Abstract:
A light output device comprises a substrate arrangement with a plurality of light source device arrangements integrated into the structure of the substrate arrangement. The plurality of light source device arrangements comprise at least first and second light source devices ( 4   a,   4   b ) which are arranged in anti-parallel. This arrangement mounts at least two light source devices in anti-parallel within an integrated light source structure, so that they can be controlled independently from shared control lines

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to light output devices, in particular using discrete light sources associated with a light transmissive substrate structure. 
       BACKGROUND OF THE INVENTION 
       [0002]    One known example of this type of lighting device is a so-called “LED in glass” device. An example is shown in  FIG. 1 . Typically a glass plate is used, with a transparent conductive coating (for example ITO) forming electrodes. The conductive coating is patterned in order to make the electrodes, that are connected to a semiconductor LED device. The assembly is completed by laminating the glass, with the LEDs inside a thermoplastic layer (for example polyvinyl butyral, PVB). The glass used may be safety glass. Applications of this type of device are shelves, showcases, facades, office partitions, wall cladding, and decorative lighting. The lighting device can be used for illumination of other objects, for display of an image, or simply for decorative purposes. 
         [0003]    One problem with this type of device is that it is difficult to provide a structure which enables individual LEDs in the glass to be turned on and off, for example in order to display an image, or a dynamic pattern. This is difficult, because a two-dimensional pattern of transparent electrodes is desired, but crossovers need to be avoided if the layer structure is to be kept simple. If individual wires are used for each LED (instead of a two dimensional pattern), this results in very high wire resistances (for example ITO electrodes), leading to high electrical losses in these wires. An alternative is to place several sheets of LEDs in glass behind each other, which adds to the cost of the device. 
         [0004]    Another problem is how to dynamically control the color of the LEDs in glass. Currently, this can again only be achieved by adding extra wires, leading again to thinner ITO wires and thus to extra electronic losses. Furthermore, LED devices are not typically suitable for ac drive voltages, as they have diode electrical characteristics. 
         [0005]    It is an object of the invention to provide independent control of the light source devices but with a simple conductor pattern. 
       SUMMARY OF THE INVENTION 
       [0006]    According to the invention, there is provided a light output device comprising a light transmissive substrate arrangement with a plurality of light source device arrangements integrated into the structure of the substrate arrangement, wherein the substrate arrangement comprises first and second substrates and an electrode arrangement sandwiched between the substrates, with the plurality of light source device arrangements connected to the electrode arrangement, wherein the plurality of light source device arrangements comprise at least a first and second light source device, each light source device comprising first and second electrical contacts and providing an electrical conduction path in a forward bias direction from the first to the second contact, and blocking electrical conduction in an opposite, reverse bias, direction, wherein the first and second light source devices are arranged in anti-parallel. 
         [0007]    This arrangement uses light source devices that transmit current in one direction only, and mounts at least two light source devices in anti-parallel, so that they can be controlled independently from shared control lines. This enables the control lines to be wider than if individual control lines are required, reducing resistive losses. 
         [0008]    By anti-parallel is meant that one is arranged to conduct only in one direction within in a circuit (which circuit includes both light source devices), and the other is arranged to conduct only in an opposite direction within the same circuit. 
         [0009]    The second electrical contact of the first light source device and the first electrical contact of the second light source device can be connected to a second common electrode. This provides two light source devices in two connected and parallel branches. The power source can then comprises a dual-polarity current source, with one polarity for each light source device. 
         [0010]    A power source is preferably provided, adapted to provide independent control of the first and second light source devices. For example, the two light source devices can be controlled simultaneously by using an ac power source with adjustable dc offset. 
         [0011]    The light transmissive substrate material may be transparent (optically clear) or a diffusive transmissive material. 
         [0012]    In another arrangement, one or more light source device arrangements comprises a third light source device having first and second electrical contacts and providing an electrical conduction path in a forward bias direction from the first to the second contact, and blocking electrical conduction in an opposite reverse bias direction, 
         [0013]    wherein the second electrical contact of the third light source device is connected to the common electrode. 
         [0014]    This provides an arrangement of three light source devices, with two in parallel and one in anti-parallel. All three share one common electrode but only two other electrodes are required to enable independent control of the three light source devices. 
         [0015]    Thus, the first electrical contact of the second light source device can be connected to a first electrode, and the first electrical contact of the third light source device can be connected to a second electrode. The first and second electrodes enable independent control of the second and third light source devices. 
         [0016]    The second electrical contact of the first light source device can be connected both to the first electrode and the second electrode without shorting the first and second electrodes. This is the device connected with opposite polarity, and either of the first and second electrodes can then be used to control the device using a polarity signal which does not turn on the other devices. 
         [0017]    One or more diodes can be provided between second electrical contacts of the first light source device and the second electrical contacts of the second and third light source devices. These ensure that the two contacts of the first light source device are not shorted together. 
         [0018]    In this arrangement, a power source can comprise first and second dual-polarity current sources, one for applying a signal between the common electrode and the first electrode and one for applying a signal between the common electrode and the second electrode. The power source may also comprise a controller for controlling the first and second dual-polarity current sources. 
         [0019]    In all embodiments, the substrate arrangement preferably comprises first and second substrates and an electrode arrangement sandwiched between the substrates, wherein the at least one light source device is connected to the electrode arrangement. 
         [0020]    The electrode arrangement can comprise an at least semi-transparent conductor arrangement, for example comprising a transparent conductive oxide such as Indium Tin Oxide, Indium Zinc Oxide, Tin Oxide or Fluorine Doped Tin Oxide. The electrical contacts of the light source devices are then connected to regions of the transparent conductor arrangement. 
         [0021]    The regions of the conductor pattern can be defined by score lines within an otherwise substantially continuous conductor layer. This provides low resistance contacts. 
         [0022]    The electrode arrangement can instead comprise a semi-transparent conductive material, for example gold, silver, copper, zinc or stainless steel. This can be in the form of an ink containing conductive particles. 
         [0023]    The light source devices preferably comprise LED devices, for example inorganic LEDs, organic LEDs, polymer LEDs or laser diodes. 
         [0024]    Each light source device arrangement can comprises at least two LED chips arranged anti-parallel. Thus, individual LED chips can be used and integrated into the substrate structure and mounted in circuit in accordance with the invention. 
         [0025]    The invention also provides a lighting system comprising a light output device as claimed in any preceding claim, and a lighting controller for controlling the signals provided by the power source. 
         [0026]    The invention also provides a method of driving a light output device, the device comprising a light transmissive substrate arrangement with a plurality of light source device arrangements integrated into the structure of the substrate arrangement, wherein the plurality of light source device arrangements comprise at least a first and second light source device, each light source device comprising first and second electrical contacts and providing an electrical conduction path in a forward bias direction from the first to the second contact, and blocking electrical conduction in an opposite, reverse bias, direction, wherein the first and second light source devices are arranged in anti-parallel, 
         [0027]    wherein the method comprises applying an ac voltage to the electrode arrangement to drive the light source devices in sequence, and providing independent control of the output intensity of the first and second light source devices. 
         [0028]    The anti-parallel arrangement enables independent control of the light source devices with shared control lines. The ac voltage can have a variable dc offset for controlling the relative intensity of the first and second light source devices, thereby providing independent control. 
         [0029]    One or more light source device arrangements may each comprise a third light source device (again having first and second electrical contacts and providing an electrical conduction path in a forward bias direction from the first to the second contact, and blocking electrical conduction in an opposite, reverse bias, direction), and the method then comprises applying two ac voltages to the electrode arrangement to drive two light source devices in a first phase of a sequence and to drive the other light source device in a second phase of the sequence, and providing independent control of the output intensity of the first second and third light source devices. 
         [0030]    It is noted that the invention relates to all possible combinations of features recited in the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]    Examples of the invention will now be described in detail with reference to the accompanying drawings, in which: 
           [0032]      FIG. 1  shows a known LED in glass illumination device; 
           [0033]      FIG. 2  shows a single LED of the device of  FIG. 1  in more detail; 
           [0034]      FIG. 3  shows a first example of circuit of the invention for controlling two LEDs; 
           [0035]      FIG. 4  is a first graph to explain how to control the light source device in 
           [0036]      FIG. 3 ; 
           [0037]      FIG. 5  is a second graph to explain how to control the light source device in 
           [0038]      FIG. 3 ; 
           [0039]      FIG. 6  shows a second example of circuit of the invention for controlling three LEDs; and 
           [0040]      FIG. 7  shows a modification to  FIG. 6 . 
       
    
    
       [0041]    The same reference numbers are used to denote similar parts in the different figures. 
       DETAILED DESCRIPTION 
       [0042]    The structure of an LED in glass illumination device is shown in  FIG. 2 . The lighting device comprises glass plates  1  and  2 . Between the glass plates are (semi-) transparent electrodes  7   a  and  7   b  (for example formed using ITO), and a LED  4  connected to the transparent electrodes  7   a  and  7   b . A layer of thermoplastic material  5  is provided between glass plates  1  and  2  (for example PVB or UV resin). 
         [0043]    The glass plates typically may have a thickness of 1.1 mm-2.1 mm. The spacing between the electrodes connecting to the LED is typically 0.01-3 mm, for example around 0.15 mm. The thermoplastic layer has a typical thickness of 0.3 mm-2 mm, and the electrical resistance of the electrodes is in the range 2-80 Ohm, or 10-30 Ohms/square. 
         [0044]    The electrodes are preferably substantially transparent, so that they are imperceptible to a viewer in normal use of the device. If the conductor arrangement does not introduce a variation in light transmission (for example because it is not patterned, or because the pattern cannot be seen), a transparency of greater than or equal to 50% may be sufficient for the system to be transparent. More preferably, the transparency is greater than 70%, more preferably 90%, and even more preferably 99%. If the conductor arrangement is patterned (for example because thin wires are used), the transparency is preferably greater than 80%, more preferably 90%, but most preferably greater than 99%. 
         [0045]    The electrodes can be made of a transparent material such as ITO or they can be made of an opaque material such as copper but be sufficiently thin so that they are not visible in normal use. Examples of suitable materials are disclosed in U.S. Pat. No. 5,218,351. 
         [0046]    The invention provides an integrated light output device having groups of at least first and second light source devices connected in anti-parallel. It then is possible to provide independent control of the first and second light source devices with shared control lines. 
         [0047]      FIG. 3  shows a first example of circuit of the invention for controlling the LEDs.  FIG. 3  shows a glass plate  7  with ITO coating, having a cut  6  in the conductive ITO coating, such that the ITO coating does not conduct electricity over this cut. Two LEDs  4   a  and  4   b  are provided, which connect to the ITO coating on both sides of the cut  6 , so that one LED terminal is coupled to one ITO region  7   a , and the other LED terminal is connected to the other ITO region  7   b . Finally, an electrical current supply source  8  is connected to the system, using wires  9 . 
         [0048]    The LEDs are connected with opposite polarity to the two ITO regions. Thus, one ITO region  7   a  connects to the cathode of LED  4   a  and the anode of LED  4   b , and the other ITO region  7   b  connects to the anode of LED  4   a  and the cathode of LED  4   b.    
         [0049]    If the current supply source  8  generates a current in direction  8   a , only LED  4   b  will turn on, because LED  4   a  is in reverse bias direction. Similarly, if the current is generated in direction  8   b , only LED  4   a  will turn on. Thus, by varying the direction of the current between directions  8   a  and  8   b , it is possible to selectively turn the LEDs  4   a  and  4   b  on. 
         [0050]    In some cases, it is desired to be able to regulate the intensity difference between two types of LEDs  4   a  and  4   b . For example, this may be used to regulate the color temperature, or to mix colors in a multi colour system (for example an RGB, or RGBA system). 
         [0051]    In order to achieve this, an AC current supply can be used which has a desired offset. By moving the offset up or down, the amount of time that a certain LED is on can be regulated, and thus the intensity of the LED can be regulated. This principle is illustrated in  FIGS. 4 and 5 . 
         [0052]    In  FIG. 4 , a comparison between different signals is shown. By shifting the alternating current drive level, the duty cycle is changed for the two LEDs. With an alternating current signal having a positive mean value as shown by plot  40   a , the LED  4   b  is brighter, as the positive duty cycle period is longer than the negative duty cycle period, whereas the LED  4   a  is dimmer. 
         [0053]    With an alternating current signal having a negative mean value as shown by plot  40   c , the LED  4   a  is brighter, as the negative duty cycle period is longer than the positive duty cycle period, whereas the LED  4   b  is dimmer. For a signal with zero offset, the brightness is the same (plot  40   b ). 
         [0054]      FIG. 5  shows the current flowing through LEDs  4   a  and  4   b , when a drive current for the bright LED  4   a  (plot  40   c ) is used. 
         [0055]    In a further embodiment, three different LEDs can be driven, as shown in  FIG. 6 . 
         [0056]    In the example of  FIG. 6 , in order to achieve one more degree of multiplexing, an extra conductor path is added, using an extra cut  6   a  in the ITO coating. This creates three ITO regions,  7   a ,  7   b ,  7   c.    
         [0057]    The drive circuitry comprises two current sources  8  and  60 , and an extra set of connector wires  9   a.    
         [0058]    The first LED  4   a  (i.e. the one with the same polarity as the first LED  4   a  in  FIG. 3 ) is connected between the first and second regions  7   a ,  7   b , with the cathode connected to the first region  7   a  and the anode connected to the second region  7   b . The LED  4   a  can in fact be connected to either of the regions  7   a ,  7   c , or to both as shown in order to reduce electrical loss (explained below). With the cathode connected to both ITO regions  7   a ,  7   c , it is connected on both sides of the ITO cut  6   a , using connector points  11   a  and  11   b.    
         [0059]    The second LED  4   b  is connected between the first and second regions  7   a ,  7   b , with the anode connected to the first region  7   a  and the cathode connected to the second region  7   b . The first and second LEDs  4   a , 4   b  are thus again in anti-parallel as in  FIG. 3 . 
         [0060]    The third LED  4   c  is parallel with the second LED  4   b , with its anode connected to the ITO region  7   c  and its cathode connected the common electrode  7   b.    
         [0061]    LEDs  4   b  and  4   c  thus share the cathode connection, but have individual anode connections. 
         [0062]    If the first current source  8  is driven in direction  8   a , the LED  4   c  will be turned on and similarly if the second current source  60  is driven in direction  8   a , the LED  4   b  will be turned on. It can thus be seen that the second and third LEDs can be driven independently. 
         [0063]    Either current source  8 ,  60  can be used to turn on LED  4   a . When both current sources are used the current can now flow through both contacts  11   a  and  11   b , due to which the total resistance of the conductor path for LED  4   a  is reduced (in this example by 50%). 
         [0064]    This arrangement thus has one common electrode for a set of three LEDs, and two other drive electrodes. Two LEDs  4   b , 4   c  are driven with the same polarity, each by a respective one of the “other drive electrodes”, whereas the other LED  4   a  can be driven with an opposite polarity signal using either one or both of the “other drive electrodes”. 
         [0065]    The connectors  11   a  and  11   b  do not make a shortcut between the layers split by ITO cut  6   a . The LED  4   a  thus requires isolated contacts  11   a ,  11   b  as part of its design. If this is not provided internally in the package of the LED, this may be achieved with additional diodes, as discussed further below. 
         [0066]    This embodiment may for example be used for mixing red-green-blue LEDs. The LED using the most current can be placed in the position of LED  4   a.    
         [0067]    The colour mixing can be carried out in two phases—a first phase for LEDs  4   b  and  4   c  with LED  4   a  turned off, and a second phase for LED  4   a , with LEDs  4   b  and  4   c  turned off. 
         [0068]    By using AC signals for the power sources, the power source signals will alternate between these two phases, and they are not perceptible to the user. Thus, the user sees simultaneous control of the brightness of all three LEDs. 
         [0069]    Using the approach described above it is also possible to mix more than 3 colors, for example mixing Red-Green-Blue-Amber. 
         [0070]    In general, by applying a voltage between regions  7   a  and  7   b , the LED  4   b  can be turned on. 
         [0071]    By applying a voltage between  7   c  and  7   b , the LED  4   c  can be turned on. 
         [0072]    By applying a reverse voltage between either regions  7   a  and  7   b  and/or between  7   c  and  7   b , the LED  4   a  will turn on. 
         [0073]    By providing suitable voltages, it is possible to control the output from all three LEDs independently as outlined above. Offset voltages can be used as described above so that the reverse voltage cycle has different amplitude to the forward voltage cycle, with the reverse cycles controlling the diode  4   a  and the forward cycles controlling the diodes  4   b ,  4   c.    
         [0074]    More diodes may be added to improve the system. For example, additional diodes can be provided near LED  4   b  and LED  4   c , in order to suppress cross-talk between the conductor paths for LEDs  4   b  and  4   c.    
         [0075]    As mentioned above, the LED  4   a  requires two isolated contact terminals if it is to be driven by either of the two drive electrodes. These may for example be added to the LED package, or they may be soldered together with the LEDs on the ITO layer. 
         [0076]      FIG. 7  represents the electrical circuit of  FIG. 6  with extra diodes  70  added to prevent short circuit between the contacts  11   a  and  11   b . The ITO resistances are shown as resistors  12 .  FIG. 7  also shows the diodes connected with opposite polarity to illustrate the fact that the polarity can be reversed without affecting the operation of the circuit. 
         [0077]    Typically, the maximum reverse voltage for a LED will be greater than the maximum reverse voltage provided in the circuits above, and thus the anti-parallel placement works well. However, in some cases it may be desirable to reduce the reverse bias voltages applied to the LEDs. Additional diodes can be added to the system, in order to increase the maximum acceptable reverse voltage, which is then a combination of the LED permitted reverse bias voltage and the added diode voltage drops. 
         [0078]    The examples above have shown individual LED groups. However, it will be understood that the invention is typically implemented as many LED LED groups, embedded in a large glass plate. A typical distance between the LED groups may be 1 cm to 10 cm, for example approximately 3 cm. 
         [0079]    The examples above use glass substrates, but it will be apparent that plastic substrates may also be used. 
         [0080]    A small number of possible materials to form the transparent (or at least semi-transparent) electrodes have been outlined above. Other examples can be found in U.S. Pat. No. 5,218,351, and include electrically conductive wires, with a diameter of approximately 0.1 mm spaced by about 10 mm or more, or with a diameter of approximately 20 um and spaced by 1 mm or more. The wires can be made from strands of gold, silver, copper, zinc or stainless steel. Alternatively, strands made of a resin such as polyester or nylon wires can be used, the outer surface of which is coated with metal by vapour deposition, metal plating or the like. Conductive films of vapour-deposited SiO2-indium alloy can also be used. 
         [0081]    Thus, wires may be used instead of contact pads as shown, and the advantage of reducing the number of lines is still obtained. 
         [0082]    A conductive ink can also be used, which can be deposited by inkjet or silkscreen printing. The ink includes fine metal particles, such as silver, and has a conductance of less than 0.1 Ohm/square/mil. A typical wire width using ink is 0.08 mm to 0.8 mm. 
         [0083]    Various modifications will be apparent to those skilled in the art.