Abstract:
An outdoor lighting fixture or luminaire includes an enclosure having within it a light source for projecting light through an opening in the enclosure and a motor train for slowly moving a transparent plastic film across the opening and over the light emitting surface of the light source, thereby preventing the light emitting surface or lens from becoming dirty over time.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims the priority of U.S. Provisional Application No. 60/234,383, entitled LIGHTING DEVICE WITH PERPTUALLY CLEAN LENS, filed on Sep. 20, 2000, the entirety of which is incorporated by reference herein. 
    
    
     FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     This invention relates to outdoor lighting fixtures and more specifically to methods of maintaining clean light emitting surfaces or lenses on such devices. 
     2. Background Art 
     Luminaires, such as outdoor street lighting fixtures, are exposed to environmental pollution and dirt and dust. Many of these luminaires are not easily accessible for cleaning. In environments, classified as very dirty, a typical luminaire can lose 50% efficiency due to dirt within 24 months unless it is periodically cleaned. This loss of efficiency represents a large amount of wasted energy in the United States and other countries every year. 
     There is thus a need for the automatic cleaning of luminaires, such as outdoor street lighting fixtures. 
     SUMMARY OF THE INVENTION 
     The human eye maintains clean lenses by a constant fluid washing from the tear ducts. This approach, using a fluid wash in front of a lens, is not practical for luminaire lenses that have a long-term exposure (&gt;1 year) to the environment without cleaning. My invention substitutes a plastic film over a lens for the same function that tears accomplish in the human eye. 
     An automatic plastic film feed device is placed in front of a light source. This plastic film is constantly moving, but at a very slow rate (˜1 inch per month). Dirt and pollution from the environment are deposited on this plastic film instead of on lenses or other clear light emitting surfaces that are now placed in front of typical light sources. 
     The buildup of dirt on a lens from the environment is a fairly slow process and the lumen dirt depreciation factor (LDD) can be approximated by a formula provided in the Illumination Engineering Society of North America Lighting Handbook, 9th Edition as follows: 
     
       
           LDD=e   −[At]     B     (Eq.1) 
       
     
     where A is a constant that represents the cleanliness of the environment and varies in a range from 0.038 to 0.396; B is a constant that represents the category of the luminaire and varies in a range from 0.53 to 0.88; and where t represents time in decimal years. 
     A lighting fixture in accordance with my invention is especially suitable for light areas where it either is inconvenient or impossible to change light sources or lenses, such as nuclear reactors or tunnels. Advantageously, this invention can be combined with a remote light source where light is provided to the apparatus via fiber optic cable, thereby allowing for the light source to be changed without access to the lighting enclosure required. Also advantageously, my invention can be powered by a spring-powered motor, such as is used in a clock or a child&#39;s wind-up toy so that neither external power supply nor batteries, which are prone to chemical leakage, are required. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 illustrates a lighting fixture in accordance with an illustrative embodiment of my invention; and 
     FIG. 2 illustrates a spring-powered motor and a gear drive that may be employed in the embodiment of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, a lighting enclosure  1  with a perpetually clean lighting surface or lens in accordance with an illustrative embodiment of my invention is shown. The lighting enclosure  1  is preferably manufactured from a material that is durable when exposed to an outdoor environment, such as aluminum or UV-resistant plastic. Such materials are well known in the art. The lighting enclosure  1  encloses a light source  20 , located therein, and has an opening  2  on one sidewall  3  that is preferably square or round-shaped although other shapes can be used as well. A thin transparent plastic film  4 , such as polyethylene, polyester, or a fluoropolymer, is strung across and completely covers the opening  2  at the inside surface of the sidewall  3 . It is preferable that the transparent plastic film  4  is provided with an antireflective coating in order to minimize fresnel reflection losses. The film should not soil easily, either through inherent anti-stick properties of fluoropolymer films, or by avoiding the attractive nature as a result of static buildup. Such buildup can be minimized by employing conductive coatings or additives in the bulk polymer. The sidewall  3  preferably provides a conductive path for the discharge of residual static buildup on plastic film  4 . It is further preferred that the transparent plastic film resists stretching. The transparent plastic film  4  is supplied in the form of a roll and is pulled from a clean film supply reel  5  to a film take-up reel  6 . 
     As depicted in FIG. 2, the film take up reel  6  is driven in a circular motion by a constant speed spring-powered motor  11  and a gear train  12  in order to pull the transparent film  4  from the clean film supply reel  5 . In this embodiment, the clean film supply reel  5  is equipped with an automating tensioning device  13  in order to maintain tension on the transparent plastic film  4  as it moves across the opening  2 . The spring-powered motor  11  and the gearing ratio of the drive gears of the gear train  12  determine the speed at which the transparent plastic film  4  is pulled across the opening  2 . The film  4  is preferably pulled at a speed to meet or exceed the LDD requirements. In one embodiment of my invention, a 25-foot long roll of transparent plastic film  4  is provided and the spring-powered motor  11  and gear train  12  are set to pull the film at 1 inch per month. The apparatus resulting from this embodiment of my invention has, therefore, a 25-year operational life. 
     In certain embodiments of my inventions, built up debris such bird droppings and ice are removed from the film  4  by using devices known in the art such as scrapers. These prior art devices are not further discussed herein. 
     Referring again to FIG. 1, in this illustrative embodiment of my invention, light is supplied to the lighting enclosure  1  via a plurality of external fiber optic cables (not shown) that are connected to an optical coupling connector  7 , which delineates light source  20  within the enclosure. Light is then fed from the fiber optic connector  7  to an array of optical waveguides  8  via connecting fiber optic cables  9 . The array of optical waveguides  8  has a flat surface  10  that allows for the movement of transparent plastic film  4  with minimal friction, the flat surface  10  serving as the light emitting surface or lens of the lighting fixture or luminaire in this embodiment. 
     In alternate embodiments of my invention, mechanisms other than optical waveguides  8  and connecting fiber optic cables, such as a direct lens system, may be used to transmit light from the optical coupling connector  7  through the transparent plastic film  4 . In even other embodiments of my invention, a long-life light source is placed within the lighting enclosure  1 , and the optical coupling connector and the connecting fiber optic cables  9  are replaced with a power connector for the light source. 
     In order to operate my invention, the roll of transparent plastic film  4  is loaded onto the clean film supply reel  5  and one end of the plastic film  4  is threaded across the opening  2  in the lighting enclosure  1  and affixed to the film take up reel  6 . The spring-powered motor  11  is wound and then allowed to rotate, causing the film take up reel to slowly rotate. The rotation speed of the film take up reel is determined by the gear train  12  and is set according to the cleanliness of the environment and the size of the opening. As noted above, the rotation speed may advantageously be set to pull the transparent plastic film across the opening at 1 inch per month. 
     Alternate embodiments may be devised without departing from the spirit or the scope of the invention.