U.S. patent application number 16/087124 was filed with the patent office on 2019-04-04 for filter.
The applicant listed for this patent is BAE Systems plc. Invention is credited to Mohammed-Asif Akhmad, Daniel Benjamin Black, Leslie Charles LAYCOCK.
Application Number | 20190101675 16/087124 |
Document ID | / |
Family ID | 56027310 |
Filed Date | 2019-04-04 |
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United States Patent
Application |
20190101675 |
Kind Code |
A1 |
Black; Daniel Benjamin ; et
al. |
April 4, 2019 |
FILTER
Abstract
A method of forming a conformable filter for a vehicle window,
comprising the steps of: --selecting at least a first wavelength
corresponding to a predetermined laser threat; --providing a
conformable photosensitive film and exposing said film to radiation
from a focused laser source of said first wavelength to create a
first filter region therein configured to substantially block
incident radiation thereon substantially only of said first
wavelength; --determining if an essential lighting source outside
or inside the vehicle includes said first wavelength and, if so,
--selecting a bandwidth corresponding to a first predetermined
wavelength band including said first wavelength and exposing said
polymeric film to radiation from one or more further laser sources
of respective different wavelengths within said first predetermined
wavelength band to create a notch filter region therein, including
said first filter region, said notch filter region being configured
to substantially block incident radiation thereon at wavelengths
within said first predetermined wavelength band whilst
substantially allowing visible wavelengths outside of said first
predetermined wavelength band to be transmitted therethrough, and
wherein said bandwidth is selected to optimise visibility through
said filter of said essential lighting source.
Inventors: |
Black; Daniel Benjamin;
(Chelmsford, Essex, GB) ; Akhmad; Mohammed-Asif;
(Chelmsford, Essex, GB) ; LAYCOCK; Leslie Charles;
(Chelmsford, Essex, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
BAE Systems plc |
London |
|
GB |
|
|
Family ID: |
56027310 |
Appl. No.: |
16/087124 |
Filed: |
March 17, 2017 |
PCT Filed: |
March 17, 2017 |
PCT NO: |
PCT/GB2017/050740 |
371 Date: |
September 21, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B 5/285 20130101;
B64C 1/14 20130101; G03H 2260/12 20130101; G02B 5/203 20130101;
A61F 9/065 20130101; E06B 3/6715 20130101; G02B 5/20 20130101; G02C
7/104 20130101; B64C 1/1476 20130101; A61F 9/022 20130101; G03H
2001/0439 20130101; G02B 5/28 20130101; G03H 2223/17 20130101; E06B
5/18 20130101; G03H 2001/0413 20130101; G02C 7/10 20130101; G03H
2223/24 20130101; B60J 3/00 20130101; B64C 1/1492 20130101; G03F
7/001 20130101; G03H 1/0402 20130101 |
International
Class: |
G02B 5/28 20060101
G02B005/28; G02C 7/10 20060101 G02C007/10; B64C 1/14 20060101
B64C001/14 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 24, 2016 |
GB |
1604995.9 |
Nov 23, 2016 |
GB |
1619763.4 |
Claims
1. A method of forming a conformable filter for a vehicle window,
comprising the steps of: selecting at least a first wavelength
corresponding to a predetermined laser threat; providing a
conformable photosensitive film and exposing said film to radiation
from a focused laser source of said first wavelength to create a
first filter region therein configured to substantially block
incident radiation thereon substantially only of said first
wavelength; determining if an essential lighting source outside or
inside the vehicle includes said first wavelength and, if so,
selecting a bandwidth corresponding to a first predetermined
wavelength band including said first wavelength and exposing said
polymeric film to radiation from one or more further laser sources
of respective different wavelengths within said first predetermined
wavelength band to create a notch filter region therein, including
said first filter region, said notch filter region being configured
to substantially block incident radiation thereon at wavelengths
within said first predetermined wavelength band whilst
substantially allowing visible wavelengths outside of said first
predetermined wavelength band to be transmitted therethrough, and
wherein said bandwidth is selected to optimise visibility through
said filter of said essential lighting source.
2. The method according to claim 1, wherein said film is formed of
a photosensitive polymer material.
3. The method according to claim 1, wherein said film has a visible
light transmission of at least 85%, and/or a thickness of 1 to 100
micrometers.
4. The method according to claim 1, further comprising the steps of
selecting an additional predetermined wavelength band corresponding
to visible light emitted by a light source internal to the vehicle,
in use, and exposing said film to radiation from one or more
focused laser sources of respective wavelength(s) within said
additional predetermined wavelength band to create a respective
notch filter region therein configured to substantially prevent
said light from said internal light source from being seen through
the filter from outside of the vehicle.
5. A conformable filter formed by: selecting at least a first
wavelength corresponding to a predetermined laser threat; providing
a conformable photosensitive film and exposing said film to
radiation from a focused laser source of said first wavelength to
create a first filter region therein configured to substantially
block incident radiation thereon substantially only of said first
wavelength; determining if an essential lighting source outside or
inside the vehicle includes said first wavelength and, if so,
selecting a bandwidth corresponding to a first predetermined
wavelength band including said first wavelength and exposing said
polymeric film to radiation from one or more further laser sources
of respective different wavelengths within said first predetermined
wavelength band to create a notch filter region therein, including
said first filter region, said notch filter region being configured
to substantially block incident radiation thereon at wavelengths
within said first predetermined wavelength band whilst
substantially allowing visible wavelengths outside of said first
predetermined wavelength band to be transmitted therethrough, and
wherein said bandwidth is selected to optimise visibility through
said filter of said essential lighting source.
6. The filter according to claim 5, wherein the first predetermined
wavelength band covers or is centred on 532 nm.
7. The filter according to claim 5, wherein the filter is
additionally for preventing transmission of radiation in a second
predetermined wavelength band, the second predetermined wavelength
band covering the wavelength of a second predetermined laser threat
and, optionally, wherein the second predetermined wavelength band
covers or is centred on 445 nm.
8. The filter according to claim 7, wherein the filter is for
additionally preventing radiation in a third predetermined visible
radiation band, the third predetermined wavelength band covering
the wavelength of a third predetermined laser threat and,
optionally, wherein the third predetermined visible wavelength band
covers or is centred on 650 nm.
9. The filter according to claim 5, wherein the filter is for
preventing radiation from two or more predetermined wavelength
bands, including at least one wavelength band corresponding to a
predetermined laser threat and at least one wavelength band
corresponding to an internal light source of the vehicle.
10. The filter according to claim 7, wherein the filter is
comprised by a single layer of material adapted or configured for
preventing the transmission of the predetermined visible wavelength
bands.
11. The filter according to claim 7 wherein the bandwidth of at
least one of the predetermined wavelength bands is between 5 nm and
10 nm.
12. The filter according to claim 5, wherein the filter has an
optical density of at least 2 at the first and/or each
predetermined wavelength band.
13. The filter according to claim 5, wherein the filter is provided
as a conformal film for coupling to a window.
14. The multi-layered window comprising a filter according to claim
5, wherein the filter is interposed between layers of the
window.
15. The window for a vehicle, the window comprising a filter
according to claim 5.
Description
[0001] The present invention relates to a filter, and also to a
window for a vehicle, the window comprising such a filter.
[0002] It is known from US2014/0293467 to provide a generally
transparent filter comprising a nanoparticle metamaterial structure
such that a particular wavelength of electromagnetic radiation may
be blocked. The use of such a filter at the windscreen (or
windshield) of an aircraft protects against laser threats, which
may damage pilot eyesight or temporarily dazzle the pilot. However,
this method of forming laser protective/blocking films complex and
costly, and typically only permits blocking of one or up to two
laser wavelength bands. Furthermore, the film is generally rigid,
and not easily conformable to a curved shape of a typical
windscreen.
[0003] It is also known from, for example, US2014/0009827, to
provide a generally transparent, conformable filter formed by
holographic exposure of a photosensitive polymeric film by a
plurality of coherent radiation sources for the purpose of forming
eyeglasses for viewing stereoscopic images. However, there are a
number of issues with the described method which make it unsuitable
for forming laser protective/blocking filters of the type described
above. Firstly, the bandwidth (or `wavelength band`) of blocked
wavelengths is inevitably relatively high which means that the
overall `colour` of the resultant film is quite pronounced and the
visible light transmission (%) is relatively low (indeed, can be as
low as 15%). This is clearly undesirable, and in many cases
unacceptable, particularly for the application referenced above--it
is self evident that the pilot must be able to see clearly through
the aircraft vehicle.
[0004] Still further, and in relation to windscreens/windshields
for aircraft in particular, it is essential that the pilot can see
the landing lights and cockpit instrument lights clearly. If the
spectral bandwidth of a laser protective/blocking filter is too
large, it can also block at least some of these types of lights,
which is clearly undesirable and, in many cases, entirely
unacceptable. It may also be required for the filter to block a
specific wavelength of radiation emanating from inside the cockpit
(such that it cannot be seen from the outside of the aircraft).
Specific examples might again include cockpit instrument lights,
but also radiation from a heads up display being used within the
cockpit by the pilot. Clearly, it is essential in such cases, i.e.
where the filter is required to block several different
wavelengths, that the respective notch filter regions can be formed
with relatively very narrow bandwidths such that the VLT and
perceived `colour` is not unacceptably affected.
[0005] Thus there is provided an alternative to metamaterial-type
optical filter in mitigating laser dazzle threats, that is
conformable to a curved shape of a typical windscreen and that
permits a number of notch filter regions to be provided very
precisely therein, each configured to block a relatively very small
spectral bandwidth of radiation around, or including, a
specifically selected wavelength.
[0006] In accordance with a first aspect of the present invention,
there is provided a method of forming a conformable filter for a
vehicle window, comprising the steps of: [0007] selecting at least
a first wavelength corresponding to a predetermined laser threat;
[0008] providing a conformable photosensitive film and exposing
said film to radiation from a focused laser source of said first
wavelength to create a first filter region therein configured to
substantially block incident radiation thereon substantially only
of said first wavelength; [0009] determining if an essential
lighting source outside or inside the vehicle includes said first
wavelength and, if so, [0010] selecting a bandwidth corresponding
to a first predetermined wavelength band including said first
wavelength and exposing said polymeric film to radiation from one
or more further laser sources of respective different wavelengths
within said first predetermined wavelength band to create a notch
filter region therein, including said first filter region, said
notch filter region being configured to substantially block
incident radiation thereon at wavelengths within said first
predetermined wavelength band whilst substantially allowing visible
wavelengths outside of said first predetermined wavelength band to
be transmitted therethrough, and wherein said bandwidth is selected
to optimise visibility through said filter of said essential
lighting source.
[0011] Thus, by selecting a (advantageously, relatively small)
bandwidth of a notch filter region that covers a first wavelength
included in, for example, the cockpit instrument lights or the
landing lights, blocking of those lights is minimised (to that very
small bandwidth) and the remaining visible light radiated thereby
can pass through such that the pilot's vision in respect of these
essential lighting sources is not significantly adversely
compromised. Advantageously, the selected bandwidth may be 10 nm or
less, so as to maximise or optimise VLT and, therefore, the
visibility through the filter of the essential lighting source.
However, the present invention is not necessarily intended to be
limited in this regard.
[0012] The method may further comprise the steps of selecting an
additional predetermined wavelength band corresponding to visible
light emitted by a light source internal to the vehicle, in use,
and exposing said film to radiation from one or more focused laser
sources of respective wavelength(s) within said additional
predetermined wavelength band to create a respective notch filter
region therein configured to substantially prevent said light from
said internal light source from being seen through the filter from
outside of the vehicle.
[0013] The first predetermined wavelength band may cover or be
centred on 532 nm.
[0014] This provision tunes the filter for mitigating attacks by a
commonly available laser, and so could provide protection in a
number of situations.
[0015] The film may be formed of a photosensitive polymer material,
which may have a visible light transmission of at least 85% and may
have a thickness of 1 to 100 micrometers.
[0016] In accordance with another aspect of the present invention,
there is provided a conformable filter formed by the method
substantially as described above.
[0017] The filter may be additionally for preventing transmission
of radiation in a second predetermined wavelength band, the second
predetermined wavelength band covering the wavelength of a second
predetermined laser threat.
[0018] The provision of such a second filter band enables the
filter to attenuate a further laser wavelength and thus can guard
against another likely threat.
[0019] The second predetermined wavelength band may cover or be
centred on 445 nm.
[0020] The filter may be for additionally preventing radiation in a
third predetermined visible radiation band, the third predetermined
wavelength band covering the wavelength of a third predetermined
laser threat.
[0021] The provision of such a third filter band enables the filter
to attenuate a further laser wavelength and thus can guard against
another likely threat.
[0022] The third predetermined visible wavelength band may cover or
be centred on 650 nm.
[0023] More generally, the filter may be for preventing radiation
from two or more predetermined wavelength bands, including at least
one wavelength band corresponding to a predetermined laser threat
and at least one wavelength band corresponding to an internal light
source of the vehicle.
[0024] The filter may be comprised by a single layer of material
adapted or configured for preventing the transmission of the
predetermined visible wavelength band or bands.
[0025] The provision of a single layer which attenuates at a
plurality of frequencies, particularly over narrow bands or
notches, allows for convenient retrofit or assembly of the layer
into or onto window/substrate structures.
[0026] The bandwidth of at least one of the predetermined bands may
be between 10 nm and 5 nm. The provision of narrow wavelength bands
tends to give, for the filter material, a higher VLT % in general,
but it is to be understood that the present invention is not
necessarily intended to be limited in this regard.
[0027] The filter may have an optical density of at least 2 at the
first predetermined wavelength band.
[0028] This has been determined to be a threshold for defence which
is suitable for various applications, but particularly aerospace
applications where threats may be at a considerable stand-off range
(e.g. 100 m).
[0029] Further, the filter may have an optical density of at least
2 at each predetermined wavelength band. However, it is to be
understood that the present invention is not necessarily intended
to be limited in this regard. In particular, lower or higher
optical densities may, in some cases, be adequate and/or more
appropriate when considering, for example, the overall VLT of the
filter.
[0030] Nevertheless, an optical density of 2 has been determined to
be a threshold for defence which is suitable for various
applications, but particularly aerospace applications where threats
may be at a considerable stand-off range and will, in many cases,
be appropriate.
[0031] The filter may be provided as a conformal film for coupling
to a window.
[0032] This further contributes to the ability of the filter to be
retrofit conveniently, or be assembled within a part
conveniently.
[0033] According to a further aspect of the invention there is
provided a multi-layered window comprising a filter according to
the first aspect of the invention, wherein the filter is interposed
between layers of the window.
[0034] According to a still further aspect of the invention there
is provided a window for a vehicle, the window comprising a filter
according to the first aspect of the invention.
[0035] So that the invention may be well understood, exemplary
embodiments thereof shall now be discussed with reference to the
following figures, of which
[0036] FIG. 1 is a schematic perspective view of a filter according
to an exemplary embodiment of the present invention applied to a
substrate;
[0037] FIG. 2 is a schematic diagram illustrating a process of
forming a filter region for use in a method according to an
exemplary embodiment of the present invention;
[0038] FIG. 3 is a schematic perspective view of the filter of FIG.
1 configured to detect radiation;
[0039] FIG. 4 is a graph in which the transmission characteristic
of the filter of FIG. 1 is plotted, and
[0040] FIG. 5 illustrates schematically the filter of FIG. 1
implemented on the windscreen of a vehicle.
[0041] With reference to FIG. 1 there is shown a layer of filter
material 10 applied to a first face of a substrate 20 to provide a
window 100 adapted for mitigating laser threats such as dazzle. The
substrate 20 is substantially transmissive of visible light (for
example it may have a visible light transmission (VLT %) of around
90% of normally incident light) and may be formed for example from
a glass or a plastics material such as polycarbonate.
[0042] The filter material 10 is an interference filter formed by
holographically exposing a photosensitive film with a plurality of
lasers having a set of predetermined wavelengths within a selected
wavelength band of bandwidth 10 nm or less.
[0043] Conformable photosensitive (e.g. polymeric) films for use in
exemplary embodiments of the present invention will be known to a
person skilled in the art, and the present invention is not
necessarily intended to be limited in this regard. Such
photosensitive polymeric films are provided having varying degrees
of inherent visible light transmission (VLT), ranging from less
than 70% (and possibly, therefore, having a coloured tinge) up to
99% or more (and being substantially colourless and transparent).
In respect of the present invention, suffice it to say that a
photosensitive flexible/conformable (e.g. polymeric) film is
selected having an inherent VLT of, for example, at least 85%. The
film typically has a thickness of 1 to 100 micrometers. Thinner,
currently known, films may not achieve useful optical densities.
Indeed, in respect of currently known photosensitive polymeric
films, the degree to which a selected radiation wavelength can be
blocked (i.e. the effectiveness of a filter region formed therein)
is determined by the thickness and refractive modulation index of
the film and, also, by the optical design. Thus, the filter region
thickness is ideally matched to the application and the potential
power of the source from which protection is required (which may be
dictated, at least to some extent, by the minimum distance from the
target platform the laser threat may realistically be located and
this, in turn, is dictated by application). In general, thicker
films and films with higher refractive modulation indices would be
selected if it were required to provide protection from higher
power radiation sources or to provide greater angular coverage, but
this might then have a detrimental effect on the inherent VLT of
the film, so a balance is selected to meet the needs of a specific
application.
[0044] Thus, once the film has been selected, the required
holographic exposure thereof is effected to form the filter regions
of a required notch filter region to be provided thereon. Referring
to FIG. 2 of the drawings, distinct filter regions defining a notch
filter region of a predetermined bandwidth (e.g. 5-10 nm) may be
formed by exposing the film to the intersection of two counter
propagating laser beams for each of a set of laser wavelengths
within the selected wavelength band having a selected spectral
bandwidth. Each laser 100 (of a wavelength within the selected
spectral bandwidth) produces a laser beam 120 which is controlled
by a shutter 140. The laser beam 120 is directed by a mirror 160
into a beam splitter 180 wherein the beam is divided into equal
beam segments 200. Each beam segment 200 passes through a
microscope objective 220 and is then reflected by a respective
mirror 360 onto the photosensitive polymer film 320. Other optical
devices (not shown) may be provided between the microscope
objective 220 and the mirror 360 to, for example, focus or diverge
the respective beam segments 200, as required. Furthermore, masking
or other limiting techniques may be utilised to limit the extent or
thickness to which the film is exposed to the beam segments 200, as
will be understood by a person skilled in the art. As a specific
(non limiting) example, if it is required to provide a notch filter
region of bandwidth 5 nm around 520 nm, then a plurality of lasers
100 may be used to produce the notch filter region of (purely by
way of example) 517.5 nm, 518 nm, 518.5 nm, 519 nm, 519.5 nm, 520
nm, 520.5 nm, 521 nm, 521.5 nm, 522 nm and 522.5 nm. The
above-described exposure process may be performed consecutively for
each of these laser wavelengths or, in other exemplary embodiments,
the exposures may be performed substantially simultaneously. Other
apparatus for forming a holographic filter region at each specified
wavelength is known and could, alternatively, be used.
[0045] Referring specifically to the present application of a
filter for a vehicle (e.g. aircraft) windscreen/windshield,
additional consideration must be given to the fact that a) the
pilot still needs to be able to see landing lights/cockpit
instrument lights, etc. clearly through the filter; and b) it may
be required to block visible light from within the cockpit (e.g.
from cockpit instrument displays, heads-up displays, etc) from
being seen from outside the vehicle. The filter of the present
invention, and the proposed method of manufacturing such a filter,
can be effectively used to additionally meet these types of
specification. Thus, in the first case of essential lighting
sources, the key here is to ensure that if one of the notch filter
regions for blocking a laser threat covers one or more wavelengths
emitted by an essential lighting source, the bandwidth of that
notch filter region should be made as small as possible to optimise
the trade off between its laser protective characteristics and its
ability to transmit sufficient essential light. Indeed, if a
particular wavelength of the overlapping bandwidth can be
identified as not being an expected laser threat but being included
in the spectral bandwidth of the essential lighting source, that
wavelength could, in theory, be omitted from the notch filter
region formation process, so as to maximise transmission of the
essential light without adversely affecting the required laser
protective characteristics. In the second case, a further notch
filter region covering the wavelength band of the internal light
source(s) to be `hidden` can be formed in the film in the manner
described above, but consideration will still need to be given to
its OD and bandwidth to ensure that the overall VLT of the filter
is not significantly adversely compromised by the addition of a
further notch filter region. The method proposed herein by the
inventors meets both of these needs.
[0046] Once the exposure process has been completed, the resultant
hologram can be fixed by, for example, a bleaching process.
[0047] The transmission characteristic (which may alternatively be
referred to as the transfer function) of visible electromagnetic
radiation incident on the filter 10 is illustrated in FIG. 3. The
transmission intensity relative to incident radiation intensity is
shown on the y-axis and the wavelength of the incident radiation is
shown on the x-axis.
[0048] As can be seen on the plot, across the range of wavelengths
the intensity of the transmitted radiation is close to 100% of that
which is incident. In general a VLT % of 90% would be acceptable if
100% was not feasible.
[0049] There are three distinct notches in the transmission
characteristic associated with three wavelength bands. These are in
particular a 10 nm band centred on 455 nm, a 10 nm band centred on
532 nm and a 10 nm band centred on 650 nm. In general any three
notches from the group consisting of 405 nm, 455 nm, 520 nm, 532
nm, and 650 nm may be selected. Further, notches may be chosen to
coincide with any expected laser threat wavelength. Still further,
the bandwidth may be 5 nm.
[0050] At the centre of each of these bands, the intensity of the
transmitted radiation is at a minimum and has an optical density of
approximately 3, which is equivalent to 0.1% of the initially
incident radiation. Additional notches may, of course, be provided
for blocking internal light from being seen outside the
vehicle.
[0051] With reference to FIG. 2 there is shown generally at 200 a
window. The window 200 comprises a transparent substrate 20 a first
face of which has been coupled a radiation detector in the form of
a detector layer 30.
[0052] Coupled to the opposite face of the detector layer 30 there
is provided a layer of the holographic filter material 10.
[0053] As such the substrate 20, detector layer 30 and filter
material 10 can be considered as a stacked multi-layer
structure.
[0054] The detector layer 30 comprises an array of photodetectors
32 distributed so as to extend substantially across the window 200.
The photodetectors 32 are sufficiently small to be substantially
invisible to the casual observer (though in practice there may be
some reduction on the VLT %). Each photodetector is electrically
connected to a processor module 34. In some embodiments, including
the present one, each photodetector is uniquely connected to a
unique port on the processor module 34.
[0055] The processor module 34 is in turn connected to an alert
module 36.
[0056] FIG. 4 shows a window 200 as shown in FIG. 2 deployed as a
windscreen on a vehicle V, which in this example is an aircraft. A
pilot P is positioned behind the windscreen and a laser beam L,
having a wavelength of 532 nm, is shown pointing at the windscreen.
Laser beam L will have some degree of divergence as the beam
propagates through the atmosphere, which will result in a certain
`spot size` observed at the windscreen.
[0057] In operation the window 200 may be used to mitigate the
effects of the laser beam L, and alert the pilot to the existence
of the laser threat.
[0058] In particular, as the laser beam L propagates onto the
window 200 it will pass through the substrate 20 and into the
detector layer 30 where some laser light will fall on one or more
of the photodetectors 32 (depending on spot size).
[0059] The laser light subsequently propagates from the detector
layer 30 and on the filter 10 where the light becomes substantially
attenuated. Assuming the filter 10 to have the transmission
characteristics shown in FIG. 3 and the laser beam L to be a green
laser of 532 nm, the laser beam L will be attenuated to 0.1% of its
original intensity.
[0060] Accordingly, the pilot P is able to look out of the
windscreen with a reduced chance of the laser beam L harming his or
her sight, or distracting him or her from flying the plane
safely.
[0061] Meanwhile, the laser light having fallen on certain
photodetectors 32, an electrical signal is generated at each
illuminated detector 32 and sent to the processing unit 34. At the
processing unit 34 the electrical signals received from the
illuminated photodetectors 32 are analysed to confirm or deny the
detection of a laser beam. In this case, the processing module 36
generates a signal confirming the presence of the laser beam and
relays this to the alert module 36.
[0062] Each photodetector 32 can have a unique location at the
filter, registered with the processor module such that signals from
each photodetector 32 can be correlated with a certain location at
the filter. Further this location can be correlated with a
particular point on the window provided the relationship between
the window and the filter is registered at the processing module.
Thus the processing module can determine, from detecting which
photodetectors are illuminated, not only the presence of a threat
but also the general dimensions of the `spot` and where on the
window the illumination is occurring. Some information relating to
the source of the threat can be derived from such measurements. If
embodiments are provided with layers of photodetectors, it may be
possible to establish more confident estimates of the threat
location.
[0063] In the present embodiment the photodetectors 32 are
configured for detecting radiation at the predetermined wavelength
or predetermined wavelengths. For example the photodetector 32
could be configured to send a signal only if 527-537 nm radiation
illuminated it. As such the system needs less noise-rejection
provisions and/or can provide fewer false positive signals.
[0064] Upon receiving the signal confirming the presence of the
laser beam, the alert module issues an alert to notify the pilot P
(or another operator) of the laser beam. Such alert could be a
visual alert (for instance on an instrument in the cockpit) and/or
an audible alert. Such alert could be a signal sent (e.g. by an RF
transmitter within the alert module) to a further aircraft or a
further element of aerospace infrastructure such as an Air Traffic
Control base.
[0065] Accordingly, should the pilot be otherwise unaware of the
laser beam (for instance because it is sufficiently attenuated by
the filter 10 to be negligible within the vehicle) the alert will
inform as to the existence of the threat and further action
(reporting to ground based security personnel, warning other
aircraft) can be taken to address or remove the threat.
[0066] As an alternative to window 200, the window 100 may be
provided as the windscreen in vehicle V. Here there is no detection
layer 30 and so there can be no automatic alert or detection of the
laser threat.
[0067] Nonetheless the holographic filter 10 will function to
attenuate the intensity of the laser beam L and thereby protect the
pilot.
[0068] The above discussion has provided an overview of how the
present invention may mitigate the threat of laser beams.
[0069] Presently various lasers are commercially available which
could be used against a number of targets at a number of different
stand-off ranges. The likely distance and the power of the laser
determine how effective the filter needs to be in order to prevent
injury to the onlooker. An intensity-at-eyeball of 0.001 W/cm.sup.2
or less should be sufficient to prevent eye damage.
[0070] Table 1 shows, for a 3 W laser with 0.5 mrad beam divergence
and no atmospheric loss at various stand-off distances, the
calculated minimum optical densities (OD) such that damage to the
eye can be avoided by blinking (i.e. damage is negligible at this
OD unless exposure is greater than 0.5 s, which is a determined
minimum multiplied by a factor of safety of 2), and such that there
is enough protection to render negligible the risk of damage from a
10 second exposure. Accordingly suggested ranges for ODs are
proposed.
TABLE-US-00001 TABLE 1 Beam `Spot` min OD min OD Example Distance
diameter Size Intensity Typical for 0.5 s for 10 s OD ranges (m)
(mm) (mm.sup.2) (W/Cm.sup.2) application exposure exposure (to
nearest 0.5) 0 3 7.1 42.4 n/a 4.03 4.63 4.5-6.0 5 6 23.8 12.6
Car/train/bus 3.50 4.10 3.5-5.5 10 8 50.3 6.0 Car/train/bus 3.18
3.78 3.5-5.5 50 28 615.8 0.5 Car/train/bus/ 2.10 2.70 2.5-4.0
aircraft 100 53 2206.2 0.1 Car/train/bus/ 1.40 2.00 1.5-3.5
aircraft 500 253 50272.6 0.006 Aircraft 0.18 0.78 0.5-2.5 1000 503
198712.8 0.002 Aircraft n/a 0.30 0-1.5
[0071] Table 2 shows, for a 1 W laser with 1.2 mrad beam divergence
and no atmospheric loss at various stand-off distances, the
calculated minimum optical densities (OD) such that damage to the
eye can be avoided by blinking (i.e. damage is negligible at this
OD unless exposure is greater than 0.5 s, which is a determined
minimum multiplied by a factor of safety of 2), and such that there
is enough protection to render negligible the risk of damage from a
10 second exposure. Accordingly suggested ranges for ODs are
proposed.
TABLE-US-00002 TABLE 2 Beam `Spot` min OD min OD Example Distance
diameter size Intensity Typical for 0.5 s for 10 s OD ranges (m)
(mm) (mm2) (W/Cm2) application exposure exposure (to nearest 0.5) 0
3 7.1 14.1 n/a 3.55 4.15 4.0-5.5 5 9 63.6 1.57 Car/train/bus 2.59
3.20 3.0-4.5 10 15 176.7 0.57 Car/train/bus 2.15 2.76 2.5-4.5 50 63
3117.3 0.03 Car/train/bus/ 0.88 1.48 1.0-3.0 aircraft 100 123
11882.3 0.008 Car/train/bus/ 0.30 0.90 0.5-2.5 aircraft 500 603
285577.8 0.0004 Aircraft n/a n/a 0.5-1.5 1000 1203 1136635.3
0.00009 Aircraft n/a n/a n/a
[0072] These experiments show that an optical density of 2 would
tend to provide sufficient attenuation for aerospace applications,
where attackers would struggle to get within 100 m of the aircraft.
Similar considerations and conclusions can be drawn from these
experimental results to provide additional notches for blocking
internal light from being seen outside the vehicle, and to optimise
the notches to maintain adequate visibility of the essential light
sources.
[0073] So that the dazzle can be prevented (dazzle being where the
vision of the operator is temporarily impaired by the laser light
but not permanently damaged) the OD values given in Table 1 or
Table 2 should be increase in each scenario by 1, or more
preferably 1.5 (i.e. and OD of 1 should become and OD of 2 or 2.5
to prevent dazzle).
[0074] In a variant of the radiation detector shown in FIG. 2, the
radiation detector may have the form of a patch arranged in the
plane of the filter, or in other words at or near a boundary of the
filter. Said patch could comprise an localised photodetector or
array thereof and would be interfaced with the processor module and
alert module in an equivalent manner. This approach would be suited
to contexts where the spot size of the laser was sufficiently large
to illuminate the periphery of the window, so that the patch need
not be positioned in the operator's view.
[0075] In a variant of the window and substrate arrangement of FIG.
1, the window may be comprised by a number of laminar substrates
between which could be positioned the filter 10.
[0076] In a variant of the window and substrate arrangement of FIG.
2, the window may be comprised by a number of laminar substrates
between which could be positioned the filter and detector.
[0077] It will be apparent to a person skilled in the art, from the
foregoing description, that modifications and variations can be
made to the described embodiments, without departing from the scope
of the invention as defined by the appended claims.
* * * * *