The present invention relates generally to filters for laser protection and, more particularly, to such filters formed by holographic exposure of a

photosensitive film or films by one or more lasers, and to a method of forming such filters.

It is known from, for example, US2014/0292467, 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, for example, protects against laser threats, which may otherwise 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. 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 (can be as low as 15%). This is clearly undesirable, and in many cases unacceptable. Furthermore, only a limited number of filter regions can be provided in a single film, without decreasing the VLT to such an extent that a user cannot see through it properly, rendering the filter entirely unfit for purpose.

Aspects of the present invention seek to address at least some of these issues and, in accordance with a first aspect of the present invention, there is provided a kit of parts comprising first and second screen members, each screen member incorporating a filter comprising a layer of filter material configured to substantially prevent transmission of radiation at, respectively, first and second predetermined visible wavelength bands covering the wavelengths of first and second respective predetermined laser threats whilst substantially allowing visible wavelengths outside of the respective wavelength band to be transmitted, wherein said first and second screen members are configured to be utilised simultaneously to afford the user, in use, with protection against said first and second predetermined laser threats.

Thus, by means of this aspect of the present invention, a complete laser protective/blocking specification can be provided using two (or more) different screen members simultaneously. This means that the filter of each screen member needs to only have a selected one or more filter regions formed therein, so as to maintain the VLT at some predetermined (high) level, as required by many applications, whilst still being able to provide a complete laser protective/blocking "package" that provides protection, in use, against multiple predetermined laser threats. The first and second screen members may each comprise one of: a screen or windscreen/windshield, a set of eyeglasses (having a single lens covering both eyes or separate respective lenses and worn on the user's head in the manner of conventional spectacles or goggles), contact lenses, a pulldown screen or visor. Thus, purely by way of example, a pilot could, for example, have an aircraft windshield that includes a layer of filter material configured to substantially block radiation at a wavelength corresponding to a first predetermined laser threat (e.g. 532nm) and could additionally wear a set of goggles having an eyepiece including a layer of filter material configured to substantially block radiation at a wavelength corresponding to a second

(different) predetermined laser threat (e.g. 455nm). In use, when the pilot is wearing the goggles and looking through the windshield, the two screen members, together, provide the pilot with protection against both predetermined laser threats.

Each layer of filter material may comprise a conformable film. 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. In this case, one or more notch filter regions may be formed in each filter by means of holographic exposure of the respective film by radiation in a wavelength band centred on, or covering, the wavelength of a respective predetermined laser threat. The bandwidth of this wavelength band can be made relatively small (e.g. 10nm or less) by means of the proposed holographic exposure process, which serves to preserve the overall VLT of the resultant filter. However, as the required filter regions are split between two (or more) separate filters, it is either possible to provide a much greater number of filter regions in the complete package, whilst maintaining a required VLT of each filter, or it is possible to provide the filter regions with a larger blocking

bandwidth without reducing the overall VLT of the package below a certain level. In an exemplary embodiment, each filter may have a VLT of at least 70%.

Optionally, and as stated above, each filter may incorporate a plurality of notch filter regions, each notch filter region covering, or being centred on, a wavelength corresponding to a (different) predetermined laser threat.

Thus, one or each filter may additionally be configured to prevent transmission of radiation in a second predetermined wavelength band covering a selected further one or more predetermined laser threats.

Advantageously, one or each filter may have an optical density of at least 2 at the first and/or second predetermined wavelength band; and in one exemplary embodiment, one or each filter may have an optical density of at least 2 at each predetermined wavelength band. However, it will be appreciated that lower optical densities for one or more of the predetermined wavelength bands may be desirable to comply with some specific specifications.

These and other aspects of the present invention will be apparent from the following specific description, in which:

Figure 1 is a schematic perspective view of a filter incorporated in a screen member according to an exemplary embodiment of the present invention applied to a substrate;

Figure 2 is a schematic diagram illustrating a process of forming a filter region for use in producing a filter for a screen member according to an exemplary embodiment of the present invention; Figure 3 is a graph in which the transmission characteristic of the filter of Figure 1 is plotted; and

Figure 4 illustrates schematically, a kit of parts according to an exemplary embodiment of the invention, embodied as a set of eyeglasses and a windscreen of a vehicle, when in use.

With reference to Figure 1 , there is shown a layer of filter material 10 applied to a first face of a substrate 20 to provide a vehicle window 100 adapted for partially mitigating a selected set of 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 plastics material such as polycarbonate.

The filter material 10 is an interference filter advantageously (but not necessarily) formed by a method hereinafter described for holographically exposing a photosensitive film with a plurality of lasers having a set of predetermined wavelengths, advantageously (but not necessarily essentially) within a selected wavelength band of bandwidth 10nm or less.

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 95% 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.

Whilst the present detailed description makes specific reference to a vehicle window, it is to be understood that the principles and techniques disclosed hereinafter are equally applicable to other applications such as eyeglasses, contact lenses, sensor protection films, screens or pull-down visors, laser designators, etc.and the present invention is not necessarily intended to be limited in this regard. It will be understood by a person skilled in the art that the following principles and techniques can be readily adapted to different laser protective/blocking applications, simply by selection of the film and the specific wavelengths to which the film is exposed to form the filter region(s), as well as selection of two or more screen members to be utilised simultaneously to provide a complete laser protection/blocking specification, as required. 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 Figure 2 of the drawings, distinct filter regions defining a notch filter region of a predetermined bandwidth (e.g. 5nm) 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 5nm around 520nm, then a plurality of lasers 100 may be used to produce the notch filter region of (purely by way of example) 517.5nm, 518nm, 518.5nm, 519nm, 519.5nm, 520nm, 520.5nm, 521 nm, 521 .5nm, 522nm and 522.5nm. 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. A plurality of notch filter regions centred on, or covering, different respective wavelengths can be formed in the same film, using the method described above, if required by the applications.

Once the exposure process has been completed, the resultant hologram can be fixed by, for example, a bleaching process.

Consider the case where, for example, it is required to provide laser threat protection in respect of three different wavelengths simultaneously, e.g. 455nm, 532nm and 650nm. In order to preserve the overall VLT provided by such laser protection, a kit of parts comprising a vehicle window and a set of eyeglasses or goggles may, for example, be provided. Thus, the notch filter region centred on, or covering, 532nm may be formed (e.g. in the manner described above) in a conformable film incorporated in the vehicle window, and two notch filter regions centred on, or covering 455nm and 650nm respectively may be formed (e.g. in the manner described above) in a conformable film incorporated in the lens(es) of the eyeglasses which are configured to be worn by a user in a conventional manner by the user or pilot of the vehicle. When the user is inside the vehicle and wearing the eyeglasses, the window and eyeglasses together provide a complete laser protection specification covering three distinct predetermined laser threats.

The transmission characteristic (which may alternatively be referred to as the transfer function) of visible electromagnetic radiation incident on the combined filter provided by the window and the eyeglasses, in use together, is illustrated in Figure 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. 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.

There are three distinct notches in the transmission characteristic associated with three wavelength bands. These are in particular a 10nm band centred on 455nm, a 10nm band centred on 532nm and a 10nm band centred on 650nm in accordance with the exemplary embodiment described above. In general any three or more notches from the group consisting of 405 nm, 455 nm, 520 nm, 532 nm, and 650 nm may, for example, be selected. Further, notches may be chosen to coincide with any expected laser threat wavelength and, if required, a third screen member (e.g. a set of contact lenses) may be added to the kit of parts to provide further additional notch filter regions and associated laser protection. The bandwidth of each of the notch filter regions may be selected as required to meet specific protection characteristics, and this is not as constrained by the need to maintain VLT (especially in the case of multiple predetermined laser threats), because not all of the notch filter regions need to be provided in the same film.

At the centre of each of the bands illustrated in Figure 3, 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. Figure 4 shows a window 201 deployed as a windscreen on a vehicle V, which in this example is an aircraft. A pilot P, wearing a pair of eyeglasses 202, 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. In operation the window 201 , which incorporates a filter including a notch filter region covering 532nm for example, may be used to mitigate the effects of the laser beam L.

In particular, as the laser beam L propagates onto the window 201 it will pass through the substrate 20 and on the filter 10 where the light becomes substantially attenuated. Assuming the filter 10 to have the transmission characteristics shown in Figure 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.

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.

Similarly, if the laser beam L has a wavelength of 650nm (or 455nm), it will pass through the window 201 to the lens(es) of the eyeglasses 202 (if the pilot is looking through the window 201 ). In this case, then, the laser beam L will be attenuated to 0.1 % of its original intensity by the filter incorporated in the eyeglasses.

The above discussion has provided an overview of how the present invention may mitigate the threat of laser beams in various applications.

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 ² or less should be sufficient to prevent eye damage.

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. Beam 'Spot' min OD min OD Example OD

Distance Intensity Typical

diameter Size for 0.5s for 10s ranges (to (m) (W/Cm ² ) Application

(mm) (mm ² ) exposure exposure 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

Car/train

50 28 615.8 0.5 2.10 2.70 2.5-4.0

/bus/aircraft

Car/train

100 53 2206.2 0.1 1.40 2.00 1.5-3.5

/bus/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

Table 1

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 2 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.

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).

In a variant of the window and substrate arrangement of Figure 1 , the window may be comprised by a number of laminar substrates between which could be positioned the filter 10.

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.