Patent Application
20110170286
None.
This invention was not made under any government contract and the United States Government has no rights under this invention.
This invention relates to light sources and more particularly to light sources utilizing light emitting diodes (LED or LEDs). Still more particularly, it relates to a light source employing an optic for focusing light emitted from the LEDs into a light guide for distribution to a remote location.
An increasing number of lighting applications have been developed utilizing LEDs because of their ruggedness (they are solid state devices), their compactness, their low power requirements and their long life. Foremost among these light applications have been the light sources used in automotive vehicles; namely, center high mount stop lights (CHMSL) and tail and brake lights. In some applications the LEDs are suitable for use as direct-view light sources, comparable to the S8 filamented lamps they replace. However, in other applications it is desirable to collect the light from the light source and concentrate and/or focus it so that it can be directed to a remote location, for example, via a light guide. Light guides do not focus or concentrate the light received by them, but merely direct it to another location. It has long been known that an optic with a parabolic surface generated by the standard formula, Z=¼f·R2, such as those used in PAR lamps, is an efficient concentrator of light and such devices have been used in the past with light emitting diodes; however, generally, each light emitting diode was utilized with an individual optic, a costly and difficult procedure compounded by alignment issues. It has been proposed to utilize a single optic with an array of LEDs for purposes of automotive headlamps, wherein a high luminance, a narrow radiation angle and a well-defined shape of radiation is used. See US Published Patent Application No 2009/0001490 A1 (Bogner, et al.). Also, direct importation of light from LEDs into a light guide or guides is known in US Published Patent Application No. 2009/0185389 A1 (Tessnow, et al.), which application is assigned to the assignee of the instant invention.
Adaptation of parabolic optics for leading light into light guides has, however, proven difficult, particularly when involving a linear array of LEDs. For example, it has been know to use a single glass compound parabolic concentrator (CPC) for each LED in a 2×3 chip array; however, such a system does not work well with a tightly spaced linear array.
Additionally, utilizing a linear array of LEDs has also proven difficult. Beginning with a linear array, one is generally limited to a particular entrance, the size of the entrance, of course, being dictated by the physical dimensions of the array itself. Developing a specific exit aperture for such an array, using a conventionally oriented CPC, has been found not practicable.
It is, therefore, an object of the invention to enhance LED light sources.
Yet another object of the invention is the improvement of LED light sources for feeding light into a light guide.
Still another object of the invention is the provision of an optic for use with a linear array of LEDs.
These object are accomplished, in one aspect of the invention, by the provision of a light assembly for directing light into a light guide, the light assembly comprising a light source having a linear array of light emitting diodes, the linear array having two opposed long sides equally disposed about a longitudinal axis and two opposed short sides and being positioned on a mounting plane and having an optical axis lying in a plane perpendicular to the mounting plane. An optic, which can a primary optic, is provided about the LEDs and has a reflecting surface associated therewith. The reflecting surface has a parabolic cross-section and a focal point and a bisector of parabolic cross-section wherein the focal point is disposed at one of the long sides of the linear array and the bisector of parabolic cross-section has an axis that is tilted with respect to the optical axis. Such a structure provides a CPC device for introducing light into a light guide. Additionally, such a structure provides an optic that has a 20 degree emission into both directions perpendicular to the optical axis, which is very efficient for emission into light guides. The parabolic cross-section need not be a true, smooth parabola, but can be approximated by polygonal or linear segments that collectively lie tangent to a parabolic cross-section.
For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims taken in conjunction with the above-described drawings.
Referring now to the drawings with greater particularity there is shown in
In a preferred embodiment the axis 283 of the bisector of parabolic cross-section 282 is tilted about 8 degrees and the linear array of LEDs 140 has 5 LEDs.
The optic 260 can be formed from a suitable metal, for example, aluminum or stainless steel, or it can be formed from a high temperature plastic such as an acrylonitrile butadiene styrene (ABS) material, with the parabolic surface 270 appropriately reflectorized. In a preferred embodiment of the invention, the optic 260 is fabricated from aluminum; however, it will be understood by those skilled in the art that the ultimate choice of material for the optic will depend upon many factors, not the least of which are cost of the materials and the environmental conditions existing where the optic is being used.
In a preferred embodiment, when the optic 260 is used with the light source described above, the optic 260 can have exit window dimensions of about 10 mm by about 14.40 mm along axes 263, 265 respectively.
Referring particularly to
A method of generating a parabolic surface 270 for use with the low profile optic 260 for use with a linear array of multiple LEDs 140 (L1-Ln) is sequentially illustrated in
Referring now to
A bisector of the parabolic cross-section 282 is created in the Z-Y plane with a focal length of 1 mm and a focal point 280 in the center of LED L1 and the axis 283 of the bisector of the parabolic cross-section 282 is aligned parallel to the axis Z1, as shown in
The axis 283 of the bisector of the parabolic cross-section 282 is tilted about the focal point 280 inwardly away from the axis Z1 and in a preferred embodiment, that tilt is 8 degrees, as shown in
The axis 283 of the bisector of parabolic cross-section 282 is then shifted along the Y axis by one half the width of LED L1 so that the focal point 280 lies on the long edge 180 of the LED array 120, as shown in
The bisector of parabolic cross-section 282 is then swept along the X axis from the center of LED L1 to the center of the last or terminal LED Ln, which has axis Z2, in the direction of arrow 284 (
The bisector of parabolic cross-section 282 is then rotated around the axis Z2 to form one half of the surface 270. These actions are then duplicated along the other long side 160 and axis Z1 to complete the parabolic surface 270, which is shown completely in
The optic 260 as constructed herein will provide, among other capabilities, a 20 degree emission into both directions perpendicular to the optical axis and an approximately 30% reduction in the width of the exit aperture, which is convenient for providing optimal coupling to a light guide.
An embodiment of the invention is shown in
Accordingly, utilizing this example it can be calculated that with the design according to an embodiment of the invention and R2=5 mm and L=4.40 mm, the exit window area A=122.5 mm2. The exit aperture has a size of 10 mm by 14.4 mm. Additionally, the entrance window has an area R1=1.64 mm and L=4.4 mm and an entrance area A=22.88 mm2; providing an area ratio whereby the exit area is 5.35 times as large as the entrance area for a height h=12.61 mm, or generally in a range of about 5.2 to about 5.4 times as large as the entrance area. This ratio is not achievable without the tilt and shift of this invention for this height.
In contrast, for an untilted/unshifted (which may be called “straight forward”) parabola with the same length and focal length and R2=7.51 mm, the exit window area A=243.3 mm2 and for an untilted/unshifted parabola with the same entrance area and R2=5.92 mm, resulting in an exit aperture of 11.84 mm by 16.24 mm, the exit window area A=162.2 mm2. Thus, it will be seen that utilizing the shift and tilt of the parabolic axis of the invention allows an exit window area that is 30% to 50% smaller than a conventional parabola.
It was considered that by using an approximately 10 mm×15 mm plastic CPC whose parabolic reflector surface was not tilted and translated as described hereinabove, that is, for example, one with a focal length of 0.434 mm centered on the chip surface at Z=0, one could use the small area as the light entrance from an array of LEDs and the large area as the emission region, but that implicated going from a large divergence angle at the entrance to a small divergence angle on the emission side and the primary optic would have to be carefully aligned with the entry to the light guide and the light guide's dimensions would have to be carefully controlled to tight tolerances; conversely if one used the large opening as the light entrance and passed the light to the smaller area, then the divergence of the light goes from a small angle to a large angle, and if light is sent at a large angle to a light guide, the light might miss the entry to the light guide. It is known that one obtains more efficiency when light goes in perpendicular to a surface, which involves about a 4% loss, in comparison to about 20% loss when light entry is at 45 degrees. If a CPC that is tilted and translated as described above is not used, then in order to more tightly control the light emission a plastic molded CPC could simply be made smaller but on the size scale involved here to inject light into a typical 10 mm×15 mm cross sectional entry area of a light guide, then the use of an inexpensive 1-shot mold process can result in sinks in the plastic, which leads to light loss; alternatively a carefully controlled molding could be made using so-called two-layer molding but that process is expensive. The use of the CPC tilted and translated as described brought the surprising advantage that one can put up with larger tolerances on the light guide, which is typically made of molded plastic for low-cost, mass produced automotive lamps, or on the alignment of the emission region of the CPC to the light guide.
While there have been shown and described what are at present considered to be the preferred embodiments of the invention, it will be apparent to those skilled in the art that various changes and modifications can be made herein without departing from the scope of the invention as defined by the appended claims.
