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Title:
DEVICE FOR STAND-OFF RAMAN SPECTROSCOPY
Document Type and Number:
WIPO Patent Application WO/2021/148771
Kind Code:
A1
Abstract:
Apparatus for stand-off Raman spectroscopy, comprising optical imaging device (e.g. a sight, e.g. of a rifle), Laser, Optical apparatus (optionally in the form of an adapter) with a wavelength band filter and optical dispersion means (e.g. grating) to disperse a Raman spectrum to an image sensor, and focusing means wherein the laser is a pulsed laser, and each of the detector elements of the array comprises a frequency band pass filter for detecting light pulses in preference to background radiation. Each detector element is provided with a first charge storage element to store charge, connected via a frequency band pass filter to a second charge storage element arranged to store charge preferentially resulting from high frequency light pulses. This may be performed in the short wave infrared spectrum. The image sensor can also have conventional detector elements, enabling viewing of a scene whilst simultaneously obtaining a Raman spectrum of a target in the scene. Advantageously this may be implemented in a rifle sight, optionally by means of an adapter arranged so that the rifle sight can be readily switched from conventional viewing mode to stand off Raman spectroscopy mode.

Inventors:
MCEWAN KENNETH (GB)
Application Number:
PCT/GB2021/000004
Publication Date:
July 29, 2021
Filing Date:
January 19, 2021
Export Citation:
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Assignee:
SECR DEFENCE (GB)
International Classes:
G01J3/02; F41G11/00; G01J3/28; G01J3/44; G01N21/65; H04N5/3745
Foreign References:
US20120120393A12012-05-17
US20090237648A12009-09-24
US20180259296A12018-09-13
US20170195600A12017-07-06
US20120261553A12012-10-18
US20120261553A12012-10-18
Other References:
JOHN M. INGRAM, EDSANTER LO: "Combining hyperspectral imaging and Raman spectroscopy for remote chemical sensing", PROC. OF SPIE 695405-1, vol. 6954, 2008, pages 695405-1 - 695405-7, XP040437126, DOI: 10.1117/12.778330
HIGH DEFINITION 10NM PITCH INGAAS DETECTOR WITH ASYNCHRONOUS LASER PULSE DETECTION MODE, January 2020 (2020-01-01), Retrieved from the Internet
Attorney, Agent or Firm:
FARNSWORTH, Alastair, Graham (GB)
Download PDF:
Claims:
CLAIMS

Claim 1. A product or kit, for performing stand-off Raman spectroscopy, comprising:

1. optical imaging device, having a front light inlet end, and having an image sensor at a proximal end comprising a first array of image sensor elements;

2. laser apparatus, operable to produce laser radiation at a transmission wavelength, and arranged to direct the laser radiation to a focused point at a location;

3. optical apparatus, arranged as part of, or adapted to be fitted to, the optical imaging device, comprising: a. a wavelength band filter, arranged to selectively block the transmission wavelength, and to transmit light in a Raman scattering wavelength band associated with the transmission wavelength; b. optical dispersion means, arranged to disperse Raman scattering photons of that wavelength band, so as to disperse a Raman spectrum across the image sensor in use; and

4. inlet optical focusing means, such as to adjust the focus of the optical imaging device to substantially match the location of the focused point.

Wherein: the laser apparatus is a pulsed laser apparatus, arranged to produce laser pulses having a predetermined duration; and each of the image sensor elements of the first array comprises:

- a respective frequency band pass filter, for discriminatively detecting light pulses having the predetermined duration, in preference to background radiation, coupled to;

- a respective charge storage element arranged to store charge in the event of such a light pulse having the predetermined duration being received by the respective detector element; wherein the charge storage elements are provided as a second array, being arranged for charges to be read off the array as a digital image, and;

Characterised in that: wherein the first and second arrays of charge storage elements are interspersed with one another, or are arranged on separate layers, in a single imaging area of a single image sensor.

Claim 2. Product or kit of claim 1, wherein the first and second arrays of charge storage elements are interspersed with one another.

Claim 3. Product or kit of claim 1, wherein: the optical imaging device is an optical sight; and each of the image sensor elements of the first array provides a respective charge storage element arranged to store charge in the event of background radiation being received by the respective image sensor element, these charge storage elements jointly providing a first array of charge storage elements arranged for charges to be read off as a first digital image; Claim 4. Product or kit of claim 1, 2 or 3, wherein the optical apparatus is arranged to be fitted to, and removed from, the optical imaging device.

Claim 5. Product or kit of claim 4, wherein the optical apparatus is arranged as an inlet filter to be fitted to cover the front light inlet end of the optical imaging device.

Claim 6. Product or kit of claim 5, wherein the optical apparatus comprises the laser apparatus, and wherein the laser apparatus is arranged and oriented to direct the laser radiation to a focused point that is in front of the optical apparatus.

Claim 7. Product or kit of claim 6, wherein the laser apparatus and optical apparatus are are arranged to have optical axes that are non-coaxial and which do not intersect at least throughout an operating distance range of the device, such that the focused point is off-center with respect to the field of view of the optical axis

Claim 8. Product or kit of claim 5, 6 or 7, wherein the optical apparatus comprises the Inlet optical focusing means by virtue of the optical apparatus providing an optical focusing element having a focal distance selected such that if the optical imaging device is adjusted to focus substantially at infinity, then the effect of the optical apparatus comprising the inlet optical focusing means is a resultant focal distance substantially matching the location of the focused point.

Claim 9. Product or kit of claim 5, 6, 7 or 8, wherein, the wavelength band filter, the optical dispersion means, and any inlet optical focusing means of the Optical apparatus are arranged as a single optical element group having at least one element, wherein the single optical element group is arranged to pivot in use between a first position covering the front light inlet end of the optical imaging device, and a second position exposing it.

Claim 10. Product or kit of any preceding claim, wherein the transmission wavelength is in the range 800 to 1400nm.

Claim 11. Product or kit of claim 10, wherein, the pulsed laser apparatus comprises a frequency doubler adapted to be selectively applied to the output of the laser, to double the frequency of output light to the transmission wavelength, and wherein the array of image sensor elements is sensitive to the near-infrared (800 to 2500nm), preferably in the range 800 to 1500nm.

Claim 12. Product or kit of any preceding claim, provided with means for automatically varying a coupling strength between the laser apparatus and detection by the second array of charge storage elements to each of a plurality of different coupling strengths, such as to detect a plurality of Raman spectra of differing signal strengths.

Claim 13. Product or kit of any preceding claim, wherein the optical sight is a rifle sight. Claim 14. Rifle sight adapter for adapting a rifle sight for use in performing Raman spectroscopy, the rifle sight having an image sensor comprising:

- an array of image sensor elements, each providing an array of respective charge storage elements, arranged to store charge in response to background radiation, and arranged for charges to be read off as a digital image, and;

- an array of charge storage elements, each coupled to a respective image sensor of the array of image sensor elements, via a respective frequency band pass filter, and arranged to discriminatively detect light pulses incident on the image sensor, in preference to background radiation, and arranged for charges to be read off as a digital image: the rifle sight adapter comprising:

- a wavelength band filter, arranged to selectively block a light wavelength band, and to transmit light in a Raman scattering wavelength band associated with that light wavelength band; and

- optical dispersion means, arranged to disperse Raman scattering photons in the Raman scattering wavelength band, so as to disperse into the rifle sight in use;

- a connector for connecting the rifle sight adapter to an inlet of a rifle sight, such as to permit the wavelength band filter and optical dispersion means to be introduced into, and removed from, an optical path of the rifle sight, such as to switch the rifle sight between a Raman spectroscopy mode, and a non Raman spectroscopy mode.

Claim 15. Rifle sight adapter of claim 14, comprising an inlet optical focusing means provided by virtue of the rifle sight adapter providing an optical focusing element, arranged to adjust the focal distance of the rifle sight from substantially infinity to a predetermined distance for performing stand-off Raman spectroscopy.

Description:
DEVICE FOR STAND-OFF RAMAN SPECTROSCOPY

The present invention relates to the field of stand-off Raman spectroscopy.

Raman spectroscopy involves directing a laser to a sample and observing a frequency spectrum of photons which scatter at lower frequencies. Raman spectroscopes are generally highly bespoke technical and complicated pieces of equipment due to the need to filter out the original laser light, which is very challenging. Filtering out everything but the desired signal is even more challenging in stand-off Raman spectroscopy as the signal of interest is weaker since less of the Raman scattered photons will be collected by the viewing apparatus, and also due to the presence of ambient light in the stand-off measurement scenario.

It is an object of the present invention to provide an improved way to perform Raman spectroscopy.

According to a first aspect of the invention there is provided a product or kit as set out in claim 1.

Although a more powerful and potentially hazardous laser may be needed, and the measurement precision would typically be lower, this avoids the need for more complex methods to discriminate the signal from background radiation, enabling the construction of the optical elements to potentially be simpler, smaller, and more robust.

With this approach it is typically not possible to accurately measure the intensity of Raman scatterred photons across the spectrum in one measurement, but rather is typically only possible to detect whether the intensity exceeded a threshold value - i.e. to produce spectral measurement at single binary value accuracy. Therefore, the quality of the spectrum will be lower, however the disadvantage of the reduction in quality is offset by the increased ease of use, speed of measurement, and robustness and simplicity of the equipment required. The type of spectroscopy primarily of interest is reflection spectroscopy. Transmission spectroscopy can also be implemented using the present invention, but is not of primary interest.

The charge storage element is typically implemented as a capacitor (a capacitive element adjacent to the detector elements forming the image sensor). The charge storage element 15 is connected via a high pass filter 16 to the detector element 17 such that if the image sensor recieves a sudden burst of light, the charge storage element records a detection bit (e.g. changes from 0 to 1). The high bandpass filter 16 preferably comprises a diode, preferably both diode and a capacitive element. The image sensor is typically an image sensor sensitive to infrared radiation, e.g. a SWIR camera (short wave infra-red camera) sensor, but image sensors that are adapted for use in the visible and/or UV spectrum are also feasible. Inlet lens assembly transparent in the wavelength band (E.g. Infrared) is generally required, and usually this is additional to any inlet filter/adapter provided.

Whilst the approach could be used as a point detector (for samples held within a housing to block ambient lighting), it can also advantageously be implemented in stand-off configuration. Either way the complexity of the equipment required to eliminate all but the photons of interest, is reduced compared with conventional approaches.

US20120261553 which is incorporated by reference, describes how to implement one type of image sensor of the required type. It describes as an imaging device, a pixelated array of semiconductor detector elements, in which each detecting element is electrically connected to an integrated circuit, the integrated circuit of each of the pixels comprising a passive signal path and a transient signal path. The passive path provides consecutive frame or scene imaging and the transient path detects the transient electromagnetic events such as laser pulses. The timing circuit described in this document is not needed for performing Raman spectroscopy, but if present may be set to a null or a preset value in order to ensure that detections are made.

Typically the optical imaging device is a magnifying device. Preferably the optical imaging device is an optical sight and each of the image sensor elements of the first array provides a respective charge storage element arranged to store charge in the event of background radiation being received by the respective image sensor element, these charge storage elements jointly providing a first array of charge storage elements arranged for charges to be read off as a first digital image;

The advantage of this is that it is possible for one device to be used for both imaging of a scene, and also for Raman spectroscopy of a point location (either alternately or simultaneously). Whilst the second array may typically only provide a binary value measurement at each point on the Raman spectrum, for pure substances this is often sufficient for determining whether they are likely to be hazardous, and therefore is advantageous when it is not practical to carry more sensitive equipment around. Additionally, the use of a sensor array where each detector element has a high frequency band pass filter, in conjunction with the use of a pulsed laser, provides a useful way to eliminate the desired signal from other photons such as those from the laser or any background light. This means that the apparatus can potentially be simpler and potentially more robust. The two arrays of the image sensor can be arranged such that for each charge storage element in the first array there is a respective adjacent charge storage element of the second array, however alternatively using 2x2 binning there may be one charge storage element in the second array for every four charge storage elements in the first array. Each charge storage element in the second array has its own electronic high frequency bandpass filter such that it only registers light if a short pulse of light was incident on it. This, along with a short pulsed transmitting laser offers a novel way to discriminate Raman scattering photons from ambient light.

Referring to figure 9, the first array of charge storage elements 15 and second array of charge storage elements 17 may be driven by the same array of detector elements (not shown), i.e. an array of detection elements are conventional image sensor elements arranged to collect charge as a result of incident light onto charge storage elements (which typically is one and the same as the respective image sensor elements - i.e. the charge may reside on the image sensor element, so that it acts as a charge storage element), and the second array of charge storage elements 17 are provided as follows: Associated with an image sensor element 15 of the array, is a respective diode 16 arranged to change state if that image sensor element of the array receives a sudden burst of light, so as to record at least one bit (typically just one bit) that indicates that there was such a burst of light. This is a simple implementation of a high band pass filter 16. Thus the charge storage elements 3', 17 which are sensitive to light pulses do not necessarily have dedicated light collection area, but rather typically operate as a derivative function of an array of conventional image sensor elements 3.

The first and second charge storage elements are in a single imaging area of a single image sensor - so that the conventional image and the image of high frequency pulsed light are collected from the same single projection of light from optical elements (the lens). This avoids the need for two image sensors (or two sensing areas of a single image sensor) and avoids having to split the light from one lens system, or to collect light via two lens systems. The first and second charge storage elements may be interspersed with one another, for example as shown in figure 9, or may be arranged on separate respective layers. Interspersing them has the advantage of image sensor simplicity, whereas separate layers may help avoid a slightly reduction in the light collection area of the image sensor.

Preferably the first and second arrays of charge storage elements are interspersed with one another. For example each of a plurality of first charge storage element 15 has an associated second charge storage element 17 adjacent to it. The relationship could be one to one, however it's also possible for a group of first charge storage elements (E.g. two, four or eight of them) to all be connected to a single second charge storage element 17. This ratio would be chosen on the basis of how many pixels or what resolution the user wants to have in a first image arising from the first charge storage elements, vs a second image of high frequency light pulses arising from the second charge storage elements. Generally for each of a plurality of fist charge storage elements 15, there is an associated (and typically adjacent) second charge storage element connected via a high frequency band pass filter. The first charge storage element optionally is shaped with an indent, to provide a space for its associated second charge storage element (see figure 9 for example where the indent is in one corner of a substantially square or rectangular first storage element 15, although the second charge storage element need not lie entirely within the indent, indeed potentially other adjacent 1 st charge storage elements 15 could be shaped such that together to accommodate the 2 nd charge storage element 17). The 1 st and 2 nd charge storage elements are in respective array, which are generally regular arrays, e.g. square/rectangular arrays, such as to provide a regular and repeating alternating pattern of * and 2 nd charge storage areas.

This has the advantage of enabling a user to see the scene in which the location being measured is visible (either as a dot due to the laser due to zero order dispersion and/or imperfect filtering of the laser frequency, or else by virtue of the location of the spectrum), without requiring a separate image sensor for the respective purposes of conventional imaging and of imaging of high frequency light pulses (the Raman spectroscopy application).

Preferably the optical apparatus is arranged to be fitted to (e.g. and removed from), the optical imaging device. The advantage of this is that by using an appropriate laser, it is possible to upgrade an imaging device to be used for Raman spectroscopy.

Preferably the optical apparatus is arranged as an inlet filter to be fitted to cover the front light inlet end of the optical imaging device. The advantage of this is that by using an appropriate laser, it is possible to upgrade an imaging device to be used for Raman spectroscopy, without modifying the imaging device. The optical apparatus may be arranged substantially as a single optical element group having one or more elements thereof, which affords greater convenience and compactness. Generally, the inlet filter fits to an inlet of an optical imaging device via a screwthread connection for convenience and reliability.

Preferably the optical apparatus comprises the laser apparatus, and wherein the laser apparatus is arranged and oriented to direct the laser radiation to a focused point that is in front of the optical apparatus. This conveniently provides a single unit which reduces the steps required for the optical magnifying apparatus to be used for Raman spectroscopy.

Preferably the laser apparatus and optical apparatus are positioned relative to one another such that in use the location of the focused point is off-axis with respect to an inlet direction of the optical imaging device, such that the zero-order of the dispersion from the dispersion means is off-axis with respect to a forward direction of the optical imaging device. This has the advantage that a wider range of the spectrum is presented to the image sensor (compared to if the zero order is directed to the middle of the image sensor). This is achieved by arranging them to have optical axes that are non-coaxial and which do not intersect at least throughout an operating distance range of the device, such that the focused point is off-center with respect to the field of view of the optical axis (if they intersect at a much larger distance this would not matter). The operating distance might for example be a particular range with its greatest extent being any value up to 1 meter, or up to 2 meters, or possibly further (this is a matter to be decided by the designer taking into account the need for a strong signal to noise ratio but a desire to keep the user and potentially dangerous sample away from each other).

Preferably the optical apparatus comprises the Inlet optical focusing means by virtue of the optical apparatus providing an optical focusing element having a focal distance selected such that if the optical imaging device is adjusted to focus substantially at infinity, then the effect of the optical apparatus comprising the inlet optical focusing means is a resultant focal distance substantially matching the location of the focused point. This conveniently provides a single unit which reduces the steps required for the optical magnifying apparatus to be used for Raman spectroscopy. An alternative is for the user to adjust the focus of the optical imaging device (typically imaging devices such as optical sights tend to have a built-in focal adjustment), e.g. manually (or via any built in automatic focal adjustment mechanism), however this either more time consuming or more complicated, and is more likely to be subject to greater error. Furthermore since many rifle sights cannot focus to nearby objects, for example being only able to focus as close as 10m, this often will not be so far away that high laser power may be required to achieve good Raman spectroscopy results. Accordingly the addition of positive magnification allows the sight to focus to nearer distances, which facilitates better quality Raman spectroscopy results without resorting to higher laser power. Accordingly it is preferable that the optical focusing element has a focal distance of less than 10m, preferably less than 2m. This enables a sight focused for use at long range for viewing a distance scene (e.g. as would be common in a rifle sight) to be readily adapted for use for nearby stand-off Raman spectroscopy (E.g. at a distance of lm) with at most minor additional focusing or movement of the device needed by the user. It also has the advantage that the laser apparatus may be adjusted to focus at a predetermined distance, and application of the optical apparatus to the imaging device will cause the imaging device to focus at the same or similar distance to the laser apparatus. Preferably the wavelength band filter, the optical dispersion means, and any inlet optical focusing means of the Optical apparatus are arranged as a single optical element group having at least one element, wherein the single optical element group is arranged to pivot in use between a first position covering the front light inlet end of the optical imaging device, and a second position exposing it. This conveniently provides a single unit which reduces the steps required for the optical magnifying apparatus to be used for Raman spectroscopy. In the case that the optical apparatus is arranged as an inlet filter to be fitted to cover the front light inlet end of the optical imaging device, the pivot preferably is about an axis which is perpendicular to an axis of the optical imaging device, such as to be operable as a flap pivotable away and to the side of the front light inlet end of the optical imaging device.

Preferably the transmission wavelength is in the near-infrared range, preferably in the range 800 to 1400nm (e.g. for use with laser illumination in the range 750-800nm) and preferably in the range 1100-1500nm (e.g. for use with laser illumination in the range 1060-1070nm), optionally 1550nm, and optionally 1064nm (e.g. 532nm frequency doubled to 1064nm), although a range including or in the ultraviolet range can also be used. This has the advantage of avoiding lower frequencies where Raman scattering is weaker, whilst avoiding the visible spectrum where harm to the user's eye is most likely.

Preferably there is provided means for automatically varying a coupling strength between the laser apparatus and detection by the second array of charge storage elements to each of a plurality of different coupling strengths, such as to detect a plurality of Raman spectra of differing signal strengths.

Collection of multiple spectra of different strengths is useful, especially if the sensor has low precision (e.g. single bit precision) so as to further narrow down the Raman spectra intensity to greater precision than is offered by a single measurement. Examples of such means include, varying the laser pulse power, varying the amount of laser intensity variation (e.g. varying the peak intensity by varying the pulse duration), variably attenuating the laser power, variably attenuating the camera inlet (attenuation also possible by applying a variable polarization filter in front of the laser, or even in front of the sensor), varying the focal sharpness of the spectra on the image sensor, for example by adjusting the focus of the laser, of the camera, or both, or by moving the laser or the camera or both with respect to object being measured, or by moving the object. Preferably the laser pulse power is varied, as reduction of the pulse power from a maximum pulse power is not only simple but also safe. A simple approach is (as in addition to, or as an alternative to any of the above other methods) to vary an electronic threshold of the (high) frequency band pass filter to adjust it's sensitivity, (the threshold is commonly adjustable to at least four different levels).

In order to maximise the strength of the spectra without increasing the laser power or the camera aperture or reducing the stand-off distance, it is desirable to use a dispersion means that maximizes one non-zero order spectrum, for example an optical prism. This ensures that all the collected Raman photons are directed into one spectrum and thus maximizes the strength of the spectrum. Another advantage is that the prism can be used to optimize the position of the focused point in the field of view of the camera, so that the spectrum can be spread across the whole image sensor to thereby maximise the frequency resolution of the spectrum measurement. Indeed as the sensor is typically square or rectangular, it may be desirable to orient the dispersion means to disperse diagonally across the sensor.

Preferably the pulsed laser apparatus comprises a frequency doubler adapted to be selectively applied to the output of the laser, to double the frequency of output light to the transmission wavelength, and wherein the image sensor elements are sensitive to the near-infrared, preferably to a frequency in the range 700 to 2500nm, preferably 700 to 1700nm, preferably 800 to 1500nm.. This has the advantage that in imaging mode the product or kit can image in the short wave infrared and is able to detect light from the pulsed laser apparatus such that it can be used as a laser designator, whereas in Raman spectroscopy mode the product or kit can be used for Raman spectroscopy using the frequency doubled pulsed laser apparatus and due to the doubled laser frequency the Raman scattering will in the detectable range. Optionally the pulsed laser apparatus is arranged to transmit through the frequency doubler when the optical apparatus is arranged to modify light entering the imaging device, and to transmit without the frequency doubler when the optical apparatus is not arranged to modify light entering the imaging device. This can be achieved for example by arranging both the frequency doubler and the optical apparatus to be jointly movable into position in front of respectively the laser apparatus and the imaging device. This can also be achieved by rotating the polarization of the fundamental beam by simply rotating a waveplate (between the laser and the frequency doubler). This provides for the imaging device to be conveniently switchable between use as an optical sight and use as a Raman spectroscope.

The pulsed laser apparatus may be arranged to generate pulses of polarized light, the pulses having consistent polarization with respect to each other, which improves the quality of the Raman spectroscopy results.

Preferably the image sensor is arranged to output an image from the second array of charge storage elements, and a processor is provided to detect a spectra from the image and to match the spectra to a known material or class thereof, based on a library of known spectra, and an audio or visual output device is provided to indicate to a user, a name or property of the known material or class thereof. As an alternative the image may be sent across a network to be analysed, to provide the name or property of the known material to the user, or 3 rd party equipment having such features may be connected to analyse the image and determine the substance or property. The audio or visual output device may include any of a screen or other digital display (E.g. in an eyepiece), or an audio warning (e.g. "warning, toxic chemical", or "warning, explosive substance").

Preferably the image sensor is adapted to output a first image of a scene from the first array of charge storage elements, and to substantially simultaneously output a second image of a Raman spectrum associated with a point in the scene from the second array of charge storage elements. This has the advantage that a user using a visual display (E.g. a screen or an eyepiece display, which optionally may be part of the optical imaging device), can view the scene and can position the focused point onto a substance or object of interest simultaneously with the optical imaging device producing a Raman spectrum, and the user can see whether the substance/object is at the right distance to produce a focused spot and adjust accordingly (this is partly because the focused point will typically be visible since the wavelength band filter will generally not be perfect, and partly because the focused point will correspond with the zero-order of the Raman spectrum which is not blocked by the wavelength band filter)

Preferably the first and second images are output as a combined image, either additively in one colour channel or in separate colour channels. This has the advantage that a user using a visual display is able to visually determine in real time, whether a strong Raman spectrum is being generated, whilst moving the focused spot around a surface or object of interest, and can adjust accordingly to maximise the quality of the Raman spectrum generated. The term 'additively' means applying any mathematical function, such as adding the values, or adding weighted values (e.g. to make the Raman spectrum more visible). If the visual display supports multiple colour channels then the second image is preferably a different colour than the first image (the colour of the first image may be any colour including greyscale).

Preferably the optical sight is a rifle sight. This has the advantage that military user in the field, carrying a rifle with a rifle sight, can use that rifle sight to perform Raman spectroscopy and thereby determine the chemical nature of substances, such as chemical agents or explosive materials. A rifle sight is sized for fitment to a rifle (such as tactical, sniper or generic rifle), and has a front lens aperture of suitable size for use in rifle shooting, and also is adapted to be resilient to the levels of physical acceleration and shock that a rifle experiences when shot.

According to a second aspect of the present invention there is provided a rifle sight as set out in claim 17. This conveniently provides a single unit which enables the aforementioned type of rifle sight to be upgraded for performing Raman spectroscopy with a pulsed laser of relevant frequency and pulse energy.

Typically the rifle sight adapter has a screwthread attachment for attaching to the objective bell of the rifle sight adjacent to the objective lens. The rifle sight normally has an optical diameter substantially matching that of the size of rifle sight to which it is designed to attach. Such diameters are generally in the range of 20mm to 50mm diameter for use in the visible spectrum, and for use in the infrared can be larger, typically 50 to 100mm. Accordingly the rifle sight adapter preferably has an optical inlet path of 20 to 100mm diameter.

Preferably the rifle sight adapter comprises an inlet optical focusing means provided by virtue of the rifle sight adapter providing an optical focusing element, arranged to adjust the focal distance of the rifle sight from substantially infinity to a predetermined distance for performing stand-off Raman spectroscopy. This conveniently adapts the focal distance of the rifle sight for use in Raman spectroscopy as soon as the adapter is set in place on the rifle sight, rather than requiring the user to adjust the focal distance of the rifle sight manually each time, and then to re-adjust the sight back for use with a rifle.

Preferably the wavelength band filter, optical dispersion means and any inlet optical focusing means thereof are provided as a single optical element group having at least one element, wherein the single optical element group is arranged to pivot in use between a first position covering a front light inlet end of a rifle sight, and a second position exposing it. This conveniently provides a single unit which enables the rifle sight to switch back and forth between rifle mode and Raman spectroscopy mode.

A preferred embodiment of the present invention will now be described by way of example only, with reference to the figures in which:

Figure 1 is a diagram illustrating a prior art approach to stand of Raman spectroscopy;

Figure 2 is a diagram illustrating an embodiment of the present invention in which a sight is used to perform Raman spectroscopy;

Figure 3 is a diagram illustrating an embodiment of the present invention with a pulsed laser attached to a sight;

Figure 4 is a diagram illustrating an embodiment of the present invention with an adapter pivoted away from a sight to enable non Raman-spectroscopic use;

Figure 5 is a diagram of an experimental set up demonstrating the effect of one embodiment of the present invention;

Figure 6a is a combined image obtained using the experimental setup shown in figure 5, being the combination of the first (conventional) and second (discriminatingly sensitive to pulsed light) images, with two enlarged views showing on the left the location of the laser spot, and on the right the location of a Raman spectrum peak intensity, corresponding to the dispersion separation expected from the Raman spectrum peak of Sulphur;

Figure 6b is an image obtained using the experimental setup shown in figure 5 but only using the output from the second array of charge storage elements (those discriminatingly sensitive to pulsed light), again with the same two enlarged views (again showing the laser spot and the Raman spectrum intensity peak for Sulphur);

Figure 7 is a diagram of a rifle with a rifle sight adapter according to an embodiment of the present invention;

Figure 8 is a diagram of a rifle sight adapter according to an embodiment of the present invention; and

Figure 9 is a diagram of charge storage elements showing an array of first charge storage elements sets interspersed with an array of second charge storage elements.

Turning to figure 1 a prior art set up for Raman spectroscopy is illustrated. This shows a laser 1 transmitting a beam 2 to a target at a distance (not shown), and an imaging sensor 18 with image sensor elements and with charge storage elements 3 behind a grating 4 and wavelength filter 5 in a housing 6. The laser beam may comprise a continuous wave laser beam, or laser pulses (long pulse illustrated schematically at 2 - not to scale). The distance to the focused spot may be up to 200m, but for reasonable quality results and convenient stand-off application, is preferably between 10cm and 4m, but preferably between 20cm and 2m, generally in the order of 50cm to 100cm is optimal.

The light returned from the sample includes both the laser frequency and some Raman scattered photons. The laser frequency is filtered out by the wavelength filter 5 and the Raman scattered photons are dispersed by the grating 4 and focused by the lens 7 onto the image sensor (dotted line shows the zero order dispersion, and solid lines show the Raman spectrum).

The image sensor has an array of charge storage elements, e.g. a ID array, and the charge collected on these elements is output 8 as an image resulting from the laser light (long pulse waveform is shown here to emphasize that the image is the result of light that is either continuous wave radiation or else not of especially short pulse duration). Features that are in common between figures 1 to 4 are referred to by the same numeric reference irrespective of being labelled in the figures.

Turning to figure 2 a similar set up is illustrated, however the laser is specifically a pulsed laser (emphasized by illustration of short pulse 2' - not to scale, and the image sensor has an arrays of charge storage elements 3' which may be a ID array, e.g. shaped as linear strips, but preferably is a 2D array.

Each of the array of charge storage elements 3' is provided with a built-in high frequency band-bass filter, such that the charge storage element will only be reactive to a fast pulse of light (multiple photons in quick succession). Thus the charge storage array behaves very differently to a conventional sensor array - a conventional sensor array is sensitive to light falling on it at any time during the exposure, whereas this sensor array is only sensitive to pulsed radiation, having a variation frequency that is fast compared to the exposure time (typically needs to be very fast) e.g. lOOus, preferably <10us, preferably lus, preferably lOOns, and ideally <10ns). Use of a faster pulse, and use of a sensor with a second array that is sensitive to such fast pulses enables the laser power to be reduced, which helps make the device safer to use.

Since the array of charge storage elements 3' is only sensitive to the amount of high frequency variation/peak in light intensity, rather than to an amount of light per se, this provides a very simple approach (albeit with a more expensive image sensor) that can filter out background radiation, potentially almost completely. However the accuracy with which the intensity of the Raman spectrum can be measured is typically lower, and typically is limited to a single bit (on/off) detection.

This typical weakness in measurement precision, can be overcome by sending pulses arranged to differently excite the charge storage elements of the array 3', and detecting the different Raman spectra produced in the respective output image 8', and from this determining the Raman spectra with greater precision. Ideally this is done with many different intensities of laser pulse, not just one. The simplest way to vary the pulses is to vary the power, however another way is to vary the peak intensity and pulse duration, and another way is to make adjustments that cause the sharpness of the dispersed spectrum on the image sensor to vary - for example by adjusting the focus of the laser, adjusting the focus of the camera, or even by moving the apparatus with respect to the surface/object being sampled or vice versa. Alternatively or additionally, the sensitivity of the frequency band pass filter(s) can be adjusted to any of multiple sensitivity levels.

A preferred approach is to vary the power of the laser, for example by providing an adjustable optical density filter. Since the laser is generally a polarized laser (generally having a consistent pulse to pulse polarization), attenuation of the laser pulse power can be achieved by positioning an adjustable polarization filter in front of the laser. Another approach is to provide an electronically/electrically tuneable waveplate with a polarization to vary the output power. Turning to figure 3, an embodiment of the present invention is shown in which the image sensor has two arrays of sensor elements 3, 3'. The first is conventional and the second is sensitive (as described above) only to pulsed radiation.

Whilst they could be separate arrays, generally they are interspersed. Typically the first array is a rectangular repeating array, and each image sensor element has adjacent to it a respective charge storage element of the second array (so the second array is therefore also a rectangular repeating array). The term rectangular here includes square, which is typically the case. Note that neither the image sensor elements nor the charge storage elements need to be rectangular/square, and may be any reasonably compact shape. Also, even though the arrays are preferably interspersed they may have different geometry, for example where each high frequency sensitive charge storage element has a respective conventional charge storage element adjacent to it, but not vice versa (or optionally the other way around) - however in a typical implementation there is one high frequency sensitive charge storage element for every four (2x2) conventional image sensor elements (e.g. each of which provide their own charge storage element for background radiation). For example the high frequency array 3' may be a ID series of strip shaped detector elements, and the conventional array 3 may be a rectangular grid.

For avoidance of doubt, generally the two arrays of charge storage elements share the same detector, and the separation of charge is done in the read out integrated circuitry (ROIC) (not shown) which is implemented as standard as with any other digital image sensor.

Features that are in common between figures 1 to 4 are referred to by the same numeric reference, irrespective of whether they are labelled in those figures.

In figure 3 laser 1 has a frequency doubler 9 in front of it, as well as focal element 10 to direct the laser pulse 2' to a focus at a predetermined location 11. The use of a frequency doubler is not necessary but is useful because without the frequency doubler the laser can be used as a laser designator in a frequency range that the image sensor elements 3 are sensitive to, but with the frequency doubler in place the frequency of the laser can be such that the Raman spectrum reflected back from the target location 11 is largely in the range of frequencies that the sensor elements 3' are sensitive to (note that usually the sensor elements 3 and 3' are sensitive to the same frequency range but this is not essential).

Using the two arrays of charge storage elements 3, 3' in conjunction with the wavelength filter 4 and dispersion means (the grating) 5, it is possible to receive back an image on the conventional charge storage elements 3 an image of the scene (admittedly an imperfect image due to the dispersion element, but nonetheless a useable image), and a Raman spectrum on the high frequency sensitive charge storage elements 3'. This is shown on the right hand side of figure 3 (and in figure 6) where a scene is visible, and a spectrum is detectable overlaid over the image of the scene.

The author is not aware of any Raman spectroscopy device which additionally can be used as a sight, or vice versa, let alone simultaneously. In this example the output from the image sensor is a combined greyscale image, and the output from the high frequency sensitive charge storage elements 3' is provided as high contrast black and white binary image, to ensure it is readily detectable over the dark greyscale image resulting from the conventional charge storage elements 3. Note that the two outputs are images, not pulses/waveforms - the different shaped waveforms are drawn onto figures 1 to 4 in a figurative (non-literal) manner merely to emphasize what types of waveforms the images result from.

In figure 3 (as in figure 4), the frequency doubler 10 and focusing means is part of an adapter 12 arranged to cover the inlet of a camera (which may be a rifle sight) 6'. The addition of the adapter to the camera 6' enables the camera to switch from a conventional viewing mode to be used for Raman spectroscopy. In this example the adapter has a lens element 13 (or indeed the wavelength filter 4 and grating 5 may be shaped as a lens element 13). The dispersion means (grating) 5 and wavelength filter 4 can usefully be arranged as an element or group, optionally with or providing the lens element 12, and this can be arranged to be moved in front of the camera (rifle sight) 6' and away to expose the camera. In this example this is facilitated by a pivot 14 as shown in figure 4. The frequency doubling crystal (which may be arranged permanently in place) may be arranged to be activated by polarisation switching with a liquid crystal waveplate. The efficiency of the frequency doubling can then be controlled by the tuneable waveplate.

Figure 4 shows the same elements as in figure 3, but with the adapter pivot oriented such that the optical elements are pivoted out of the optical inlet path of the camera/sight 6' (in this case the filter 4, dispersion means 5, and optional lens element/feature 13, and optional frequency doubler 9 and laser focusing element 10 (or alternatively a laser may be provided instead of the frequency doubler 9). This adapter enables the camera having both types of charge storage element 3, 3' to switch between a conventional viewing mode, as shown in figure 4, and a stand-off Raman-spectroscopy mode (optionally showing a combined view and spectrum output), as shown in figure 3.

Turning to figure 5, a diagram of an experimental setup is shown, which the author has used to verify that the approach works. This shows a laser with polarizer and frequency doubler focused through 800nm short pass filter, directed to focus on a sample (in this case Sulphur which is known to provide a clear Raman spectrum). Also directed to and focusing on the sample is a camera and lens, with a grating and 800nm long pass filter. The grating disperses the light to present a spectrum onto the camera sensor.

Figure 6a shows an image generated using the experimental setup of figure 5. The image results from the combination of a first array of conventional pixels (dark greyscale image showing a bench with a target about 1 meter away) and a second array of pixels each of which has a built in electronic high frequency bandpass filter arranged to provide a binary response detection specific to high frequency light pulses. Since the laser pulse was a very short pulse, this shows up clearly as a black and white image showing the first order Raman spectrum of Sulphur (which can be seen to the side of a bright dot representing the zero order of the Raman spectrum and also representing the focused spot of the laser pulse). Two enlarged views are shown below in which the zero order and first order peak can be shown. Figure 6b shows an image generated from the same experimental setup of figure 5, but only showing the output from the second array of charge storage elements - i.e. those that are discriminatively sensitive to pulsed light. Here the zero order (on the left) and first order (on the right) peaks are clearly visible, showing that using this kind of sensor, and a pulsed laser, stand-off Raman spectrum identification of a Sulphur sample is readily achievable.

To the author's knowledge this is the first example of an imaging device producing both a useable image of a scene, and a useable Raman spectrum of a sample within the scene. And also represents an advance in simplicity of the equipment required for Raman spectroscopy. By repeating the process with successively weaker laser pulses, the intensity of the Raman spectrum can be determined, rather than relying on a binary image in which intensity is limited to one bit.

Additionally or alternatively, improved precision in measuring the intensity across the spectrum can be achieved by measuring the neighboring charge storage elements (those away from the line of the spectrum - in this case immediately above and below the line), as when those charge storage elements are triggered to store charge, this indicates a stronger intensity than if only one charge storage element is triggered to store charge).

Furthermore, by arranging the focused point off-axis and using a stronger dispersion element (grating with more closely spaced lines, e.g. 600 lines/mm rather than 300), a larger part of the Raman spectrum can be measured, and indeed to maximise this further it is advantageous to arrange the dispersion diagonally across the rectangular field of view of the image sensor.

Turning to figure 7, a tactical rifle is shown according to an embodiment of the invention, illustrating the positioning of a rifle sight according to an embodiment of the invention, which is advantageously of the type described above. A conventional sight (having an image sensor of the type described above) can be used with the adapter according to an embodiment of the invention, as described above.

Finally figure 8 shows an adapter (inside dotted line box) according to an embodiment of the present invention. This example includes an optional laser, an optional focusing element, an optional pivot, and means for attaching it to a rifle sight (which may optionally be present).

As an example of a suitable sensor which has the required characteristics, reference is directed to the products Cardinal 640 ™ and Cardinal 1280 ™. Technical details of the Cardinal 1280 ™ are available at time of writing, in the publication "High Definition lOpm pitch InGaAs detector with Asynchronous Laser Pulse Detection mode" which was available at https://scdusa-ir.com/wp-content/uploads/2017/08/Cardinal-HD .pdf and the same paper also available at https://hobbydocbox.com/Photography/91005284-High-definition -10um-pitch-ingaas-detector- with-asynchronous-laser-pulse-detection-mode.html

Accessed Jan 2020, and 13 th Jan 2021, and incorporated herein by reference. This describes that in the Cardinal 1280 ™ "The image data is converted by the ADCs at 13 bit resolution and packed with the ALPD bit to form a 14 bit word . ALPD function is common to a group of 4 pixels as depicted in the figure. Each one of the 4 diodes can detect the laser pulse and activate the ALPD bit (and of course it can be done by more than one diode simultaneously)... [whereas] in the Cardinal 640 ™ it was implement for each pixel..." This document is incorporated herein by reference.

Further detail regarding the operation and sensitivity of such an image sensor according to one embodiment is described in: https://www.imagesensors.org/Past%20Workshops/2017%20Worksho p/2017%20Papers/P39.pdf Accessed 13 th Jan 2021 and incorporated herein by reference.

Suitable sensors have been available on the market, as described at: https://www.laserfocusworld.com/detectors-imaging/article/16 558806/sierraolympic-to-showcase- visibleswir-video-camera-at-spie-photonics-west-2016

Accessed Jan 2020 and 13 th Jan 2021.

These examples are sensors that are sensitive in the short wave infrared band (and therefore will also be sensitive to higher frequencies of light (e.g. visible light) if not filtered out). However the approach could equally well be applied in other parts of the spectrum, such as additionally or alternatively in the visible spectrum and additionally or alternatively in the ultraviolet spectrum.

More generally speaking, apparatus is provided for stand-off Raman spectroscopy, comprising optical imaging device (e.g. a sight, e.g. of a rifle), Laser, Optical apparatus (optionally in the form of an adapter) with a wavelength band filter and optical dispersion means (E.g. grating) to disperse a Raman spectrum to an image sensor, and focusing means, characterized in that the laser is a pulsed laser, and each of the charge storage elements of the array comprises a frequency band pass filter for detecting light pulses in preference to background radiation. This may be performed in the short wave infrared spectrum. The image sensor can also have conventional charge storage elements, enabling viewing of a scene whilst simultaneously obtaining a Raman spectrum of a target in the scene. Advantageously this may be implemented in a rifle sight, optionally by means of an adapter arranged so that the rifle sight can be readily switched from conventional viewing mode to stand off Raman spectroscopy mode.