A multi-resolution optical filter

Abstract

A multi-resolution optical filter, comprising: a substrate; and more than one linear variable filter region formed on the substrate, wherein each linear variable filter region comprises a designated spectral slope corresponding to a designated spectral range of the linear variable filter region, and wherein each linear variable filter region comprises: a first mirrored layer formed on the substrate; a spacer layer formed on the first mirrored layer; and a second mirrored layer formed on the spacer layer, wherein the designated spectral slope is constant for the linear variable filter region and defined by change in the spacer layer thickness along one axis.

Description

A multi-resolution optical filterTechnical Field

[0001] The present invention relates to a multi-resolution optical filter; an imaging system comprising a multi-resolution optical filter and an optical detector array, optically coupled to the multi-resolution optical filter, to detect optical radiation from the multiresolution optical filter; and a method of fabricating a multi-resolution optical filter.Background of Invention

[0002] A Linear Variable Filter (LVF) is a two-dimensional optical filter characterised by an increase in optical thickness across one axis. By linearly increasing the optical thickness, the wavelength of light that is transmitted through each position of the filter also increases linearly.

[0003] An LVF, when placed in front of a Focal Plane Array (FPA), enables a means of generating hyperspectral imagery from a compact system. In its simplest form, the filter is formed through two reflective mirrors and a wedged spacer deposited on a glass substrate providing the increase in optical thickness along one axis.

[0004] Typically, LVF's are characterised using the following specifications:• Spectral Resolution (nm) - the width of the transmitted mode at each wavelength;• Transmission Efficiency (%) - the peak optical efficiency at each wavelength; and• Spectral Slope (nm / mm) - the change in transmitted wavelength across the filter.

[0005] For example, an LVF filter may allow both shorter wavelengths (e.g. blue light) and longer wavelengths (e.g. red light) to pass through with high transmission efficiency. This continuum of spectral samples is used to generate hyperspectral imagery, but there are drawbacks associated with LVF's. Spectral cross-talk may occur when adjacent spectral bands overlap and a LVF may have a limited wavelength range and resolution.

[0006] A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that the document or matter was known or that the information it contains was part of the common general knowledge at the priority date ofany of the disclosure or claims herein. Such discussion of prior art in this specification is included to explain the context of the present invention in terms of the inventors' knowledge and experience.Summary of Invention

[0007] According to one aspect of the present invention, there is provided a multiresolution optical filter, comprising: a substrate; and more than one linear variable filter region formed on the substrate, wherein each linear variable filter region comprises a designated spectral slope corresponding to a designated spectral range of the linear variable filter region, and wherein each linear variable filter region comprises: a first mirrored layer formed on the substrate; a spacer layer formed on the first mirrored layer; and a second mirrored layer formed on the spacer layer, wherein the designated spectral slope is constant for the linear variable filter region and defined by change in the spacer layer thickness along one axis.

[0008] In an embodiment, the spectral slope is shallower for a spectral range of interest for the multi-resolution optical filter than for a spectral range that is of less interest.

[0009] In addition, or in an alternative embodiment, the spectral slope is further defined by change in the first mirrored layer thickness and or the second mirrored layer thickness along the one axis.

[0010] The multi-resolution optical filter is thus a composition of multiple linear variable filters (LVF's) that each have designated constant spectral slopes that are defined by a linear change in the spacer layer thickness along one axis. Each of the multiple LVFs are linear variable filter regions as they are continuously formed on the substrate.

[0011] The use of multiple linear regions allows controlled spectral sampling across selected wavelength ranges, allowing regions of key importance to be sampled with lower linear slopes. This configuration supports improved signal-to-noise ratio (SNR) performance in, for example, hyperspectral imaging systems through a simple post-processing (e.g. running averages).

[0012] Benefits of the multi-resolution optical filter further include the downstream benefits in tailored hyperspectral imaging - specifically the improvement in system SNR in critical bands.

[0013] Preferably, the spacer layer for each linear variable filter region is formed continuously on the first mirrored layer. The first mirrored layer for each linear variable filter region is formed continuously on the substrate. The second mirrored layer for each linear variable filter region is formed continuously on the spacer layer.

[0014] In an embodiment, the first mirrored layer and the second mirrored layer comprise Silver. It will be appreciated by those persons skilled in the art that the first mirrored layers may comprise alternative materials, such as Aluminium or Gold.

[0015] In an embodiment, each linear variable filter region further comprises a Nichrome layer (NiCr) formed on the first mirrored layer to protect the Silver in the first mirrored layer when forming the spacer layer.

[0016] The spacer layer may comprise Niobium Oxide (NbzOs). It will also be appreciated that the spacer layer may comprise alternative materials, preferably being a dielectric material with a high refractive index between 1.6 - 2.5, such as include oxides or nitrides of Aluminium, Titanium, Tantalum, and Niobium.

[0017] In an embodiment, each linear variable filter region further comprises a Silicon Dioxide layer (SiOz) formed on the second mirrored layer to protect the Silver in the second mirrored layer.

[0018] According to another aspect of the present invention, there is provided an imaging system, comprising: the above multi-resolution optical filter; and an optical detector array optically coupled to the multi-resolution optical filter to detect optical radiation from the multi-resolution optical filter.

[0019] Preferably, a composite spectral range of the designated spectral range of each linear variable filter region comprises a hyperspectral range and the imaging system produces a hyperspectral image.

[0020] In an alternative embodiment, a composite spectral range of the designated spectral range of each linear variable filter region comprises a multispectral range and the imaging system produces a multispectral image. It will be appreciated by those persons skilled in the art that a multispectral range is a non-continuous spectral range.

[0021] In an embodiment, the imaging system signal-to-noise ratio (SNR) is improved by the multi-resolution optical filter through averaging over the composite spectral range.

[0022] In an embodiment, the optical detector array comprises a Focal Plane Array (FPA).

[0023] In an embodiment, the imaging system further comprises an Order Sorting Filter(OSF) optically coupled to the multi-resolution optical filter to filter selected transmission modes of the optical radiation from the multi-resolution optical filter.

[0024] According to another aspect of the present invention, there is provided a method of fabricating the multi-resolution optical filter, comprising: positioning a mask relative to the substrate; depositing the spacer layer on the first mirrored layer using Physical Vapor Deposition (PVD) in a vacuum environment; and dynamically adjusting the speed of the mask translation while depositing the spacer layer to produce the change in the spacer layer thickness along the one axis on the substrate.

[0025] In an embodiment, the speed of the mask translation is programmable prior to depositing the spacer layer.

[0026] In an embodiment, the mask is mechanically coupled to a high-precision stepper motor via a series of speed reducers to provide precise linear motion of the mask.

[0027] In an embodiment, the method further comprises positioning the mask adjacent the substrate. For example, the distance is 4mm. The distance could potentially be reduced by redesigning a new masking system or by introducing a more robust cooling system to protect the first mirrored layer from heat radiation damage.

[0028] In an embodiment, the method further comprises continuously cooling the substrate with a vacuum-compatible water chiller.

[0029] Preferably, the method further comprises continuously forming the more than one linear variable filter region on the substrate. The method further comprises depositing the spacer layer continuously on the first mirrored layer for each linear variable filter region. The method further comprises depositing the first mirrored layer for each linear variable filter region continuously on the substrate. The method further comprises depositing the second mirrored layer continuously on the spacer layer for each linear variable filter region.

[0030] The multi-resolution filter has more than one region of different linear spectral slope. In the embodiment, by depositing the filter with a shallower spectral slope over key wavelengths regions, the effective instrument signal-to-noise ratio (SNR) can be improved in this region through averaging, thus providing an extra degree of design freedom in the development of a hyperspectral imaging system.Brief Description of Drawings

[0031] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

[0032] Figure 1 shows a multi-resolution optical filter according to an embodiment of the present invention;

[0033] Figure 2 shows a multi-resolution optical filter fabrication masking system according to an embodiment of the present invention;

[0034] Figure 3 shows a multi-resolution optical filter deposition specification according to an embodiment of the present invention;

[0035] Figure 4 shows a spectral scan of a multi-resolution optical filter showing variable spectral slopes according to an embodiment of the present invention;

[0036] Figure 5 shows transmission curves for the multi-resolution optical filter of Figure 4;

[0037] Figure 6 shows a multi-resolution optical filter design for measuring algal blooms according to an embodiment of the present invention;

[0038] Figure 7 shows a multi-resolution optical filter design for measuring crop stress according to an embodiment of the present invention;

[0039] Figure 8 shows a flow chart of a method of fabrication a multi-resolution optical filter according to an embodiment of the present invention;

[0040] Figure 9 shows a schematic of an imaging system comprising a multi-resolution optical filter according to an embodiment of the present invention;

[0041] Figure 10a shows a theoretical spectral response for a traditional linear filter

[0042] Figure 10b shows a theoretical spectral response for a prototype filter according to an embodiment of the present invention; and

[0043] Figure 11 shows a comparison of the predicted SNR of two instrument configurations.Detailed Description

[0044] An embodiment prototype of a multi-resolution optical filter 10 is shown in Figure 1. The filter 10 comprises a substrate 12 and a first linear variable filter region 14 and a second linear variable filter region 16 formed on the substrate 12. The first linear variable filter region 14 comprises a first designated spectral slope 18 corresponding to a designated spectral range of the first linear variable filter region 14. The second linear variable filter region 16 comprises a second designated spectral slope 20 corresponding to a designated spectral range of the second linear variable filter region 16. As mentioned above, it will be appreciated by those persons skilled in the art that more than two linear variable filter regions can also be provided on a multi-resolution optical filter.

[0045] The prototype multi-resolution optical filter 10 is thus a multi-resolution Linear Variable Filter (LVF) and it has an increase in optical thickness across one axis for more than one region, each having different linear spectral slopes.

[0046] Each of the first linear variable filter region 14 and the second linear variable filter region 16 comprises a first mirrored layer 22 formed on the substrate 12, a spacer layer 24 formed on the first mirrored layer 22, and a second mirrored layer 26 formed on the spacerlayer 24. The designated spectral slopes 18 20 are defined by a change in the spacer layer 24 thickness along one axis. The thickness of the first 22 and second 26 mirrored layers in the protype, on the other hand, is relatively constant. The spacer layer 24 is formed continuously on the first mirrored layer 22 and its thickness changes along one axis by adjusting the speed of mask translation while depositing the spacer layer 24 for each linear variable filter region.

[0047] That is, the first designated spectral slope 18 and the second designated spectral slope 20 are slopes that are constant and are defined by different linear changes in the spacer layer 24 thickness along one axis.

[0048] Multiple materials can be used for both the mirror and spacer layers depending on both system requirements and deposition capabilities. For the mirror layers, using thin (10 - lOOnm) metallic coatings can enhance the optical transmission of the LVF, example materials include Silver (Ag), Aluminium (Al), or Gold (Au). For the spacer layer, a dielectric material with a high refractive index between 1.6 - 2.5 is preferred, examples include oxides or nitrides of aluminium, titanium, tantalum, and niobium.

[0049] In the prototype filter 10, the first 22 and second 26 mirrored layers use Silver (Ag), and Niobium Oxide (NbzOs) as the spacer layer 24. A further Nichrome (NiCr) layer is formed on the first mirrored layer 22 to protect the Silver during the deposition of Niobium Oxide in the spacer layer 24. A further Silicon Dioxide (SiO2) layer is formed on the second mirrored layer 26 to prevent oxidization of the Silver in the second mirrored layer 26. The two separately sloped linear variable filter regions were fabricated in the protype to demonstrate the feasibility of manufacturing this filter; however, it should be noted that more spectral slopes can be deposited if required by the application.

[0050] More specific parameters of the protype filter 10 are provided below. The first designated spectral slope 18 is 16.3 nm / mm which corresponds to a designated spectral range of 420 - 550nm of the first linear variable filter region 14. The second designated spectral slope 20 is 40.5 nm / mm which corresponds to a designated spectral range of 550 - 750nm of the second linear variable filter region 16. The spectral slope of the of the first linear variable filter region 14 is shallower than the slope of the second linear variable filter region 16. The filter 10 is thus designed with the spectral range of 420 - 550nm being of more interest than the spectral range of 550 - 750nm. The effective signal-to-noise ratio(SNR) is improved in the 420 - 550nm region through averaging; thus, providing an extra degree of design freedom in the development of a hyperspectral imaging system as shown in Figure 9. The prototype filter 10 had a spectral resolution between 10-20 nm and transmission efficiency of 15-20%. Measurements were conducted using a Bentham DTMc300 monochromator and integrating sphere

[0051] A summary of a method 28 of fabricating a multi-resolution optical filter 10 is show in Figure 8. The method 28 comprises positioning 30 a mask relative to the substrate 22, depositing 32 the spacer layer 24 on the first mirrored layer 22 using Physical Vapor Deposition (PVD) in a vacuum environment, and dynamically adjusting 34 the speed of the mask translation while depositing the spacer layer 24 to produce the change in the spacer layer 24 thickness along the one axis on the substrate 12.

[0052] In an example, the filter 10 is fabricated utilising the above method 28 using PVD, which is a highly controlled thin-film deposition technique conducted in a vacuum environment.

[0053] In the example, the method 28 employs a 3-inch target as a source material and the mask is positioned 4mm above the substrate 12.

[0054] Figure 2 shows an example of a multi-resolution optical filter fabricating masking system 25 performing the method 28. It will be appreciated by those persons skilled in the art that further aspects of the method 28 will be apparent from the below description of the system 25.

[0055] A custom-designed rectangular mask 27 shown in Figure 2 is mechanically coupled to a high-precision stepper 29 motor via a series of speed reducers. The speed reducers, which may include gear systems or other mechanical advantage devices, serve to convert the relatively high-speed rotation of the stepper motor into slower, more precise linear motion of the mask 27.

[0056] The mask 27 of the masking system 25 is strategically positioned in front of the substrate 12, maintaining a critical gap of 4mm between the substrate 12 and the mask 27, with the substrate 12 continuously cooled with a vacuum-compatible copper-piped water chiller. This gap is optimised to balance the need for precise deposition control and substrate stability with a minimum heat constraint.

[0057] The stepper motor 29 is connected to an external controller 31 through a specialised vacuum feedthrough, which is designed to maintain the integrity of the vacuum while allowing for precise electrical and mechanical control from outside the chamber. Persons skilled in the art will appreciate that steps of the method 28 are embodied in software, or program code, that can be supplied to the controller 31 in a number of ways, such as on a memory. The software could also be supplied by any possible data transfer method, such as downloading.

[0058] The method 28 of fabricating a multi-resolution optical filter, such as the protype multi-resolution optical filter 10, continuously forms each of the filter regions 14 16 on the substrate 12 using the masking system 25. The method 28 does so by depositing the first mirrored layer 12 for each linear variable filter region continuously on the substrate 12, depositing the spacer layer 24 continuously on the first mirrored layer 22 for each linear variable filter region, and depositing the second mirrored layer 26 continuously on the spacer layer 24 for each linear variable filter region.

[0059] The speed of the mask translation controlled by the controller 31 is not only programmable prior to deposition but also dynamically adjustable during the deposition process. This feature produces variable deposition slopes during the deposition process to produce a multi-resolution linear variable filter. The ability to modify the deposition slope in situ provides predictable control over the filter's spectral properties, enabling the creation ofhigh resolution of images in the hyperspectral ranges when the filter is used in an imaging system shown in Figure 9.

[0060] The deposition design of an embodiment of a multi-resolution optical filter is shown in Figure 3. In this design specification, two distinct deposition gradients were specified based on the correlation between peak wavelength displacement and physical distance: 12 nm / mm in the shorter wavelengths and a steeper gradient of 28 nm / mm in the long wavelength region, which is of less interest, over a total substrate length of 20mm.

[0061] Another prototype multi-resolution optical filter, e.g. a multi-resolution LVF, was developed and the corresponding optical scan produced by the filter is shown in Figure 4. In the design specification of this prototype, the change of slope between linear variable filter regions occurred at 8mm along the filter. The scan shown in Figure 4 clearly shows a 'knee' present at 8mm on the filter corresponding to the change in slope. The filter shows a peak transmission of ~20% and a spectral resolution of 10 - 20 nm across the spectral range. Each line on the plot corresponds to a different transmission mode. When used in a hyperspectral imaging system, all but one would be filtered using a separate order sorting filter (OSF).

[0062] In this prototype, the filter was positioned in front of a focal plane array (FPA), allowing the spectral response to be measured at discrete wavelengths. The monochromator was swept from 400 nm - 900 nm, with 10 frames capture at each wavelength using a long exposure time (300 ms) and high gain (10 dB) to compensate for the low source intensity. The resulting measurements are shown in Figure 4. The mapping of input wavelengths to position highlights the various constant slopes for each region.

[0063] The use of multiple linear regions thus allows controlled spectral sampling across selected wavelength ranges and provides improved signal-to-noise ratio (SNR) performance hyperspectral imaging systems using multi-resolution optical filter 10.

[0064] By looking at the transmission curves of the primary transmission mode in Figure 5, with a 40nm spacing, the change in spectral sampling can be seen in the lower slope regions (left) against the higher slope regions (right). The blacked dashed line is the transition between the two sloped regions.The results show that this prototype filter exhibits multiple modes as a two-mirror LVF design. These secondary modes would need to be suppressed in a final system using an order-sorting filter (OSF). The primary transmission mode spans approximately 220-750 nm and displays two distinct spectral slopes, with a designed transition point (or "knee") around 600 nm. This behaviour is also evident in the instrument SRFs, sampled at uniform spectral intervals, which show varying spacing around the knee point. From these curves the spectral resolution, defined as the full width at half maximum (FWHM), is measured to range between 10 - 20 nm across the spectral range.

[0065] Two further example filter designs tailored for specific environmental monitoring applications are described with reference to Figure 6 and 7. If manufactured, these filter designs could be integrated with a standard silicon Focal Plance Array (FPA) and lens to form a tailored hyperspectral imaging system.

[0066] In the examples referenced in Figure 6 and 7, the spectral range, slopes, and materials proposed are comparable to those produced in the prototype filter shown in Figure 1.

[0067] The filter design of Figure 6 is suited to the detection of potentially harmful algal blooms. The filter design could also be used to monitor crop stress. Potentially harmful algal blooms are commonly caused by the rapid growth of cyanobacteria in inland waterbodies, and the detection of these blooms in their early development are crucial to maintaining safe drinking and recreational waters. Cyanobacteria is commonly identified through critical phycocyanin and chlorophyll-a pigments. This can be seen at 625nm, 650nm, 670nm, 724nm in Figure 6. Other ancillary bands at 412nm, 555nm, 748nm, 865nm are captured as they are critical to support measuring the confounding water quality parameters and performing atmospheric correction.

[0068] In the filter design shown in Figure 6, the designated slope of the region comprising the 625nm, 650nm, 670nm, 724nm bands is shallower than the surrounding regions, and extends from a position around 2.8mm to 7.8mm on the filter. The filter size is approximately 10mm.

[0069] The filter design of Figure 7 is suited to detecting crop stress. Active monitoring of crop stress allows for the early detection of plant health issues, enabling timely intervention to minimize loss in the agricultural industry. Stress is commonly monitored using the red-edge (687nm, 700 - 740nm) but there are other ancillary bands (515nm, 550nm, 570nm, 650nm, 760nm, 855nm) that are also used to support measurements of critical pigments and total biomass. This design focuses on capturing the red-edge in finer detail whilst still providing those key ancillary bands.

[0070] In the filter design shown in Figure 7, the designated slope of the region comprising the 687nm, and 700 - 740nm bands is shallower than the surrounding regions, and extends from a position around 2.2mm to 7.2mm on the filter. The filter size is approximately 10mm.

[0071] These filter designs could be implemented in an imaging system to provide hyperspectral images to detect algal blooms and crop stress. An example of an imaging system 36 is shown in Figure 9. The imaging system 36 comprises an multi-resolution optical filter, such as the protype multi-resolution optical filter 10 described above, and an optical detector array 38 optically coupled to the multi-resolution optical filter 10 to detect optical radiation from the multi-resolution optical filter 10. The optical detector array 38 comprises a Focal Plane Array (FPA). The imaging system 36 further a lens 8 optically coupled to the multi-resolution optical filter 10 to focus incoming radiation on the filter 10.

[0072] The imaging system 36 produces a hyperspectral range that is a composite spectral range of the designated spectral range of each linear variable filter region. The imaging system 36 thus produces a hyperspectral image.

[0073] The imaging system 36 further comprises an Order Sorting Filter (OSF) 40 optically coupled to the multi-resolution optical filter 10 to filter selected transmission modes of the optical radiation from the multi-resolution optical filter 10 before transmission to the FPA.

[0074] The prototype filter 10 has been developed with two spectral deposition slopes that could be integrated with the FPA, lens, and OSF to produce a hyperspectral imaging system 36. While the slopes of the prototype filter 10 were arbitrarily selected to prove thefeasibility of manufacture, more tailored slopes could be designated and programmed for the controller 31 for a design depending on the end-application of the filter.

[0075] Further, the imaging system 36 signal-to-noise ratio (SNR) is improved by the multi-resolution optical filter 10 through averaging over the composite spectral range.

[0076] Embodiments of the imaging system 36 are integrated into drone and aircraft platforms, and may be used with advanced inversion algorithms for environmental monitoring.

[0077] Performance validation results from simulation studies indicate a ~40% improvement in chlorophyll-a retrieval accuracy using multi-resolution LVF designs. These simulations utilized Hydrolight and MODTRAN to model water-leaving radiance and atmospheric effects.

[0078] In an embodiment, a case-study simulation that examines the potential benefits of using a multi-resolution LVF for measuring chlorophyll-a in inland waterbodies was conducted. Chlorophyll-a is a key indicator of water quality that can be monitored using optical Earth Observation instruments. Elevated concentrations are typically associated with increased algal biomass and a heightened risk of harmful algal blooms.

[0079] Instrument performance was assessed using radiative transfer modelling to generate representative remote sensing reflectance (Rrs) spectra. Input inherent optical properties (lOPs), representative of Australian inland drinking water reservoirs were used to parameterise a four-component optical model in Hydrolight 6.0. Rrs spectra were produced across the 25th - 75th quantile ranges, assuming a solar zenith angle (SZA) of 30°.

[0080] The reflectance spectra were converted to top-of-atmosphere (TOA) radiance using MODTRAN-6. This atmospheric radiative transfer model accounts for molecular and aerosol absorption and scattering, as well as relative solar illumination, to simulate the signal received by the sensor. A mid-latitude summer atmospheric profile was assumed, with a solar zenith angle of 30°, clear-sky conditions, and high visibility.

[0081] Instrument performance is simulated using a black-box model developed in Python, which converts input TOA radiance to a measured radiance, incorporatinginstrument-induced noise. The model is parameterised using a basic set of input variables, with key parameters listed in the Table below.*Specified at 550km altitude

[0082] The model represents the instrument as a point spectrometer and does not account for spatial effects such as vignetting, distortion, or pixel non-uniformity. While such effects are considered during instrument design, this simulation focuses solely on radiometric performance; therefore, these spatial effects are not included, as they are typically not limiting factors in this context.

[0083] In this analysis, the instrument is modelled as a visible to near-infrared imaging spectrometer operating in low Earth orbit, with a GSD of 50m, representative of a CubeSat class instrument.

[0084] Two filter configurations are considered, as shown in Figure 10a and 10b. These figures show theoretical spectral response functions for a traditional linear filter in figure 10a, and a filter with multiple deposition slopes and oversampling from 500 - 600 nm in Figure 10b.

[0085] The multi-resolution LVF configuration provides increased spectral sampling across key chlorophyll-a absorption features spanning 500-600 nm, while also capturing ancillary spectral information below 500 nm and into the near-infrared. These additional bands are critical for resolving confounding water quality parameters and enabling robust atmospheric correction.

[0086] As a quantitative assessment, a comparison of the predicted SNR of the two instrument configurations using 6 nm spectral binning is shown in Figure 11. While the traditional linear response filter provides relatively uniform SNR across the spectral range, the multi-resolution filter exhibits significantly higher SNR in the oversampled bands of interest. This targeted enhancement allows greater flexibility in optimising instrument performance for specific application needs.

[0087] To assess the ability of each instrument configuration to detect changes in chlorophyll-a concentration, a linear retrieval algorithm is applied centred around the median chlorophyll-a value. For small perturbations in radiance, retrieval is optimised using a matched filter algorithm. This approach enables direct quantification of the impact of instrument configuration on the lo measurement error.

[0088] This analysis accounts for instrument noise but does not incorporate other sources of uncertainty typically associated with satellite-derived data products, such as radiometric calibration errors, atmospheric correction residuals, or retrieval algorithm limitations. These factors are expected to increase measurement uncertainty in real-world scenarios. Therefore, the errors reported here represent a best-case, driven solely by instrument induced noise.

[0089] The results of this analysis, presented in the Table below, show that oversampling the spectral response in key bands used for chlorophyll-a retrieval leads to an approximate 40% improvement in measurement accuracy compared to the traditional LVF configuration.

[0090] Where any or all of the terms "comprise", "comprises", "comprised" or "comprising" are used in this specification (including the claims) they are to be interpreted as specifying the presence of the stated features, integers, steps or components, but not precluding the presence of one or more other features, integers, steps or components.

[0091] Finally, while the invention has been described in conjunction with a limited number of embodiments, it will be appreciated by those skilled in the art that many alternative modifications and variations in light of the foregoing description are possible. Accordingly, the present invention is intended to embrace all such alternative, modifications and variations as may fall within the spirit and scope of the invention as disclosed, such as different materials for each of the components of the multi-resolution optical filter.

Claims

The claims defining the invention are as follows:

1. A multi-resolution optical filter, comprising: a substrate; and more than one linear variable filter region formed on the substrate, wherein each linear variable filter region comprises a designated spectral slope corresponding to a designated spectral range of the linear variable filter region, and wherein each linear variable filter region comprises: a first mirrored layer formed on the substrate; a spacer layer formed on the first mirrored layer; and a second mirrored layer formed on the spacer layer, wherein the designated spectral slope is constant for the linear variable filter region and defined by change in the spacer layer thickness along one axis.

2. A multi-resolution optical filter of claim 1, wherein the spectral slope is shallower for a spectral range of interest for the multi-resolution optical filter than for a spectral range that is of less interest.

3. A multi-resolution optical filter of claims 1 or 2, wherein the first mirrored layer and the second mirrored layer comprise Silver.

4. A multi-resolution optical filter of claim 3, wherein each linear variable filter region further comprises a Nichrome layer (NiCr) formed on the first mirrored layer to protect the Silver in the first mirrored layer when forming the spacer layer.

5. A multi-resolution optical filter of claim 4, wherein the spacer layer comprises Niobium Oxide (NbOs).

6. A multi-resolution optical filter of any one of claims 3 to 5, where each linear variable filter region further comprises a Silicon Dioxide layer (SiOz) formed on the second mirrored layer to protect the Silver in the second mirrored layer.

7. A multi-resolution optical filter of any one of claims 1 to 6, wherein the more than one linear variable filter region is continuously formed on the substrate.

8. A multi-resolution optical filter of claim 7, wherein the first mirrored layer for each linear variable filter region is formed continuously on the substrate.

9. A multi-resolution optical filter of claim 8, wherein the spacer layer for each linear variable filter region is formed continuously on the first mirrored layer.

10. A multi-resolution optical filter of claim 9, wherein the second mirrored layer for each linear variable filter region is formed continuously on the spacer layer.

11. An imaging system, comprising: a multi-resolution optical filter of any one of the above claims; and an optical detector array optically coupled to the multi-resolution optical filter to detect optical radiation from the multi-resolution optical filter.

12. An imaging system of claim 11, wherein a composite spectral range of the designated spectral range of each linear variable filter region comprises a hyperspectral range and the imaging system produces a hyperspectral image.

13. An imaging system of any one of claim 11 or 12, wherein the imaging system signa I- to-noise ratio (SNR) is improved by the multi-resolution optical filter through averaging over the composite spectral range.

14. An imaging system of any one of claims 11 to 13, wherein the imaging system further comprises an Order Sorting Filter (OSF) optically coupled to the multi-resolution optical filter to filter selected transmission modes of the optical radiation from the multi-resolution optical filter.

15. A method of fabricating a multi-resolution optical filter of any one of claims 1 to 10, comprising: positioning a mask relative to the substrate; depositing the spacer layer on the first mirrored layer using Physical Vapor Deposition (PVD) in a vacuum environment; and dynamically adjusting the speed of the mask translation while depositing the spacer layer to produce the change in the spacer layer thickness along the one axis on the substrate.

16. A method of claim 15, wherein the speed of the mask translation is programmable prior to depositing the spacer layer.

17. A method of claim 15 or 16, wherein the mask is mechanically coupled to a high- precision stepper motor via a series of speed reducers to provide precise linear motion of the mask.

18. A method of any one of claims 15 to 17, further comprising continuously cooling the substrate with a vacuum-compatible water chiller.

19. A method of any one of claims 15 to 18, further comprising continuously forming the more than one linear variable filter region on the substrate.

20. A method of claim 19, further comprising depositing the spacer layer continuously on the first mirrored layer for each linear variable filter region; depositing the first mirrored layer for each linear variable filter region continuously on the substrate; and depositing the second mirrored layer continuously on the spacer layer for each linear variable filter region.

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