US20010038736A1 - High efficiency reflector for directing collimated light into light guides - Google Patents

Abstract

A reflector for reflecting light from an elongate light source into the input end of a light guide having a diameter “D”. The light source is inserted through the narrow end of a collimating reflector which has a wide end with a diameter exceeding “D” through which light is emitted into the guide. The wide end of an output reflector circumferentially surrounds the collimating reflector's wide end. The output reflector's narrow end circumferentially surrounds the light guide's input end. The wide end of an input reflector circumferentially surrounds the collimating reflector's narrow end. The reflectors are cylindrically symmetrical about a common axis. Light passing from the light source to the collimating reflector is reflected, producing an output beam whose width varies as a function of distance along the axis. The light guide's input end is positioned along the axis to minimize the width of the output light beam.

Description

REFERENCE TO RELATED APPLICATION
• This is a continuation-in-part of U.S. patent application Ser. No. 09/264,247 filed Mar. 8, 1999.[0001]
TECHNICAL FIELD
• This invention pertains to efficient reflective coupling of collimated light into the input end of a light guide.[0002]
BACKGROUND
• FIG. 1 schematically depicts the way in which light is conventionally emitted into the input end of a light guide of the type disclosed in, U.S. Pat. Nos. 4,615,579; 4,750,798; 4,787,708; 4,834,495; or, 4,850,665. An elongate (i.e. non-point) light source such as an arc-type metal[0003]halide light bulb10 mounted in asocket12 is slidably inserted through anaperture14 in aparaboloidal collimator16. The circumferential rim at the wide end ofcollimator16 is flanged as shown at18. A matingcircumferential flange20 is provided around the input end of hollowprism light guide22.Flanges18,20 are connected together to mechanicallycouple collimator16 to the input end oflight guide22. Whenlight bulb10 is energized, light rays emitted by the bulb'sarc portion24 are reflected and collimated bycollimator16 toward the input end oflight guide22 which confines the rays and distributes them uniformly along the light guide.
• Prism light guides work best when the input light is collimated within a half angle of about 30°. This requirement limits the range of suitable light sources, because the efficiency with which light can be emitted into the input end of the light guide decreases rapidly as the size of the light bulb increases. Arc-type metal halide light bulbs of the type shown in FIG. 1 are reasonably practical light sources for illuminating light guides, due to their high efficiency, compact size and reasonably long lamp life.[0004]
• The maximum efficiency with which prior art[0005]paraboloidal collimator16 can couple collimated light intolight guide22 depends on the ratio of the light guide's diameter “D”, to the length “L” of the light bulb's arc. For a typical prior art D:L ratio of about 6:1, reflector efficiencies of about 70% can be achieved. Thus, about 30% of the light emitted bylight bulb10 is typically lost, in the sense that it does not enter the input end oflight guide22. It is desirable to reduce the ratio D:L, since this would enable the use of larger, higher output, more efficient metal halide arc lamps. But, even modest reductions in the ratio D:L substantially reduce the efficiency with which light emitted by the light bulb can be collimated and coupled into the input end of the light guide. For example, a D:L ratio of about 4:1 typically yields a light guide input coupling efficiency of only about 50%, meaning that about 50% of the light emitted by the light bulb is lost, in the sense that it does not enter the input end of the light guide as collimated light.
• This invention overcomes the coupling efficiency problems associated with conventional light guide systems. For example, the invention permits efficient collimation and coupling of light emitted by a one kW metal halide light bulb having a 12,000 hour rated life into a 25 cm diameter light guide. Such systems are advantageous in general lighting situations in which high efficiency linear lighting is required and in which maintenance of multiple point source or fluorescent fixtures is problematic.[0006]
SUMMARY OF INVENTION
• The invention provides a reflector for reflecting light emitted by an elongate light source into the input end of a light guide having a light guide diameter “D”. All embodiments of the invention incorporate a collimating reflector having a narrow apertured end through which the light source is insertably positionable; and, a wide apertured end having a diameter exceeding “D”, through which light is emitted into the light guide. All embodiments of the invention also incorporate an “output end” annular reflector, or an “input end annular reflector, or both.[0007]
• The output end annular reflector has a wide apertured end which circumferentially surrounds the collimating reflector near its wide apertured end, and a narrow apertured end which circumferentially surrounds the input end of the light guide. The input end annular reflector has a narrow apertured end through which the light source is insertably positionable, and a wide apertured end which circumferentially surrounds the collimating reflector near its narrow apertured end. All of the reflectors are cylindrically symmetrical about a common longitudinal axis. Light rays emitted by the light source which pass to the collimating reflector are reflected by the collimating reflector and produce an output light beam having a beam width which varies as a function of distance along the aforementioned axis. The light guide's input end is positioned at a selected distance along the axis at which the output light beam has a minimum beam width value.[0008]
• Advantageously, the input end annular reflector further has a curved surface, such that, in any cross-sectional plane containing the aforementioned axis, the curved surface defines a first arc on one side of the axis, and, a second arc on an opposed side of the axis. The first arc has a first center of curvature located on the one side of the axis, and further located within a cylinder which is symmetrical about the axis and which has a diameter not greater than the diameter of the collimating reflector's narrow apertured end. The second arc has a second center of curvature located on the opposed side of the axis and further located within the aforementioned cylinder.[0009]
• The collimating reflector is preferably an off-axis paraboloidal cross-section, cylindrically symmetrical reflector having a focal point f. The output end annular reflector preferably forms a spherical arc section having a center of curvature near the collimating reflector's focal point f The light source is typically a metal halide light bulb having a light emitting arc having one end near the focal point f and an opposed end near the narrow apertured end of the collimating reflector.[0010]
BRIEF DESCRIPTION OF DRAWINGS
• FIG. 1 is a schematic side elevation view of a prior art paraboloidal collimator mounted to reflect light emitted by an arc-type metal halide light bulb into the input end of a light guide.[0011]
• FIG. 2 is similar to FIG. 1, except that the light guide diameter “D” is less than the diameter of the outward end of the paraboloidal collimator.[0012]
• FIG. 3 is a schematic side elevation view of a reflector constructed in accordance with the invention.[0013]
• FIG. 4 is similar to FIG. 3, but shows a longer portion of the light guide.[0014]
• FIG. 5 is a schematic fragmented cross-section showing the geometric characteristics of the preferred off-axis paraboloidal collimator.[0015]
• FIG. 6 is a schematic side elevation view of a reflector constructed in accordance with the invention, showing how the paraboloidal collimator reflects into the light guide substantially all light rays which pass directly from the center “C” of the light source to the paraboloidal collimator.[0016]
• FIG. 7 is a schematic side elevation view showing how the paraboloidal collimator reflects light rays which pass through the center, rearward (left) end, and forward (right) end of the light source.[0017]
• FIG. 8 graphically depicts the variation of the width of the output beam produced at the wide apertured end of the reflector as a function of distance d along the reflector's longitudinal axis.[0018]
DESCRIPTION
• As shown in FIGS. 3 and 4, reflectors constructed in accordance with the invention include a collimating reflector such as conventional[0019]paraboloidal collimator16 A, which is preferably an off-axis paraboloidal cross-section cylindrically symmetric reflector. Arc-type metalhalide light bulb10A mounted insocket12 A is slidably inserted throughaperture14 A in the narrow end ofparaboloidal collimator16 A, as in the case ofprior art reflector16 described above. Unlike prior art reflectors, reflectors constructed in accordance with the invention include not onlyparaboloidal collimator16 A, but alsoannular reflectors26 and28.
• As seen in FIG. 1, the diameter of prior art[0020]paraboloidal collimator16 at its wide end (i.e. the end bearing circumferential flange18) is substantially equal to the diameter D oflight guide22. However, the wide end diameter ofparaboloidal collimator16 A shown in FIGS. 3 and 4 exceeds the diameter D oflight guide22 A. This facilitates collimation of light emitted by a metalhalide light bulb10 A which is larger thanlight bulb10, assuminglight guides22,22 A to be of equal diameter.
• If a larger light bulb were used in a prior art reflector system like that of FIG. 1, the diameter of[0021]aperture14 in the narrow end ofparaboloidal collimator16 would have to be increased to facilitate insertion of the larger bulb. Increasing the diameter ofaperture14 increases the amount of light which is lost due to rearward emission (i.e. to the left, as viewed in the drawings) outsideparaboloidal collimator16 and away fromlight guide22. This problem is solved by the provision ofannular reflector26, as hereinafter explained. Increasing the wide end diameter of the paraboloidal collimator would also cause a problem in a prior art reflector system. Specifically, light rays which encounter the annular region of overlap25 (FIG. 2) betweenparaboloidal collimator16 andlight guide22 are lost, in that they do not enter the input end oflight guide22. This problem is solved by the provision of anotherannular reflector28, as hereinafter explained.
• Input end”[0022]annular reflector26 is an aspherical arc section reflector having a wide apertured end30 (FIG. 3) which circumferentially surroundsparaboloidal collimator16A, near the narrow apertured (i.e. “input”) end ofparaboloidal collimator16A. (Reflector26 is “annular” in the sense that it forms a ring aroundaxis27 when viewed from the right alongaxis27.)Light bulb10A is slidably inserted through the narrow aperturedend32 ofannular reflector26.Annular reflector26 intercepts much of the light which is emitted rearwardly (i.e. to the left, as viewed in the drawings) bylight bulb arc24A and reflects a substantial portion of such rearwardly emitted light back throughaperture14A andlight bulb10A for further reflection byparaboloidal collimator16A intolight guide22A.
• The aspherical characteristic of reflector[0023]26 (i.e. the shape ofreflector26 deviates slightly from a perfectly spherical shape) ensures that most light rays reflected byreflector26 are unlikely to pass througharc24A, thus avoiding reabsorption of such rays byarc24A. More particularly,reflector26 is preferably toroidal in the sense that, when viewed in cross-section as shown in FIGS. 3 and 4,curved portions26A and26B ofreflector26 form circular arcs having centers which lie slightly off thelongitudinal axis27 about whichparaboloidal collimator16A,light guide22A,reflectors26,28 andlight bulb10A are respectively cylindrically symmetrical. Thus, the circular arc formed bycurved reflector portion26A has a center C1 which is located aboveaxis27, within the diameter ofaperture14A at the narrow (i.e. “input”) end ofparaboloidal collimator16A. Similarly, the center C2 of the circular arc formed bycurved reflector portion26B is beneathaxis27, within the diameter ofaperture14A. Centers C1, C2 need not lie on lines which are diameters ofaperture14A, although such positioning is preferred, as shown in FIG. 3. Generally, it is sufficient to locate centers C1, C2 within a cylinder which is symmetric aboutaxis27 and which has a diameter equal to the diameter ofaperture14A. The inner reflecting surface ofreflector26 is preferably slightly diffuse (i.e. non-specular) to further reduce possible reabsorption by are24A of light rays reflected byreflector26.
• As shown in FIG. 3, the end of[0024]arc24A closest to the wide end ofparaboloidal collimator16A is positioned to coincide with, or at least be located near to the focal point f ofparaboloidal collimator16A. Light rays emitted byarc24A near focal point f which reachparaboloidal collimator16A are reflected byparaboloidal collimator16A through a range of angles toward the input end oflight guide22A. If the input end oflight guide22A were moved away from the output end ofparaboloidal collimator16A (i.e. moved to the right, as viewed in the drawings) then more of the light rays reflected byparaboloidal collimator16A could be coupled into the input end oflight guide22A. However, this would again leave an annular region or gap through which light rays passing directly fromarc24A would be lost.Annular reflector28 solves this problem as well as the aforementioned problem respecting loss of light rays atannular region25.
• “Output end”[0025]annular reflector28 is a spherical arc section reflector having a center coinciding with focal point f More particularly,reflector28 has a wide apertured end34 (FIG. 4) which circumferentially surroundsparaboloidal collimator16A near the wide apertured (i.e. “output”) end ofparaboloidal collimator16A.Reflector28 also has a narrowapertured end36 which circumferentially surrounds the input end oflight guide22A. (Reflector28 is “annular” in the sense that it forms a ring aroundaxis27 when viewed from the left alongaxis27.) Light rays which would otherwise be lost throughannular region25 as aforesaid are reflected byreflector28 for possible re-reflection byparaboloidal collimator16A intolight guide22A. The efficiency with whichreflector28 reflects light is not particularly high due to a number of factors. In particular, some rays may be lost due to absorption byreflector28 itself (collimator16A andreflector26 are also absorptive to some degree); some rays may be lost due to reflection onto and reabsorption byarc24A; some rays may be lost due to reflection throughaperture14A; and, in general, efficiency is reduced whenever multiple reflections occur, such as reflection byreflector28 followed by further reflection byparaboloidal collimator16A. However, only a relatively small portion of the light rays emitted byarc24A reach reflector28, so losses inherent in its comparative reduced efficiency are acceptable. The inner reflecting surface ofreflector28 may be made slightly diffuse (i.e. non-specular) to further reduce possible reabsorption byarc24A of light rays reflected byreflector28, although this is not essential.
• FIG. 5 illustrates the preferred geometric characteristics of off-[0026]axis paraboloidal collimator16A. As shown in FIG. 6, these characteristics enablecollimator16A to direct into the input end oflight guide22A, as collimated light, substantially all light rays which pass directly fromarc24A toparaboloidal collimator16A. Persons skilled in the art conventionally use the term “collimated”, as does this application, to describe a light beam containing light rays which trace paths defining a limited range of directions, as opposed to a non-collimated light beam containing rays which trace paths in all directions. Thus, persons skilled in the art will understand thatparaboloidal collimator16A is a “collimating” reflector and that light rays40 depicted in FIG. 6 are “collimated”, even thoughrays40 do not represent theoretically perfect collimation. “Perfect” collimation in which light rays propagate in parallel relationship without any crossing or intersection of the rays' paths, can never be attained in practice, as is well known to persons skilled in the art. See for exampleThe Photonics Dictionary,45th ed., 1999, Laurin Publ. Co. Inc., Pittsfield, which defines collimated radiation as follows “Radiation in which every ray from any given object point can be considered to be parallel to every other. This is never completely the case: The light from a star is really diverging, and all collimators have aberrations.”
• [0027]Paraboloidal collimator16A consists of two parabolic sections located on opposite sides of axis27 (i.e. the upper and lower sides ofaxis27, as viewed in the drawings). The upper parabolic section is formed by rotating aparabola16A-1 through a 180° arc about and aboveaxis27, as viewed in FIG. 5.Parabola16A-1 is an “off-axis” parabola, in that its vertex V is located above, not on,axis27. Theaxis38 ofparabola16A-1intersects axis27 at an angle Θ, with the parabola's focus f being at the point of intersection ofaxes27,38. The focal length f, ofparabola16A-1 is the segment ofaxis38 extending between the parabola's vertex V and focus f. In an embodiment of the invention in which collimated light is coupled into the input end of a light guide having a diameter of 25 cm,parabola16A-1 is defined by the equation:$y=\frac{x^2}{4f}-f$

  
• where y is measured along[0028]axis38 as shown in FIG. 5; x is measured along theperpendicular axis39 passing through the parabola's focus f Θ33.2°; and, f 8.862 cm. The lower parabolic section ofparaboloidal collimator16A is identical to the upper parabolic section, except that it is formed by rotating another off-axis parabola through a 180° arc about and belowaxis27, as viewed in FIG. 5.
• As shown in FIG. 6, the above-described off-axis, cylindrically symmetric characteristics of[0029]paraboloidal collimator16A ensure that substantially all of the light rays40 which pass directly fromlight bulb arc24A (more particularly, all light rays originating or passing through the center “C” oflight bulb arc24A) toparaboloidal collimator16A are reflected into the input end oflight guide22A. Accordingly, rays which would otherwise be lost in annular region25 (FIG. 2) if a prior art, non off-axis paraboloidal reflector16 having an output diameter greater than the light guide's input diameter were used, are collimated and directed into the input end oflight guide22A by off-axis paraboloidal collimator16A.
• [0030]Light bulb arc24A is elongate and emits light rays along its entire length “L”, not just from its center “C”. As previously explained, this has an important bearing on the efficiency with which collimated light can be coupled into the light guide. FIG. 7, which is similar to FIG. 6 but omitsreflectors26,28 andlight guide22A, shows not only light rays which originate at the center “C” ofarc24A, but also light rays which originate near the rearward (left) and forward (right) ends ofarc24A. As can be seen, the width of the light beam produced byreflector16A varies as a function of distance d alongaxis27. More particularly, and as is graphically depicted in FIG. 8, at the wide apertured end ofreflector16A the output beam's width is essentially equal to the diameter Dr of the wide apertured end ofreflector16A. The beam width then decreases as d increases, eventually reaching a minimum value D_o(d); and, thereafter, the beam width continues to increase as d increases. By structuringreflector16A to produce a light beam having a minimum beam width equal to the diameter of the light guide's input end; and, by positioning the light guide's input end to coincide with the point at which the beam attains such minimum width, one may ensure that a maximal fraction of the lightrays exiting reflector16A are directed into the light guide's input end. Conversely, if the input end of the light guide were positioned to the left or to the right of the point at which beam attains its minimum width, then some light rays would be lost.
• As previously explained, as a light guide's D:L ratio increases, it becomes progressively more difficult to achieve high efficiency coupling of collimated light from a light source into the light guide. For light guides having a given D:L ratio, the invention provides much higher efficiency in coupling of collimated light into such light guides than can be provided by prior art reflectors in respect of light guides having the same D:L ratio.[0031]
• As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. For example, although the invention has been described in relation to “prism” light guides, those skilled in the art will appreciate that the invention can be applied to any light guide having input collimation requirements and D:L ratio as described above. As another example, it is not necessary that the narrow[0032]apertured end36 of output endannular reflector28 coincide precisely with the input end oflight guide22A. In some cases, end36 may project a short distance inside the input end of the light guide. This construction, which is convenient if the light guide happens to be out-of-round at its input end, is still considered to “circumferentially surround” the input end of the light guide, within the meaning of the claims. There should not however be a gap between the narrowapertured end36 ofoutput end reflector28 and the input end of the light guide, because this would result in loss of light through the gap. Persons skilled in the art will also understand that the invention is not confined to the dimensional or geometrical relationships depicted in the drawings. The scope of the invention is to be construed in accordance with the substance defined by the following claims.

Claims (13)

What is claimed is:

1. A reflector for reflecting light emitted by an elongate light source into an input end of a light guide having a light guide diameter “D”, said reflector comprising:

(a) a collimating reflector having:

(i) a narrow apertured end for insertable positioning of said light source within said collimating reflector;

(ii) a wide apertured end for emitting light into said light guide, said wide apertured end of said collimating reflector having a diameter exceeding said light guide diameter “D”;

(b) an output end annular reflector having:

(i) a wide apertured end circumferentially surrounding said collimating reflector near said wide apertured end of said collimating reflector;

(ii) a narrow apertured end circumferentially surrounding said input end of said light guide;

wherein:

(1) said reflectors are cylindrically symmetrical about a common longitudinal axis;

(2) light rays emitted by said light source which pass to said collimating reflector are reflected by said collimating reflector and produce an output light beam having a beam width which varies as a function of distance along said axis; and,

(3) said light guide input end is positioned at a selected distance along said axis at which said output light beam has a minimum beam width value.

2. A reflector for reflecting light emitted by a light source into an input end of a light guide having a light guide diameter “D”, said reflector comprising:

(a) a collimating reflector having:

(i) a narrow apertured end for insertable positioning of said light source within said collimating reflector;

(ii) a wide apertured end for emitting light into said light guide, said wide apertured end of said collimating reflector having a diameter exceeding said light guide diameter “D”;

(b) an input end annular reflector having:

(i) a narrow apertured end for insertable positioning of said light source through said input end annular reflector;

(ii) a wide apertured end circumferentially surrounding said collimating reflector near said narrow apertured end of said collimating reflector;

(c) an output end annular reflector having:

(i) a wide apertured end circumferentially surrounding said collimating reflector near said wide apertured end of said collimating reflector; and,

(ii) a narrow apertured end circumferentially surrounding said input end of said light guide;

wherein said reflectors are cylindrically symmetrical about a common longitudinal axis.

3. A reflector as defined in

claim 1

, wherein said collimating reflector is an off-axis paraboloidal cross-section, cylindrically symmetrical reflector.

4. A reflector as defined in

claim 2

, wherein said collimating reflector is an off-axis paraboloidal cross-section, cylindrically symmetrical reflector.

5. A reflector for reflecting light emitted by a light source into an input end of a light guide, said reflector comprising:

(a) a collimating reflector having:

(i) a narrow apertured end for insertable positioning of said light source within said collimating reflector;

(ii) a wide apertured end for emitting light into said light guide;

(b) an input end annular reflector having:

(i) a narrow apertured end for insertable positioning of said light source through said input end annular reflector; and,

(ii) a wide apertured end circumferentially surrounding said collimating reflector near said narrow apertured end of said collimating reflector;

wherein said reflectors are cylindrically symmetrical about a common longitudinal axis,

(c) said input end annular reflector further comprising a curved surface, wherein, in any cross-sectional plane containing said axis, said curved surface defines:

(i) a first arc on one side of said axis, said first arc having a first center of curvature located on said one side of said axis, and located within a cylinder which is symmetrical about said axis and which has a diameter not greater than the diameter of said narrow apertured end of said collimating reflector; and,

(ii) a second arc on an opposed side of said axis, said second arc having a second center of curvature located on said opposed side of said axis, and located within said cylinder.

6. A reflector as defined in

claim 3

, said input end annular reflector further comprising a curved surface, wherein in any cross-sectional plane containing said axis, said curved surface defines:

(i) a first arc on one side of said axis, said first arc having a first center of curvature located on said one side of said axis, and located within a cylinder which is symmetrical about said axis and which has a diameter not greater than the diameter of said narrow apertured end of said collimating reflector; and,

(ii) a second arc on an opposed side of said axis, said second arc having a second center of curvature located on said opposed side of said axis, and located within said cylinder.

7. A reflector as defined in

claim 1

, wherein:

(a) said collimating reflector has a focal point f, and,

(b) said output end annular reflector further comprises a spherical arc section having a center of curvature near said focal point f.

8. A reflector as defined in

claim 2

, wherein:

(a) said collimating reflector has a focal point f; and,

(b) said output end annular reflector further comprises a spherical arc section having a center of curvature near said focal point f.

9. A reflector as defined in

claim 3

, wherein said light source is a metal halide light bulb having an elongate light emitting arc.

10. A reflector as defined in

claim 9

, wherein said light emitting arc has one end near said focal point f and an opposed end near said narrow apertured end of said collimating reflector.

11. A reflector as defined in

claim 2

, wherein:

(a) light rays emitted by said light source which pass to said collimating reflector are reflected by said collimating reflector and produce an output light beam having a beam width which varies as a function of distance along said axis; and,

(b) said light guide input end is positioned at a selected distance along said axis at which said output light beam has a minimum beam width value.

12. A reflector as defined in

claim 4

, wherein:

(a) light rays emitted by said light source which pass to said collimating reflector are reflected by said collimating reflector and produce an output light beam having a beam width which varies as a function of distance along said axis; and,

(b) said light guide input end is positioned at a selected distance along said axis at which said output light beam has a minimum beam width value.

13. A reflector as defined in

claim 5

, wherein:

(a) light rays emitted by said light source which pass to said collimating reflector are reflected by said collimating reflector and produce an output light beam having a beam width which varies as a function of distance along said axis; and,

(b) said light guide input end is positioned at a selected distance along said axis at which said output light beam has a minimum beam width value.

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