Aircraft lighting means compatible with a night vision...

Details Aircraft lighting means compatible with a night vision... Aircraft lighting means compatible with a night vision...

2000-08-10

2004-07-06

Cariaso, Alan (Department: 2875)

Illumination

Supported by vehicle structure

Aircraft

C362S230000, C362S231000, C362S800000

Reexamination Certificate

active

06758586

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a lighting means compatible with a light intensifier night vision imaging system.
The present invention relates especially but not exclusively to lighting systems, lighting means and lighting or lit objects present in or on aircraft, for example instrument panel lighting systems, light scattering devices for ambient light in pilot's cockpits, light indicators, luminous graphics display systems, position or navigation lights, landing lights, flight training lights, anti-collision lights, etc.
Recent years have seen the emergence in the market of aeronautical equipment for night vision systems to facilitate night flying and overcome the lack of the sensitivity of the human eye to light in the infrared range, namely at wavelengths of more than about 770 nm. These systems, generally known as NVIS (night vision imaging systems) are highly sensitive to infrared radiation up to wavelengths of about 900 nm. They usually take the form of goggles comprising two light intensifiers, each intensifier being comparable to a miniature video camera delivering an electronic image of the external environment. Even in the greatest darkness, a night imaging system delivers a clear, high-contrast monochromatic image of the external environment.
Very briefly, it may be recalled with reference to
FIG. 1
that a light intensifier comprises a vacuum tube
2
having a photocathode
3
at its first end that converts the photons from the image received on its external face into an electron beam whose density and distribution are a function of the image. The electron beam is sent to a phosphor screen
4
positioned at the other end of the tube
2
by means of an amplifier plate
5
. The amplifier plate
5
has numerous microchannels
6
covered with a secondary high-emission coating whose role is to greatly increase the number of electrons sent by the photocathode
3
. The amplifier plate is driven by a circuit
7
called an “automatic gain control” circuit. This circuit
7
is a feedback circuit that optimizes the gain, namely the intensifying level, as a function of the ambient luminosity and gives a result comparable to the closing or opening of a diaphragm. Without this protection circuit, an increase in the ambient light energy in the band of sensitivity of the photocathode would prompt an immediate increase in the flow of electrons and would lower the sensitivity and resolution. The circuit
7
prompts the total extinction of the intensifier tube when there is a sharp variation in the radiant energy.
As light intensifiers have undergone various improvements since their appearance, there are now two types of light intensifiers using concurrent technologies on the market. These are GEN
2
(second generation) and GEN
3
(third generation) intensifiers. GEN
3
tubes have a gallium arsenide photocathode and can be distinguished by their very high sensitivity to radiant energy, namely a sensitivity of about 1200 to 1800 &mgr;A/lm depending on the models, and a fairly selective passband ranging from 600 nm (at the borderline limit between the yellow and the red wavelengths) to 900 nm. The GEN
2
tubes have a tri-alkaline photocathode with lower sensitivity, of about 500 to 800 &mgr;A/lm, and a wider passband ranging from 400 to 900 nm and covering the visible spectrum. For a clearer picture, the curves
10
and
11
of
FIG. 2
respectively show the gain G of the tubes GEN
2
a GEN
3
as a function of the wavelength &lgr;. Despite their lower sensitivity, tri-alkaline photocathodes have a better signal-to-noise ratio than gallium arsenide photocathodes so that there are GEN
2
type night vision systems that are equal to GEN
3
type night vision systems in terms of resolution and image quality.
SUMMARY OF THE INVENTION
In practice, an essential goal to be achieved is that aircraft pilots should be able to use night vision goggles while continuing to be able to consult their panel instruments. This goal, which is essentially ergonomical, requires that two conditions should be met:
firstly, the intensifier tubes should not entirely mask the pilot's visual field,
secondly, the lighting of the aircraft should not disturb the intensifier tubes by giving rise to parasitic halos or ghost images due to the reflection of illuminated objects on the windows of the cockpit.
With regard to the first condition, various ergonomical studies conducted in recent years have given rise to two types of night imaging goggles known as type I and type II goggles according to the MIL-L-85762A standard to which reference is made herein purely for reasons of convenience, the above-mentioned classification being frequently used by those skilled in the art. The I type goggles, designed for piloting helicopters, are fixed to the pilot's helmet so that the two phosphor screens are before the pilot's eyes at a minimum distance enabling him to see the panel instruments when he looks down. The type II goggles, designed for fixed-wing aircraft, work like a head-up display unit: the image delivered by the phosphor screens is projected before the pilot's eyes by transparent lenses through which the panel instruments can be viewed in simultaneous juxtaposition.
Furthermore, the risks of interference between the light sources of the aircraft and the night vision goggles are eliminated by a retrofitting of the aircraft lighting system. This retrofitting operation essentially consists in securing all the light sources to a monochromatic color that is as far as possible from the red wavelengths band. Indeed, as can be seen in
FIG. 2
, the GEN
2
or GEN
3
type night imaging goggles do not have a passband limited to the infrared and have high sensitivity to the wavelengths in the red range, in a band that substantially covers 600 to 770 nm (herein with a view to simplicity, it is assumed that the red band also covers the orange and yellow wavelengths since, in practice, there is no purely monochromatic light: any orange or yellow light source inevitably includes a red component). In the prior art, the red wavelengths band is thus considered to be a critical band in which any emission of light is likely to greatly disturb night vision goggles by causing the activation of the automatic gain control circuit (namely the closing the electronic shutter). In particular, white incandescent lights are not allowed since they contain a high proportion of red and infrared light.
Thus, in practice, the retrofit of an aircraft illumination system consists in encapsulating the incandescent white lamps with lowpass attenuator filters and replacing the other white incandescent white lamps by light-emitting diodes or light-emitting panels scattering a narrow and green colored light also called an “aviation green” centered on the 555 nm. In general, the white incandescent lamps to be encapsulated are the yellow, orange and red lamps of warning indicators and alarm indicators. The incandescent white lamps need to be replaced by green light-emitting diodes and are for example green indicator lamps, used for the lighting of the instrument panel as well as backlighting lamps which, by transparency, reveal luminous graphic characters on an instrument panel. Finally, the lights used for ambient illumination are generally replaced by green light-emitting panels with which a mechanical scattering device with swiveling shutters is associated.
This kind of retrofitting of lighting means for an aircraft has various drawbacks. Firstly, it gives a greenish ambient light that very substantially attenuates the readability of the panel instruments and dilutes the colors. Thus, for example, the white or yellow, orange and red paint on the panel instrument packs (used for example to define and demarcate the operating modes of an engine) are respectively seen as green, light brown or dark brown. Again, the green lighting makes it difficult and tiresome to read the navigation maps. Furthermore, the red alarm indicators and those that have a red component like the yellow and orange indicators have medi

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