Abstract:
An aircraft position light employs selected colored LEDs to produce a white appearing warning light with a reduced light component in the amplification spectrum of night vision imaging equipment. A combination of amber and cyan LEDs are selected to produce approximately three amber flux units for every cyan flux unit resulting in a white composite light. Both of the selected LEDs have dominant wavelengths of less than 600 nm.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates generally to external navigation lighting for aircraft and more particularly to an LED aircraft taillight that appears white but emits little or no light in the upper (red) portion of visible light spectrum and the just above visible spectrum (near infra red). 
   2. Description of the Related Art 
   Civilian air traffic control agencies such as the FAA specify standards for aircraft external lighting. Aircraft operating at night in civilian airspace are required to display lights to attract the attention of other aircraft operating in the same airspace. These external aircraft lights include flashing anticollision lights mounted on the aircraft&#39;s upper and lower fuselage, as well as steady burn position/navigation lights on the tail and the wing tips. The location, color, intensity and light radiation pattern for each particular light is typically specified by the relevant regulation. 
   Night vision imaging systems (NVIS) were introduced in the late 1960&#39;s and are now widely used by military and law enforcement personnel to enhance the effectiveness of aircraft during night operations. Night vision goggles (NVGs) are binocular-looking devices that can be attached to a flight helmet or a combat helmet and suspended in front of the eyes. NVGs are now widely appreciated as one of the best and most cost-effective ways to permit night operations in aircraft. 
   NVGs operate by greatly multiplying very low levels of existing light and then presenting the operator with a scene in front of his eyes that closely resembles a daytime scene. NVGs have made great advances in recent years in both their power to multiply existing light and their visual acuity. Modern NVGs amplify light in the upper visible (red) spectrum and the just-above-visible (near infrared) spectrum. This area of amplification will be referred to as the “NVG amplification spectrum.” While NVG sensitivity is greatest in the just-above-visible/near infrared region, there is a significant sensitivity in the visible red region as well. Consequently, there is an overlap between the light frequencies that the unaided human eye sees and those light frequencies that NVGs respond to, e.g., the NVG amplification spectrum. 
   Most modern NVGs have a built-in filter that limits this overlap by making a cleaner NVG cutoff at the lower end of the NVG amplification spectrum. These NVGs are not sensitive to blue and green colored light. NVGs may also employ an automatic brightness control feature, which acts to decrease light amplification as input light levels increase. Automatic brightness control was designed to maintain a constant NVG scene brightness. Brightness control is initiated above a certain input light level threshold and reduces the light sensitivity of the goggles causing a corresponding decrease in system gain, and an overall loss of NVG-aided visual acuity. Higher levels of input light in the NVG amplification spectrum can reduce NVG performance and thus reduce the operator&#39;s ability to see a low light level nighttime scene. 
   Aircraft external lighting have previously been provided by “strobe” lights or incandescent lamps. An unfiltered incandescent bulb emits strongly in the near infrared spectrum. Strobe lights also produce a broad spectrum of light having significant components in the NVG amplification spectrum. Incandescent and strobe lamps also suffer from relatively short service life. 
   With advances in the intensity of light output from light emitting diodes (LEDs), it is now possible to replace incandescent and strobe lamps with LED light sources. LED light sources are attractive because of their extremely long service life and relatively low power consumption. Several high flux LEDs such as the Luxeon™ emitter from LUMILEDS™ of San Jose, Calif., in certain configurations, can achieve the required light output and radiation pattern for an aircraft position light. Recently developed white LEDs make it possible to produce a chromatically white LED aircraft position light meeting the requirements for intensity and radiation pattern. However, white LEDs emit significant light in the NVG amplification spectrum, and in particular display a spike in light emission at a wavelength of approximately 610 nm, very close to the red portion of the spectrum. As a result, aircraft position lights employing white LEDs are unacceptably bright in the NVG amplification spectrum. 
   Aircraft flying in civilian airspace are required to turn on their navigation lights. Navigation lights employing incandescent or LED light sources impair the effectiveness of aircrews equipped with NVGs due to the intensity of light emitted in the NVG amplification spectrum. FAA regulations requiring external aircraft lighting have been modified to permit military aircraft operation in military airspace without external lighting in recognition of this problem. It is possible to equip such navigation lights with filters that reduce the light output in the NVG amplification spectrum. However, these filters usually reduce the visible light output below the intensity specified by the relevant regulations. The filters also tend to shift the color chromaticity coordinates outside those permitted by the relevant regulations. In extreme cases it is known to turn off external lights, placing aircraft, aircrews and the public at risk. 
   There is a need in the art for an aircraft navigation light that provides the required chromaticity, light intensity and radiation pattern for a white position light and emits little or no light in the red and near infrared light spectrum. 
   SUMMARY OF THE INVENTION 
   An exemplary aircraft taillight in accordance with the present invention uses a combination of colored LED light sources to provide a position light that appears white to a viewer. The light radiated by the LEDs combines to produce a hue of light that appears white but does not include a significant component in the NVG amplification spectrum. One effective combination of colored LEDs is amber and cyan. The LEDs are combined to produce a amber and cyan light in a flux ratio of approximately three amber flux units for each cyan flux unit. An LED array employing amber and cyan LEDs in this ratio is then configured to provide the light intensity and radiation pattern specified for the particular position light. 
   One example is a position light configured for use as an aircraft taillight. An aircraft taillight is required to be a white light having a specified chromaticity (see  FIG. 2 , at  70 ) at an intensity of at least 20 candela over an arc of 140° centered on the tail of the aircraft. To comply with this standard, a support block includes two angled support surfaces to which PC boards are mounted. Each PC board includes a plurality of high flux LEDs mounted in a linear array. The linear array includes one amber LED on either side of a cyan LED. The LEDs are of the lambertian (high dome) lens configuration with a viewing angle of approximately 140°. Viewing angle is the off axis angle from the emitter optical axis where the luminous intensity is ½ of its peak value. Viewing angle is an industry standard indication of the radiation pattern of an LED emitter. A large viewing angle indicates a wide radiation pattern. 
   The support block fixes the PC boards at an included angle of approximately 90°. This angle may be an internal angle, producing a concave angled orientation (valley), or may be an external angle, producing a convex angled orientation (peak). In combination, the viewing angle of the LEDs and the included angle of the support block result in a radiation pattern that extends over an arc of 140° centered on the taillight. Each of the circuit boards includes two amber high-flux LEDs and one cyan high-flux LED. This combination of light sources meets FM requirements for intensity and approximates the required white color without significant light radiation having a wavelength in the NVG amplification spectrum. 
   The resulting position light is “NVG friendly” in that it does not emit large amounts of light that will degrade NVG performance when within the field of view of NVG equipped aircrew. This is particularly important for an aircraft taillight because, when aircraft fly in formation, the taillight will necessarily be in the field of view of those piloting aircraft in the rear of the formation. If the position lights in the field of view of the following aircraft emit excessive light in the NVG amplification spectrum, they must be switched off to avoid blinding NVG equipped aircrew. 
   An object of the present invention is to provide a new and improved aircraft position light that complies with FAA requirements for color, intensity and radiation pattern and does not emit significant light in the NVG amplification spectrum. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This and other objects, features and advantages of the invention will become readily apparent to those skilled in the art upon reading the detailed description of the exemplary embodiment in conjunction with the accompanying drawings, in which: 
       FIG. 1  is an exploded perspective view of an exemplary aircraft taillight illustrative of aspects of the present invention; 
       FIG. 2  is a perspective assembly view of the aircraft taillight of  FIG. 1 ; 
       FIG. 3  is a CIE 1931 Chromaticity diagram annotated to indicate the chromaticity of light produced by LEDs in the aircraft taillight and the resulting composite light with respect to the required (specified) taillight color; 
       FIG. 4  is a graphical illustration of the relative spectral power distribution of LEDs of various colors; 
       FIG. 5  is a graphical illustration of the relative spectral power distribution for a typical white LED; 
       FIGS. 6   a  and  6   b  are exterior side and top views, respectively of an aircraft showing the locations for required external lights; 
       FIG. 7  is a top view of the support block and LED arrays illustrating the light emission pattern of an exemplary aircraft taillight; 
       FIG. 8  is a partially exploded perspective view of an alternative aircraft taillight exemplary of aspects of the present invention; 
       FIG. 9  is another, more completely exploded perspective view of light emitting component of the aircraft taillight of  FIG. 8 ; 
       FIG. 10  is a top end view, partly in phantom, of the light emitting component of  FIG. 9 ; 
       FIG. 11  is an exploded rear perspective view of the light emitting component of  FIGS. 9 and 10 ; and 
       FIG. 12  is a schematic of a representative driver circuit for the light emitting units of  FIGS. 1 and 9 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   An exemplary embodiment  10  of a white position taillight for an aircraft is illustrated in  FIGS. 1 and 2 . With reference to  FIGS. 6   a  and  6   b , an aircraft taillight must be placed on the rearward most part of the aircraft as shown at  106 . FM standards require a light radiation pattern that extends 70° (degrees) to the right and left of the aircraft centerline as shown in  FIG. 6   b . The standard requires a minimum light intensity of 20 candela over this arc in the plane of the aircraft. The required light intensity falls as the pattern moves further above or below the plane of the aircraft centerline as shown in  FIG. 6   a . The relevant aviation lighting standards also specify the chromaticity (apparent color) for the aircraft taillight. 
   An aircraft taillight must be “aviation white” with a chromaticity in the range bounded by line  70 ′ in  FIG. 3 . In the CIE chromaticity chart of  FIG. 3 , the lower right-hand corner represents the overlap between the NVG amplification spectrum and the visible light spectrum. The chromaticity indicated as “bluish white” at the left-hand end of the aviation white range  70 ,  70 ′ is the composite chromaticity of white LEDs presently available.  FIG. 5  illustrates the relative spectral power distribution for a Luxeon™ white emitter manufactured by Lumileds™ of San Jose, Calif.  FIG. 5  illustrates a spike  72  of light output at approximately 610 nm and the significant light output  74  of the white LED in a light wavelength above 610 nm. The light output pattern of a white LED causes it to appear very bright to an NVG. This brightness results in activation of the automatic brightness control feature of the NVG, effectively blinding NVG equipped aircrews. 
   LEDs typically emit light having a relatively narrow spectral band, with white LEDs being a notable exception.  FIG. 4  illustrates the light output of various Luxeon™ emitters manufactured by Lumileds™ of San Jose, Calif. Each of the various colored LEDs emits a “pure” light, e.g., light within a narrow spectral band. An aspect of the invention relates to combining the light output of two colored LEDs having a dominant wavelength below 600 nm to produce a white LED position light that emits little or no light in the NVG amplification spectrum. In an exemplary embodiment, this goal is achieved by combining amber and cyan Lumiled® emitters in a ratio of two amber emitters to each cyan emitter. The current version of an amber Lumiled® emitter has approximately 1.4 times the radiometric power, or luminous flux, of a cyan Lumiled® emitter. The two to one LED ratio therefore produces a light flux ratio of approximately three amber flux units for every cyan flux unit. In this ratio of flux, these wavelengths produce a composite light hue at  71  on  FIG. 3 . As can be seen, this composite light hue  71  falls within the color range established by both the SAE and FAR for “white”. 
   With reference to  FIG. 3  it will be understood that the amber LED  52  has a chromaticity indicated at yellow (approximately 590 nm), while the cyan LED  54  has a chromaticity indicated in the blue-green range (approximately 492 nm). Combining LEDs having these colors in a pattern that results composite flux ratio of approximately three amber flux units for each cyan flux unit results in a composite light having a chromaticity  71  within the chromaticity range  70 ,  70 ′ specified for aviation white by both the SAE (Society of Automotive Engineers) and the FAR (Federal Aviation Regulations). The composite light has a chromaticity center tolerance of approximately X=0.418 and Y=0.397. Since the component LEDs emit light in narrow spectral bands that do not include significant emission above 600 nm, the resultant composite light can be described as “NVG friendly.” The term “NVG friendly” is intended to describe a light that, while visible to an NVG equipped aircrew, does not appear so bright as to reduce the sensitivity of the NVG by activating the automatic brightness control feature. 
   It will be understood that there is a natural variation in the dominant wavelength of light emitted and the radiometric power (total luminous flux) produced by any given LED selected from a production lot of LEDs. For example, a cyan LED dominant wavelength may vary from closer to a green color to closer to a blue color and its total luminous flux may vary from a low value to a high value. LED manufacturers mitigate this variability by sorting or “binning” their LEDs into subsets having similar dominant wavelengths and luminous power. Therefore, the values used for dominant wavelength and luminous flux used in this application should be understood to be representative values. Further, the amber/cyan flux ratio of three to one is given as an approximate value. This flux ratio can be expected to vary from 2.5 to as much as 3.5 amber flux units for every cyan flux unit because of the variations in the radiometric power of particular LEDs. Careful selection of LEDs can reduce this variability to acceptable levels. Process or material changes in LED manufacture will affect the performance of the product and may require altering the number, placement, type and/or numbers of LEDs for a particular composite light. 
   While amber and cyan LEDs are disclosed in the context of this application, other combinations of LEDs with light output below 600 nm are consistent with the invention described herein. For example, green and amber or blue and amber LEDs in a different ratio may produce a white appearing light with little or no output in the NVG amplification spectrum and are intended to be encompassed by this application. 
   With reference to  FIGS. 1 and 2 , a first exemplary embodiment of an aircraft position light  10  includes a thermally conductive support block  40  to which are mounted PC boards  50  bearing arrays of LEDs  52 ,  54 . Since the light output pattern of the position light is specified to be symmetrical with respect to the aircraft centerline, the overall configuration of the exemplary position light  10  is also symmetrical. For example, the thermally conductive support block  40  defines two substantially planar surfaces  42 . The planar surfaces  42  are arranged at an angle α (approximately 45°) to a vertical plane  41  passing through the support block  40 . The vertical plane  41  passing through the support block corresponds to a vertical plane passing through the center of the aircraft. 
   The angular orientation of the surfaces  42  of the support block relative to this vertical plane  42  are selected to complement the light radiation pattern of the LEDs  52 ,  54 . As best seen in  FIG. 7 , the light radiation pattern of each of the LEDs  52 ,  54  is in the form of a half-globe. The light radiation pattern for an LED depends on the lens shape and other factors. The first exemplary embodiment  10  utilizes a Luxeon™ emitter with a lambertian lens shape. This emitter has a viewing angle of 140. The term “viewing angle” describes the angle relative to the optical axis of the LED where the luminous intensity is one-half (½) of the peak value. 
   The standard for the aircraft taillight requires a light output of 20 candela over a range of 140 centered on the longitudinal axis of the aircraft (see  FIG. 6   b ). What is important to note about the exemplary LED position light  10  is that the support surfaces  42  for the PC boards  50  are calculated to complement the radiation pattern of the LEDs used. These relationships are illustrated in  FIG. 7 . The support block angled surfaces of the exemplary embodiment define an included angle β of approximately 90°. This included angle β cooperates with the light radiation pattern from the LEDs to produce an overall light radiation pattern that meets FM specifications. As can be seen in  FIG. 7 , the wide angle light from each LED overlaps at the vertical plane of the aircraft. Even though the light output of individual LEDs at this relatively large angle from the optical axis of the LED is weak, the overlap in light emission allows a relatively few LEDs to produce the required 20 candela over the entire arc of 140°. In other words, the overlap in light emission from the two LED arrays improves the uniformity of light output directly behind the aircraft. Other types of LEDs having different light radiation patterns will likely require a support block with support surfaces tailored to those LEDs. The number and/or position of LEDs will vary depending on the intensity and light output pattern of the LEDs. 
     FIG. 1  is an exploded view showing the primary components of the exemplary position light  10 . A metal base plate  46  and support block  40  provide structural support for the position light components. PC boards  50  configured to match the shape of the angled support block surfaces  42  carry LEDs  52 ,  54  and electrical circuitry for delivering electrical current to the LEDs  53 ,  54 . In the exemplary embodiment each PC board  50  carries three LEDs. More specifically, each PC board  50  carries two amber Luxeon™ emitters  52  and one cyan Luxeon™ emitter  54 . A connector PC board  62  is configured to mount to one end of the support block  40  to electrically connect the driver PC board  60  to the PC boards  50  mounted to the angled surfaces  42 . Such electrical connection could also be accomplished by other known means, such as wires or the like. 
   The driver PC board  60  includes circuitry for producing current pulses for energizing the LEDs. An exemplary driver circuit is illustrated in  FIG. 12 . An LM317S integrated circuit is configured as a constant current source to deliver approximately 350 mA to the LEDs. The LEDs are connected in series. The bottom of the support block and base plate define a cavity (shown in  FIG. 11 ) for receiving the driver PC board  60 . Rigid electrical leads  64  extending from the driver PC board  60  protrude through an aperture  48  in the support block  40  to engage the connector PC board  62 . The integrated circuit  66  generates heat and is arranged with a heat transfer surface in thermally conductive contact with the support block lower surface (as shown in  FIG. 11 ). A thermally conductive gasket  167  provides a thermal pathway between the integrated circuit  66  and the support block  40 . In the illustrated embodiment fasteners extend through the driver PC board  60  to compress the PC board, integrated circuit  66  and gasket  167  against the thermally conductive support block  40  to enhance heat transfer. The cavity  170  for the driver board  60 ,  160  will typically be filled with potting material to seal the components against vibration and moisture intrusion. Electrical connections between PC boards  50 ,  150  connector PC board  62 ,  162  and driver PC board  60 ,  160  are soldered connections that are highly resistant to vibration. 
   The assembly is completed by installation of a gasket  30  and lens  20  that cover and seal the position light against the weather. The base plate  46  includes at least one aperture to relieve pressure beneath the lens  20 . It will be noted that the base plate  46  extends radially beyond the support block  40  and is exposed to airflow around the aircraft. This support plate configuration provides a path for heat generated by the position light components to escape the assembly. 
   The resulting aircraft position light  10  is an extremely durable, energy efficient and low maintenance assembly that meets the requirements for an aircraft position taillight. Exemplary position taillight  10  has the further significant advantage that it emits little or no light in the NVG amplification spectrum and therefore does not blind NVG equipped aircrew. This reduces the temptation of aircraft employing NVGs to turn off position lights, enhancing the safety of civilian and military aircraft. 
   A second embodiment  100  of an aircraft taillight according to aspects of the present invention is illustrated in  FIGS. 8–11 . The principal difference between the taillight of  FIGS. 1 and 2  and the taillight of  FIGS. 8–11  is that the surfaces  142  bearing the PC board mounted LEDs are oriented to form a valley rather than a peak. This configuration addresses the need for a sharp cut off of light radiation at the outward lateral ends of the pattern illustrated in  FIG. 6   b . In the second embodiment  100 , the support block  140  includes wings  180  projecting away from the base plate  146  along the outward sides of the valley supporting the PC boards  150 . The wings  180  are positioned to block light from the LEDs  52 ,  54 ,  56  having a trajectory beyond the required cutoff at 70° to the right and left of the aircraft centerline. Up to the cutoff, the light from all the LEDs blends together to meet the radiation requirements for the taillight  100 . 
     FIG. 8  illustrates the lens  120  and gasket  130  that seal the outward part of the taillight  100  against the weather. A teflon tube  165  extends through the base plate  146  into the area covered and sealed by the lens  120  and gasket  130  to relieve pressure changes caused by changes in aircraft elevation. The connector board  162  has a configuration complementary to the valley shape of the support block  140 .  FIG. 10  is an end view of taillight  100  illustrating the included angle β of approximately 90° between surfaces  142 . Electrical leads  164  from the driver board  160  extend through aperture  148  to meet connector board  162 . The embodiment of the taillight illustrated in  FIGS. 8–10  may employ white LEDs  56 . The illustrated taillight  100  meets the FM light radiation requirements with only 6 (six) Luxeon™ emitters. 
   While exemplary embodiments of the foregoing invention have been set forth for purposes of illustration, the foregoing description should not be deemed a limitation of the invention herein. Accordingly, various modifications, adaptations, and alternatives may occur to one skilled in the art without departing from the spirit and the scope of the present invention.