Reflector based optical design

[0001] This application claims priority from U.S. Provisional Patent Application Ser. No. 60/595,316 filed Jun. 22, 2005 which is hereby incorporated by reference.

[0002] This invention relates to novel optical design based on a conical reflector (1) centered within an upward facing circular array of LEDs (8).

[0003] Navigational light beacons typically emit a fan beam that is vertically narrow and broad in the horizontal plane. Lights of this type must have uniform output around the horizontal plane.

[0004] Since the advent of high brightness light emitting diodes (LED), a plethora of beacons have been designed to take advantage of the LED. The majority of these beacons utilize a plurality of narrow beam 5 mm LEDs in a circular array, where the axis of maximum intensity is directed outward and lies in the horizontal plane. The light output from the LEDs is typically collimated by an additional refractive optical element. A high intensity beacon requires a large number of these LEDs to produce the appropriate amount of light. The individual beam profiles of these LEDs are often seen as ripples in the horizontal uniformity. Adding a diffusion filter that spreads the light horizontally to smooth out the beam profile can eliminate these ripples, but may attenuate the light intensity. Recent innovations in LED technology have created dramatically brighter LEDs. These new LEDs facilitate the creation of high intensity beacons with substantially fewer LEDs. There are at least two difficulties in utilizing these new LEDs for beacons. The newer LEDs have wide (lambertian) beam patterns which makes collimating the LED's light difficult. In addition, the reduced number of LEDs can lead to non-uniform horizontal output. Manufacturing a beacon utilizing a plurality of Lambertian LEDs in a circular array, where the axis of maximum intensity is directed outward and lies in the horizontal plane is difficult.

[0005] The present invention provides light beacon reflector arrangement that emits a horizontal fan beam of light and a method for providing a desired intensity distribution for the beam of light.

[0006] The invention relies on the use of a plurality of wide angle (Lambertian) LEDs in a circular array, and a curved reflector in concentric relationship with the circular array. The reflector may extend from the plane in which the LEDs lie to a point outside the diameter of the circular array and the LEDs are arranged such that each LED's axis of maximum intensity is perpendicular to the plane in which the circular array lies.

[0007] The LEDs and the reflector may all be mounted on a planar circuit board. A beacon design utilizing a planar circuit board is desirable due to its suitability for automated production. This design eliminates the requirement for a diffusion filter to smooth out the ripples in many applications, as ripples are reduced to an acceptable level.

[0008] In one aspect of the invention, the reflector comprises a plurality of contiguous conical surface segments where each surface is designed to reflect a portion of the LEDs' light within a specific angular width, thereby facilitating the matching of the reflection characteristic to the desired intensity distribution by the selection of the location and reflection angle of each segment.

[0009] In another aspect of the invention the plurality of conical surfaces can be replaced by a smooth curved surface, where the curve is a spline that follows the plurality of segments.

[0010] In yet another aspect of the invention, there is provided a transparent cover that protects the reflector and the LEDs from moisture and other outdoor contaminants. Another aspect of the invention is a self-contained solar powered beacon utilizing this optical design.

[0011] Other aspects of the invention will be appreciated by reference to the description of the various embodiments of the invention that follow and from the claims.

[0026] FIG. 1 depicts a beacon 50 according to an embodiment of the invention including a reflector 1, wide-angle LEDs 8 and one or more solar panels 51. The beacon 50 may be utilized in applications that require a narrow beam of light such as marine or aviation navigation.

[0027] FIG. 2a depicts a beam pattern 5 of the typical wide-angle LED 8 including the axis of maximum intensity 4. FIG. 2b depicts a narrow beam pattern 6 of the typical 5 mm LED 3.

[0028] Referring to FIGS. 3, 4 and 5, there is shown a reflector arrangement according to the invention. A plurality of wide-angle (Lambertian) LEDs (8) are arranged in a circular array, pointing up at a curved or substantially conical reflector (1) concentric with the ring of LEDs 8. Both the LEDs and the reflector are mounted to a planar circuit board 9. The reflector is designed to reflect rays directed upward above some maximum angle 14 shown in FIG. 4, and rays inward 17 (see FIG. 5) toward the middle of the ring so that they go outward 18 from the ring within some specified angular width 12 above and/or below the horizontal plane.

[0029] The reflector comprises a surface revolved about the radial axis of the circular array of LEDs to form a truncated conic section. The reflector comprises a base, shown as the top portion in FIG. 3, and a vertex truncated where the reflector is secured to the circuit board 9. The diameter of the base of the reflector is larger than the diameter of the circular array of LEDs such that the top edge of the reflector overlaps the circular array. The diameter of the vertex is less than the diameter of the circular array.

[0030] The reflector 1 may be constructed from metal and the reflective surface 10 may be polished to a mirror finish, or the reflector may be made out of plastic and the reflective surface 10 may be coated with a reflective material such as aluminum or silver. The coating may then be coated again to prevent corrosion. A transparent cover 16 may protect the assembly from the outdoor environment.

[0031] Typically the light emitted by the beacon must meet some specification (such as that presented in an aviation or marine standard) for intensity over some angular range about the horizontal plane. An example of such a specified intensity distribution (square dots) is shown in FIG. 6 together with a simulated output from the reflector (smooth trace). The design in FIG. 9 meets or exceeds the specification detailed in FIG. 6. In order to direct the light in such a way as to meet required intensity specifications the shape of the reflector surface 10 is selected so as to direct the reflected light rays into specific angular segments from various parts of the reflector surface 10 as illustrated in FIG. 4. Each linear segment can be designed to direct light rays into a specific angular beam width around the horizontal plane, with this beam width being proportional to the length of the segment 13. The angle of the segment relative to the horizontal plane 11 determines the overall direction of this beam. The additive sum of the individual beams from each segment constitutes the output beam of the beacon. This provides a means of customizing the reflector to meet various specifications by modifying the location, length and angle of each segment 15. The desired intensity distribution is ascertained. The intensity distribution is then segregated into discrete adjacent segments wherein a direction and beam width representative of each segment is determined. From such specifications, the length and angle of nominal flat reflective surfaces that are required to achieve the desired reflection direction and beam width are determined. This determination takes into account the relative positions of the LEDs. A reflector is then provided that consists of a plurality of flat adjacent segments corresponding to the nominal reflective surfaces. Each flat segment is revolved about the array axis to yield a segment of a right circular cone.

[0032] In order to meet a specified intensity distribution as efficiently as possible it is desirable to be able to direct rays reflected by particular parts of the reflector surface 10 into a beam with the minimum possible width. The minimum angular beam width that can be produced by this design is limited by several factors. The finite size of the emitting area within the LED 8 introduces an inherent angular size as any reflecting point on the reflector surface 10 receives light rays from a distributed source and thus the reflected rays have a corresponding angular width. Making the reflector surface 10 larger in size relative to the LEDs 8 can reduce this limitation. Once a plurality of segments have been defined to provide the desired beam profile, a spline 19 may be fit to the series of segments 20 and to create a curved rather than faceted profile (FIG. 7). This will further tighten the beam spread, while maintaining the intended profile.

[0033] Typically the beam emitted by the beacon will be designed for rotational uniformity, i.e. equal intensity at a given vertical angle for all azimuthal angles. The use of a finite number of LEDs 8 around the reflector results in some rotational variation in beam intensity. Rotational variations may be more pronounced at certain vertical angles depending on the design of the reflector surface 10. Design can reduce rotational variations at critical angles such as peak intensity angle where some minimum intensity may be specified, while allowing greater rotational variation at angles where it does not violate any specification.

[0034] Increasing the number of LEDs 8 in the ring increases cost and complexity but can reduce rotational variation. 8 LEDs 8 gives reasonably low rotational variation when the proportions suggested by FIGS. 8 and 9 are used. Use of LEDs 8 with narrower beam width would increase rotational variation requiring more LEDs (8) in the ring. However this will also tend to reduce vertical beam spread and allow more efficient light collection.

[0035] The reflector surface 10 collects all light rays from the LEDs 8 directed inward and upward above some minimum upward angle. Rays directed outward from the ring and below this minimum upward angle 14 may escape unreflected. Ideally the reflector surface 10 will extend out far enough to collect all upward rays that are above the required vertical angular coverage for the light. However this may require excessive large diameter for the reflector as the reflector surface 10 diameter expands rapidly as the collection angle is increased. In one example rays above 30° can be collected and the reflector diameter is about 13 cm. For a Lambertian emitter the half power points typically lie at about 30° above the horizontal so that such a reflector surface 10 will collect most of the emitter light.

[0036] Light rays directed in towards the lower portion of the reflector surface 17 will be reflected back out by the reflector surface 10, as illustrated in FIG. 5, however some of them may impinge on the LEDs and be lost by absorption or scattered in useless directions. These losses are typically small for Lambertian emitters where most of the light is emitted above the horizontal plane so that the reflected rays mostly go over the top of the emitters.

[0037] Typically, at least one flat segment of the segmented reflector embodiment will have a diameter about the radial axis of the reflector that is greater than the diameter of the circular array of LEDs while at least one other flat segment will have a smaller diameter than that of the circular array.

[0038] FIGS. 8 and 9 describe two of the possible profiles of the reflector surface 10. The surface is described relative to the center axis 22 of the reflector surface 10 and to the radial location of the LEDs 8. The angles shown 24 describe the angle of the facets 15 relative to the vertical center axis 22. The vertical measurements 23 describe the vertical location of the lowest point of each facet relative to the focal point 21 of one of the LEDs 8. The horizontal measurements 25 describe the horizontal location of the focal point 21 of the LEDs 8 relative to the center axis 22 of the reflector 1 and the horizontal location of the lowest point of the lowest facet. The embodiment of FIG. 8 will create a narrow beam centered on the horizon. The embodiment of FIG. 9 will create beam centered above the horizon according to the specifications provided in FIG. 6.

[0039] FIGS. 10 and 11 depict the reflectors of FIGS. 8 and 9 respectively with the position 102 of the LED 8 indicated as a number of units along the X-axis. The measurement values in FIG. 10 and FIG. 11 are unit-less, as the designs will work provided that the specified proportions are followed. The position of the junction points of the individual facets indicated are also indicated in X-Y coordinates 103, 23. Some deviation from the ideal position of these junction points will still result in acceptable performance of the reflector 1. For example, a 20% relative deviation in the position of the points 101a, 101b may result in an acceptable performance for general purpose applications. A smaller deviation in the position of the points (such as 10%, 5%, 2%, etc.) may result in acceptable performance for more precise applications. In addition, variation in the position of the points may be more critical for some parts of the reflector 1 than others depending on the application.

[0040] It will be appreciated that alternate reflectors may be produced by changing the position of the facet junction points. The tables below shows the facet junction points for two possible alternate embodiments which are combinations of the embodiments shown in FIGS. 10 and 11.

[0041]

		Distance of facet from
		light source in X
Y	X	direction
Facet Junction Points (Alternate Embodiment 1)
0.995	2.602	1.697
0.380	1.121	0.217
0.310	0.977	0.072
0.220	0.896	−0.008
0.175	0.844	−0.061
0.120	0.822	−0.082
0.080	0.803	−0.101
0.050	0.793	−0.111
Facet Junction Points (Alternate Embodiment 2)
0.995	2.602	1.697
0.590	1.338	0.432
0.310	0.977	0.072
0.250	0.888	−0.018
0.175	0.844	−0.061
0.150	0.822	−0.084
0.110	0.800	−0.106
0.060	0.794	−0.112

[0042] FIG. 12 depicts the reflector 1 in spline configuration and shows various points on the reflector 1 with X-Y coordinates 103, 23. This configuration may produce an acceptable flat beam of light for general purpose applications if the points on the reflector are within 20% of those shown. A smaller deviation in the position of the points in this implementation of the reflector 1 may result in acceptable performance in more precise applications.

[0043] The X-Y coordinates shown in FIGS. 10-12 are unitless. In other words, the reflector will function as expected as long as the relative positions of the points on the lens with respect to the light source are maintained. One embodiment for example may be realized with the dimensions shown in inches. Another embodiment may be realized with the dimensions shown in centimeters. Other usable embodiments may be realized with the dimensions shown being any unit of measure between half centimeters and two inches per unit.

[0044] It will be appreciated by those skilled in the art that the preferred and alternative embodiments have been described in some detail but that certain modifications may be practiced without departing from the principles of the invention.