Aircraft lighting system

Abstract: An aircraft lighting system is disclosed in which conventional visible light sources at the exterior of an aircraft are provided with optical filters which block infra red light. In this way, it is made possible to use visible external lighting (required by Civil Aviation Authority regulations) during training exercises involving infra red sensitive night vision goggles, which would otherwise be dazzled by the infra red output of the visible lighting. Also disclosed are means of selectively controlling both visible and I.R. light sources using a single existing cable.

Description: The present invention relates to external aircraft lighting systems, and in particular to such systems for use with night vision goggles. Night vision goggles (NVGs) are used in military aircraft. By detecting ambient infra red light ("infra red light" should be understood to include near infra red light throughout this document) and interpreting this to produce a display visible to the pilot, NVGs considerably enhance a pilot's ability to "see" in the dark. One of the many advantages thus made possible is that external aircraft lighting in the visible spectrum becomes (in certain contexts) unnecessary, and military aircraft can thus be operated in a covert mode in which no such external lighting is provided, making the aircraft harder to detect. By providing low intensity external infra red lighting, it can be ensured that aircraft remain visible to each other, e.g for collision avoidance, formation flying, location and orientation, while still providing no illumination visible to the naked eye. Several problems arise in providing the type of external aircraft lighting needed to best utilise night vision systems. Perhaps the most significant of these problems arises during training. Regulations imposed, for example, by the Civil and Federal Aviation Authorities require aircraft in Civil airspace to carry visible external lighting. Thus, even during training in the use of night vision goggles, this visible lighting must be provided. Unfortunately, conventional lights (using filament bulbs) emit strongly in the IR part of the spectrum and thus can dazzle and effectively incapacitate NVGs, making training difficult or impossible. In addition, many existing aircraft are not equipped with external infra red lighting, and a retro-fit lighting system is therefore needed. Difficulty arises because wiring of the lighting must extend from the cockpit to extremities of the aircraft including wingtips and tail. The cost of adding fresh sets of wiring to provide for signals to two sets of lights (visible and IR) instead of the conventional single set of visible lights is considerable, since it requires disassembly of numerous parts of the aircraft to provide access to the routes taken by the wiring. Solutions to both of the above described problems are objects of the present invention. DE-A-4117289, in the name of Hella KG Hueck & Co., concerns an external aircraft light having a filter but does not address the problems arising with night vision goggles. US-A-5031080, Aikens et al, teaches provision of a portable light for use inside an aeroplane cockpit which has a filtered output compatible with NVGs but does not in any way address the problems referred to above. In accordance with a first aspect of the present invention, there is provided an external lighting assembly for an aircraft, said assembly comprising a source of visible and infra red light and being characterised by a filter which is at least substantially transparent to visible light and allows visible light from the source to be emitted by the lighting assembly while substantially reducing emission of selected infra red frequencies, thereby preventing or reducing dazzle of night vision systems in use. Preferably said source of visible light is mounted in a housing having a window through which visible light is emitted by sy said visible light source, said filter being disposed in said housing between the visible light source and said window such that visible light from said visible light source can only reach said window via said filter. Preferably, said filter is an interference type filter which selectively reflects the infra red frequencies. Preferably, said lighting assembly further comprises electrically powered infra red emitting means. Said infra red emitting means may take the form of one or more infra red diodes disposed externally on said housing. In such a lighting assembly, selected infra red frequencies can be blocked by the filter and thus removed from the output of the visible light source. By removing or substantially removing infra red frequencies to which the night vision system is sensitive, the visible light source can be made to appear dimmer to the night vision system (thereby preventing the NVG from being dazzled) or can even be made substantially invisible to the night vision system, while still giving the visible signal (that is, visible to the naked eye) required by regulations. Thus, training in the use of night vision systems in civil airspace is made possible by the present invention. The additional infra red emitter can be used to provide a relatively low intensity IR signal to others using night vision systems (to enable formation flying etc) without producing visible light. Preferably, the lighting assembly is provided with voltage sensing means adapted to monitor a supply voltage to said visible light source and to connect said supply voltage to the infra-red emitting means when said supply voltage is less than a predetermined threshold. In this way, the single wire conventionally used to drive the source of visible light (earth being provided by the airframe itself) can be used to selectively energise the visible or infra red light sources, as will be explained below. In accordance with a second aspect of the present invention, the external lighting assembly in accordance with the first aspect is incorporated in a lighting system for an aircraft which further comprises at least one current supply cable mounted in the aircraft fuselage/wings, a coder at a cockpit end of said cable for coding power signals applied to that cable, and decoding means at an opposite, remote end of said cable for selectively energising the infra red emitting means and/or the visible light source. In a particularly preferred embodiment, the coder enables a range of voltage levels to be selectively applied to said current supply cable and the decoder includes a voltage sensing means which is adapted to selectively couple the visible light source and/or the infra red emitting means to the current supply depending on the voltage level detected. Preferably, the decoder is adapted to connect the infra red emitting means to the current supply only when the voltage level detected lies below a predetermined threshold. In an alternative embodiment, the coder is adapted to enable a voltage source to be connected selectively to said current supply cable in either of a forward (+) polarity or a reverse (-) polarity, the decoder comprising two diodes connected respectively to the visible light source and the infra red emitting means with opposite orientations, whereby one diode is conductive to the forward polarity supply to energise the visible light source and the other diode is conductive to the reverse polarity supply to energise the infra red emitting means. In a further preferred embodiment, the coder is adapted to apply to said current supply cable a coded digital signal and the decoder is adapted to decode the latter signal and selectively connect the visible light source and/or the infra red emitting means to the current supply cable in dependence upon the coded signal. In accordance with a third aspect of the present invention, there is provided a filter unit adapted to be fitted to an external aircraft lighting assembly having a source of visible and infrared light disposed within a housing and a housing window through which visible light is emitted, the filter unit being adapted to partition the housing of the aircraft lighting assembly such as to separate the light source within the housing from the housing window and comprising a filter through which light from the source can reach the window, the filter being at least substantially transparent to visible light and being such as to remove or substantially remove selected infra red frequencies from the radiation emitted by the light source. In this way, existing light assemblies can be adapted for the above described compatibility with night vision systems. A preferred lighting system according to the present invention comprises an external lighting assembly of the above described type provided with a control means, a variable voltage supply means and a conductor via which the voltage supply means is connected to the visible light source, the control means being sensitive to the voltage supplied by the voltage supply means through the conductor and being adapted to connect the infra red emitter means to the conductor to apply a drive voltage thereto only when the voltage is below a predetermined threshold, the visible light source being such as to be dimmed by voltages below the threshold so that either the visible light source or the infra red emitter can be activated. Using such a system, a single wire can be used to selectively switch on either the IR emitter or the visible light source. Preferably, the voltage supply means is connected to a cockpit end of the conductor and the control means is disposed in the vicinity of the infra red emitter. Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying Figures in which:- Fig. 1 is a perspective illustration of a navigation light in accordance with the present invention intended to be mounted at an air intake of a jet fighter aircraft; Figs. 2a and 2b are, respectively, side and plan views of a formation light in accordance with the present invention intended to be mounted on the upper fuselage of such an aircraft; Fig. 3 illustrates in a simplified generalised format a modified aircraft wiring system in accordance with the present invention; and Figs. 4 to 7 show specific circuits in accordance with the present invention and applicable to the generalised format of Fig. 3. The light illustrated in Fig. 1 comprises a substantially conventional tear drop shaped fairing 1 with a shaped, tinted forward window 3 within which is disposed a conventional light bulb 5. Such lights are in use in existing aircraft. In accordance with one aspect of the present invention, the light has been modified by insertion of a filter 7 within the forward window. The filter 7 comprise an arched, opaque cage 9 which is placed around the bulb 5. The cage 9 is penetrated by several apertures 11 which are covered by filter elements 13, and the front of the cage (which would otherwise be open) is covered by an arch shaped filter element 15. This construction permits the use of conventional, flat, filter elements 13,15. The filter elements 13 and 15, while at least substantially transparent to visible light, are at least substantially opaque to the IR wavelengths detected by NVGs, so that little or no light at these wavelengths produced by the bulb 5 is emitted by the light. The filter elements must be heat resistant, to withstand heat from the incandescent bulb, and are (according to the present embodiment) in the form of interference type glass filters which selectively reflect infra red, preventing heat damage. However, the light has been provided with an infra red emitter 17 (eg an infra red emitting diode) at a rear portion thereof. "Infra red emitter", in the present context, implies a device which can be driven to emit infra red or near infra red light while generating little or no visible light. Two modes of operation are made possible by the modified light. Where appropriate (eg during training exercises, when visible external light is required) the bulb 5 is illuminated. Because infra red light detectable by NVGs is not emitted or is emitted at only very low intensity, the modified lights cannot dazzle NVGs. In fact, where a small amount of I.R. light is allowed to escape from the visible light source, this can be used to effectively mimic the appearance (to NVGs) of the I.R. lighting used in covert operations, which is useful for training exercises. Of course, the modified light retains, in visible mode, all the facilities of standard lighting such as flashing and dimming. During covert operations, the bulb 5 is turned off, the IR emitter 17 is (if desired) turned on, and the light becomes visible only to those wearing NVGs. Where a light of the above described type is fitted to an existing aircraft, only a single electrical supply line (for supply/control of a single type of light - a visible light) is normally provided. To provide for control of both visible and infra red light sources, the present exemplary embodiment can incorporate a voltage sensitive "sniffer" circuit 40 (see Fig. 7). While the bulb 5 is connected directly to the single supply line 113d, the IR emitter 17 is connectable to the supply line 113d via detection circuitry 42 within the sniffer circuit 40. The sniffer circuit 40 operates as a voltage sensitive switch which opens (and so applies a voltage across the IR emitter 17) only when the applied supply voltage on the supply line 113d lies below a predetermined level of approximately 5 volts. Now, the bulb 5 is designed to operate at between 50v (dim mode) and 110v (bright mode). When the bulb 5 is lit by a voltage in this range, the detection circuitry 42 in the sniffer circuit 40 turns the IR emitter 17 off. If the voltage at the supply line is reduced to 5V via supply control circuitry 44, however, not only is the IR emitter 17 switched on by the detection circuitry 42 in the sniffer circuit 40, but in addition the bulb 5 is naturally dimmed so that it produces little or no visible light, any residual infra red or near infra red light emitted being blocked by the filter 7. The detection circuitry 42 in the sniffer circuit 40 can be straightforwardly achieved using transistor/zener diode devices. The control electronics 44 provided at the cockpit end of the supply line can selectively set the supply voltage at either 5 volts or 50-110 volts and so to switch between IR (covert) lighting only and visible lighting only. Fig. 2 shows a further light - in this case a formation light for mounting at the upper surface of a fuselage - modified in accordance with the present invention. The formation light is based on a circular housing 20 on the outer surface of which is disposed a tear drop shaped fairing 22, having an opaque rear portion 24 and a transparent forward window 26. Components within the housing are shown in dotted lines and comprise a main bulb 28, a filter partition 30 and a re-fuelling bulb 32. The re-fuelling bulb 32 is used only during refuelling under visible lighting and is of no interest for present purposes. The main bulb 28 is used in normal flight to provide visible lighting, its light emission being filtered (to remove IR frequencies, as in the previously described embodiment) by the filter partition 30 before escaping through the forward window 26. The filter partition 30 can be retro-fitted to an existing light. Two infra red emitters 31 are provided on the housing 20 for use in covert flying, and visible and IR lighting can be switched as in the previously described embodiment. While the above described method of controlling the visible/I.R. lighting assembly (using a variable supply voltage and a sniffer circuit) is particularly advantageous, other methods can be used, without departing from the scope of the invention, to control the two light sources using existing wiring. This aspect of the present invention will now be considered in more detail, with reference to Figs. 3 to 6. Referring to Fig. 3, there is shown a generalised view of a modified aircraft wiring system in accordance with the present invention. The system can be considered to comprise three distinct sections, namely a cockpit section 110, a wing tips and/or tail fin section 112, both of which are relatively easily accessible and modifiable, and a section 114 consisting of existing wiring 113 which runs through the aircraft fuselage/wings between sections 110 and 112. It is desirable to avoid modification of the existing wiring in section 114, since this requires extensive disassembly of the aircraft. For this latter purpose, the system of Fig. 3 includes in the cockpit section 110 a coder unit 116 controlled by a cockpit switch 118 preferably positioned near to the cockpit lighting switch, and in the wing tips/tail fin section 112 one or more corresponding decoders 120, and also a visible light source 124 and an I.R. light source 122. The coder/decoder system can power and selectively activate the two exterior lights (visible/I.R.) using only one existing power wire. If necessary, further lights could even be controlled. Fig. 3 shows by way of example three sets of existing wiring 113a, 113b, 113c comprising respectively the existing wiring through the wing to the port wing tip, the existing wiring through the wing to the starboard wing tip and the existing wiring through the fuselage to the tail fin. Figs. 4 to 6 show examples of possible coding means applicable to the generalised system of Fig. 3. In the system shown in Fig. 4, a first diode D1 is connected in series with the visible light source 124a and a second diode D2 is connected in series with a newly fitted I.R. source 122a. It does not matter which way round the diodes D1,D2 are fitted as long as they are fitted in the opposite directions to each other. The coder at the cockpit end consists in this case of a supply inverter 116a which enables the existing cabling 113a to be supplied either with a positive or negative supply voltage. In this case, if a positive supply is selected, the visible source 124 only will be energised, and if the negative supply is selected, only the I.R. source 122a will be energised. Fig. 5 shows an example where the detector is a linear voltage detector 120b which responds to the magnitude of the input voltage thereto to select different loads, ie. different light sources 122b, 124b. In this case, the cockpit coder enables different levels of voltage to be selected for application to the existing cabling 113b. In the simple case illustrated, this is achieved by selecting either a direct connection to a +28v supply or a proportion (in this case 50%) of that supply determined by the value of a switched resistor R. In this case more than two light sources could be controlled by splitting the supply voltage further. Fig. 6 shows an example where a small digital signal is superimposed on the supply voltage on the existing wiring 113c by means of a digital coder 116. This small digital signal is picked up by a digital decoder 120c in section 112 and can supply any number of additional light sources (not shown) in addition to or in place of the existing visible source 24c.

Claims: An external lighting assembly for an aircraft, said assembly comprising a source of visible and infra red light (5;28) and being characterised by a filter (13;15;30) which is at least substantially transparent to visible light and allows visible light from the source to be emitted by the lighting assembly while substantially reducing emission of selected infra red frequencies, thereby preventing or reducing dazzle of night vision systems in use. An external lighting assembly as claimed in claim 1, wherein the light source is mounted in a housing (1) having a window (3) through which visible light is emitted by the light source, the filter being disposed in the housing between the light source and the window such that visible light from the light source can only reach the window via the filter. An external lighting assembly as claimed in claim 1 or 2, wherein the filter is an interference type filter which selectively reflects infra red radiation. An external lighting assembly as claimed in any preceding claim comprising an opaque arched cage (9) around the light source, the cage being penetrated by apertures (11) which are covered by flat filter elements (13). An external lighting assembly as claimed in any preceding claim, further comprising electrically powered infra red emitter means (17;31). An external lighting assembly as claimed in claim 5, further comprising voltage sensing means (40) adapted to monitor a supply voltage to the light source and to connect the supply voltage to the infra-red emitter means when the supply voltage is less than a predetermined threshold. A lighting system for an aircraft, comprising an external lighting assembly as claimed in claim 5, at least one current supply cable (113) mounted in the aircraft fuselage/wings, a coder (116) at a cockpit end of said cable for coding power signals applied to that cable, and decoding means (120) at an opposite, remote end of the cable for selectively energising the infra red emitter means and/or the light source. A lighting system for an aircraft as claimed in claim 7, wherein the coder is adapted to enable a voltage source to be connected selectively to the current supply cable in either of a forward (+) polarity or a reverse (-) polarity, the decoder comprising two diodes (D 1 ,D 2 ) connected respectively to the visible light source and the infra red emitter means with opposite orientations, whereby one diode is conductive to the forward polarity supply to energise the visible light source and the other diode is conductive to the reverse polarity supply to energise the infra red emitter means. A lighting system for an aircraft as claimed in claim 7, wherein the coder enables a range of voltage levels to be selectively applied to the current supply cable and the decoder includes a voltage sensing means which is adapted to selectively couple the visible light source and/or the infra red emitter means to the current supply depending on the voltage level detected. A lighting system for an aircraft as claimed in claim 9, wherein the decoder is adapted to connect the infra red emitter means to the current supply only when the voltage level detected lies below a predetermined threshold. A lighting system for an aircraft as claimed in claim 7, wherein the coder is adapted to apply to the current supply cable a coded digital signal and the decoder is adapted to decode the latter signal and selectively connect the visible light source and/or the infra red emitter means to the current supply cable in dependence upon the coded signal. A lighting system for an aircraft, comprising an external lighting assembly as claimed in claim 5 provided with control means (40), a variable voltage supply means and a conductor (113d) via which the voltage supply means is connected to the visible light source, the control means being sensitive to the voltage supplied by the voltage supply means through the conductor and being adapted to connect the infra red emitter means to the conductor to apply a drive voltage thereto only when the voltage is below a predetermined threshold, the visible light source being such as to be dimmed by voltages below the threshold so that either the visible light source or the infra red emitter can be activated. A filter unit adapted to be fitted to an external aircraft lighting assembly having a source of visible and infrared light (5, 28) disposed within a housing (1) and a housing window (13) through which visible light is emitted, the filter unit being adapted to partition the housing of the aircraft lighting assembly such as to separate the light source within the housing from the housing window and comprising a filter (13, 15, 30) through which light from the source can reach the window, the filter being at least substantially transparent to visible light and being such as to remove or substantially remove selected infra red frequencies from the radiation emitted by the light source. A filter unit as claimed in claim 13 comprising an opaque arched cage for placement around the light source, the cage being penetrated by apertures which are covered by flat filter elements.

Inventors: BUSHELL TIMOTHY GEORGE, EDWARDS GEOFFREY STEPHEN, LOGAN PHILLIP, BUSHELL, TIMOTHY GEORGE, EDWARDS, GEOFFREY STEPHEN, LOGAN, PHILLIP
Applicants: OXLEY DEV CO LTD, OXLEY DEVELOPMENTS CO., LTD.
Application year: 1996
Main classification: B64D47
IPC codes: B64D47