Abstract:
A Precision Approach Path Indicator (PAPI) unit comprises first and second light sources and a projection lens assembly. Light emitted by the first and second light sources is collected by first and second solid waveguides respectively and is guided by said waveguides to an intermediate plane. The intermediate plane is located in the focal plane of the projection lens assembly.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. Provisional Patent Application No. 60/500,705 filed on Jun. 24, 2011 and claims priority from United Kingdom Patent Application No. 1103731.4 filed on Mar. 4, 2011, both of which are incorporated herein in their entirety by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    This invention relates in general to a visual navigational aid, and in particular to a Precision Approach Path Indicator (PAPI) system. 
       BACKGROUND 
       [0003]    Precision Approach Path Indicator (PAPI) systems are used to assist the pilot of an aircraft on a landing approach to an airfield. In particular, the PAPI provides a visual indication of the correct glide slope. Typically, a PAPI system comprises four units that are positioned in a row alongside and perpendicular to the runway. Each unit transmits a beam of light that has two differently-colored components, referred to as sectors, typically white above the horizontal center line and red below. Two of the units are directed at an angle slightly greater than the optimum approach angle, and two of the units are directed at an angle that is slightly lower than the optimum approach angle. A pilot on the correct glide slope will see two red beams and two white beams. If the approach path is too steep, the beams all appear white; if the approach path is too low, the beams all appear red. Thus, the pilot is able to adjust the aircraft&#39;s altitude in order to maintain the desired combination of red and white beams, thereby optimizing the angle of approach to landing. In certain circumstances, the use of a so-called “abbreviated” PAPI may be permitted, comprising just two units, but the principle of operation is the same as that of the normal, four-unit PAPI. 
         [0004]    Authorities such as the Federal Aviation Authority (FAA) and International Civil Aviation Organization (ICAO) apply strict standards to PAPI systems, imposing stringent requirements on such parameters as the dimensions and intensity of the transmitted light beams, and most importantly on the angular range over which the transition from white to red occurs. 
         [0005]    Conventional PAPI systems have used incandescent light sources, or in some cases fluorescent or arc lamps. Such lamp-based systems suffer, however, from numerous disadvantages. Notable amongst these are the relatively short life span of the lamps, as well inefficient energy usage. 
         [0006]    More recently, PAPI systems using light emitting diode (LED) light sources have been proposed. However, the use of such light sources is also not without problems. In particular, LED light sources emit light over a considerable range of angles and efficient collimation of the light into an effective beam is difficult. In consequence, there is an ongoing need for improved PAPI systems, in particular those based on LED or other non-incandescent light sources. 
       SUMMARY 
       [0007]    There has now been devised an improved PAPI system which addresses the above-mentioned and/or other disadvantages associated with the prior art. 
         [0008]    According to a first aspect of the invention, there is provided a Precision Approach Path Indicator (PAPI) unit, said unit comprising first and second light sources and a projection lens assembly, wherein light emitted by the first and second light sources is collected by first and second solid waveguides respectively and is guided by said waveguides to an intermediate plane, said intermediate plane being located in the focal plane of the projection lens assembly. 
         [0009]    In a further aspect of the invention, there is provided a Precision Approach Path Indicator (PAPI) system comprising a plurality of PAPI units according to the first aspect of the invention. 
         [0010]    In the PAPI unit and system according to the invention, solid waveguides are used to channel light emitted by the light sources to an intermediate plane. The intermediate plane coincides with the focal plane of the projection lens assembly, which transmits the image formed at the intermediate plane into the far field, transforming position in the intermediate plane into angle in the far field. The use of solid waveguides to collect and channel light to the intermediate plane significantly reduces alignment tolerances of the light sources and associated optical components, greatly simplifying manufacture. The units are also easier to maintain, as they require little or no critical optical setup, so reducing downtime and leading to time and cost savings. 
         [0011]    The dimensions of the waveguides may be such that their distal ends lie in the intermediate plane, i.e., the image at the intermediate plane that is projected into the far field by the projection lens assembly may be the image at the distal ends of the waveguides. In alternative embodiments, the intermediate plane may lie beyond the distal ends of the waveguides. In the latter case, it will generally be necessary for the light beams from the first and second light sources to be kept apart between the waveguides and the intermediate plane, e.g. by means of a physical partition. 
         [0012]    As is conventional, the PAPI unit of the invention includes first and second light sources, corresponding to the two sectors that are fundamental to the operation of a PAPI system. The two sectors are visually distinguishable. In a PAPI system intended for conventional operation, the first and second light sources will emit differently colored light, e.g. red and white light. The first and second light sources may be light sources that emit differently colored light, or they may be similar or identical light sources, e.g. that emit white light, light from one or both being passed through a filter to create a visually distinguishable color difference between the two sectors. For the avoidance of doubt, it should be made clear that more than two light sources may be employed, though for the operation of a conventional PAPI system it is only two light sources that are required. 
         [0013]    Other forms of differentiation between the two sectors may also be employed. For instance, the light sources in one sector may emit continuously, while those in the other are intermittent. Such an arrangement may be useful in a PAPI system intended for night use, wherein the light sources are infra-red and are visualized using night vision equipment. 
         [0014]    The light sources used in the PAPI unit of the invention are preferably non-incandescent light sources. Most preferably, the light sources are light emitting diodes (LEDs), and most commonly each light source will comprise a plurality of LEDs. The number of LEDs in each light source is not critical, but the first light source most preferably comprises a plurality of red light-emitting diodes, e.g. 2 to 10, or 4 to 8, such LEDs. Similarly, the second light source most preferably comprises a plurality of white light-emitting diodes, e.g. 2 to 10, or 4 to 8, such LEDs. The LEDs of each light source are preferably arranged in a row, though other arrangements are also possible. 
         [0015]    The light generated by the light sources will generally be compliant with regulations and specifications stipulated by the relevant authorities such as the FAA and the ICAO. The white light generated by the white LEDs will generally have a color temperature of between 2750K and 10000K, e.g. between 2750K and 4500K. The red LEDs will generally produce light of wavelength between 620.5 nm and 645 nm. 
         [0016]    Suitable LEDs for use in the PAPI unit of the invention are available from Luminus Devices, Inc., 1100 Technology Park Drive—Unit 2, Billerica, Mass. 01821, USA, e.g. the white LEDs available under the product code SST-90W and the red LEDs available under the product code SST-90R. 
         [0017]    Light from the light sources is channeled by means of solid waveguides to the intermediate plane. The waveguides are typically of such a shape and size that light is collected efficiently from the light sources, and that the distal (output) faces of the waveguides define an appropriately shaped field in the intermediate plane. In a typical arrangement, in which each light source comprises a row of five LEDs, the individual LEDs are typically arranged on 25 mm centers and so the waveguide typically has a rectangular cross-section, with a width of 125-200 mm and a height (thickness) of 20-30 mm. Thus, the waveguide usually has a width of 100-250 mm, more commonly 125-200 mm, and a thickness of 15-50 mm, more commonly 20-30 mm. The length of the waveguide is not critical, but should be sufficient that the intermediate plane, i.e. the distal face of the waveguide, is fully illuminated by the light propagated through the waveguide. Typically, the waveguide has a length of 100-200 mm, e.g. about 150 mm. Longer waveguides may be used, but at the cost of increasing the overall size of the unit, which may be undesirable. 
         [0018]    The waveguides may be of glass, but for reasons of cost the waveguides are preferably formed of synthetic plastics material. The material chosen needs to be sufficiently transmissive to light and to be stable over the operating temperature range of the unit. One suitable plastics material is acrylic. Suitable acrylic waveguides may be prepared by machining of cast acrylic. It is preferred that at least some of the surfaces of the waveguides should be highly uniform, to minimize optical losses. Polishing of the waveguide surfaces may therefore be desirable. 
         [0019]    To ensure that the waveguides do not touch and that there is sufficient space to accommodate the light sources and any associated optical components, the waveguides are preferably not disposed parallel to each other, but are slightly angled towards each other. Typically, the angle between the two waveguides is between 0.5° and 4°, e.g. approximately 2°. Because of this, the distal faces of the waveguides are not perfectly coplanar, but the deviation from planarity is so small that it is not significant. To ensure a distinct transition between the red and white sectors, the output faces of the two waveguides are slightly separated, the separation generally being 1 mm or less, more commonly 0.5 mm or less, say 0.05 to 0.5 mm, e.g. about 0.1 mm. In a typical embodiment of the invention, such a gap corresponds to 1 arcminute and so is well within the angle of 3 arcminutes normally specified for the transition between sectors (i.e. from white to red). In practice, it has been found that the edges of the waveguides may be permitted to touch, imperfections in the edges giving an effective separation of an appropriate magnitude. 
         [0020]    In order to maximize the performance of the unit, it is important that as much light as possible is channeled from each LED into the associated waveguide. Light from each LED may be directly coupled into the waveguide. Alternatively, a lens may be used to capture light from each LED for coupling into the waveguide. Tapering of the waveguide may also be used to enhance coupling of light from the LEDs. 
         [0021]    In the typical arrangement in which the waveguide has a cross-section of approximately 150×25 mm and the light source is made up of a row of five LEDs, as much light as possible from each LED needs to be channeled into an area of about 25×25 mm. This requires an input light cone angle of about 10°, which may be achieved by means of a collimating lens associated with each LED. Such a lens is typically hemispherical in form, which produces a circular input light field. However, as the light is unconfined as it propagates through the waveguide, the light field at the distal face of the waveguide is more uniform. This also has the benefit that the positioning of the LEDs and the collimating lenses is less critical, leading to greater tolerances and hence easier manufacture. 
         [0022]    The proximal (input) face of the waveguide may be planar, in which case the light sources (e.g. LEDs) and associated optical components (e.g. collimating lenses) are preferably arranged at angles to the proximal (input) face of the waveguide. For reasons of manufacturing simplicity, however, it is preferred for the light sources and associated optical components to be aligned linearly, transverse to the longitudinal axis of the waveguide. In such a case, the proximal (input) face of the waveguide is preferably faceted, the number of facets matching the number of LEDs. Thus, for instance, where there are five LEDs, the input face of the waveguide has five facets. The central facet is parallel to the output face of the waveguide (and to the row of LEDs), the facets on each side of the central facet are disposed at a first angle to the central facet (typically of the order of 2-5°, e.g. 4°), and the outermost facets are disposed at a second, greater angle (typically of the order of 8-15°, e.g. 10°). 
         [0023]    Applicable standards for PAPI systems generally require the intensity profile of the transmitted light beam to satisfy certain criteria. The intensity profile may be modified by some or all of the following measures: 
         [0024]    a) The light sources and associated optical elements (e.g. collimating lenses) may be slightly offset from the center of the respective waveguides. 
         [0025]    b) Further control of the intensity profile can be achieved electronically, by varying the output of the individual light sources. 
         [0026]    c) Imperfections may be introduced into some or all of the surfaces of the waveguides, so as to cause localized optical losses from the waveguides. For instance, some or all of the faces of the waveguide may have roughened surfaces. In one embodiment, where the waveguide has a rectangular cross-section, one major face of the waveguide has a highly polished surface, to minimize optical losses at that surface, while the opposite face and sides are roughened, e.g. by sand-blasting or a similar process. 
         [0027]    d) Positioning of an optical diffuser between the light source and part of the proximal face of the associated waveguide. 
         [0028]    The projection lens assembly conveys the image in its focal plane (i.e. the light field at the distal (output) faces of the waveguides) into the far field, i.e. in use towards an inbound aircraft. Because the light fields of the two sectors are very close together in the intermediate plane, it is possible to use one projection lens to convey both sectors. 
         [0029]    Generally, the diameter of the projection lens will be about 50 mm or more, but should be no more than about 200 mm, e.g. about 150 mm or 120 mm, simply for reasons of cost and compactness. Because simple spherical lenses perform best only at high f-number (the f-number of a lens being the ratio of the focal length to the diameter), the projection lens assembly is preferably a composite lens assembly, most preferably comprising three lens elements. The focal length of the projection lens assembly is preferably in the range 200-400 mm, e.g. about 350 mm. 
         [0030]    Because the projection lens inverts the image at the intermediate plane, where (as is conventional) the upper sector of the transmitted light beam is white and the lower sector is red, the image at the intermediate plane must be the converse, i.e. the image at the output face of the upper waveguide is red and that at the output face of the lower waveguide is white. 
         [0031]    The light sources (LEDs), associated collimating lenses and waveguides may form part of a sub-assembly that is referred to herein as the “light engine”. That sub-assembly and the projection lens assembly may be mounted on an optical bench with formations that cooperate with the components mounted upon to it facilitate correct alignment and spacing of those components. 
         [0032]    The PAPI unit preferably incorporates means for leveling the unit and aiming the output light beam at the correct angle. Most conveniently, the unit is provided with legs that are independently height-adjustable for this purpose. 
         [0033]    The PAPI unit according to the invention preferably includes a tilt fault detection system consisting of a tilt switch assembly or a clinometer to indicate any deviation from the proper leveling of the unit. 
         [0034]    The PAPI unit preferably includes a weatherproof and corrosion-resistant housing. The unit preferably has a weight and dimensions that are such that it can be lifted and installed in position by a single operator. 
         [0035]    In the majority of cases, where the PAPI unit according to the invention is installed permanently at a commercial or military airfield, the unit will be powered by the existing main electricity supply by which landing lights and other electrical equipment associated with the runway are powered. In other circumstances, however, e.g. installation at temporary airfields, the units may be battery-powered, or may be powered by alternative energy sources, such as wind or solar power. 
         [0036]    The PAPI unit may comprise a thermostatically controlled heater to prevent ice formation on the lenses. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0037]    The various features, advantages and other uses of the present apparatus will become more apparent by referring to the following detailed description and drawing in which: 
           [0038]      FIGS. 1A and 1B  illustrate the principle of operation of a PAPI system; 
           [0039]      FIG. 2  is an exploded view of a PAPI unit in accordance with the present invention; 
           [0040]      FIG. 3  is an exploded view a light engine forming part of the PAPI unit of  FIG. 2 ; 
           [0041]      FIG. 4  is a schematic plan view of the PAPI unit of  FIG. 2 ; 
           [0042]      FIG. 5  is a schematic side view of the PAPI unit of  FIG. 2 ; 
           [0043]      FIG. 6  shows a first alternative waveguide configuration that could be employed in a PAPI unit according to the invention; and 
           [0044]      FIG. 7  shows a second alternative waveguide configuration that could be employed in a PAPI unit according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0045]    Referring first to the general principle of operation of a PAPI system,  FIG. 1(   a ) shows (schematically and not to scale) an aircraft approaching an airfield runway. The aircraft is shown on its optimal glide angle of 3° to the horizontal, but in practice, without visual guidance, the glide angle may deviate slightly from that optimal value. Such deviations may only be of the order of 0.5° or less, but even such apparently small changes can lead to difficulties in landing the aircraft. This problem is addressed by the PAPI system. 
         [0046]      FIG. 1(   b ) shows schematically a pilot&#39;s eye view of such a system. The PAPI system comprises a row of four (or, in the case of an abbreviated PAPI, two) units installed alongside the runway. Each unit emits a horizontally split beam of light, consisting of white light in the upper sector and red light in the lower sector. Two of the units are arranged to transmit at angles slightly greater than the optimal approach angle, and the other two at angles that are slightly smaller than the optimal approach angle. Consequently, when the aircraft is on the correct approach path, the pilot sees two red beams (black circles in  FIG. 1(   b )) and two white beams (open circles in  FIG. 1(   b )). This is indicated by the middle representation in  FIG. 1(   b ). If the approach path is too steep, however, the pilot sees three or four white beams (upper representations in  FIG. 1(   b )); if the approach path is too shallow, the pilot sees three or four red beams (lower representations in  FIG. 1(   b )). 
         [0047]    Turning now to  FIG. 2 , a PAPI unit according to the invention, generally designated  1 , comprises a weatherproof enclosure formed from an optical bench  10  and housing shell  20  that is supported by adjustable legs  50 . The sides and top of the housing shell  20  project beyond the front wall, to form an overhang which inhibits entry of rain into the interior. The front edge of the top of the housing shell  20  is formed with an upstanding deflection plate  23 , the purpose of which is to prevent an approaching pilot seeing light reflected off the top of the unit  1 . 
         [0048]      FIG. 2  shows the PAPI unit  1  in exploded form, revealing the following major sub-assemblies, both of which are carried on the optical bench  10 :
       a “light engine”  30 , the structure of which is described more fully with reference to  FIG. 3 ; and   a projection lens assembly  40 .       
 
         [0051]    The unit  1  is, in use, engaged with a ground fixing plate  60  that is generally trapezoidal in shape and is intended to be fixed to an appropriate support, typically a concrete pad, alongside an airfield runway, with the front edge  61  of the plate  60  arranged perpendicular to the runway. The plate  60  is formed with keyhole-shaped locating holes  62  for the feet of the legs  50 , such that the unit  1  can be engaged with the plate  60  by insertion of the feet into the enlarged forward parts of the holes  62  and pressed backwards. Similarly, the unit  1  can be easily released from the plate  60 , e.g. for maintenance. Nonetheless, when engaged with the plate  60 , the unit  1  is held securely in a fixed position and orientation. 
         [0052]    The housing shell  20  is downwardly open and cooperates with the optical bench  10  to form a substantially fully enclosed housing. The front wall of the housing shell  20  is formed with a circular opening  21  through which, in use, a light beam is emitted from the unit  1 . A pair of handles  22  is fitted to the upper wall of the housing shell  20 , to facilitate handling of the unit  1 , e.g. during engagement with, or disengagement from, the fixing plate  60 . 
         [0053]    The housing shell  20  encloses two principal sub-assemblies, both of which are carried by the optical bench  10 . These sub-assemblies are the “light engine”  30  and the projection lens assembly  40 . 
         [0054]    The light engine  30  is shown in exploded view in  FIG. 3 . It comprises a substantially cuboidal box formed from a base assembly  31 , side walls  32 , a top plate  33 , a front plate  34  including an aperture  35  and a rear plate/LED mounting assembly  36 . The base assembly  31  includes a pair of spacer plates  311  that are bolted to the mounting base  10 . 
         [0055]    The rear plate/LED mounting assembly  36  includes a printed circuit board (PCB)  361  on which two rows of five light emitting diodes (LEDs; not visible in  FIG. 3 ) are mounted. The upper row of LEDs emit red light, and the lower row emit white light. As is explained below with reference to  FIGS. 4 and 5 , each LED is associated with a collimating lens  362 , each collimating lens  362  being held in a fixed spaced-apart position relative to the LEDs by a lens holder  363 . A heat sink  364  with associated fans is mounted behind the PCB  361 . 
         [0056]    The light engine  30  includes two waveguides  37 , one of which is associated with the upper row of (red) LEDs and the other with the lower row of (white) LEDs. The two waveguides  37  are identical and are formed of cast acrylic material. Each waveguide  37  is of rectangular cross-section and is generally cuboidal in form, save that the rear face that is disposed in juxtaposition to the associated collimating lenses has five facets. The central facet is parallel to the front face of the waveguide  37 , the facets either side of the central facet are disposed at 4° to the central facet, and the outermost facets at 10° (as is most clearly seen in  FIG. 4 ). The waveguides  37  are approximately 180 mm in width, with a length of approximately 150 mm and a thickness of approximately 25 mm. 
         [0057]    For convenience, the upper row of (red) LEDs with their associated collimating lenses  362  and waveguide  37  are referred to herein as the “upper sector” or “red sector” of the light engine  30 , and the lower row of (white) LEDs and their associated collimating lenses  362  and waveguide  37  as the “lower sector” or “white sector”. 
         [0058]    The lens holder  363  includes a slotted, forwardly-projecting shelf  364  in which is located a divider  365  that is intended to block stray light that might otherwise be transmitted from one sector to another (i.e. between the upper (red) sector and the lower (white) sector). 
         [0059]    The construction of the light engine  30  is shown in more simplified fashion in  FIGS. 4 and 5 .  FIG. 4  is a schematic view of the light engine from above, i.e. showing the red sector, and  FIG. 5  a similarly schematic view from the side. 
         [0060]      FIG. 4  shows the PCB  361  with the upper row of five red LEDs  366 , each with an associated collimating lens  362 . The collimating lenses  362  are identical and essentially hemispherical. A waveguide  37  is positioned in front of the collimating lenses  362 , with the five facets of its rear face in juxtaposition with the collimating lenses. 
         [0061]    The arrangement of the lower sector is substantially the same as for the upper sector. As can be seen from  FIG. 5 , the white LEDs  367  are each associated with a collimating lens  362  and a waveguide  37  is positioned in front of the collimating lenses, as in the upper sector. The divider  365  is positioned between the waveguides  37  and prevents transmission of any stray light from the red sector to the white sector, or vice versa. 
         [0062]    Light emitted by the LEDs  366 , 367  is directed by the collimating lenses  362  onto the rear face of the waveguides  37 , and propagates through the waveguides  37  by total internal reflection. The light is allowed to spread unconfined in the transverse direction, which reduces the required tolerances in the positioning of the LEDs  366 , 367  and of the collimating lenses  362 . In the vertical direction, the light is confined by the depth of the waveguide  37 , and the length of the waveguide  37  is sufficient that its distal end is illuminated fully and uniformly by the light from the LEDs  366 , 367 . In the illustrated embodiment, the dimensions of the waveguides  37  are approximately 15 cm×18 cm×2.5 cm (length×width×depth). 
         [0063]    The LEDs  366 , 367  and the collimating lenses  362  are slightly offset from the center of the respective waveguides  37 , in order to achieve the required intensity profile of the transmitted light beam. Further control of the intensity profile can be achieved electronically, by varying the output of the individual LEDs  366 , 367 . In addition, referring to the upper (as viewed in  FIG. 3 ) waveguide  37 , the top surface and each side of the waveguide  37  are roughened by sand-blasting, whereas the other faces are highly polished. This results in greater optical losses at the top and sides of the waveguide, which affects the intensity profile of the light beam at the distal face of the waveguide  37 . The lower (as viewed in  FIG. 3 ) waveguide  37  is identical, save that it is the bottom face and sides that are roughened, rather than the top and sides. 
         [0064]    To ensure that the waveguides  37  do not touch and that there is sufficient space to mount the collimating lenses  362 , the waveguides  37  are not disposed parallel to each other, but are slightly angled towards each other. In  FIG. 5 , the angle is exaggerated; the angle between the two waveguides is approximately 2°. Because of this, the faces of the waveguides  37  that are remote from the LEDs  366 , 367  are not perfectly coplanar, but the deviation from planarity is so small that it is not significant and the end faces of the waveguides  37  can be treated as an optical plane. In  FIG. 5 , a gap is shown between the distal ends of the waveguides  37 . Again, that gap is exaggerated in the drawing. In reality, the gap is approximately 100 μm, which corresponds to 1 arcminute and so is well within the angle of 3 arcminutes normally specified for the transition between sectors (i.e. from white to red). In practice, it has been found that the edges of the waveguides  37  may be permitted to touch, imperfections in the edges giving an effective separation of an appropriate magnitude. 
         [0065]    The effect of the waveguides  37  is to channel light emitted by the LEDs  366 , 367  to the optical plane defined by the distal ends of the waveguides  37 . That plane is adjacent to the front plate  34  of the light engine  30 , which contains the window  35 . The shape of the window  35  defines the shape (i.e. the width and height) of the image at the intermediate plane that is transmitted by the projection lens assembly  40 . 
         [0066]    The optical plane defined by the ends of the waveguides  37  lies in the focal plane of the projection lens assembly  40 . The projection lens assembly  40  includes a three component lens of conventional form. The lens has a diameter of 120 mm and a focal length of 350 mm. 
         [0067]    In use, the PAPI unit  1  of the invention is used in a similar manner to in which a conventional PAPI unit is used. Briefly summarized, four PAPI units  1  are installed alongside and perpendicular to an airfield runway. Each PAPI unit  1  projects a beam of light that has a white upper sector and a red lower sector. Two of the units  1  are aligned so that the center line of the projected beam is above the optimum glide slope for incoming aircraft, and two are aligned so that the center line of their projected beam is slightly below that glide slope. In the unit  1  of the invention, the light beams are formed by light emitted by the red LEDs  366  and the white LEDs  367  being channeled by their respective waveguides  37  to an intermediate plane defined by the distal ends of the waveguides  37 . The image that is formed at that plane comprises a rectangular beam in the upper sector and a rectangular white output beam in the lower sector. That image lies at the focal plane of the projection lens assembly  40 , which inverts the image and projects the beams towards incoming aircraft. 
         [0068]    A feature of the PAPI unit  1  that has not hitherto been described is the presence of alternative light sources for use at night. These light sources are auxiliary LEDs  368  in the upper sector of the light engine  30  (see  FIG. 4 ) and auxiliary LEDs  369  in the lower sector (see  FIG. 5 ). The auxiliary LEDs emit infra-red light that can be observed with night vision equipment (e.g. night vision goggles). In order to provide the necessary differentiation between the two sectors, the auxiliary LEDs  368  in the upper sector are occulting (i.e. intermittent) whereas the auxiliary LEDs  369  in the lower sector operate continuously. 
         [0069]    Finally,  FIGS. 6 and 7  illustrate waveguide configurations that are alternatives to the waveguide configuration shown in  FIGS. 4 and 5 , i.e. a configuration in which the LEDs and collimating lenses are positioned linearly and the input face of the waveguide is faceted. 
         [0070]    In the arrangement shown in  FIG. 6 , the waveguide  77  is tapered. The taper acts as an efficient way of reducing the angular output from the LED  76  along the waveguide  77 . This works most effectively, the closer the LED  76  can be placed to the input face of the waveguide  77  and therefore the smaller the input face can be. However, LEDs conventionally are fitted with silicone domes that increase the light output of the LED, typically by about 25% for white LEDs and considerably more for red LEDs. This increases the separation of the LED  76  and the input face of the waveguide  77 , and hence the required size of the input face. This decreases the efficiency of the tapered waveguide compared to the faceted waveguide of  FIGS. 4 and 5 . Also, the tapered waveguide  77  requires more complex mountings, and the contact between those mountings and the waveguide  77  itself couples light out of the waveguide and so reduces the amount of light transmitted. 
         [0071]      FIG. 7  shows a waveguide  87  with a planar input face, but where the LEDs  86  and associated collimating lenses  82  are arranged at angles to the input face. The central LED  86  and collimating lens  82  are disposed parallel to the input face, but the adjacent LEDs  86  and collimating lenses  82  are disposed at a first angle to the central LED and lens, and the outermost LEDs and lenses are disposed at a second, greater, angle. This arrangement is optically substantially equivalent to that of  FIGS. 4 and 5 , but is mechanically more complex. 
         [0072]    While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiments but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, which scope is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures as is permitted under the law.