Patent Publication Number: US-6910785-B2

Title: Industrial luminaire with prismatic refractor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application is related to co-pending U.S. Design patent application Ser. No. TBD (“Prismatic Refractor With Circumferential Prisms”), filed on Jan. 22, 2003, and co-pending U.S. Design patent application Ser. No. TBD (“Prismatic Refractor with Circumferential Prisms and Lens), filed on Jan. 22, 2003, which are both incorporated by reference here in their entirety. 

   TECHNICAL FIELD 
   The subject matter disclosed here generally relates to illumination, and, more particularly, to a light source support and modifier including a refractor. 
   BACKGROUND 
   The “INESA Lighting Handbook,” ninth edition, published by the Illuminating Engineering Society of North America, is incorporated by reference here in its entirety. As discussed in chapter seven of that book, a “luminaire” is a device for producing, controlling, and distributing light. It is typically a complete lighting unit consisting of one or more lamps, sockets for positioning and protecting the lamps and for connecting the lamps to a supply of electric power, optical devices for distributing the light, and mechanical components for supporting or attaching the luminaire. Luminaires are also sometimes referred to as “light fixtures.” 
   Luminaires are usually classified by their application, such as residential, commercial, or industrial. However, a particular luminaire can often be used in more than one application, depending upon its performance characteristics. For example, so-called “high bay” and “low bay” luminaires are often used for general lighting in industrial and other settings. Low bay luminaires are generally designed to provide adequate vertical illumination for spaces with mounting heights that are less than about 20 feet. High bay luminaires, on the other hand, are generally used in applications with mounting heights that are greater than 20 feet. However, high bay luminaires can also be used at lower mounting heights to produce light distributions that vary from narrow to wide, depending on the application and the need for vertical illumination. 
   For high bay, low bay, and other luminaires, the light distribution is often controlled using a “refractor.” Refractors are light control devices that take advantage of the change in direction that light undergoes as it passes through the boundary of materials having different optical densities (or indices of refraction), such as air to glass or air to plastic. This redirecting is typically accomplished with two-or three-dimensional prisms that are raised from, or embossed into, the surface of a translucent material, such as acrylic plastic, polycarbonate, or glass. When the prisms are formed on the surface of a substantially flat sheet of material, then the sheet is sometimes referred to as a “prismatic lens.” 
   The material that is used in a refractor may also have reflective characteristics, such as those that produce a phenomenon known as “total internal reflection.” In this phenomenon, light passes through the first surface of the refractive material and is mostly reflected from the second surface, back into the material, and out the first surface. Due to this and other reflective properties of many light-transmitting materials, the term “reflector” is sometimes loosely interchanged with the term “refractor” in connection with light distribution devices in luminaires. 
   Luminaire performance is typically described in terms of a combination of electrical, photometric, mechanical, thermal, and/or other characteristics. Photometric performance refers to the efficiency and effectiveness with which a luminaire delivers light to an intended target. The mechanical performance of a luminaire refers to its behavior under environmental extremes such as water spray, moisture, or dust, while thermal performance describes the behavior of the luminaire at elevated temperatures. 
   Photometric performance is often described in terms of various light distribution characteristics of a luminaire. For example, a “luminous intensity distribution curve” may be used to represent the variation of luminous intensity in a plane through the light center of the luminaire. (The light center is the center of the smallest sphere that would completely contain the light-omitting element of the lamp, often simply the center of the arc tube.) Indoor luminaires are typically described in terms of a vertical intensity distribution curve that is obtained by taking measurements at various angles of elevation about a source in a vertical plane through the light center. Unless otherwise specified, the vertical distribution curve is assumed to represent an average such as would be obtained by rotating the luminaire about its vertical axis where the origin, or “nadir,” is downward through the light center. 
   Luminous intensity values are typically recorded at various vertical elevations between 0° to 90° or 180°. The total number of lumens emitted by the luminaire can then be calculated from the luminous intensity distribution. Dividing this total number of lumens produced by the luminaire by the number of lumens that are omitted by the lamp then provides the overall efficiency of the luminaire. Luminous intensity can also be determined for nested, solid angle, cones having apexes at the luminaire photometric center. Each cone then defines a conic zone and the lumens within each zone are referred to as “zonal lumens.” This zonal lumen data is often presented for each zone as a percentage of the total lumens produced by the fixture (“% Fixture” or “% F”) and/or a percentage of the total lumens produced by the lamp, without the fixture (“% Lamp” or “% L”). 
   Photometric performance can also be described in terms of a luminaire spacing criterion, or “SC.” The SC of a luminaire is an estimated maximum ratio of spacing to mounting height above the work plane for a regular array of luminaires such that the work plane illumination will be acceptably uniform. SC is often loosely used interchangeably with spacing-to-mounting-height ratio, or “SMH” which is now a disfavored term. For luminaires with an adjustable SC, it is generally preferred that the beam width be adjustable over as broad a range as possible. 
   The mechanical and thermal performance characteristics of a luminaire can also be described in a variety of ways. For example, as noted above, certain applications require luminaires which are resistant to liquid and/or vapor infiltration. With regard to thermal performance, the effect of lamp heat on the luminaire materials can also be quite important because various refractor materials exhibit poor heat resistance above certain temperatures. 
   For example, although acrylic refractors can be formed using a wide variety of common manufacturing techniques, they generally have poor resistance to heat when used in lighting applications at temperatures above 90° C. Consequently, the maximum lamp wattage for a particular luminaire will often be limited by the temperature at which the acrylic refractor starts to discolor. Although so-called “high heat acrylics” or polycarbonate materials can sometimes be used to allow for higher-wattage lamps, they are generally more-expensive and can only withstand about an additional 20-50° C. increase in continuous operating temperature. 
   Cooper Lighting of Peachtree City, Ga. offers a wide variety of industrial luminaires under its LUMARK® brand. For example, Cooper&#39;s SS series of prismatic high-bay products and FP series of prismatic food processing luminaires are provided with acrylic prismatic refractors and have multiple field-adjustable lamp positions for providing various light-distribution patterns. The beam dispersion of these luminaires can be increased, or decreased, by positioning the lamp closer, or further, from the opening of the refractor. Cooper Lighting&#39;s SS, FP and HB Series of products are optionally provided with a prismatic drop lens for covering the opening of the refractor. Cooper Lighting and/or its parent, Cooper Industries, Inc. also own a variety of patents covering various aspects of industrial lighting including U.S. Pat. No. 4,403,277 which is incorporated by reference here in its entirety. 
   LexaLite International Corporation of Charlevoix, Mich. offers a line of acrylic and polycarbonate refractors that it refers to as its “800 Series Prismatic Reflexor®.” For example, information about LexaLite&#39;s Model 822 is available at www.lexalite.com/822.html, including images, zonal lumen, and other data. Conical lenses and drop lenses are also available for Lexalite&#39;s Model 822. These and other of LexaLite products are allegedly covered by U.S. Pat. Nos. 4,839,781, 5,446,606, and D367,337, each of which is also incorporated by reference here in its entirety. 
   U.S. Pat. No. 4,839,781 to Barnes et al. discloses a reflector/refractor device that is generally shaped as an inverted bowl. The body of the device is defined by a series of sectional zones that are formed as frustro-toroidal segments. The outside surface of the body is formed with a plurality of reflective/refractive prism elements that consist of curved and angled surfaces. Internal rays impinging on these surfaces will be reflected or refracted as the incident angle is greater than or less than the critical angle of the transparent material forming the body. 
   Each of the Barnes et al. zones has a predetermined radius with an origin that is offset from the vertical axis in order to create the bowl-shaped profile of the body. The light distribution characteristics for the top and bottom zones are selectively variable by vertical movement of the light source so as to increase or decrease the incident angle to the inner surface of the body. This, in turn, increases or decreases the internal incident angle to the corresponding prism element with respect to the critical angle of the material so as to reflect or transmit, through refraction, individual rays. 
   U.S. Pat. No. 5,444,606, also to Barnes et al., discloses a combination of a prismatic reflector and a prismatic lens for use with lighting fixtures. The reflector body has a substantially parabolic contour defining an interior cavity and includes a plurality of prisms for receiving, transmitting, and reflecting light. An inside surface of the reflector includes a smooth light-receiving, lower surface portion and an optional light depressing prism portion. The prisms provide modest or slight light spreading for additional rays near nadir. Rays which are omitted from the lamp to near the top, middle, and bottom of the reflector are illustrated as striking the surface at near normal angles in order to reduce first surface reflections and refraction losses. 
   U.S. Pat. No. 4,903,180 to Taylor et al. (assigned at issuance to General Electric Company) is also incorporated by reference here and discloses a luminaire with a protected prismatic reflector. The dome-shaped reflector includes a series of superimposed integrally-connected sections, each of a truncated conical form that tapers to a progressively greater extent than the one beneath it. This patent also notes that a substantial amount of light will pass through the reflector even though the reflecting surfaces of the prisms are clean. According to the patent, one reason for this effect is that the molds used for making such reflectors are not precise enough to achieve mathematical precision of the reflecting surfaces all the way to the apexes of the prisms and to the nadir of the valleys between them. Additional light leakage can also occur at points of defects in the prism surfaces. 
   SUMMARY 
   Various drawbacks of these and other conventional technologies are addressed here by providing an industrial luminaire with a refractor including an interior surface formed as an open-ended surface of revolution of a plane curve about a rotational axis. The plane curve has a plurality of segments corresponding to segments on a reference curve which, for each segment of the reference curve, the corresponding segment on the plane curve has been incrementally rotated with respect to a reference point on a reference axis for the reference curve. For example, the reference curve may be a portion of a conic section and the reference point on the reference axis of the reference curve may be a focus of the conic section. The reference axis for the reference curve may also extend substantially perpendicular to a directrix of the conic section and the conic section may be an ellipse, a parabola, and a hyperbola. 
   In addition, a portion of the interior surface formed by at least one of the segments of the plane curve may include a plurality of circumferential prisms and each of the circumferential prisms may include at least one circumferential wall that is displaced from that segment of the plane curve toward an interior of the refractor. The circumferential wall may be arranged parallel to an opening of the refractor and/or arranged substantially horizontally. 
   The refractor may also have an exterior surface generally conforming to the surface of revolution that includes a plurality of exterior prisms arranged substantially parallel to the planar curve. Each of the exterior prisms may include a V-shaped groove extending substantially over the length of the plane curve and at least same of the V-shaped grooves may have a width that changes over the length of the plane curve. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other aspects of this technology will now be described with reference to the following drawings. Various features in each figure have been drawn to scale relative to other features in the same figure and like reference numerals have been used to designate corresponding parts throughout the several views. 
       FIG. 1  is a side view of one embodiment of an industrial luminaire with a prismatic refractor. 
       FIG. 2  is an enlarged side view of the refractor shown in FIG.  1 . 
       FIG. 3  is a schematic, cutaway isometric view comparing the interior and exterior surfaces of the refractor in FIG.  2 . 
       FIG. 4  is a bottom isometric view of the refractor shown in FIG.  2 . 
       FIG. 5  is a partial top view of the refractor of FIG.  2 . 
       FIG. 6  is an enlarged top view of a portion of the refractor shown in FIG.  5 . 
       FIG. 7  is a partial, cross-sectional view of the reflector taken along section line VII—VII in FIG.  6 . 
       FIG. 8  is a partial, cross-sectional view of the wall refractor taken along section line VIII—VIII in FIG.  5 . 
       FIGS. 9A through 9D  are a schematic illustration of segment positions for a rotated reference parabola. 
       FIG. 10  is a comparison plot of bracket position verse spacing criteria for the refractor shown in  FIG. 2 and a  conventional refractor. 
       FIGS. 11A-11D  are comparison plots of zonal lumen data for various bracket positions of the luminaire shown in  FIG. 1 and a  modified luminaire including a conventional refractor. 
       FIG. 12  is a side view of the refractor shown in  FIG. 2  with a drop lens. 
       FIGS. 13A-13D  are schematic views of a mold for use in making the refractor shown in FIG.  2 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  is a side view of one embodiment of an industrial luminaire  20 . The luminaire  20  is depicted here as a high-bay luminaire; however, a variety of other luminaires may also be used with the technology described below, including, but not limited to, low-bay luminaires and non-industrial luminaires. 
   Although not shown in  FIG. 1 , the bottom of the luminaire  20  may be provided with a lens, such as a flat lens, conical lens, drop lens, bubble lens and/or other bottom enclosure, including those available from Lexalite Corporation of Charlevoix, Mich. (and www.lexalite.com), or as shown in the related co-pending design patent applications identified above. 
   The illustrated luminaire  20  is provided with a mounting box  22  for securing the luminaire to an overhead support. However, a variety of other support mechanisms may also be provided as is known in the art including fixture hooks, fixture loops, and hooks with safety screws. A housing  24  is connected to the mounting box  22  and contains various electrical components and/or connections for providing power to the luminaire  20 . A ballast  26  is arranged below the housing  24 . However, some or all of these components and connections may be arranged in the mounting box  22 . Alternatively, the housing  24  and mounting box  22  may be combined as in the BENCHMARK HB Prismatic High Bay product available from Cooper Lighting. 
   In  FIG. 1 , a bracket  28  secures the lamp socket  30  and the refractor  40  to the rest of the luminaire  20 . A lamp  32 , or other illumination source, is arranged in the lamp socket  30 , and preferably disposed along a central axis of the refractor  40  as discussed in more detail below. For example, a high intensity discharge lamp, such as, for example, a high pressure sodium, metal halide or mercury vapor lamp can be used for the light source  32 . However, a variety of other commercially-available lamps may also be used be employed, including one or more compact fluorescent lamps or incandescent lamps. 
   The bracket  28  is preferably adjustable for vertically-positioning the lamp relative to the refractor  40  in order to control the width of the light distribution from the refractor  40 . For example, Cooper Lighting offers various industrial luminaires with brackets that are adjustable in ⅜″ increments identified as positions “A” through “L” on the side of the bracket. Other brackets with fewer positions simply identify those increments as corresponding to “concentrated,” “medium,” and “wide” beam dispersions corresponding to high, medium, and low positions of the lamp  32  in the refractor  40 . Although the refractor  40  is configured in  FIG. 1  to provide mostly direct, concentrated downlight, the refractor may also be inverted in order to provide indirect lighting and/or uplight, or directed toward a vertical wall to produce a “wall wash” effect. 
   The refractor  40  is preferably formed as a unitary body of translucent or transparent polymeric or glass material. For, example, the refractor  40  may be injection molded and/or machined from acrylic, polycarbonate, or other plastic material. However, other materials may also be used including, but not limited to, opaque materials such as aluminum, brass, and epoxy resin materials. Other manufacturing processes may also be used including casting, stamping, rotary molding, extrusion, coating and/or other processes. 
   As shown in  FIG. 2 , refractor  40  has an upper rim  42  for securing to the bracket  28 , or other supporting hardware, and a lower rim  44  for receiving a drop lens (shown in FIG.  12 ). The upper rim also includes an optional gasket ring  46  for receiving a gasket (not shown). In this configuration, the gasket can be used to seal the upper rim  42  of the refractor in order to prevent the infiltration of dust and/or water into the interior cavity of the refractor  40 . For example, appropriate hardware for sealing the upper rim  42  of the refractor  40  is available, for example, from Cooper Lighting as Model Nos. ENPD and FP for so-called “enclosed and gasketed” luminaires. 
     FIG. 2  illustrates a translucent embodiment of the refractor  40  where interior features of the refractor can be seen through the exterior surface.  FIG. 3  is a schematic, cutaway isometric view of the refractor  40  from  FIG. 2  comparing the interior surface  50  to the exterior surface  70  of the refractor. In other words,  FIG. 3  shows the interior surface of the refractor  40  in only the cutaway portion, and, in this regard, corresponds to what the refractor  40  in  FIG. 2  would generally look like if made from an opaque material. 
     FIG. 3  illustrates a plurality of interior, circumferential prisms  52  arranged on the interior surface or wall  50 , and a plurality of exterior, longitudinal prisms  72  arranged on the exterior surface or wall  70 . Although the interior prisms  52  are illustrated in  FIGS. 2 and 3  as being arranged substantially horizontal, and the exterior prisms  72  are illustrated as being arranged substantially vertical, they may, in fact, be arranged at other angles. For example,  FIG. 4  is a bottom isometric view of the refractor  40  shown in  FIG. 2  which the axis shown by the centerline has been rotated approximately 45° from nadir so that the interior prisms  52  and exterior prisms  72  are neither horizontal nor vertical. Similarly, although the interior prisms  52  and exterior prisms  72  have been illustrated as being arranged substantially perpendicular to each other, other relative angles may also be used between the interior and exterior prisms including 0°, 5°, 10°, 15°, 30°, 45°, 60°, and 75° and/or combinations thereof. 
   The location of the interior prisms  52  and exterior prisms  72  may also be reversed so that interior, circumferential prisms  52  are arranged on the exterior wall  70  while longitudinal prisms  72  are arranged on the interior wall  50 . Alternatively, some or all of the circumferential and longitudinal prisms may be arranged on the same interior surface  50  and/or exterior surface  70  of the refractor  40 . In yet another alternative, the circumferential and/or longitudinal prisms may be arranged beneath the interior and/or exterior surfaces  50 ,  70  of the refractor  40 . For example, one or both of the interior surface  50  and exterior surface  70  may be covered with a coating or additional layer of reflective, non-reflective, refractive, non-refractive and/or other materials. 
   The refractor  40  is generally shaped as an open-ended, shell of a surface of revolution. The term “surface of revolution” is used here in its mathematical sense to describe a surface generated by rotating a two-dimensional “plane curve” about an axis. Examples of surfaces of revolution include the apple, cone (excluding the base), conical frustum (excluding the ends), cylinder (excluding the ends), Darwin-de Sitter spheroid, Gabriel&#39;s horn, hyperboloid, lemon, oblate spheroid, paraboloid, prolate spheroid, pseudosphere, sphere, spheroid, and torus (and its generalization, the toroid). 
   As discussed in more detail below with regard to  FIG. 8 , the plane curve that is used to generate the surface of revolution for the refractor  40  preferably includes one or more conic sections. In simple terms, conic sections, or “conics,” are formed by the intersection of a right circular cone and a plane. The cone may have any vertex angle; however, it is the angle of intersection between the cone and the plane that determines whether the resulting curve is an ellipse, parabola, or hyperbola. 
   In mathematical terms, a conic can also be defined in terms of a “directrix” line D, and a “focus” point F not on D, where the conic is the locus of points P such that the distance from P to F divided by the distance from P to D is a constant referred to as the “eccentricity.” If the eccentricity is equal to 1, the resulting conic is a parabola, while eccentricities less than 1 result in an ellipse and eccentricities greater than 1 result in a hyperbola. The ellipse and hyperbola are also known as “central conics.” 
   The solid formed by the revolution of a conic section about its axis is referred to as a “conoid.” If the conic section is a parabola, the resulting solid is a parabolic conoid or “paraboloid.” Similarly, if the conoid is a hyperbola, then the solid is referred to as a hyperbolic conoid, or “hyperboloid,” while an ellipse forms an elliptic conoid, also known as an “ellipsoid.” The term “conoid” is used here to include truncated conoids, and/or other partial conoids. The term “conoid shell” is used here to generally refer to a thin wall of substantially uniform thickness including an external surface of a conoid. 
     FIG. 5  is a partial top view of the translucent refractor  40  shown in  FIGS. 2 and 4  where the upper rim  42 , lower rim  44 , and gasket ring  46  have been removed for simplification.  FIG. 6  is an enlarged view of a portion of the exterior surface  70  of the refractor  40  shown in FIG.  5 .  FIG. 7  is a partial cross-section taken along section line VII—VII in FIG.  6 . 
   As best illustrated in  FIGS. 5-7 , the exterior surface  70  of the refractor  40  includes a plurality of flutes configured as V-shaped exterior prism elements  72 .  FIG. 5  illustrates  360 , longitudinal V-shaped prism elements  72  that are equally spaced around the circumference of the refractor  40  near the bottom rim  44 . However, other numbers, configurations, and shapes of elements may also be used including U-shaped and channel-shaped prism elements. The exterior prism elements may also be clustered in just certain sectors of the exterior surface  70 , for example, to provide substantially rectangular, square, elliptical, and/or other non-circular light distribution patterns. 
   In the figures, the exterior prism elements  72  are labeled at the bottom apex of each V-shaped groove. All of the illustrated exterior prism elements  72  extend from near the top rim  42  (not shown in  FIG. 5 ) to near the bottom rim  44  (also not shown in  FIG. 5 ) of the refractor  40 . Alternatively, some or all of the exterior prisms  72  may extend across only part of the longitude of the exterior surface  70  of the refractor  40 . 
   As best shown in  FIG. 6 , some of the exterior prism elements  72  have a constant width while others have a variable width. More specifically, constant-width prism elements  721  are alternated with blended, or variable-width, prism elements  722 . However, other ratios and/or arrangements of constant-width prism elements  721  and variable-width prism elements  722  may also be used including the use of only constant-width prism elements or only variable-width prism elements. 
   As the name implies, the variable-width prism elements  722  have a peak to peak width that increases with the length of the prism element, along the longitude of the refractor  40  from the top to the bottom of the refractor. Alternatively, some or all of the variable-width prism elements  722  may have a width that decreases from the top to the bottom of the refractor  40 . Furthermore, although the rate of change of the width of the variable-width prism elements  722  is illustrated in the drawings as being substantially constant over the longitude to the refractor  40 , variable and/or discontinuous rates of width change may also be used. 
   For the embodiment illustrated in the figures, the constant-width prism elements  721  are spaced every 2° near the top rim  42  of the refractor  40  so that 180 constant-width prism elements  721  are equally spaced near the top of the refractor. However, other numbers of prism elements and/or spacing intervals may also be used. Near the lower rim  44  of the refractor  40 , the constant-width prism elements  721  and the variable-width prism elements  722  have substantially the same peak to peak width so that 360 exterior prism elements  72  are evenly spaced at 1° intervals. As best illustrated in  FIG. 7 , the peak of each leg of the exterior prism elements  72  is preferably angled at 45° from a radial line  723  extending perpendicular from the central axis of the refractor  40  shown in FIG.  4 . Since the exterior circumference of the refractor  40  is curved, this configuration results in the troughs of the prism elements  72  having legs that are angled at slightly more than 45° from a radial centerline, or slightly more than 90° from each other. However, other angular configurations may also be used so that light rays impinging thereon will be reflected or refracted as the incident angle is greater than or less than the critical angle of the translucent material. 
     FIG. 8  is a partial, cross-sectional view of the wall, or shell, of the refractor  40  taken along section line VIII—VIII in FIG.  5 . In  FIG. 8 , the solid line on the left corresponds to the interior surface  50 , the solid line in the middle corresponds to the base of the V-shaped groove forming constant width prisms  721 , and the broken line corresponds the base of the V-shaped groove forming variable-width prisms  722 . On the exterior wall  70 , the variable-width prism elements  722  start from zero, or near zero, depth near the top rim  42  of the refractor  40  and becomes deeper and wider as the prism  722  extends longitudinally downward over the external surface  70 . At the bottom rim  44 , the depth and width of the variable width prism  722  are substantially the same as that of the adjoining constant-width prism  721 . 
   As illustrated in  FIG. 8 , the plane curve forming the internal surface (of revolution)  50  of the refractor  40  is divided into several segments  53 - 59 , several of which include one or more of the circumferential prisms  52 . The segments  53 - 59  are portions of the planar curve that was used to provide the surface of revolution. Although seven parabolic segments are illustrated in the figures, any number of parabolic, hyperbolic, elliptic, and/or other conic section segments may also be used. 
   Each of the parabolic segments  53 - 59  is taken from a corresponding segment of a reference parabola where each adjacent segment has been incrementally tilted, or rotated, about a reference point as is schematically illustrated in  FIGS. 9A-9E . For example,  FIG. 9A  illustrates an un-rotated reference parabola having one side that has been divided into a plurality of segments shown by dashed lines. In  FIG. 9B , the reference parabola from  FIG. 9A  has been rotated clockwise 5° in order to obtain the orientation of the first segment shown in bold.  FIG. 9C  shows the reference parabola rotated another 5° in order to obtain the orientation of the next adjacent segment of the parabola. Similarly,  FIGS. 9D and 9E  show the reference parabola from  FIG. 9A  rotated 15° and 20°, respectfully, in order to obtain the orientation of the third and fourth segments, respectfully, also shown in bold. A plane curve similar to the one that was used to form the interior surface  50  and exterior surface  70  of the refractor  40  can then be made by joining consecutive rotated parabola segments shown in bold in  FIGS. 9B-9E . The reference parabola in  FIG. 9A  is preferably rotated about the light center point on the center line in  FIGS. 9B-9E . However, other rotational reference points may also be used, including the focus of the parabola. 
   Returning to  FIG. 8 , segments  53 ,  54 ,  55 ,  56 ,  57 ,  58 , and  59  correspond to segments of a reference parabola which has been rotated about its focus 49.3267°, 49.3200°, 45°, 40°, 35°, 30°, and 25°, respectively. However, other rotations may also be used. For a nominal 22-inch refractor  40  having bottom opening of about 21.5 inches, a top opening of about 10 inches, and a vertical height of about 13.5 inches, the lower segment  53  is approximately 0.75 inches in length. 
   As noted above, segments  55 - 59  in  FIG. 8  are further provided with circumferential prism elements  52 . For the nominal 22-inch refractor illustrated in the figures, the circumferential prism elements  52  are preferably about ½ inch in vertical width. However, other widths may also be used and the prism elements  52  may extend only part way around the circumference of the interior surface  50 . One or more of the interior prism elements  52  may also be broken or discontinuous around the circumference. 
   Each prism element  52  is rotated by about 1-10°, and preferably about 5°, from the segmented parabolic curve forming the interior surface of revolution. More specifically, the top edge of each prism element  52  is fixed while the bottom edge of each prism element is rotated about 5° toward the interior of the refractor  40 . However, other rotational configurations may also be used. For example, each prism element  52  may be rotated about another point on the element, such as its center-point. 
   The bottom, inwardly-extending wall  60  of each of the interior prism elements  52  is preferably arranged to form a surface which is substantially parallel to the opening of the reflector  40 . For the configuration illustrated in the side view of  FIG. 1 , the walls  60  will be arranged substantially horizontal to the surface being illuminated. However, other arrangements of the walls  60  may also be used depending upon the location of the illumination surface relative to the opening of the refractor  40 . 
   Table 1 below provides zonal, efficiency and SC data for various bracket positions of the luminaire  20  with the refractor  40  described above. 
                   TABLE 1                  Refractor 40                                             0-30°   0-40°   0-60°   0-90°   90-180°   0-180°           Efficiency   Efficiency   Efficiency   Efficiency   Efficiency   Efficiency                                                                     Pos.   SC   % L   % F   % L   % F   % L   % F   % L   % F   % L   % F   % L   % F                                                                             A   0.6   34.8   41.2   46.8   55.2   56.0   66.2   62.7   74.1   22.0   25.9   84.6   100       B   0.7   35.4   41.4   48.2   56.5   58.5   68.5   65.2   76.4   20.1   23.6   85.4   100       C   0.7   35.1   40.7   48.8   56.6   60.2   69.9   67.1   77.9   19.0   22.1   86.2   100       D   0.8   34.1   39.4   48.5   55.9   61.5   71.0   68.6   79.2   18.0   20.8   86.6   100       E   0.9   32.8   37.5   47.7   54.6   62.6   71.6   70.0   80.0   17.5   20.0   87.5   100       F   1   30.8   35.0   46.3   52.6   63.1   71.7   70.9   80.5   17.1   19.5   88.0   100       G   1.2   29.4   33.0   45.3   50.9   63.7   71.7   71.9   80.9   17.0   19.1   88.9   100       H   1.4   26.7   29.9   42.9   48.0   63.7   71.3   72.4   81.1   16.8   18.9   89.3   100       J   1.6   24.0   26.7   40.4   45.0   63.6   70.7   73.1   81.3   19.8   18.7   89.9   100       K   1.8   20.1   22.5   36.2   40.5   61.9   69.2   72.6   81.3   16.8   18.7   89.4   100       L   2   17.5   19.3   33.0   36.4   60.7   66.9   73.0   80.5   17.7   19.5   90.7   100                    
Table 2 provides similar data for a few of the same bracket positions of a luminaire  20  where the refractor  40  has been replaced with a Model 822 prismatic REFLEXOR® from LexaLite Corporation.
 
   
     
       
         
             
           
             
               TABLE 2 
             
           
          
             
                 
             
             
               Model 822 REFLEXOR ® from LexaLite Corporation 
             
          
         
         
             
             
             
             
             
             
             
          
             
                 
               0-30° 
               0-40° 
               0-60° 
               0-90° 
               90-180° 
               0-180° 
             
             
                 
               Efficiency 
               Efficiency 
               Efficiency 
               Efficiency 
               Efficiency 
               Efficiency 
             
          
         
         
             
             
             
             
             
             
             
             
             
             
             
             
             
             
          
             
               Pos. 
               SC 
               % L 
               % F 
               % L 
               % F 
               % L 
               % F 
               % L 
               % F 
               % L 
               % F 
               % L 
               % F 
             
             
                 
             
             
               A 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
             
             
               B 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
             
             
               C 
               1.1 
               31.9 
               36.6 
               47.0 
               53.9 
               58.4 
               66.9 
               66.7 
               76.5 
               20.5 
               23.5 
               87.3 
               100 
             
             
               D 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
             
             
               E 
               1.3 
               29.1 
               32.8 
               46.0 
               51.8 
               60.5 
               68.2 
               69.6 
               78.5 
               19.1 
               21.5 
               88.7 
               100 
             
             
               F 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
             
             
               G 
               1.6 
               24.7 
               27.2 
               43.1 
               47.3 
               62.2 
               68.4 
               72.3 
               79.4 
               18.7 
               20.6 
               91.0 
               100 
             
             
               H 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
             
             
               J 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
             
             
               K 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
             
             
               L 
               2.1 
               14.8 
               15.9 
               30.2 
               32.5 
               59.2 
               63.5 
               73.1 
               78.5 
               20.1 
               21.5 
               93.2 
               100 
             
             
                 
             
          
         
       
     
   
     FIG. 10  is a comparison plot of bracket position versus spacing criteria for the luminaire shown in  FIG. 2  with Refractor  40  (Table 1) and one that has been modified by replacing the refractor  40  with a 22-inch, Model 822 prismatic REFLEXOR® from LexaLite Corporation.  FIG. 10  illustrates that the range of spacing criteria provided by the refractor  40  for positions C-L is greater than the range of spacing criteria for the conventional refractor. This shows that the refractor  40  has a greater range of adjustability, at least between positions C and L. Moreover, it is expected that additional testing of the conventional refractor at positions A and B would simply extend the plot for the conventional refractor at substantially the same slope. 
     FIGS. 11A-11D  are comparison plots of zonal lumen data for various bracket positions of a luminaire  20  with the refractor  40  shown in  FIG. 1 and a  modified luminaire with the 22-inch, Model 822 prismatic REFLEXOR®) from LexaLite Corporation.  FIGS. 11A-11D  correspond to bracket positions C, E, G, and L, respectively. In each of these plots, it will be noted that the refractor  40  provides a slightly higher overall efficiency for the 0-180° zone as compared to the conventional refractor. However, the refractor  40  provides significantly higher efficiencies for zones that are less than about 0-90°. 
   In fact, this is the preferred result for many industrial applications where a narrow light distribution pattern is preferred. For example, in many applications for high-bay industrial luminaires, the light which is distributed at the higher angles is undesirable because all of the human activity takes place near the floor. Consequently, light which is omitted at the higher angled zones is not very useful and should be avoided in any refractor design. 
     FIG. 12  is a side view of the refractor  40  fitted with a drop down lens  80 . A variety of drop down lens may be used including the models  622  and  630  available as part of the LexaLite 600 Series of indoor and outdoor area lighting. Conical lenses may also be used. The lens  80  may be secured to the refractor  40  in a variety of ways including the use of various bands, fasteners, adhesives, and/or other devices as are known in the art. 
     FIGS. 13A-13D  are schematic views of a mold for use in making the refractor shown in FIG.  2 .  FIGS. 13A and 13B  are side and top views, respectively, of the exterior of the mold in its closed position while  FIGS. 13C and 13D , on the other hand, are side and top views, respectively, of the exterior of the mold in an open position.  FIGS. 13C and 13D  illustrate how various sectors of the exterior mold may be removed and/or replaced. For example, if a different pattern of exterior prism elements  72  is required for one section of the exterior of the refractor  40 , then that section of the mold may be removed and/or replaced. In this way, various radial distributions of light may be engineered for only certain sections of the refractor  40 . 
   In order to test the refractor  40 , three acrylic prototypes were constructed using a variety of manufacturing specifications as discussed in more detail below. It was found that by increasing the surface finish of tool that was used to shape the interior surface  50  (core side of the mold shown in  FIGS. 13A-13D ) about 12 microns to 8 microns, the overall efficiency of the luminaire at position G increased from 80.4% to 89.6%. At position H, the overall efficiency was found to increase from 83.4% to 89.9% while at position J the efficiency increased from 85.8% to 89.9%. 
   In addition, it was found that increasing the surface finish of the tool to 6 microns and making the walls  60  ( FIG. 8 ) substantially horizontal, further decreased the overall efficiency at position G from 89.6% to 88.9%. However, along with this decrease in overall efficiency came an increase in efficiency at the lower angles. For example, at 0° to 30° for position G the zonal efficiency increased from 28.4% to 29.4%, and at 0° to 40° the efficiency increased from 44.3% to 45.3%. Furthermore, at position J, the overall efficiency of the third prototype was substantially the same as the second prototype while the efficiencies at the lower angles were still higher for the third prototype than the second prototype. 
   This latter data showed that for higher spacing criteria (bracket positions later in the alphabet), the overall luminaire efficiency stayed substantially the same with improved tool finish and more-precise cutting of the walls  60  (FIG.  8 ). Nonetheless, zonal efficiency at lower angles continued to improve with the surface finish of the tool and improved tolerances for the internal prism elements  52 . 
   Photometric data for position L, the highest spacing criteria that was tested, is set forth in Table 3 below for a luminaire  20  with the refractor  40  a similar luminaire that has been modified with the LexaLite Model 822. 
                   TABLE 3                  Refractor 40 and Conventional Refractor at Position J                                         0-30°   0-40°   0-60°   0-90°   0-180°           Efficiency    Efficiency   Efficiency   Efficiency   Efficiency                                                             Refractor   SC   % L   % F   % L   % F   % L   % F   % L   % F   % L   % F                                                                     40   2   17.5   19.3   33.0   36.4   60.7   66.9   73.0   80.5   90.7   100       Conv.   2.1   14.8   15.9   30.2   32.5   59.2   63.5   73.1   78.5   93.2   100                    
In Table 3, the overall efficiency of the luminaire  20  with the refractor  40  (SC=2) was 90.7% while the efficiency was 93.2% with the conventional refractor (SC=2.1). However, at 0-30° the zonal efficiencies for the refractor  40  provided 17.5% zonal efficiency while the conventional refractor provided only 14.8% zonal efficiency. Although these zonal efficiency differences become less pronounced with larger zones, they continue into the 0-90° zone. This illustrates a significant improvement over the conventional refractor because the smaller zones are where the light is often most useful for luminaires which are mounted at greater heights.
 
   Various thermal tests were also performed on the luminaire  20  with refractor  40  and it was found that with a 450 Watt, M144 lamp in a 55° C. environment, the top of a nominal 22-inch, acrylic, refractor  40  did not exceed 90° C. even when the bracket was in position A. This exceptional thermal performance was thought to be due to increased turbulence near the interior wall  50  of the luminaire  40  caused by the sharper angles provided by horizontal walls  60  of the prism elements  52 . 
   It should be emphasized that the various embodiments of the technology described above and, particularly, any “preferred” embodiments, are merely examples of various implementations that have been used here to set forth for a clear understanding of the technology. Many variations and modifications may be made to these embodiments without departing substantially from the spirit and principles of the invention defined by the following claims.