FIELD

The present disclosure relates to devices for suppressing specular reflection, such as anti glint or anti-narcissus devices, containing a Volume Holographic Grating (VHG), methods of manufacturing such devices, and methods of suppressing specular reflection using such devices.

BACKGROUND

Optical filters are used in a variety of applications, such as laser protection (including laser eye protection), equipment protection (such as to prevent camera sensors from saturation), and spectroscopy (where strong unwanted optical signals should be supressed in order for weaker signals of higher interest to be captured). The filters are often required to be narrowband, in order to selectively filter wavelength bands of optical radiation while maintaining a high transmission outside of the filter band.

Filters made with dyes absorb the wavelength bands of interest, and are usually wideband, i.e. they will also tend to filter out potentially useful wavelengths. Filters based on subwavelength structures such as rugate filters, dielectric deposition filters, holographic filters, are better suitable for narrowband filtering as they rely on resonance conditions (constructive and destructive interference) to filter out the unwanted wavelengths. Such structured, grating-based filters are usually reflective. Depending on their designed optical density (OD), i.e. the amount of light that is filtered on a log scale, virtually all the light can be reflected in a specular fashion. As an example, an OD 1 filter will reflect 90% of the incoming light and an OD 2 filter will reflect 99% of the incoming light, at the wavelength bands of interest.

As an example shown in Figure 1, specular reflection is mirror-like reflection 14 of an incidence beam 12, where the angle of reflection a-R off a surface 10 is approximately equal to the angle of incidence a-l. Specular reflection can cause a number of problems depending on the application of the filter. For example, in laser filters for protecting equipment or a wearer’s eyes from a hazardous laser beam, specular reflection would reflect the hazardous laser beam in another direction with the potential to harm other equipment or another person. A specular reflect of a laser beam can also be seen over long distances and could reveal the location of the wearer of laser protection filter. As another example, in light sensing applications such as cameras or spectroscopes, specular reflected light may be redirected to other sensitive equipment or be reflected off other surfaces to impinge on the filter a second time. These effects may damage equipment or distort readings.

Specular reflection can be reduced to a certain extent by using sun-shades or slats, such as on a traffic light. However, such devices do not work well for sensors or viewing from behind since they will limit the field of view (FOV) looking out through the device. Further, such devices are unable to protect against specular reflection at, or near, normal incidence.

SUMMARY

According to an aspect of the present disclosure, a filter device for suppressing specular reflection is provided. The filter device comprises a filter component configured to reflect a first bandwidth of electromagnetic radiation received at a first range of angles of incidence and to transmit electromagnetic radiation outside of the first bandwidth of the electromagnetic radiation. The filter device also comprises a material layer comprising a volume holographic grating, wherein the volume holographic grating comprises periodic grating features spaced along a grating direction by a spacing, wherein the grating direction forms a slant angle with respect to a direction normal to a surface of the material layer; wherein the volume holographic grating is configured to reflect, at a first angle of reflection, incident electromagnetic radiation received at a first angle of incidence, wherein the first angle of reflection is different to the first angle of incidence, wherein the first angle of incidence is in the first range of angles of incidence.

The filter component may have an optical density (OD) of OD3, OD4 or OD5 (equivalent to 0.1%, 0.01% and 0.001% transmission, respectively, with the rest reflected) at the first range of angles of incidence for the first bandwidth. The volume holographic grating may have an optical density of OD1 or OD2 (equivalent to 10% or 1% transmission, respectively, with the rest reflected) at the first angle of incidence. Accordingly, the term reflecting does not necessarily imply 100% reflection but, in general, reflecting a significant portion of incident radiation and transmitting a minority portion of the incident radiation. The volume holographic grating and filter component may be arranged such that the filter component is arranged to reflect a portion of incident radiation transmitted by the volume holographic grating.

The filter component may reflect the electromagnetic radiation in a specular way (i.e. with angle of incidence substantially equal to angle of reflection), whereas the volume holographic grating reflects light in a non-specular way since the angles of incidence and reflection are different. The non-specular reflection may be diffuse or directed at a particular angle.

In general, the volume holographic grating may be configured to reflect radiation as described above at least in the first bandwidth for which the filter component is configured to reflect. The volume holographic grating may be configured to reflect incident radiation outside of the first bandwidth as well.

The filter device may be configured so that the volume holographic grating is arranged to receive incoming electromagnetic radiation before the filter component, e.g. the volume holographic grating is closer to the ‘outside’ of the filter device than the filter component.

In other words, the filter component is closer to the viewing side of the filter device where a viewer (an eye or an imaging sensor) would be located in use. Accordingly, the volume holographic grating would reflect a major portion of the incident radiation in way which does not contribute to glint (e.g. reflect diffusely or directed towards a baffle), whereas the filter component would reflect a remaining portion which was transmitted by the volume holographic grating (or at least a very high proportion thereof).

The device may comprise at least one opaque baffle for blocking electromagnetic radiation and wherein the slant angle of the material layer is such that electromagnetic radiation incident on the material layer within the first range of angles of incidence is reflected onto the at least one opaque baffle. To block the reflected electromagnetic radiation, the at least one opaque baffle may be positioned adjacent to the material layer and extend at least partially in a direction perpendicular to the surface of the material layer.

The at least one opaque baffle may comprise a plurality of opaque baffles arranged in array. The plurality of opaque baffles may comprise a plurality of plates. Additionally or alternatively, the plurality of opaque baffles may comprise a honeycomb arrangement, e.g. form a plurality of contiguous hexagonal cylinders where in the plurality of baffles are the sides of the hexagonal cylinders. The slant angle may be between 5 and 30 degrees and the range of angles of incidence may be any angle less than or equal to the slant angle. Optionally, the slant angle may be between 11 and 13 degrees or, further optionally, the slant angle may be 12 degrees. The slant angle may be greater at positions further from the at least one opaque baffle than positions closer to the at least one opaque baffle.

The slant angle of the volume holographic grating may vary across locations on the material layer such that volume holographic grating diffusely reflects incident electromagnetic radiation. The volume holographic grating may have a plurality of regions each having a respective uniform slant angle, wherein each of the plurality of regions is no greater in size in any direction than a first length. The first length may be 1 mm. Alternatively the first length may be 1 cm, 1.5 cm or 2 cm. In general, the first length may be less than a beam width of the incident electromagnetic radiation. The plurality of regions may be a plurality of contiguous regions in a regular or irregular arrangement.

The filter component may be a grating in the material layer, for example another volume holographic grating. The filter component may be part of the first volume holographic grating, i.e. the filter component grating features and the grating features which cause specular reflection suppression may be in a superposition in the material layer. Alternatively, the filter component may be formed in a different part of the material layer to the volume holographic grating. The filter component may be in a different layer to the first volume holographic layer. Multiple volume holographic components may be paired with a single filter component to supress the specular reflection of the filter component.

The filter device may further comprise a second filter component configured to reflect a second bandwidth of electromagnetic radiation at the first range of angles of incidence and to transmit electromagnetic radiation outside of the second bandwidth of the electromagnetic radiation; and a second material layer comprising a second volume holographic grating, wherein the second volume holographic grating is configured to reflect incident electromagnetic radiation in the second bandwidth at an angle of reflection different to an angle of incidence of the incident electromagnetic radiation. Analogously to the first volume holographic grating, the second volume holographic grating comprises periodic grating features spaced along a second grating direction by a second spacing, wherein the second grating direction forms a second slant angle with respect to a direction normal to a surface of the second material layer. In general, the properties (e.g. grating directions and spacings) of the first and second volume holographic gratings can be different.

The second volume holographic grating may be configured to reflect incident radiation as described above at least in the second bandwidth for which the second filter component is configured to reflect. The volume holographic grating may be configured to reflect incident radiation outside of the second bandwidth as well.

The second filter component may be a grating in the second material layer. As explained above for the first filter component and the first material layer, the second filter component may be another volume holographic grating or may be part of the second volume holographic grating.

According to an aspect of the present disclosure, there is provided a laser protection device comprising: a filter device for suppressing specular reflection according to any filter device with features as described above, wherein the substrate is one of: a window, a visor, eyewear, a scope, a recessed viewport, and a sample holder. The filter device may be configured so that the volume holographic grating is arranged to receive an incoming laser beam before the filter component, e.g. the volume holographic grating is closer to the ‘outside’ of the substrate than the filter component and the filter component is closer to the viewing side of the substrate where a viewer (an eye or an imaging sensor) would be located in use.

According to an aspect of the present disclosure, there is provided a method of manufacturing a laser protection device as described above, wherein the method comprises: mounting the filter device for suppressing specular reflection onto the transparent substrate.

According to an aspect of the present disclosure, there is provided a method of supressing specular reflection from incident electromagnetic radiation within a range of angles of incidence. The method comprises receiving, at a material layer, incident electromagnetic radiation within the range of angles of incidence; wherein the material layer comprises a volume holographic grating, wherein the volume holographic grating comprises periodic grating features spaced along a grating direction by a spacing, wherein the grating direction forms a slant angle with respect to a surface of the material layer. The method comprises reflecting, by a filter component, a first bandwidth of the incident electromagnetic radiation. The method comprises reflecting, by the volume holographic grating, the incident electromagnetic radiation at an angle of reflection different to an angle of incidence of the incident electromagnetic radiation. The method may further comprise transmitting, by the filter component, electromagnetic radiation outside of the first bandwidth of the electromagnetic radiation. The reflecting, by the filter component, may be reflecting a portion of the incident electromagnetic radiation which was not reflected (i.e. was transmitted) by the volume holographic grating.

The reflecting, by the volume holographic grating, may comprise reflecting the electromagnetic radiation towards at least one opaque baffle for blocking electromagnetic radiation; and the method may further comprise blocking, at the at least one opaque baffle, the electromagnetic radiation reflected by the volume holographic grating.

The reflecting, by the volume holographic grating, may comprise diffusely reflecting the incident electromagnetic radiation.

The method of supressing specular reflection may be performed using any filter device or laser protection device having features as described above. Likewise, any filter device or laser protection device having features as described above may be configured to perform the above features of the method of supressing specular reflection.

The reflecting by the filter component may be primarily specular reflection, whereas the reflecting by the volume holographic grating is substantially non-specular reflection.

BRIEF DESCRIPTION OF THE FIGURES

Specific embodiments are now described by way of example and with reference to the accompanying drawings, in which:

Figure 1 shows an example of specular reflection off a surface;

Figure 2 shows specular reflection off a conformal filter device and non-specular reflection off a slant-grating filter device;

Figure 3 shows an illustration of stray-light reflection;

Figure 4 shows an illustration of anti-glint arrangements;

Figure 5 shows a diagram defining parameters used to describe reflection;

Figure 6 shows a polar plot of which incident angles have reflections blocked by a honeycomb anti-glint arrangement;

Figure 7 shows schematic of a device for suppressing specular reflection (not to scale); Figure 8 shows a polar plot of which incident angles have reflections blocked by the device shown in figure 7;

Figure 9 shows results of a device for suppressing specular reflection;

Figure 10 shows a device for supressing specular reflection by diffuse reflection, from above and from a side;

Figure 11 shows an example of a filter device;

Figure 12 shows a diagram of a laser protection device;

Figure 13 shows a method of manufacturing a laser protection device; and Figure 14 shows a method of supressing specular reflection.

DETAILED DESCRIPTION

In overview, filter devices for suppressing specular reflection and laser protection devices are disclosed, as well as methods of manufacturing such devices, and methods of suppressing specular reflection using such devices.

Overview of VHGs

Volume Holographic Gratings (VHGs), also known as volume Bragg gratings, are periodic patterns created inside a medium. For example, the periodic patterns may be variations in the refractive index of the medium. The periodic patterns cause reflection or refraction of incident light when satisfying certain criteria for wavelength and angle of incidence. VHGs differ from diffraction gratings made on the surface of an optical medium, since the periodic pattern is disposed throughout the bulk (i.e. volume) of the medium, rather than along its surface. VHGs can be used in various kinds of optical devices for light manipulation.

VHGs are typically made by irradiating a photosensitive material with two near- monochromatic light beams with different propagation directions. The beams superimpose inside the material to produce an interference pattern, i.e. a pattern of varying intensity through the thickness of the medium. The exposure of the photosensitive material with this interference pattern creates a refractive index pattern with fundamental similarity to the intensity pattern inside the material.

With respect to Figures 2A and 2B, an optical device 20 comprises a VHG implemented in a photosensitive material 15. The VHG can be locally described by the grating direction G and spacing L of a periodic or near periodic modulation of the refractive index of the photosensitive material. Along the grating direction of the VHG, the refractive index alternates between high and low values, i.e. locally the VHG comprises planes 12 of alternating high and low refractive index that are spaced by A. With respect to Figure 2A, a conformal reflective VHG has these grating planes parallel to the surface of the material, i.e. grating direction G is parallel to a direction normal to the surface. With respect to Figure 2B, a slanted VHG has the grating planes slanted by a slant angle with respect to the surface material, and the grating direction G is at the same angle with respect to a direction normal to the surface. Additionally, the projection of the grating direction G of a slanted VHG onto the surface of the material provides the azimuthal direction of the slanted VHG, measured by the azimuthal slant angle that is enclosed by the azimuthal direction and a fundamental direction parallel to the surface of the material. For a conformal VHG with effectively no slant angle (or a nominal slant angle of 0), the azimuthal slant angle is undefined.

Details of how to manufacture VHGs, including how to control the VHG parameters including spacing A and slant angle, are provided in published PCT international application WO 2019/243554. In summary, the process includes irradiating a photosensitive film with two (or more) beams of light from a light source coherent enough to be capable of creating a stable interference pattern over a length scale of the thickness of the film. An example of a suitable photosensitive film is Covestro™ Bayfol™ film, or one of its derivatives. One of the beams may be a reflection of the other. The photosensitive film is positioned in the interference pattern of the two beams so that variation of intensity in the interference pattern is translated into a variation in refractive index of the photosensitive film. The photosensitive film can be positioned at any angle with respect to the interference pattern (either by moving the film or the beams of light) to achieve the desired properties of the VHG such as slant angle. The spacing of the VHG is generally determined by the wavelength of the two beams of light and the angle between the beams, since these determine the spacing between intensity maxima of the interference pattern. The beams can be scanned over the photosensitive film to create a VHG larger than just the beam width. The VHG parameters can also be varied as the beams are scanned across the photosensitive film to create a VHG with different slant angle and/or spacing at different locations of the photosensitive film, as explained in depth in WO 2019/243554.

Reflection from VHGs

VHGs as described above will reflect electromagnetic radiation having a particular wavelength (or bandwidth of wavelengths) and angle of incidence a-l to satisfy the Bragg condition. With reference to Figure 2A, conformal VHGs will reflect electromagnetic radiation at an angle of reflection a-R equal to the angle if incidence a-l. With reference to Figure 2B, slant VHGs will reflect electromagnetic radiation with a different angle of reflection a-R to the angle of incidence a-l. Because the grating features in the material layer are at a slant angle, the electromagnetic radiation has a different effective angle of incidence inside the material layer. Accordingly, the angle of reflection a-R (with respect to the normal of the surface of the material layer) is in principle equal to the sum of the angle of incidence a-l (with respect to the normal of the surface of the material layer) and the slant angle.

With reference to Figure 3, VHGs can be used as a filter with a high attenuation for a narrow bandwidth of electromagnetic radiation and for specific angles of incidence. As such, VHGs are useful in laser protection devices as light at a wavelength particular to a specific laser type can be reflected whereas other wavelengths are not attenuated so as to not adversely affect the optical function of the laser protection device. This is shown in Figure 3 as the VHG filter 110 reflects an incoming laser beam 120 of a particular colour.

A potential downside of this is that the VHG filter 110 will reflect ambient light that the particular colour that the VHG filter protects against. This leads to a stray-light reflection 130. Stray light reflections exist constantly, even if no laser is present. Such stray light reflections can reveal the location of object or person to which the filter is fitted to, to the detriment of any camouflage measures. Stray light reflection is type of glint, which is the specular reflection off a shiny surface (often a window or view port) towards the outside world and the visible signature that arises from it.

The same effect also exists in the reverse direction, where an individual’s vision of the outside world from the inside can disturbed by an unwanted stray light reflection off a laser-protection filter from light sources located on the inside of e.g. a vehicle. The problem is known as narcissus effect or narcissism in the opposite direction, where the back-reflection towards the eye side of a window/view port can cause a distraction.

With reference to Figure 4A, a shade 140 can mitigate glint for large angles of incidence from certain directions but need to be unpractically large to mitigate small-angle glint from all directions. With reference to Figure 4B, channel plate structures 150 (e.g. honeycomb) can achieve glint mitigation for most incidence angles, but restrict the field of view accordingly, and do not mitigate at normal incidence. With reference to Figure 5, the angles for which a visible signature escapes from an arrangement of channel plate structures of height H, separated by length L and width W, are the angles satisfying the equation:

VL ² + W ² tan a £ - - -

H

As an example, a honeycomb arrangement has visible signature for angles as shown in Figure 6. The radial axis shows angle of incidence on incoming electromagnetic radiation, with concentric rings indicating angle of incidence of 10, 20 and 30 degrees from normal (sometimes called the polar angle). The angular position from vertical axis labelled 0 degrees is the azimuthal angle of incoming electromagnetic radiation. The shaded area of the plot shows the ranges of angles for which there is no visible signature reflected by the honeycomb arrangement. The unshaded region shows where there is a visible reflected signature. For a particular example as shown in Figure 6, the dimensions (height, width, length) of the honeycomb arrangement is such that any incoming electromagnetic radiation at greater than approximately 10 degrees polar angle of incidence will not have a visible reflected signature, i.e. the honeycomb arrangement blocks the radiation.

Filter device for suppressing specular reflection

With reference to Figure 7, a filter device 200 comprises a filter component 210 is configured to reflect a certain bandwidth of incoming electromagnetic radiation. On the filter component 210 there is a material layer 220 comprising slant VHG reflects light in the certain bandwidth in an anomalous direction. In combination with mechanical anti-glint devices, in this example a plurality of baffles 230, the filter device can fully mitigate glint from narrow-band reflective coatings, e.g. laser-protection interference filters. Further, this approach provides less FOV- restriction compared to other arrangements such as channel-plate structures, because the baffles can be shorter.

When incoming electromagnetic radiation approaches the filter device, some angles of incidence will be blocked by the plurality of baffles 230. At low angles of incidence not blocked by the baffles 230, the electromagnetic radiation reaches the material layer 220 comprising the slant VHG. The slant VHG reflects electromagnetic radiation at the relevant bandwidth for which the filter device is designed. However, unlike specular reflection, the slant VHG reflects at a different angle of reflection to the angle of incidence, and therefore the electromagnetic radiation is reflected towards one or more of the plurality of baffles 230 and is thereby blocked. Therefore there is no glint even at low angles of incidence. Electromagnetic radiation at other wavelengths do not meet the Bragg condition and so pass through the material layer so as to not compromise visibility behind the filter device.

In practice, the slant VHG in the material layer 220 will not reflect all of the electromagnetic radiation in the bandwidth but will reflect enough to satisfactorily supress glint such that there is no significant visible signature returned from the filter device. For example, the material layer 220 may have an optical density of OD1 or OD2. While this is enough to avoid a significant visible signature reflected by the filter device, it may not be sufficient to prevent a dangerous power level of electromagnetic radiation passing through from a laser beam, e.g. in a laser attack. However, the filter component 210 behind the material layer 220 has a higher attenuation, e.g. OD3 to OD5, which provides protection against higher power electromagnetic radiation in the relevant bandwidth. The amount reflected by the filter component 210 may be reflected at the same angle of reflection as angle of incidence but is attenuated enough to not cause a significant reflected visible signature. Therefore the filter device 200 provides both high protection against incoming radiation at the bandwidth to be filtered but does not produce glint.

The filter component 210 and material layer may be further mounted onto a substrate to make a laser protection device, either directly or via one or more support layer.

An example of the results of an anti-glint filter device as described above is shown in Figure 8 as a polar diagram. Superimposed on the signature pattern for a honeycomb pattern (as shown in Figure 6) is the effect of a material layer comprising a slant VHG having a slant angle of 12 degrees. The range of incident directions and angles of incidence which are reflected into the honeycomb arrangements form two curved bands on either side of the vertical axis. This covers the range of angles of incidence for which the honeycomb arrangement would otherwise produce a visible signature. Accordingly, the filter device suppresses specular reflection (i.e. it is anti-glint) for all possible directions and angles of incidence. The VHG visible signature is suppressed by the honeycomb and the honeycomb visible signature is suppressed by the VHG.

Another example of the results of an anti-glint filter as described above is shown in Figure 9 as polar diagrams. A visible signature (VLR) of a conformal VHG filter component, without a material layer comprising the VHG configured to reflect at an angle of reflection different to angle of incidence, is shown in Figure 9A. A visible signature (VLR) of a conformal VHG filter component with a material layer (or layers) comprising two slant VHGs applied is shown in Figure 9B. The radial axis extends from 0 to 60 degrees and the azimuthal angle ranges for the full 360 degrees. The slant design is optimized to redirect the light reflected by the conformal filter near normal incidence, significantly reducing the VLR of the conformal reflections. The darker areas show directions and angles of incidence which are less reflected by the filter device. Accordingly, the filter device with at least one slant VHG suppresses specular reflection. Additional baffles can restrict the FOV to suppress the visible signature of the slant VHGs.

With reference to Figure 10, in some arrangements the slant angle of the VHG in the material layer varies across locations on the material layer such that the VHG diffusely reflects incident electromagnetic radiation. This produces an anti-glint effect, either as an alternative to using baffles or in combination with baffles as described above.

With reference to Figure 10A, the material layer 220 comprising the slant VHG has a plurality of contiguous regions 225 arranged across the material layer 220. Although in Figure 10A the contiguous regions 225 are rectangles, other shapes are possible, e.g. hexagonal regions or irregular shaped regions. The regions may vary in size or shape across the material layer 220. The slant VHG has different properties in different regions 225 of the plurality of contiguous regions. In particular, the slant angle differs between regions. This has the effect that incident electromagnetic radiation at any given angle of incidence will be reflected at different angles of reflection by different regions 225. The size of each region is such that they are no greater than a first length in any direction. For rectangular regions, this means that the length and width of each region is less than the first length. The first length is chosen to be small enough that electromagnetic radiation reflected does not produce a significant visible signature, i.e. the width of a beam reflected in any particular direction is small enough to not be visible at the relevant required distance away from the device (or other conditions as required). For example, the first length may be 1 mm, so that no radiation reflected in any given direction has a beam width of more than 1 mm. In other arrangements, the first length is 5 mm, 1 cm, 1.5 cm or 2 cm. The first length may also be chosen to be smaller than an expected beam size of a potential laser attack, e.g. based on a likely distance from which an attack would occur and typical divergence of the type of laser being protected against.

When incoming electromagnetic radiation is incident on the filter device 200, electromagnetic radiation incident on any part of the material layer 220 will be reflected into multiple directions, i.e. diffuse reflection rather than specular reflection. This is because adjacent portions of the incoming radiation are incident on different regions 225 of the VHG in the material layer, which have different slant angles. The pseudo-random nature of the reflections will scatter the reflected electromagnetic radiation so that there is no effective visible signature which could be noticed. Accordingly, the anti-glint material layer 210 of the filter device 200 supresses specular reflection, while the filter component 210 still provides the high level of attenuation required to reflect an intense beam of electromagnetic radiation, e.g. from a laser attack.

In other arrangements, a variation in the slant angle of the VHG in the material layer is arranged to re-shape the reflection visible signature from the filter device, so that the filter device is not recognisable.

With reference to Figure 11, in some examples, the filter device 200 comprises a second filter component 310 and a second material layer 320 comprising a second VHG in addition to the first filter component 210 and first material layer 310. The second filter component 310 and material layer 320 may have any of the features described above for the first filter component 210 and first material layer 220. The angles of reflection at which the second VHG is configured to reflect incident electromagnetic radiation may be the same or different to the first VHG or, in examples wherein the slant angle varies, the second VHG may have the same or different variation as the first VHG.

In these examples, the pairs of filter components and material layers may be designed to protect against different typical wavelengths of laser light, and therefore protect against more kinds of laser attack. In some arrangements, the filter device 200 comprises further filter components and corresponding material layers with volume holographic gratings for yet further bandwidths of electromagnetic radiation.

A filter device as described above may be integrated into a wafer stack (e.g. a silicon-on- insulator stack) in order to provide the anti-glint and filtering functions described above (or perform the methods described above) for an optical component.

Further examples

In any of the example filter device as described herein, the first or second filter component may be a separate layer to the respective first or second material layer. Alternatively, the first or second filter component may themselves be a VHG formed in the respective material layer such that the filter and anti-glint effects are achieved by a single layer. In other examples, the filter component may be a dye filter or any other type of filter suitable for providing the required filtering as meets the need of the implementation. The first bandwidth may be 512 nm to 552 nm, or 531 to 533 nm, or any other range of wavelengths covering a common laser wavelength. The bandwidths of electromagnetic radiation for which the filter and material layers are designed may be outside of the visible light range, for example in the infrared wavelengths, depending on the requirements of the specific implementation. In some examples, if the optical density of the anti-glint slant VHG is large enough, it may itself serve as the laser protection filter. This can be extended to an assembly of multiple slant VHGs where the combination of layers serves as the laser protection filter.

In some arrangements, a combination of techniques can be used to produce the anti-glint effect. For example, a combination of baffles (for example a honeycomb structure) and varying slant angle of the VHG can be used. One such example would be for there to be a greater slant angle of the VHG further away from a baffle, as the VHG needs to reflect electromagnetic radiation at a greater angle of reflection in order to direct it into the baffle. Conversely, the VHG in the region nearer a baffle only needs to reflect electromagnetic radiation at a lesser angle of reflection in order for the reflected radiation to be blocked by the baffle. In other examples, a combination of baffles to block radiation reflected of the VHG and having a plurality of regions with differed slant angles to reflect radiation diffusely can be used.

Laser protection device

With reference to Figure 12, a laser protection device 400 comprises a filter device 200 as previously described (according to any of the examples described herein) and a substrate 410. As an example, the substrate may be any of a window, a visor, eyewear, a scope, a recessed viewport, and a sample holder. These may be used according to the required implementation, such as to protect a wearer’s eyes from a laser attack, or to prevent bleaching a signal when analysing a spectrum from a sample illuminated by a laser beam.

With reference to Figure 13, a method of manufacturing a laser protection device as described above includes mounting the filter device for suppressing specular reflection onto the transparent substrate. Mounting may be according to any suitable attachment technique as fits the type of substrate and its purpose.

Method of suppressing specular reflection

With reference to Figure 14, a method 600 for supressing specular reflection from incident electromagnetic radiation within a range of angles of incidence comprises receiving 610, at a material layer, incident electromagnetic radiation within the range of angles of incidence. The method also comprises reflecting 620, by a filter component, a first bandwidth of the incident electromagnetic radiation and reflecting 630, by the volume holographic grating, the incident electromagnetic radiation at an angle of reflection different to an angle of incidence of the incident electromagnetic radiation.

The method 600 may be performed by a filter device 200 or laser protection device 400 as described above according to any of the examples described herein.

It is to be understood that the above description is intended to be illustrative, and not restrictive. Many other implementations will be apparent to those of skill in the art upon reading and understanding the above description. Although the present disclosure has been described with reference to specific example implementations, it will be recognized that the disclosure is not limited to the implementations described but can be practiced with modification and alteration within the scope of the appended claims. Accordingly, the specification and drawings are to be regarded in an illustrative sense rather than a restrictive sense. Although various features of the approach of the present disclosure have been presented separately (e.g., in separate figures), the skilled person will understand that, unless they are presented as mutually exclusive, they may each be combined with any other feature or combination of features of the present disclosure.