GB2197941A

GB2197941A - Nvg and sunlight readable indicators

Info

Publication number: GB2197941A
Application number: GB08628016A
Authority: GB
Inventors: Andrew John Hulme; Charles Frederick Joh Underhay; Matthew Alexander Lowery
Original assignee: British Aerospace PLC
Current assignee: BAE Systems PLC
Priority date: 1986-11-24
Filing date: 1986-11-24
Publication date: 1988-06-02
Anticipated expiration: 2006-11-24
Also published as: GB8628016D0; JPS63279999A; GB2197941B; DE3739643A1; FR2607247A1

Description

2197941 NVG AND SUNLIGHT READABLE INDICAMRS This invention relates to

illuminated displays for use on equipment such as consoles in aircraft and which need to be visible at day and night.

Illuminated displays are often built into the front face of function switches and buttons and either show the switch is on or else convey a message concerning the current function of the switch. The displays must be bright enough to be visible in strong sunlight but this can cause problems during night flying if a pilot is wearing night vision goggles. Night vision goggles usually comprise a detector similar to a small T.V. camera connected to two small T.V. monitors. The T.V. camera at the front is photo electrically responsive to infra-red wavelengths and to some visible wavelengths in the red region of the light spectrum. The night vision goggles detect the infra-red light from an external scene and present an image which is visible to the human eye in the monitors. The glare from brightly lit switches and their reflections from the inside of a canopy can blind night vision equipment. If the displays are directly in the field of view of the goggles they can cause overload/white out of the I.R. goggle display, during which all images are erased.

Even if the displays are not directly in the f ield of view of the goggles their reflections can reduce the contrast and resolution of objects viewed with the goggles or else cause ambient I.R. light to reach such a level that scattering occurs in the objective lenses of the night vision goggles again reducing their effectiveness.

Even when using night vision goggles the pilot still needs to be able to read the displays (it is usual practice for the pilot to lift the goggles and peep below them to view the displays) Consequently it would not be practical to turn the displays off or dim them. if the displays are dimmed to an extent that they are no longer visible to night vision goggles then they would no longer be visible to the naked eye. A difficult problem arises of how to modify the display so it is visible to the naked eye in sunlight and also visible to the naked eye at night time without blinding night vision goggles which are many times more sensitive than the naked eye to I.R. radiations.

It is known to use filters to reduce glare from displays and interference with night vision can be reduced or eliminated by placing an I.R. filter over the front of a display to attenuate the infra-red wavelengths emitted from the source. Although I.R. filters are suitable for use with night vision goggles they reduce the intensity of transmitted light by as much as 50% which makes the displays dim and difficult to read in daytime. Generally the unfiltered displays which are available for daytime viewing by the unaided eye are not suitable for use in a cockpit when a pilot is using night vision goggles and the filtered displays which are compatible with night vision goggles are not suitable for ordinary daytime use. It is an object of the present invention to provide a dual mode lighting system in which filtered and unfiltered light sources can be selected to illuminate a display for night time and sunlight time viewing respectively.

1 According to one aspect of this invention there is provided means for illuminating display legends including at least two light channels, each associated with at least one light source for providing light within said channels, means for directing light output from each of said light channels onto a con=n legend bearing area, means for ensuring that light output from one of said channels is of a broad frequency spread predominantly falling outside the infra-red area and within the photopic area of the electro-magnetic frequency spectrum and that light output from another of said channels has a broad frequency spread covering at least the photopic area of the electro-mgnetic frequency spectrum, and means for selectively energising said light sources.

The light sources may be incandescent filament bulbs in which case one light is provided with an infra-red filter. The light outputs from two channels may then be selectively directed through separate faces of a prism and dispersed by a dispersive medium at an exit face of the prism onto a legend bearing surface.

At least one light source may be an incandescent filament bulb or bulbs and another at least one light source may be a blue or green light emitting diode which obviates the need for any I.R. filter in one of the channels.

The means for selectively energising may switch both sources on in daytime and only the source providing light to the I.R. excluded channel in night.

A specific embodiment of the invention will now be described by way of example only and with reference to the following drawings of which:

Figure 1 is a graph showing the photopic response of the human eye and of night vision goggles against wavelength; Figures 2 to 3 show the spectral distribution of a filtered incandescent light source and an L.E.D. light source respectively.

Figure 4 is a schematic section view through one form of night vision/sunlight compatible display; Figure 5 is a schematic diagram of a further form of night vision compatible display.

Figure 6 is a perspective view of yet another form of night vision/sunlight compatible display.

Referring to figure 1, two graphs 1 and 2 are plotted beside each other, graph 1 represents the response of the human eye and figure 2 represents the electrical response of night vision goggles. Graph 1 represents the photopic response of cones in the human eye during daylight, this graph shifts slightly to the right during night time as rods in the human eye play a more prominent role and the eye becomes slightly more sensitive to longer wavelengths. The night vision goggles operate over the infra-red region of the spectrum under graph 2 and part of the visible spectrum of the eye indicated by the area of overlap 3 of the two graphs.

Figure 2 illustrates the spectral distribution of an incandescent light source which has been modified by filtering so that it is ctible with night vision goggles. Graph 4 represents a modified incandescent source, the graph is roughly the same shape as the response of the human eye in Figure 1 but the longer infra-red wavelengths have been removed by filtering so that they do not impair the performance of night vision goggles. Graph 2 represents the spectral response of a pair of night vision goggles. The spectrum of the source 4 extends from wavelengths at point c and ends abruptly at wavelengths point D the infra-red filter absorbs all wavelengths beyond D and prevents interference of the night vision goggles 2 by the source 4. The dotted outline which extends from graph 4 represents the spectral distribution of the incandescent source before filtering, it can be seen that the graph would usually extend from point C to E and overlap with the graph 2 i.e. the incandescent source would interfere with the night vision goggles. Filtering narrow s the bandwidth of the incandescent source from CE to CD and prevents the source from interfering with the night vision 'goggles but in doing so the brightness of the display is halved making it dimmer. The present invention overcomes the limitations of filtering by combining two sources in a dual mode display. A broadband, unfiltered, incandescent source is used to illuminate the display during the hours of sunlight and a narrow band, filtered, source is used to illuminate the display during night.

As an alternative to filtering an incandescent light source light emitting diodes (L.E.D.1s) can be used which have a very narrow wave band. By using green or yellow L.E.D.Is a display can be illuminated at night without interfering with night vision goggles. Figures 3 illustrates the spectral distribution of an L.E.D. source 6 compared with an incandescent source 4 (shown in dotted outline). The narrow band L.E.D. emissions do not overlap with the range of the night vision goggles 2 whereas the incandescent source 4 does. On 6 - their own the L.E.D. Is would not be bright enough to illuminate a display sufficiently for viewing in bright sunlight. But in the present invention the L.E.D.'s are supplemented with an incandescent source during sunlight conditions.

Construction of a dual mode display is illustrated in Figure 4. The display comprises a transparent or translucent front panel 7 which has a legend or symbol marked on it and two incandescent light sources 8 and 9 behind the panel and enclosed in separate light proof enclosures 10 and 11 one of the enclosures 11 contains a filter 12 which absorbs infra-red wavelengths of light. The two enclosures 10 and 11 are positioned behind adjacent faces 13 and 14 of a prism 15 so that light from each enclosure emerges through the same face 16 of the prism to illuminate the front panel 7. The power supply to the two sources can be routed with electrical switches so that in the night vision mode only source 9 can be activated behind the display; whereas in the daylight mode both sources 8 and 9 can be activated. The source 9 is not really necessary for daylight viewing but is used to supplement the brightness of the display. In an alternative arrangement the power supply could be switched so that when the sunlight mode is selected only the source 8 can be activated behind the display.

In the night vision mode light from source 9 passes through filter 12 and has the I.R. wavelengths filtered out as shown by figure 2 in the region DE of graph 4. The filtered beam passes into the prism where it is reflected and refracted and finally diffused over the rear surface of the front panel 7. The light transmitted f rom the display does not contain any I.R. wavelengths which could interfere with night vision goggles. In the sunlight mode source 8 can be activated; unfiltered light from the source passes directly into the prism through face 13 and is reflected and refracted to finally emerge through face 16 of the prism to illuminate the rear surface of the front plate 7. 'The unfiltered light from source 8 remains bright enough to be visible in sunlight. Source 9 is activated at the same time as source 8 to supplement it output.

Figure 5 shows a second embodiment of the invention. Two light sources 8 and 9 are contained in closures 10 and 11 respectively, the closures each have a face plate 17 and 18 at one end. One of the closures 10 has a filter 12 inside it positioned between the source 8 and face plate 17. During night time only source 8 can be activated, light from the source 8 passes through filter 12 and the infra-red wavelengths are removed, the resulting light beam illuminates the whole of the front plate 17. During day time source 9 is switched on and unfiltered light passes along the closure 11 to illuminate front plate 18. Because the light is unfiltered it emerges bright enough for daylight viewing.

Figure 6 shows another embodiment of display in which an incandescent source 8 is mounted inside a light tight closure 10 for daytime illumination and light emitting diodes 19 are placed around the periphery of the display for night viewing. In the night mode only L.E.D.1 can be activated behind the display this corresponds to Figure 3 graph 6. In the sunlight mode the incandescent source 8 is activated corresponding to graph 4 in Figure 3.

Although only three specific arrangements of the N.V.G./sunlight. compatible display have been described other dual mode arrangements could be devised without departing from the scope of the invention.

Claims

Means for illuminating display legends including at least two light channels, each associated with at least one light source for providing light within said channels. means for directing light output from each of said light channels onto a con=n legend bearing area, means for ensuring that light output from one of said channels is of a frequency spread predominantly falling outside the infra-red area and within the photopic area of the electro-magnetic frequency spectrum and that light output from another of said channels has a broad frequency spread covering at least the photopic area of the electro-magnetic frequency spectrumt and means for selectively energising said light sources.