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You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION 
     The present invention relates to skylights, and more particularly to tubular skylights, which include a reflective tube extending downwardly from the dome. 
     Tubular skylights have acquired increasing popularity as a means of introducing natural light into a building interior. These skylights include a dome mounted on the building roof, a light diffuser mounted in the building ceiling, and a reflective tube interconnecting the dome and the diffuser. Natural light entering the skylight through the dome reflects downwardly through the tube to the diffuser. The tube in a sense acts as a gigantic optical fiber. Typically, the domes are fabricated of acrylic; and the tube is fabricated of reflective aluminum. A tubular skylight of this type is sold by ODL, Incorporated, the assignee of the present invention, under the trademark EZ LIGHT. 
     The efficiency of such skylights (i.e. the amount of natural light reaching the building interior) is primarily a function of the amount of light passing through the dome into the tube and of the reflective efficiency of the tube. It is desirable to channel or steer as much light as possible downwardly through the tube to illuminate the building interior. One such approach includes positioning a reflector inside the dome above the roof to reflect light downwardly into the tube. A tubular skylight of this type is illustrated in U.S. Pat. No. 5,099,622, issued Mar. 31, 1992, to Sutton, and entitled “Skylight.” However, this approach is relatively complex structurally, relatively expensive, and aesthetically deficient. Further, the reflector may actually decrease the performance of the skylight when no direct sunlight is present, because the reflector blocks a portion of the ambient light. 
     SUMMARY OF THE INVENTION 
     The aforementioned problems are overcome in the present invention wherein the dome of a tubular skylight includes an exterior surface having a prismatic portion to reflect light downwardly into the tube. Preferably, the prismatic portion is located on the northern portion of the dome. Consequently, sunlight entering the southern portion, and to a lesser extent the eastern and western portions, of the dome at relatively low angles is reflected by the prismatic surface downwardly into the tube. 
     In the disclosed embodiment, the dome is hemispherical including a base and an apex. The prismatic surface includes a plurality of vertical grooves each extending between the base and the apex along great circles passing through the apex. The grooves begin at the base and terminate short of the apex. Further preferably, the grooves are located in an angular segment of the hemispherical dome. 
     The prismatic surface increases the amount of light directed or steered downwardly through the tube. The enhancement is most notable when the sun is relatively low in the sky as in the morning, the late afternoon and evening, and during the winter months. The dome also increases both the portion of the day and the number of seasons during which the skylight provides effective interior lighting. 
     These and other objects, advantages, and features of the invention will be more fully understood and appreciated by reference to the detailed description of the preferred embodiment and the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view showing a tubular skylight having the dome of the present invention mounted within a building; 
     FIG. 2 is a perspective exploded view of the tubular skylight; 
     FIG. 3 is a top plan view of the dome; 
     FIG. 4 is a sectional view of the dome taken along line IV—IV in FIG. 3; 
     FIG. 5 is a fragmentary sectional view of the prismatic portion of the dome showing the grooves in the exterior surface; 
     FIG. 6 is a schematic illustration of noon-day sun rays at the vernal and autumnal equinoxes; and 
     FIG. 7 is a schematic illustration of morning sun rays at the vernal and autumnal equinoxes. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A tubular skylight constructed in accordance with a preferred embodiment of the invention is illustrated in FIGS. 1 and 2 and generally designated  10 . As perhaps most clearly illustrated in FIG. 2, the skylight includes a dome assembly  12 , a diffuser assembly  14 , and a tube assembly  16  interconnecting the dome and diffuser assemblies. The skylight  10  is installed in a building B having roof R and ceiling C. More particularly, the dome assembly  12  is mounted within the roof R; and the diffuser assembly  14  is mounted within the ceiling C. The tubular assembly  16  extends between the dome assembly  12  and the diffuser assembly  14  to channel light from the dome to the diffuser. With the exception of the dome, the skylight  10  is generally well known to those skilled in the art. 
     The dome assembly  12  includes a dome  20  and a roof flashing  22 . The dome  20 , which is new, will be described in greater detail below. The flashing  22  mounts within a building roof R to provide a structural support for the dome  20 . The roof flashing  20  includes a curb  24  and an integral flashing flange  26  extending therefrom. The roof flashing  22  is available in a variety of constructions to accommodate shingle roofs, tile roofs, and other selected applications. 
     The diffuser assembly  14  includes a diffuser  30 , a ceiling trim ring  32 , and a tube/ring seal  34 . The diffuser  30  is a prismatic light diffuser. The ceiling trim ring  32  supports the diffuser  30  within the ceiling C. The tube/ring seal  34  fits about the tube assembly  16  as will be described and provides a mechanical interlock between the tube assembly and the diffuser assembly  14 . 
     The tube assembly includes upper and lower adjustable tubes  40  and  42 , respectively, and an interconnecting adjustable tube  44 . The upper adjustable tube  40  fits within the roof flashing  22 , and the lower adjustable tube  42  connects to the diffuser assembly  14  by way of the tube/ring seal  34  as will be described. The adjustable tube  44  telescopically interfits with both of the adjustable tubes to accommodate a variety of heights of the roof R above the ceiling C. Additional adjustable tubes  44  can be used as necessary to accommodate unusual heights between the roof and the ceiling. 
     Again, as thus far described, the tubular skylight components are conventional and generally well known to those in the relevant art. The novelty of the present invention resides in the dome  20  to be described hereinafter. 
     The dome  20  is illustrated most clearly in FIGS. 3 and 4. The dome includes a circular base  60  and a generally hemispherical portion  62  extending upwardly therefrom and reaching an apex  63 . The base  60  includes steps  64  and  65  that fit over and receive the stepped curb  24  of the roof flashing  22  (see FIG.  2 ). The second step  65  rests on the top of the curb and is defined by four pairs of fingers  66  located at 90° intervals around the circumference of the base  60 . Holes  67  are provided to receive fasteners to secure the dome to the curb. 
     The generally hemispherical dome portion  60  includes an interior surface  70  and an exterior surface  72 . The exterior surface includes a prismatic surface or portion  68  and a nonprismatic surface or portion  69 . The prismatic portion is illustrated perhaps most clearly in FIG.  3  and includes the patterned surface covering somewhat less than the left half of the dome in a pattern as described below. The interior surface  70  has a radius  72 , and the exterior surface  74  has a radius  76  in the nonprismatic portion and a radium  78  in the prismatic portion. The radius  76  is slightly greater than the radius  78 . The nonprismatic portion  69  is generally uniform in thickness between the inner surface  70  and the exterior surface  74 . The dome portion  60  has an increased thickness in the prismatic portion  68 . Because the prismatic surface is uneven (i.e. grooved) the distance between the interior surface and the exterior surface varies. The minimum thickness in the prismatic portion  68  is approximately equal to the thickness in the nonprismatic portion  69 , and the maximum thickness in the prismatic portion is approximately twice the thickness in the nonprismatic portion. 
     The shape and configuration of the prismatic portion  68  is perhaps best illustrated in FIG.  3 . The prismatic portion  68  includes a plurality of grooves  71  that molded, cut, or otherwise formed in the exterior surface  74 . Each of the grooves  71  extends along a great circle passing through the apex  63 . Each of the grooves  71  extends from the base  64  to a location short of the apex  63 . In the preferred embodiment, 37 first grooves  71  are formed at 4° intervals, and 38 second grooves  71  are formed at 4° intervals offset 2° from the first set of grooves so that each first groove is bracketed by a pair of second grooves. As currently contemplated, the grooves  71  are formed by molding; however, other forming techniques, such as cutting, can be used. 
     The exterior angle between the walls of a groove  71  when using the preferred material is preferably in the range of 86° to 94°, with the most preferred angle being 92°. The groove angles may change with other materials depending on their indices of refraction. The angle is selected so that direct light from the dome interior is reflected by the internal reflection of the prism—not refracted—as it strikes the interior side of the groove walls. The structure and effect of the described technique is disclosed in U.S. Pat. No. 4,839,781, issued Jun. 13, 1989 to Barnes et al, and entitled “Reflector/Refractor.” 
     The prismatic portion  68  comprises an angular segment of the hemispherical dome. In the preferred embodiment, this segment is approximately 148° of the 360° circumference. The grooves  71  stopping short of the apex leaves a pie-shaped portion  82  surrounding the apex of the dome that is part of the nonprismatic portion  69  of the exterior surface. 
     The entire dome  20  is fabricated of a single piece of acrylic. The currently preferred material is that sold under the designation V825UVA-5A by Rohm &amp; Haas. For a dome 10 inches in diameter, the dome portion  60  is 0.114 inch thick in the nonprismatic portion and up to 0.204 inch thick in the prismatic portion. Other materials suitable for skylight domes may be used and includes polycarbonates and nylons. Other materials may be used if they provide the light transmittance and strength characteristics required in skylight domes. 
     The particular pattern of the prism will depend on the performance desired and the anticipated location of the skylight. The illustrated dome has been designed for use at 40° latitude as representative of a “normal” U.S. location. The pattern follows the highest path of the sun, which of course occurs during the summer. 
     The light reflectance provided by the prismatic portion  68  is perhaps best illustrated in FIG.  5 . Each of the grooves  71  provides two apparent reflective surfaces to light rays striking the surfaces from inside the dome because of the high index of refraction. Consequently, light impinging on the grooves  71  from the interior of the dome are reflected back into the interior of the dome. 
     Turning specifically to FIG. 5, a light ray L from the interior of the dome passes through the interior surface  70 , then reflects off the surfaces of two grooves  71  to be returned to the dome interior. Consequently, light at low angles which would pass directly through the dome is instead reflected back into the dome interior. 
     The prismatic portion  68  does not significantly block ambient light from passing through the dome. Therefore, the dome does not significantly reduce the amount of ambient light; and the dome does not decrease the amount of direct light passing into the skylight. The only losses (approximately 8% in the preferred material) are due to the material from which the dome is fabricated. 
     Assembly and Operation 
     The tubular skylight  10  is installed within a building in conventional fashion. Vertically aligned holes are cut in the roof R and the ceiling C. The roof flashing  22  is installed in the roof. The upper adjustable tube  40  is fitted within the curb  24  of the roof flashing  22  and slid downwardly until the upper edges of both are aligned. The dome  20  is fitted over the curb  24  (with the upper adjustable tube  40  fitted therein) and secured in position using screws (not shown). 
     The ceiling trim ring  32  is secured to the underside of the ceiling C. The tube/ring seal  34  is placed over the lower adjustable tube  42 , and the assembly is pushed into the ceiling trim ring from above the ceiling C. The extension tube  44  is then slid as necessary to a connecting position between the upper and lower adjustable tubes  40  and  42 . All seams are taped with duct tape. Finally, the diffuser  30  is installed within the trim ring  32  using a partial-turn coupling. 
     FIGS. 6 and 7 illustrate the functional performance of the new dome  20 . Turning first to FIG. 6, the dome  20  and tube assembly  16  are schematically illustrated. Direct light rays  90  are shown entering the skylight at a 50° angle from the horizontal, which is the angle of the sun at 40° latitude and 98° longitude on the vernal and autumnal equinoxes. When the sun is at this angle, virtually all of the direct rays  90  pass through the nonprismatic portion of the dome  20  to enter the skylight  10  in conventional fashion. The reflected rays  90 ′ are illustrated in dotted lines and illustrate how the light is reflected downwardly through the skylight assembly. 
     FIG. 7 illustrates the performance of the skylight dome when the sun is relatively low in the sky. Specifically, the direct sunlight rays  90  arrive at the skylight dome basically on the horizontal. The direct rays  90  pass directly through the nonprismatic portion  69 . Without the prismatic portion  68  of the present invention, the direct rays  90  would continue to pass through the skylight dome so that none of those rays would pass downwardly into the tube. Instead, the prismatic portion  68  reflects the direct rays  90  downwardly through the dome at a variety of angles. The reflected rays  90 ′ are illustrated as dashed lines and pass downwardly at a variety of reflected angles. 
     The object of the present invention is to direct light downwardly through the tube at virtually any angle on the premise that all of the downwardly direct light enhances, even in some small fashion, the light exiting the bottom of the tube. Additionally, as can be seen, some of the rays pass directly through the tube without further reflection off the tube wall to the bottom of the tube assembly  16 . 
     The present invention greatly enhances the performance of the tubular skylight by directing or steering a larger percentage of the available light downwardly through the tube. The prismatic portion  68  enhances the performance of the skylight tube particularly when the sun is relatively low in the sky as occurs in the morning, the late afternoon, and the winter. 
     The above description is that of a preferred embodiment of the invention. Various alterations and changes can be made without departing from the spirit and broader aspects of the invention as defined in the appended claims, which are to be interpreted in accordance with the principles of patent law including the doctrine of equivalents.

Summary:
A tubular skylight having an improved dome improving the efficiency of the skylight. The dome includes an integral prism in a portion of its outer surface to reflect light downwardly through the skylight. The dome is hemispherical, and the prism includes a plurality of grooves extending along great circles that pass through the apex of the dome. The prism covers only an angular segment of the hemispherical dome; and the grooves stop short of the apex of the dome.