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Internally illuminated sign

Internally illuminated sign description/claims

This application is a continuation of co-pending U.S. application Ser. No. 11/374,833, which is a continuation in part, and claims the benefit of priority to PCT application No. PCT/IL2004/000850, filed Sep. 14, 2004, which in turn claims the benefit of priority to U.S. provisional application No. 60/504,945, filed Sep. 15, 2003. The above identified applications are incorporated herein by reference in their entirety.

The present invention relates to the field of internally illuminated signs, and especially those having uniform illumination over their operative area and having a very low energy requirement for their operation.

Numerous applications exist which require the use of an illuminated sign for night-time visibility in situations where access to an electric mains supply is either unavailable or inconvenient to supply. Some such applications are in house signs, road signs, directional signs, road milestones, camping sites, emergency signs such as in aircraft or in underground car parks, display signs for advertising purposes or for providing other information. Such signs should ideally have uniform illumination over the whole of the area in which the sign's information is to be displayed, should be visible by day as well as by night, and should provide a level of illumination visible at some tens of meters at night with a minimal electrical power input.

In the prior art, many different types of such signs exist, and in many of them, the illumination source is generally internal to the sign structure, and is powered by a secondary cell or battery, which is maintained charged by solar power. One of the main problems to be solved in all such internally illuminated signs is the provision of uniform illumination without any regions of higher or lower intensity. Reflectors and/or diffusers are generally necessary in order to improve uniformity. Large areas arrays of fluorescent tubes are quite effective to this end, but are very energy consumptive.

A number of prior art, internally illuminated signs have been described using light emitting diodes (LED's), which are generally more energy saving than incandescent or fluorescent sources, because of their ability to localize their emission in the direction of desired illumination. Some examples of such prior art signs are to be found in U.S. Pat. Nos. 5,105,568, 5,539,623, 5,388,357, 5,729,925 and 4,952,023, and in International Publication No. WO 03/010738, all of which are hereby incorporated by reference, each in its entirety. In such signs, reflective surfaces are generally used within a light box to direct the illumination towards the sign panel. The reflective surfaces can be either on the back of the character panel, or on other internal reflective surfaces, or usually on both. Commercial products are available for facilitating these functions, such as those supplied by the 3M Company of St. Paul, Minn. 55144, which provides a range of products designed for enhancement of the performance of internally illuminated signs. One such product is called a blockout film, products 3635-20B and No. 3635-22B, and is designed for preparing the indices of such signs. These films have a black matte surface on one side, which is used as the outer side of the sign, and a white surface on the other side, used to reflect light internally from the back of the indices.

Many of the above-described prior art signs have a number of disadvantages. Firstly, they often use a source located at one end of a light box with internal reflective surfaces, and although this is a convenient arrangement for such signs, the design of such prior art signs does not generally provide sufficiently uniform illumination on the sign panel. Secondly, they generally use flat panels, and a source located at one or both ends of a light box may generate an illumination gradient down the length of such a flat panel sign. Thirdly, the manufacturing method of the panel with the display character, such as is described in WO 03/010738, is complicated, generally requiring at least two silk screen printed layers, one light blocking layer and one diffusively reflective layer, and may possibly not provide an optimally illuminative effect. Also, most of the above-described prior art signs are single-sided, and signs visible from both sides have to be constructed by placing a pair of signs back-to-back.

There therefore exists a need for an improved internally illuminated sign, which overcomes many of the disadvantages of such prior art signs. Besides the primary aim of improvement in the illumination effect, the sign should preferably have good performance under limited solar illumination levels, should be reliable even in winter time, and should provide an indication of low battery charge capacity and continued operation even when the battery charge capacity is low. The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.

The present invention seeks to provide a new method and apparatus for providing uniform illumination on the display screen of an internally illuminated sign, such as is generally illuminated from an end by means of an optical source. The method is based on the application of a curvature to the display screen, which compensates for the effect of the fall-off of illumination with distance from the source. For any uniform source, having an approximately uniform angular intensity profile over its useable angular range, each angular segment provides the same luminous flux. However, angular segments illuminating regions of the screen further from the source provide a lower level of illumination on the screen, because of the increased area which the divergence of the angular segment must illuminate. In order to compensate for this decreased illumination, the display screen, according to the present invention, is curved inwards towards the illuminating beam such that the further the point of impingement of the beam from the source, the more close to normal is the angle of incidence of the impinging beam on the screen, so as to maintain the same length of screen illuminated by the same angular segment. In this manner, the curvature of the screen is able to compensate for the fall off of the illumination with distance from the source. In order to clarify the nomenclature used in this application, it should be noted that the term illumination is often used in its popular sense, to mean the luminous flux per unit area of a surface, which is also known as the luminance or brightness of the surface. However, occasional use has also been made of terms such as intensity, or intensity of illumination, or likewise, and it is to be understood that the meaning, when related to the display surface, is intended to mean the luminous flux per unit area of the surface, or the brightness of the surface.

Such a curved display screen has a number of applications in the field of low power, internally illuminated signs, such as are used for house numbering at night, for street signs, for advertising, for emergency lighting, and other similar applications. When such a curved screen is used in such signs, a more uniform level of illumination is achieved than with prior art signs using flat screens to display their information. Furthermore, because the curved display screen allows compensation of the geometric illumination fall-off effect, it is possible to provide more direct illumination on the display screen itself, thus placing less reliance for the screen illumination on energy-robbing reflection effects. As a consequence, the luminous efficiency of signs constructed with such curved display screens is enhanced.

The light propagating within the sign is preferably reflected by a high reflectivity coating, most preferably specularly reflective, applied to the rear side of the opaque parts of the display characters. Though the term character generally means letter or digit, it is to be understood that the present invention is equally applicable for use with any form displayed on the illuminated screen, whether textual or graphic. The term character is thus used throughout this application, and is also thuswise claimed, to mean any such letter, digit, symbol or any other graphic form. The light reflected from the rear of the reflective display panel can preferably be returned to the display screen by means of a reflective back-plate, which essentially generates a mirror image of the rear of the reflective display panel. The use of a highly reflective rear surface for the panel indices is a significant advantage over prior art display panels using diffusively reflective back surface indices, because the increased reflectivity increases the light utilization. This is particularly significant for the direct illumination of the curved display panel of the present invention.

According to further preferred embodiments of the present invention, there are provided double sided signs, using a pair of curved display screens illuminated uniformly by sources disposed between the display screens and either at one end of the sign, or in the center of the sign, without the need for any additional optically active surfaces other than the display panels themselves on the two sides of the sign.

The present invention also seeks to provide new methods and systems for providing efficient power management in solar charged illuminated signs in general. A battery discharge system is provided which adjusts itself to the solar illumination level by measuring the battery depth of discharge (DOD) or state of charge (SOC). It also provides mutual battery back up to ensure use of the full service life of the batteries, and provides indication of the impending end of battery function, but without resulting in the extinction of the sign.

In order to accomplish these features of the present invention, firstly, in a multi-character sign made up of several single character signs, all of the batteries in the separate units making up the composite sign are connected in parallel, thus providing back-up functionality to each other. Secondly, the electronic system checks the charge level of the batteries, either continuously or at predetermined intervals. When this level drops below a certain predetermined level, a circuit in the sign is operative to reduce the illuminating power. If this reduction of power does not cause recovery of charge level of battery, power is further reduced, until a balance is achieved between the available solar charging level, and the illumination level of the sign, taking into account the current condition of the battery. In this way, the sign is provided with a power management system which adjusts the sign's performance level in accordance with the solar illumination conditions at the site of mounting of the sign, whether based on seasonal or even daily variation of the solar intensity, or on the initial installation location of the sign, and also taking into account the charge holding capacity of the battery.

When the battery is close to the end of its useful life, typically after several years, its charge level even when fully charged drops below a certain predetermined level, and an indication is given that the battery life is close to its end, such as by causing the sign to start blinking. By this means, warning is given that the battery charge holding capacity is low, and that the battery or batteries have to be exchanged, without stopping the sign from fulfilling its function.

There is thus provided in accordance with a preferred embodiment of the present invention, a sign, internally illuminated by at least one source of light, comprising a first display surface illuminated by the light, the display surface comprising regions generally opaque to the light and regions at least partly transmissive to the light, the regions at least partly transmissive to the light having the form of at least one character to be displayed by the sign, wherein the at least one source of light is mounted in the sign such that the light is at least partially directed at the display surface, and wherein the display surface has a curved contour, concave to the at least one source, such that light from the at least one source impinges at a point on the display surface at an angle of incidence which varies according to the distance of the point of impingement from the source.

Additionally and preferably, the above mentioned sign may also comprise at least one reflective surface disposed adjacent to and facing the first display surface. In such a case, this at least one reflective surface disposed adjacent to and facing the first display surface may preferably be a second display surface illuminated by the light, the second display surface comprising regions generally opaque to the light and regions at least partly transmissive to the light, the regions at least partly transmissive to the light having the form of at least one character to be displayed by the sign, and the regions generally opaque to the light having a reflective back facing the first display surface.

In accordance with still another preferred embodiment of the present invention, in any of the above-mentioned internally illuminated signs, the curved contour may preferably be shaped such that variations in the luminous flux per unit area of the light impinging on at least the first display surface, as a function of the distance of the point of impingement from the at least one source, is reduced compared with variations in the luminous flux per unit area impinging on a flat display surface. Alternatively and preferably, the curved contour may be shaped such that the luminous flux per unit area of the light impinging on at least the first display surface is essentially independent of the distance of the point of impingement from the at least one source of light.

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