisticated systems can measure the force of a touch.
In the prior art, U.S. Pat. No. 6,433,846 (Omar et al.) discloses the combination of a liquid crystal display overlying a touch screen, wherein both are back lit by a gas discharge lamp and a light guide. U.S. Pat. No. 6,822,640 (Derocher) discloses a touch screen back lit by an electroluminescent (EL) lamp. U.S. Pat. No. 6,842,170 (Akins et al.) discloses a liquid crystal display, an EL lamp, and a touch screen arranged in several permutations. Published Patent Application 2004/0075779 (Paukshto et al.) discloses a touch screen overlying a back lit liquid crystal display. A back lit piezo-resistive touch screen is currently an impossibility because known piezo-resistive materials are opaque. A back lit touch screen must be transparent. Edge lit sheets, or light guides, for back lighting have long been known in the art; e.g. U.S. Pat. Nos. 3,027,669 (Hardesty) and 3,356,839 (Mehess et al.). Edge lit keypads are also known in the art. For example, U.S. Pat. No. 4,247,747 (Swatten) discloses light emitting diodes (LEDs) optically coupled to a polyester sheet having a thickness of seven mils (0.18 mm). U.S. Pat. No. 5,975,711 (Parker et al.) discloses a light conductive panel coupled to a light source.
Edge lit sheets conduct light from an edge, relying mostly on what is known as total internal reflection. (There is also some direct transmission). Total internal reflection is obtained when light is incident upon a surface at greater than a critical angle from perpendicular. The critical angle is given by the following equation, in which n2 is the index of refraction of the surrounding medium and n1 is the index of refraction of the sheet.
Obviously, n1≧n2 because the limits of positive arcsine are 0 and +1. Air (n2=1) is typically used around light guides but providing separation makes construction awkward. It is known to use a reflecting layer, such as aluminum, as a “cladding layer” to simulate total internal reflection in light guides. Published Patent Application 2005/0210643 (Mezei et al.) discloses using white or “reflecting” paint for a reflector. Published Patent Application 2004/0251567 (Cappellini et al.) discloses Superflex™ 2500 resin as a cladding material for optical fiber and discloses that the resin has a refractive index of 1.35-1.41.
A light extracting feature is anything that interferes with total internal reflection. One or more such features are located on a major surface of a sheet for lighting discrete areas. Alternatively, an entire surface can be used to extract light. U.S. Pat. No. 4,183,628 (Laesser et al.) discloses a watch that is back lit by a light source coupled to the edge of a “frost glass” having scratches on its lower surface to redirect light up through a display. U.S. Pat. No. 5,550,676 (Ohe et al.) discloses an edge lit light guide having graduated features to compensate for distance from a light source.
Screen printing is long known in the art for making a variety of products, such as electroluminescent (EL) lamps. The inks used for making EL lamps include a binder, a solvent, and a filler, wherein the filler determines the nature of the ink (e.g. carbon for conductive layers, barium titanate for dielectric layers). The inks are printed and cured (dried) to form each layer in an EL lamp.
Screen printed panels larger than 45-60 cm. on a side are rare. Within the size limitation, screen printing is a low cost, well developed art. The opportunity exists to make touch screens at reduced cost by employing screen printing technology. It may seem straightforward to screen print a touch screen on a light guide and have a luminous touch screen for a liquid crystal display. Unfortunately, the reality is somewhat different. The electrical requirements of a touch screen may not be compatible with a liquid crystal display and neither may be compatible with the optical requirements of a light guide.
As used herein, “transparent” does not imply a particular level of light transmission. The amount of light transmission that is suitable depends, for example, on the size (largest dimension) of the light guide or the contrast of a graphic.
In view of the foregoing, it is therefore an object of the invention to provide improved lighting for touch screens.
Another object of the invention is to provide a combined luminous sheet and touch screen that is more easily constructed than in the prior art.
The foregoing objects are achieved by this invention in which a luminous touch screen includes a light guide having at least one light extracting feature on a first major surface and an interstitial layer on a second major surface. A touch screen overlies the interstitial layer. The light guide has an index of refraction, n1, the interstitial layer has an index of refraction, n2, and n1>n2. The interstitial layer is preferably a layer of resin bearing ink, which is cured or dried after deposition.
A more complete understanding of the invention can be obtained by considering the following detailed description in conjunction with the accompanying drawings, in which:
In
In accordance with a first aspect of the invention, light extracting features, such as feature 13, are imprinted on a first major surface of light guide 11. Light guide 11 includes a second major surface joined to the first major surface along an edge adjacent LED 12. In the embodiment illustrated in
In accordance with a second aspect of the invention, interstitial layer 15 is a cured layer of resin bearing ink on a second major surface of light guide 11. Interstitial layer 15 has a lower index of refraction than light guide 11, preferably at least 0.1 lower. As known in the art, amorphous polymers have a lower index of refraction than partially or completely crystalline polymers, in which the molecules are more densely packed. Thus, amorphous forms of, for example, polytetrafluorethylene (PTFE) are preferred for the interstitial layer. In one embodiment of the invention a layer of Superflex™ 2500 resin having a thickness of approximately 5 μm was used as an interstitial layer. Thinner layers are not preferred. There is no operational or functional upper limit to thickness. Practical considerations, such as cost and ease of manufacture, provide an upper limit.
Touch screen 17 is preferably what is known as a capacitive touch screen, although any type of touch screen can be used in the embodiment illustrated in
Optional transparent layer 18 overlies light guide 11 and is back lit by light extracting features 13. Layer 18 is preferably the same material as interstitial layer 15 and has graphics printed thereon. Alternatively, layer 18 is an interstitial layer between a sheet (not shown) containing graphics and light guide 11. This allows a sheet containing graphics to have any index of refraction without concern for it matching the index of refraction of light guide 11. As another alternative, layer 18 is not the same material as interstitial layer 15 but has an index of refraction lower than the index of refraction of light guide 11.
In
In
The embodiments illustrated in
Overlying interstitial layer 52 is touch screen 40, including front electrode layer 41. Layer 41 is screen printed in a pattern of columns across the surface of interstitial layer 52. Layer 41 is a transparent, conductive layer made from PEDOT (poly-3,4-ethylenedioxythiophene), for example. Overlying layer 41 is bus bar layer 42, printed from a conductive ink containing carbon or silver. Overlying bus bar 42 is clear, UV curable insulating layer 44.
Rear electrode layer 46 and rear bus bar layer 47 overlie insulating layer 44. Insulating or protective layer 49 overlies rear electrode layer 47. Layers 46 and 47 are screen printed in a pattern of rows, providing an X-Y grid for the touch screen. Interstitial layer 52 isolates front electrode layer 41 from light guide 51 and improves the efficiency of the light guide; that is, light guide 51 provides more light at the light extraction features with interstitial layer 52 than without layer 52.
The invention thus provides an improved luminous touch screen that is constructed more easily and at lower cost than in the prior art.
Having thus described the invention, it will be apparent to those of skill in the art that various modifications can be made within the scope of the invention. For example, diffusing agents, such as barium titanate, or dyes, or other agents, can be added to the light extracting features to affect the extracted light. Optical coupling can be used to gather more light into the light guide. Depending upon need, e.g. because of limited space, light can be reflected from a source prior to entering the light guide. By extending the light guide past the interstitial layer and the touch screen, one can inject light through the second major surface.
