Vehicle LED bulb with polygonal light guide

An automotive LED lamp having a light guide (28) having a single central bore (29) defined by a cylindrical wall (26) and an outer surface (31) defined by a plurality of outer wall segments (35) bounding, as seen in cross-section transverse the optical axis (18) a regular polygonal shape having more than four sides. The outer polygonal shape has between five sides and sixteen sides, preferably a ten-sided decagon.

CROSS REFERENCE TO RELATED APPLICATIONS

TECHNICAL FIELD

The present disclosure relates to light sources employing light emitting diodes (LED or LEDs) and more particularly to light sources useful in the automotive field such as for taillights, stoplights, fog lights and turn signals. More particularly, it relates to such light sources employing a light guide made of light transmissive material.

BACKGROUND

Vehicle lamps similar to those depicted in U.S. Pat. No. 7,110,656 or U.S. Des. 610,544 have been manufactured and marketed by Osram Sylvania Inc. in the United States under the trade designation L1224R lamp, which has a hollow light guide that, transverse along its major longitudinal extent, is tubular in cross-section having both inner and outer smooth cylindrical walls, and with four (4) LEDs (reference numeral24) spaced equidistant in a circular pattern in register with the wall thickness “T”, as shown in FIG. 2 of the aforementioned U.S. Pat. No. '656. Over a majority of its length the cylindrical light guide has an inner cavity with an inside diameter of about 10 mm, an outer diameter of 16.5 mm, and thus a wall thickness of about 3.25 mm.

For a thorough understanding of the present disclosure, reference is made to the following detailed description, including the appended claims, in connection with the above-described drawings. Although the present disclosure is described in connection with exemplary embodiments, the disclosure is not intended to be limited to the specific forms set forth herein. It is understood that various omissions and substitutions of equivalents are contemplated as circumstances may suggest or render expedient. Also, it should be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting.

DETAILED DESCRIPTION INCLUDING BEST MODE OF A PREFERRED EMBODIMENT

FIG. 3andFIG. 4Bshow a prior art LED lamp having light guide82indicated thereon that is built according to U.S. Pat. No. 7,110,656 and marketed in the US by Osram Sylvania Inc. under, for example, the trade designation L1224-R. The reference numerals onFIG. 3indicate parts as described in the U.S. Pat. No. '656. The prior art light guide82receives light from four (4) LEDs spaced equidistant on an imaginary circle. Light from the LEDs is guided and mixed within the light guide and then redirected by the reflector ring38to fill another reflector optic (mounted on the chassis) that forms the tail lamp beam. Light guide82is, over a majority of its length, cylindrical with an inner cylindrical cavity having an inside diameter of about 10 mm, an outer diameter of 16.5 mm, and thus a wall thickness of about 3.25 mm. Applicants herein recognized that some light does not hit the reflector ring38and exits directly from the light guide. In certain directions there are peaks visible, one peak (or set of peaks) corresponding to each LED, for example four peaks visible, which are significantly higher than the intensity at surrounding exit angles. These four peaks exceed the relevant maximum specification for the bulb type LR1 in the harmonized United Nations Economic Commission for Europe (“ECE”) regulation No. 128. The ECE regulation 128 (“ECE R128”) contains uniform provisions concerning the approval of light emitting diode (LED) light sources for lamp units on motor vehicles. Regulations concerning LED light source category LR1 are contained in Annex 1 to UN Regulation No. 128 (available from UN ECE, Palais des Nations, CH-1211, Geneva 10, Switzerland).

FIG. 6shows a plot of an emission pattern as a Mercator plot of a prior art lamp and light guide ofFIG. 3illustrating the peaks that exceed, or nearly exceed, the ECE R128. The graphic inFIG. 7shows the orientation of the polar angle referenced to optical axis18as depicted on the improved lamp10of the present application which has a 10-sided light guide28, reference hereto for explanatory purposes relative toFIG. 6and prior artFIG. 3merely to illustrate the orientation of the direction indicated as “polar angle” declining from the optical axis and thus having a range of 0 to 180 degrees and the direction indicated as “azimuthal angle” as being in a plane perpendicular to the plane in which the polar angle is indicated and sweeping one revolution around optical axis18, the azimuthal angle thus having a range from 0 to 360 degrees. With reference toFIG. 6, the present Applicants determined, such as through computer simulation, that the prior art lamp and light guide arrangement exhibits a primary peak70at circa 55 degrees polar angle that has an intensity, as measured in cd/klm (candela/kilo-lumens), in excess of 115 cd/klm maximum allowed in ECE R128. Applicants also determined a secondary peak72at circa 126 degrees polar angle that tends to exceed the 200 cd/klm maximum allowed at that location in ECE R128. Applicants herein determined through simulation that the peaks are due to an unfortunate superposition of reflected images inside light guide82. The peaks70(or sets of peaks70,72) are 90 degrees apart (azimuthal angle) because there are four LEDs spaced equidistantly around an imaginary circle at the light entrance to light guide82.

FIG. 5contains an emission pattern of the knownFIG. 3light guide arrangement whose peaks are present in aboveFIG. 6as a cross-section through the peak, plotting intensity (candelas per kilo-lumens) vs. polar angle. The knownFIG. 3arrangement is presented as the curve in large dashes, it being noted that a peak at circa 55 degrees exceeds a limit imposed by the curve, shown in fine dashed line, of the ECE regulatory maximum.

FIGS. 1 and 2show an LED light source10of the present application comprising a housing12having a base14. A hollow core16projects from the base14and is arrayed about a longitudinally extending optical axis18. A printed circuit board20is positioned in the base14at one end22of the hollow core16and has a plurality of LEDs24operatively fixed thereto about the center thereof, in a preferred embodiment of the present disclosure the hollow core16is tubular and the array of LEDs is circular. A light guide28with a body30that is, in a preferred embodiment, cup-shaped (closed-end tubular) is shown inFIGS. 2 and 4A. A light guide is sometimes also referred to as a light pipe. The light guide28is positioned in the hollow core16and has at the light guide's first end a light entrance region32in operative relation with the plurality of LEDs24and a second end34projecting beyond hollow core16. The radial wall thickness of light guide28is at least large enough to encompass the emitting area of the LEDs that are employed with it; in practice this wall thickness is about 3 mm. Preferably, light entrance region32is located close to and optically formed to capture a substantial portion, preferably all, of the emitted light from the LEDs24for conduction to the light guide28. Further construction details are seen in U.S. Pat. No. 7,110,656, which is hereby incorporated by reference as if fully set forth herein.

In light guide28, light guide body30extends between light entrance region32and light exit region33. Light guide body30defines a hollow interior formed by a single, centrally located void or bore29. Central bore29is aligned with optical axis18. The plurality of wall segments35collectively define outer peripheral surface31of light guide body30. As seen in a cross section perpendicular to optical axis18, outer wall segments35define a closed polygon shape, preferably a regular polygon. In a regular polygon the wall segments are of equal length to one another, and the interior angle (sometimes also called “internal angle”) defined between adjacent wall segments are equal. In any polygon having a number of sides a (where n is an integer), the sum of interior angles is (n−2)×180 degrees. In a regular polygon of a sides, each interior angle is (n−2)×180 degrees/n. Thus in light guide28having a 10-sided (also called decagon) regular polygon cross sectional shape shown inFIG. 1, 2, 4A, or7, the interior angle is 144 degrees.

The wall segments35extend in planes that are parallel, or substantially parallel, to a plane containing optical axis18. That is, for a polygon of an even number of sides, such as a 10-sided regular polygon (decagon) of light guide28, the wall segments35that are diametrally opposite one another on opposite sides of optical axis18are parallel. That is, preferably wall segments35, as seen in a longitudinal direction extending in a direction of optical axis18, are not, in an optically relevant sense, mutually inclined to one another or tapered along their length, that is wall segments35maintain a substantially constant distance from optical axis18. This is understood in an optically relevant sense, ignoring the small draft angle of typically less than 1 degree, or more commonly one-half to one-quarter degree, used as a convenience to remove molded plastic parts from their molds.

A heat sink36is positioned in a heat-transferring relationship with the printed circuit board20and a first reflector38is attached to the second end34of the light guide28. A suitable heat sink is shown U.S. Pat. No. 7,059,748 and assigned to the assignee of the present disclosure, the teachings of which are hereby incorporated by reference.

The preferred light guide28has a projecting second end34formed with an outside diameter D1that is less that the outside diameter D of the body30and the first reflector38is provided with a depression40that encompasses the diameter D1so as to be mateable with the second end34of the light guide28in a press-fit

In an alternate embodiment, not shown, of the present disclosure a light guide28is tubular and a first reflector38is formed with a protrusion that fits mateably within the open end of the light guide28, details of which are more fully set forth in U.S. Pat. No. 7,110,656 at FIGS. 4b and 4c therein and are hereby incorporated by reference.

The light source10is formed to fit within an aperture in a second reflector42and the second reflector42has its reflective surface44facing a reflective surface46on the first reflector38. Second reflector42is coupled to a chassis of a vehicle. A resilient elastomeric reflector gasket80is positioned about core16as an environmental seal.

The LEDs24preferably are arranged in an imaginary circle, preferably spaced equidistant on the imaginary circle, to disperse them one from the other and thereby facilitate cooling. Typically there are four LEDs24spaced equidistant in a circular pattern. At the same time the light guide28is formed as a tube, such as closed-end tube shown inFIG. 2, with optical axis18. The light guide28has, formed at the first end of light guide28, light entrance region32that bridges the LEDs24and thereby captures the light emitted directly and from the sides of the LEDs in one light guide.

Light guide28has a peripheral seat50formed adjacent light entrance region32to insure the proper spacing relationship to the LEDs24. Additionally, the light guide's first end has a flange52formed thereabout that receives gasket54between the flange52and the one end22of the hollow core16.

The outer surface of the hollow core16can be provided with fastening ears56for engaging the light source10within the second reflector42, as is known. Also, a connector58can be provided on the body16containing electrical contacts60for providing the necessary power from an outside source to the printed circuit board20and thence to the LEDs24.

As shown in the drawings, in operation, light from the LEDs24is fed through the common light guide28to the first reflector38. The light is reflected from reflective surface46back to reflective surface44of the second reflector42and thence forward to the area external to the vehicle to be illuminated. The reflective surfaces44and46can be optically tuned to determine a preferred output beam pattern. Each can be an interchangeable part specific to a particular beam pattern, while the remaining lamp parts are standard components.

However, if desired the reflective cap38can be eliminated and the light can be emitted directly out of the light exit region33of light guide28. The LED lamp10can then be employed with lenses or other structures to customize the light output.

The light guide28is made from a light-transmissive material such as molded acrylic materials such as Albis Opitx CA-41 or CA-75 or polymethylmethacrylate (PMMA). The material is chosen to be light transmissive in the light wavelength range of interest, as is known in the art. The utilization of the single light guide facilitates construction and the optical alignment, without the need to individually align and support multiple guides.

As a practical matter, to improve upon the performance of a known LED lamp model L1224R generally known inFIGS. 3 and 4B, and for purposes of compatibility with the size of lamp securement apertures in known tail lamp reflectors44, it was chosen to design particular embodiments of the light guide28of the present invention based on the overall size of known light guide82in particular on its existing outside diameter of 16.5 mm. Applicants then selected an internal bore29defined by a cylindrical wall26since if internal bore29were to have corners, then light losses become very high, but that with a cylindrical inner bore and polygonal outer wall, light guide28still functions according to principles of total internal reflection (TIR). Having corners defined by the interior angle of adjacent wall segments35allows for more random light mixing, and shifts the light from the four LEDs24so that it does not overlap, Applicants thereby achieving the surprising result that the ECE R128 could be maintained without peaks in excess of the regulatory limits. Applicants performed simulations on polygonal shapes inscribed within the 16.5 mm O.D. circular cross section of known light guide82to model light guide28. Applicants simulated body30having in cross section a regular polygon between five (5) sides up to twenty-three (23) sides, in increments of one (1) side, and determined through ray tracing that a range of from five (5) sides up to sixteen (16) sides avoided peaks in excess of the ECE regulation, and would thus be compliant.

A preferred embodiment of light guide28has ten (10) wall segments35defining body30that is a regular decagon in cross section. On an emission plot of the type shown inFIG. 6, the lamp10having light guide28of the present disclosure showed no peak in excess of the ECE R128 regulation, and as shown inFIG. 5the emission pattern, shown in solid line, exhibits peaks below the ECE maximum.

Applicants determined through simulation that using an inscribed polygon of less than five sides led to inadequate size of light entrance region32to capture all light emitted from LEDs24, so too much light would by-pass the light guide. Having a polygon shape defined by five or six sides would advantageously produce no peaks exceeding, the ECE regulation, but due to the sharpness of the internal angle (below 120 degrees) the pentagon or hexagon was less preferred due to light inefficiency. Having a greater number of sides reduces light losses and inefficiency, Applicants also determined that a regular polygon having up to sixteen (16) sides still remained within the ECE and sixteen was generally considered the most number of wall segments35since it then resulted in the minimum amount of tolerance or safety factor to stay within the ECE limit. Simulation on polygons of seventeen to twenty-three sides revealed peaks in excess of the ECE regulation. A ten-sided polygon (decagon) was seen as a good trade-off between light losses and inefficiency and comfortably staying below the ECE regulatory maximum given tolerances in manufacturing and alignment with LEDs24.

It is pointed out that as shown inFIG. 5the relevant measurement is the normalization of intensity (in candela) divided by total flux (in kilo-lumens), and this normalization creates a specification for the light distribution which is independent of the LED flux, thus a person of skill in the art in attempting to reduce peaks of the prior art (FIG. 3) lamp might consider simply reducing the LED current, but that would not eliminate the peaks that were in excess of ECE R128, since even if one halved the flux of the LED, then the total flux out of the lamp would be half and the intensity of the emitted light in any angle would be half as well, and therefore the ratio of intensity (cd)/bulb flux (klm) would remain the same. The present Applicants determined the surprising result that an advantageous range of regular polygon shapes for outer surface31of light guide28can satisfy the ECE regulation.

While several embodiments of the present disclosure have been described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other means and/or structures for performing the functions and/or obtaining the results and/or one or more of the advantages described herein, and each of such variations and/or modifications is deemed to be within the scope of the present disclosure. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and/or configurations will depend upon the specific application or applications for which the teachings of the present disclosure is/are used.

The indefinite articles “a” and “an,” as used herein in the specification and in the claims, unless clearly indicated to the contrary, are understood to mean “at least one.”

An abstract is submitted herewith. It is pointed out that this abstract is being provided to comply with the rule requiring an abstract that will allow examiners and other searchers to quickly ascertain the general subject matter of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims, as set forth in the rules of the U.S. Patent and Trademark Office.

The following is a non-limiting list of reference numerals used in the specification:10LED lamp12housing14base16hollow core18optical axis20printed circuit board22an end of hollow core24LEDs26inner cylindrical wall28light guide29bore of light guide30body of light guide31outer peripheral surface of light guide32light entrance region33light exit region34second end35outer wall segments36heat sink38first reflector40depression42second reflector44reflective surface of second reflector46reflective surface of first reflector50peripheral seat52flange54gasket56fastening ear or bayonet lugs58connector60electrical contacts62backup light white LED assembly64electric wires to backup light66power supply to PCB1268power plug70peak72secondary peak80reflector gasket82prior art light guide of U.S. Pat. No. 7,110,656