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This is a continuation of and claims priority to U.S. patent application Ser. No. 11/438,178, filed May 22, 2006. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to skylights. 
     BACKGROUND OF THE INVENTION 
     In U.S. Pat. Nos. 5,896,713 and 6,035,593, both of which are owned by the same assignee as is the present invention and both of which are incorporated herein by reference, tubular skylights are disclosed. Both of the skylights can use the skylight dome disclosed in U.S. Pat. No. 5,896,712 also owned by the same assignee as is the present invention and also incorporated herein by reference. These inventions represent advances over the prior art and one or more of them has found commercial success. 
     Briefly, a tubular skylight such as those mentioned above includes a tube assembly mounted between the roof and ceiling of a building. The top end of the tube assembly is covered by a roof-mounted dome or cover, such as the one disclosed in the above-mentioned &#39;712 patent, while the bottom end of the tube assembly is covered by a ceiling-mounted diffuser plate. With this combination, natural light external to the building is directed through the tube assembly into the interior of the building to illuminate the interior. 
     Tubular skylights use a near specular finish reflective surface to transport sunlight down the tube from the roof to the interior ceiling. “Specular” means that reflected direct rays of sunlight maintain a near parallel beam of light as they reflect down the tube if the tube sides are parallel and the specular reflective surface is maintained. 
     The present invention understands that sunlight enters the tube at various incident angles based on time of day/year, latitude, and tube opening plane location. Despite the fixed position of the tube, direct beam sunlight reflects down the perimeter of the tube at approximately the same elevation angle as it enters the tube. As understood herein, this can result in the following undesirable outcomes. First, the parallel beam sunlight can converge at concentrated focal points at various locations down the tube, resulting in potentially dangerous hot spots that can cause fires particularly in the presence of combustible materials. Second, uneven illumination results at the base diffuser of the tube, because the perimeter path of the light rays in combination with the focal points can cause partial and non-uniform illumination of the diffuser. The result is poor illumination performance and glare from the diffuser. Additionally, the direct light beams passing through a prism in the diffuser can cause the separation of the wavelengths and project rainbows into the interior. 
     Accordingly, the present invention makes the following critical observations. Direct beam sunlight reflected from a specular surface in the approximate shape of a tube can create hot spots that are unsafe and that reduce product performance due to non-uniform illumination. As further understood herein, simply reducing the specularity of the tube results in reduced light transmission. Likewise, installing a diffuser above the tube to address glare and hot spots reduces the total system performance due to the transmission loss of this extra diffuser and the increased tube reflections caused by the light spread. Moreover, attempting to remedy the above-noted shortcomings using a random patterned reflector results in the light being diffused in a hemispherical shape that may send greater than 50% of the light back up the tube, therefore once again reducing performance. With the above observations in mind, the invention herein is provided. 
     SUMMARY OF THE INVENTION 
     The interior of a skylight tube is formed with structures that change sunlight beam angles to prevent the formation of a focal point, and/or that mix the light to uniformly illuminate the base diffuser and eliminate glare and color separation, and/or that control light direction to prevent retro-reflection and excessive reflections. 
     Accordingly, in a first aspect a skylight assembly has a transparent dome and a skylight shaft substrate extending away from the dome to convey light entering the dome through the shaft substrate. Surface irregularities are formed on the shaft substrate. The surface irregularities can be dimples or longitudinal corrugations. 
     In some implementations the shaft substrate is metal and is polished to provide a reflective surface without any reflective film incorporated into the assembly. In other implementations the shaft substrate is metal and a reflective metal is deposited as by vapor deposition directly on the inside surface of the shaft substrate to render the assembly internally reflective without incorporating an adhesive into the assembly. In yet other implementations the shaft substrate may be a reflective film with a metallic substance being vapor-deposited onto the film or with a reflective multi-layer polymer composite being adhered to the substrate. 
     When the surface irregularities are longitudinal corrugations, each corrugation defines a midline and opposed edges, and a first angle is formed between transverse tangents to edges of first corrugations. Also, a second angle different from the first angle is formed between transverse tangents to edges of second corrugations. The first and second corrugations alternate around the circumference of the shaft substrate, and may be V-shaped or U-shaped in transverse cross-section. 
     When the surface irregularities are plural dimples, each dimple defines a center and a periphery. A tangent to the periphery establishes an angle with respect to a tangent to the center of no more than two degrees. 
     In another aspect, a skylight assembly includes a transparent dome and a skylight shaft substrate extending away from the dome to convey light entering the dome through the shaft substrate. Surface irregularities are formed on the shaft substrate. In this aspect, the shaft substrate is metal, and a reflective metal is deposited directly on the inside surface of the shaft substrate to render the assembly internally reflective without incorporating an adhesive into the assembly. 
     In yet another aspect, a skylight assembly includes a transparent dome and a skylight shaft substrate extending away from the dome to convey light entering the dome through the shaft substrate. Surface irregularities are formed on the shaft substrate. In this aspect, the shaft substrate is metal and is polished to provide a reflective surface without any reflective film incorporated into the assembly. 
     In still another aspect, a skylight assembly includes a transparent dome and a skylight shaft substrate extending away from the dome to convey light entering the dome through the shaft substrate. Surface irregularities are formed on the shaft substrate. In this aspect, the shaft substrate is a reflective film with a metallic substance being vapor-deposited onto the film. 
     In another aspect, a skylight assembly includes a transparent dome and a skylight shaft substrate extending away from the dome to convey light entering the dome through the shaft substrate. Surface irregularities are formed on the shaft substrate. In this aspect, the shaft substrate is a reflective film including a reflective multi-layer polymer composite. 
     The details of the present invention, both as to its structure and operation, can best be understood in reference to the accompanying drawings, in which like reference numerals refer to like parts, and in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view in partial cross-section of the tubular skylight of the present invention; 
         FIG. 2  is a perspective view of the present tube with longitudinal corrugations for reducing focal points; 
         FIG. 3  is a side elevational view of the inside of the tube shown in  FIG. 2 ; 
         FIG. 4  is a top plan view of the tube shown in  FIG. 2 ; 
         FIG. 5  is a detail of part of the circumference of the tube in circle “ 5 ” of  FIG. 4 ; 
         FIG. 6  is a perspective view of an alternate tube with dimples for reducing focal points; 
         FIG. 7  is a top plan view of the tube shown in  FIG. 6 ; 
         FIG. 8  is a side elevational view of the tube as seen along the line  8 - 8  in  FIG. 7 ; and 
         FIG. 9  is a detail of a dimple in circle “ 9 ” of  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring initially to  FIG. 1 , a tubular skylight made in accordance with the present invention is shown, generally designated  10 , for lighting, with natural sunlight, an interior room  12  having a ceiling dry wall  14  in a building, generally designated  16 .  FIG. 1  shows that the building  16  has a roof  18  and one or more joists  20  that support the roof  18  and ceiling dry wall  14 . 
     As shown in  FIG. 1 , the skylight  10  includes a rigid hard plastic or glass roof-mounted cover  21 . The cover  21  is optically transmissive and preferably is transparent. In one embodiment, the cover  21  can be the cover disclosed in the above-mentioned &#39;712 patent. Or, the cover  21  can be other suitable covers, such as the covers marketed under the trade name “Solatube” by the present assignee. 
     The cover  21  can be mounted to the roof  18  by means of a ring-like metal flashing  22  that is attached to the roof  18  by means well-known in the art. The metal flashing  22  can be angled as appropriate for the cant of the roof  18  to engage and hold the cover  21  in the generally vertically upright orientation shown. 
     As further shown in  FIG. 1 , an internally reflective hollow shaft assembly, generally designated  24 , is connected to the flashing  22 . The cross-section of the assembly  24  can be cylindrical, rectangular, triangular, etc. Accordingly, while the word “tube” may be used from time to time herein, it is to be understood that the principles of the present invention are not to be limited to a cylinder per se unless otherwise specified. 
     The shaft assembly  24  extends to the ceiling  14  of the interior room  12 . Per the present invention, the shaft assembly  24  directs light that enters the shaft assembly  24  downwardly to a light diffuser assembly, generally designated  26 , that is disposed in the room  12  and that is mounted to the ceiling  14  or to a joist  20  as described in the above-mentioned &#39;593 patent. 
     The shaft assembly  24  can be made of a metal such as an alloy of aluminum or steel, or the shaft assembly  24  can be made of plastic or other appropriate material. The interior of the shaft assembly  24  may be rendered reflective by means of, e.g., electroplating, anodizing, metalized plastic film coating, or other suitable means. In one preferred embodiment, the shaft assembly  24  is rendered internally reflective by laminating the inside surface of the shaft assembly with a multi-ply polymeric film made by Minnesota Mining and Manufacturing (3M). A single ply of such film is transparent, but when hundreds of layers are positioned flush together and then thermally laminated to the interior surface of the shaft assembly  24 , the combination is specularly reflective. 
     Thus, in non-limiting implementations the shaft may be made of a composite of a metal substrate, e.g., aluminum or steel, with a reflective film adhered to the shaft using an adhesive. The shaft substrate can also be made of a polymer with the reflective film bonded to it. 
     Alternatively, if the shaft is metal, the shaft substrate can be polished to provide a reflective surface or have a highly reflective metal such as silver or aluminum vapor deposited directly to its surface without the need for a separate adhesive. 
     In yet other implementations a reflective film itself may be used as a shaft substrate. In such an implementation, metal is vapor-deposited onto the film surface. Or, the film can include a reflective multi-layer polymer composite. 
     In one preferred embodiment, the shaft assembly  24  is established by a single tube. However, as shown in  FIG. 1 , if desired, the shaft assembly  24  can include multiple segments, each one of which is internally reflective in accordance with present principles. Specifically, the shaft assembly  24  can include an upper shaft  28  that is engaged with the flashing  22  and that is covered by the cover  21 . Also, the shaft assembly  24  can include an upper intermediate shaft  30  that is contiguous to the upper shaft  28  and that can be angled relative thereto at an elbow  31  if desired. Moreover, the shaft assembly  24  can include a lower intermediate shaft  32  that, if desired, may be slidably engaged with the upper intermediate shaft  30  for absorbing thermal stresses in the shaft assembly  24 . And, a lower shaft  34  can be contiguous to the lower intermediate shaft  32  and join the lower intermediate shaft  32  at an elbow  35 , with the bottom of the lower shaft  34  being covered by the diffuser assembly  26 . The elbow  35  may be angled as appropriate for the building  16  such that the shaft assembly  24  connects the roof-mounted cover  21  to the ceiling-mounted diffuser assembly  26 . It is to be understood that where appropriate, certain joints between shafts can be mechanically fastened and covered with tape in accordance with principles known in the art. 
     In any case, the present shaft assembly  24  is formed with surface irregularities on part or all of its inner surface for reflecting light in a way that minimizes the chance of hot spots while nonetheless maximizing light throughput. The surface irregularities may be those described in the present assignee&#39;s U.S. patent publication no. 2003/0061775, incorporated herein by reference, or the corrugations or dimples disclosed herein. 
     More specifically,  FIGS. 2-5  show a hollow shaft  40  that may be used as any one of the shafts or shaft segments shown in  FIG. 1  and that is formed throughout its length and circumference with linear longitudinal corrugations  42 , it being understood that the corrugations  42  may be formed in only part of the length and/or circumference of the shaft  40 . As shown best in  FIG. 5 , the corrugations  42  can have a V-shape in transverse cross-section, although alternately they may be U-shaped. 
     In general, each corrugation can vary in its included angle from less than one hundred eighty degrees to greater than one degree, and preferably the included angle is greater than one hundred twenty degrees to minimize reflections. Also, the angle can vary around the shaft to provide a greater amount of mixing of the light before it reaches the base diffuser, as will be made clearer momentarily in reference to  FIG. 5 . 
     In addition to the controlled light spreading features, the corrugations  42  also allow easier bending of the substrate, when metal, to form a cylindrical shaft from a sheet, and to increase the lateral strength of the shaft  40  due to the increased moment of inertia this geometry provides. These two features allow easier assembly of small diameter tubing and the use of reduced caliper metal due to the increased strength. 
     With greater attention to preferred non-limiting details in  FIG. 5 , each corrugation  42  defines a midline  44  and opposed edges  46 ,  48 , with adjacent edges of adjacent corrugations joining each other as shown. While for illustration the midlines  44  in  FIG. 5  are defined to be the parts of the corrugations that are radially inset from the edges, hence establishing concave corrugations, the skilled artisan will readily appreciate that the convention can be reversed, i.e., that midlines can be defined to be the radially outside parts of the corrugations, hence establishing convex corrugations. 
     In any case,  FIG. 5  shows that an angle α is formed between transverse tangents to the edges  46 ,  48  of each corrugation. When the corrugation is V-shaped as shown, the transverse tangents are simply lines extending in the transverse dimension from the midline  44  to each edge  46 ,  48  as shown, i.e., the angle α is the angle of the “V”. As further shown in  FIG. 5 , odd-numbered corrugations can have a first angle α, e.g., one hundred twenty degrees, whereas even-numbered corrugations can form a second angle α, e.g., one hundred twenty five degrees, with the odd-numbered corrugations alternating around the circumference of the shaft  40  with the even-numbered corrugations. This geometry results as shown in an angle of two and a half degrees being established between the normal  50  to an edge  48  and the normal  52  to the adjacent midline  44 , and an angle of five degrees being established between the normal  52  to a midline  44  and the normal  54  to the next successive midline  44 . 
     Alternatively to corrugations,  FIGS. 6-9  show a dimpled hollow shaft  60  that may be used as any one of the shafts or shaft segments shown in  FIG. 1 . Specifically, the shaft  60  is formed throughout its length and circumference with dimples  62 , it being understood that the dimples  62  may be formed in only part of the length and/or circumference of the shaft  60 . 
     A dimple  62  may have any suitable shape, e.g., one of the myriad of shapes of golf ball dimples, e.g., spherical, elliptical, parabolic, hyperbolic, etc. In the illustrative non-limiting embodiment shown best in  FIG. 9 , each dimple  62  has a concave (or convex, depending on the perspective, i.e., outside or inside) saucer-like shape and hence can establish what might be thought of as the very bottom portion of spherical or parabolic or other bowl that is curvilinear throughout its surface. 
     Accordingly, in the non-limiting implementation shown, each dimple  62  defines a center  64  and a preferably circular periphery  66 . A tangent  68  to the periphery  66  establishes an angle β with respect to a tangent  70  to the center  64  of no more than two degrees (to give greater resolution for seeing the angle β in  FIG. 5 , the tangent  70  to the center  64  has been offset to the right of the center  62 .) Additionally, the distance in the radial dimension between the center  64  and periphery  66  preferably is less than one half inch (&lt;0.5 inches, or less than approximately one and one-quarter centimeters). By limiting the angle β to less than two degrees, the loss of low elevation sunlight is prevented. By limiting the distance in the radial dimension between the center  64  and periphery  66  to less than one-half inch, excessive reflections are prevented, which could otherwise eventually lead to sunlight being reflected back up and out of the dome  21  ( FIG. 1 ) and hence decrease the light throughput of the shaft  60 . 
     In non-limiting embodiments the corrugations and/or dimples described above can be formed in any appropriate way. In one non-limiting example the metal shaft, or the reflective film, or the adhesive can be formed or patterned with an embossing roller. In another example the metal shaft, film, or adhesive can be extruded or coated with the required profile in the extrusion or coating die. Yet again, the metal shaft, film, or adhesive can be molded with the required pattern in a mold tool. 
     While the particular SKYLIGHT TUBE WITH REFLECTIVE STRUCTURED SURFACE is herein shown and described in detail, the invention is to be limited by nothing except the appended claims.

Summary:
A skylight shaft substrate is rendered internally reflected in several ways. To limit the formation of focal points that can lead to hot spots as light travels down the shaft, the shaft substrate is formed with dimples or longitudinal corrugations.