Integral Field Spectral Imager

An integral field spectral imager has a plurality of optical homogenizers. Each optical homogenizer is in-register with a corresponding different superpixel in a superpixel array and is configured to spatially homogenize incident EMR and to pass the spatially homogenized EMR to a spectral filter in an array of spectral filters, thence to the in-register, corresponding different superpixel. Baffles are included to maximize confinement of the spatially homogenized EMR passed by a single optical homogenizer to the in-register, corresponding different superpixel so as to minimize crosstalk between superpixels. Optical homogenizers and baffles are designed to produce a pattern of homogenized EMR on a superpixel, regardless of where incident EMR is received on an optical homogenizer. Methods for using embodiments of the spectral imager in a variety of spectral bands in the EMR spectrum enable determining spectral information about incident EMR.

TECHNICAL FIELD

The invention relates to spectral imaging, in particular to an integral field spectral imager and to methods for using the spectral imager to determine spectral information about incident electromagnetic radiation received by the imager.

GENERAL DESCRIPTION

Spectral imaging is used in a variety of scientific applications in which specific wavelengths of electromagnetic radiation (EMR) reflecting from or emitted by an object can provide useful information about the object, such as by way of example only, its material composition, its material classification (e.g., paper vs. plastic), as well as quantitative knowledge about associated lighting conditions during measurements.

Chip-scale spectral imagers are commercially available, but many are compromised by performance challenges. Many chip-scale spectral imagers are traditionally mosaic imagers, in which each pixel in the mosaic has a spectral filter that samples the spectral content of the electromagnetic spectrum from a scene, at a single spatial location. Because each pixel measures a different part of the spectrum and a different spatial section of the scene, the resulting spectral datacube is undersampled, and interpreting the data typically requires a demosaicing algorithm. However, if a scene has high spatial frequency content, demosaicing algorithms often fail, producing artifacts that corrupt subsequent analysis.

Intentionally defocusing the camera is often proposed for addressing problems associated with undersampling in mosaic systems, e.g., unwanted aliasing effects in which spatial variations in a scene can couple into erroneous spectral variation in a measured datacube. However, defocusing can reduce the acquisition of spatial information, thereby eliminating the advantages of increased spatial information. Furthermore, because demosaicing algorithms may assign spectral measurements to incorrect spatial origins or otherwise cause poor, non-reproducible, or incomplete sampling of the scene, defocusing typically does not eliminate erroneous spectral variations in the datacube. Numerous types of imaging systems have been employed in unsuccessful attempts to address undersampling errors, but the fundamental problem remains any time a camera uses a mosaic.

DETAILED DESCRIPTION

Reference will now be made in detail to certain exemplary embodiments, some of which are illustrated in the accompanying drawings. Certain terms used in the application are first defined. Additional definitions may be provided throughout the application.

The symbol “˜” which means “approximately”, and the terms “about” or “approximately” are defined as being close to, as would be understood by one of ordinary skill in the art. In an exemplary non-limiting embodiment, the terms may be used to mean within 10%, within 5%, within 1%, or within 0.5% of a stated value. For example, “about 4” or “˜ 4” may mean from 3.6-4.4 inclusive of the endpoints 3.6 and 4.4, and “about 1 nm” may mean from 0.9 nm to 1.1 nm inclusive of the endpoints 0.9 nm and 1.1 nm. All ranges described herein are inclusive of the lower and upper limit values.

As used herein, the term “equal” and its relationship to the values or characteristics that are “substantially equal” would be understood by one of skill in the art. Typically, “substantially equal” can mean that the values or characteristics referred to may not be mathematically equal but would function as described in the specification and/or claims. As used herein, “substantially” may mean “largely but not wholly”. The terms “substantially” and “approximately” may account for industry-accepted tolerance for the corresponding term and/or relativity between items.

As used herein, the phrases “at least one of A or B”, “one or more of A or B”, “at least one of A and B”, and “one or more of A and B” are each meant to include one or more of only A, one or more of only B, or any combination and number of A and B. Any combinations having a plurality of one or more of any of the elements or steps listed are also meant to be included by the use of these phrases. For example, the combinations of1A and1B,2A and1B,2B and1A, and2B and2A are included. Similar phrases for longer lists of elements or steps (e.g., “at least one of A, B, or C” and “at least one of A, B, and C”) are also contemplated to indicate one or more of either element or step alone or any combination including one or more of any of the elements or steps listed. As used herein, “one or more of” means “one or more than one of”.

Embodiments described herein include an integral field spectral imager100and methods of fabricating embodiments of the spectral imager that provide simpler manufacturing and assembly and more intuitive operation over previously described spectral imaging technology. Some embodiments are directed to addressing the aforementioned problems associated with undersampling and demosaicing and employ a strategy that is contrary to traditional approaches. Rather than seeking higher spatial resolution or defocusing an optic to reduce spatial-spectral artifacts in images, spectral imager embodiments described herein are configured to intentionally coarsen the spatial resolution at the detector (image sensor101) surface to prevent the need for antialiasing filters and enable more usual prescriptions and types of foreoptics. Resulting images may appear less appealing to the eye, but also may be more useful with machine perception systems. Furthermore, spectral imager embodiments described herein do not require a spatial filter in the form of an entrance slit, thereby greatly improving fill factor. The novel strategy allows for more flexible integration with external optics. Spectral imager100is typically more amenable to the use of faster (lower f-number) optics that may not require precision matching to micro-optical elements.

In many embodiments, an integral field spectral imager may comprise an image sensor comprising a superpixel array having a plurality of superpixels, each superpixel comprising at least four pixels; a plurality of optical homogenizers, each optical homogenizer being in-register with a corresponding different superpixel in the superpixel array, being positioned to receive electromagnetic radiation (EMR), and being configured to spatially homogenize the received electromagnetic radiation and to pass the spatially homogenized electromagnetic radiation to the in-register, corresponding different superpixel; an optical filter array positioned between each optical homogenizer and the in-register, corresponding different superpixel and comprising a plurality of spectral filters, wherein at least four spectral filters in the plurality of spectral filters are configured to spectrally filter the spatially homogenized electromagnetic radiation differently from one another and to pass the spectrally filtered, spatially homogenized electromagnetic radiation to the in-register, corresponding different superpixel, and wherein each of the at least four pixels is in-register with a different one of the at least four differently configured spectral filters and is positioned to receive the spectrally filtered, spatially homogenized electromagnetic radiation passed by the in-register spectral filter; and, a plurality of baffles, each baffle being configured and positioned to maximize confinement of the spatially homogenized radiation passed to the in-register, corresponding different superpixel. Spectral filters that are configured to spectrally filter the spatially homogenized electromagnetic radiation differently from one another are considered to be differently configured spectral filters.

In some embodiments, a method of determining spectral information about incident EMR received by spectral imager100comprises exposing spectral imager100to incident EMR, measuring an electrical response of the at least four pixels in each corresponding different superpixel to the spectrally filtered, spatially homogenized electromagnetic radiation passed to each of the at least four pixels, and based on analysis of the electrical responses of the at least four pixels in each corresponding different superpixel determining spectral information about the electromagnetic radiation received by the optical homogenizer and passed to the in-register corresponding different superpixel.

FIGS.1A-1Eshow schematic views of an exemplary embodiment of a spectral imager100and exemplary embodiments of associated elements and structures. Spectral imager100comprises image sensor101, the image sensor101comprising a superpixel array102having a plurality of superpixels103, each superpixel103comprising at least four pixels104(FIGS.1A-1C). Spectral imager100comprises a plurality113(FIG.1A) of optical homogenizers106(e.g.,106a,106b,106c,106dinFIG.1A;106a,106bin FIG.-1B). Each optical homogenizer106in the plurality of optical homogenizers113, is in-register with a corresponding different superpixel103in superpixel array102, is positioned to receive incident EMR107(FIG.1B) and is configured to spatially homogenize the received incident EMR107and to pass the spatially homogenized EMR109(FIG.1B) to the in-register corresponding different superpixel103. In the exemplary embodiments shown inFIGS.1A-1B, optical homogenizer106ais in-register with superpixel103aand optical homogenizer106bis in-register with superpixel103b. For an optical homogenizer106that is in-register with a corresponding different superpixel103it may also be said that the corresponding different superpixel103is in-register with the corresponding optical homogenizer106. That is for example, optical homogenizer106ais in-register with superpixel103a, and superpixel103ais in-register with optical homogenizer106a. An optical homogenizer106and a corresponding different superpixel103that are in-register with each other may be referred to as “in-register optical homogenizer” and “in-register corresponding different superpixel” respectively.

An exemplary superpixel103comprising a 5×5 array of pixels104is enlarged in the lower part ofFIG.1C. Typically, superpixel103comprises at least four pixels104, for example at least a 2×2 array of pixels104. InFIG.1C, pixels104are labeled to correspond to their position in the superpixel103, by row number and column number in the superpixel, with the prefix2. For example, the pixel at row1, column1is designated π11and the pixel at row5, column1is designated λ51. InFIG.1C, pixels104a,1046,104c,104d,104e, and104fcorrespond to pixels104at positions λ31, λ41, λ32, λ42, λ33, and λ43respectively. In some aspects, it may be preferred that superpixel103comprises a larger-sized mosaic superpixel that is prone to experience more significant undersampling effects with currently available imaging systems, such as for example six pixels in a 2×3 pixel array, or twenty-five pixels in a 5×5 pixel array, or other large mosaic configurations comprising four or more pixels.

Image sensor101further comprises an optical filter array110(FIGS.1A-1B), positioned between each optical homogenizer106(106a,106binFIG.1B) and the in-register corresponding different superpixel (103a,103b, respectively inFIG.1B).FIG.1Dis an exploded view that schematically depicts a 5×5 optical filter array110comprising a plurality of spectral filters105. (For clarity, spectral filters105in optical filter array110are shown for one in-register superpixel103inFIG.1A.) At least four spectral filters105in the plurality of spectral filters shown in optical filter array110are configured to spectrally filter the spatially homogenized EMR109differently from one another, as indicated by the different shading patterns for different spectral filters105(FIGS.1D and1E), and are configured to pass the spectrally filtered, spatially homogenized EMR109to the in-register, corresponding different superpixel103. In embodiments of image sensor101, each of the at least four pixels104in superpixel103is in-register with a different one of the at least four differently configured spectral filters105and is positioned to receive spectrally filtered, spatially homogenized EMR109passed by the in-register spectral filter105.

By way of example, referring toFIGS.1D-1E, the at least four spectral filters105ab,105c,105e, and105fin the plurality of spectral filters105are configured to spectrally filter spatially homogenized EMR109differently from one another as represented by different shading patterns for each of those spectral filters. Each of the at least four pixels104in superpixel103, here pixels104aat231,104cat232,104eat233, and104fat243, are in-register with a different one of the at least four differently configured spectral filters105in optical filter array110, here the spectral filters105ab,105c,105e, and105f, respectively. Pixel104ais in-register with spectral filter105ab, pixel104cis in-register with spectral filter105c, pixel104eis in-register with spectral filter105e, and pixel104fis in-register with spectral filter105f. In addition, each pixel104is positioned to receive spectrally filtered, spatially homogenized EMR109passed by the in-register spectral filter105. For a pixel104that is in-register with one of the four differently configured spectral filters105, it may also be said that the corresponding one of the four differently configured spectral filters105is in-register with the corresponding pixel104. That is for example, pixel104cis in-register with spectral filter105c, and spectral filter105cis in-register with pixel104c. A pixel104and a spectral filter105that are in-register with each other may be referred to as “in-register pixel” and “in-register spectral filter” respectively.

A spectral filter105in optical filter array110may be in-register with one or more than one selected pixels104in superpixel103. In some embodiments, as exemplified inFIGS.1D and1E, a single spectral filter105in optical filter array110may be sized and positioned to pass spectrally filtered, spatially homogenized EMR to two or more pixels104in the in-register corresponding superpixel103. Here, spectral filter105abis sized and positioned to pass spectrally filtered, spatially homogenized EMR109to pixels104aand104b, corresponding to the pixels at positions231and241respectively, in superpixel103. In these aspects, spectral filter105abis said to be “in-register” with each of the pixels104aand104b. Similarly, pixel104ais in-register with spectral filter105ab, and pixel104bis in-register with spectral filter105ab.

In some embodiments, two different pixels104in superpixel103may each be in-register with a separate spectral filter105, wherein the separate spectral filters105are configured to have the same spectral filtering characteristics. For example, referring toFIGS.1D-1E, pixels104cand104dat positions232and242in superpixel103are each in-register with a single spectral filter,105cand105drespectively. Both spectral filters105cand105dare configured to have the same spectral filtering characteristics as indicated by their having the same shading pattern.

In embodiments described herein spectral imager100comprises a plurality of baffles108(FIGS.1A-1B), each baffle108being configured and positioned to maximize confinement of the spatially homogenized EMR, passed by a single optical homogenizer106in the plurality of optical homogenizers to the in-register, corresponding different superpixel103.

As used herein, “optical filter array” may also be referred to as “array of optical filters”, “filter mosaic”, or “optical filter mosaic” and refers to the optical filter array110positioned between each optical homogenizer106and the in-register, corresponding different superpixel103. In some aspects, optical filter array110may comprise one or more different “types” of spectral filters105, i.e., one or more spectral filters105in optical filter array110may be based on any of a variety of different technologies, and a filter type may be selected based on the specific application of spectral imager100, among other considerations. A wide range of spectral filter105types and technologies are compatible with embodiments described herein and include, by way of example only, resonant dispersion filters, bandpass filters, metasurface filters, and notch filters, among other filter technologies represent different “types” of filters.

In various embodiments, optical filter array110may take any of a variety of configurations, provided that the filter mosaic pattern is contiguous in a superpixel103pattern and can be readily tiled. In some embodiments, it may be preferred that pixels104in a superpixel103be of the same size and shape, but this is not a requirement. In some aspects, it may be advantageous that superpixel array102comprises a plurality of superpixels103, each superpixel103having a different configuration, such as for example to accommodate specific optical configurations. By way of example only, a spectral imager100that comprises an external optic, such as for example a fisheye lens, may have a plurality of superpixels103with some superpixels having a different dimension and/or shape from the other superpixels in the plurality to account for image warp effects.

In some embodiments, one or more spectral filters105may also be configured to polarimetrically filter spatially homogenized EMR109, in addition to spectrally filtering spatially homogenized EMR109. In some aspects, optical filter array110may further comprise one or more polarization filters111(e.g.,111a,111binFIGS.1D and1E) for polarimetric measurements, each polarization filter111being configured to polarimetrically filter spatially homogenized EMR109and being in-register with one or more than one pixel104in a superpixel103. In some aspects, a polarization filter may also be configured for spectral filtering and, by way of example, may pass selected wavelengths of spatially homogenized EMR109, while preventing the passage of other wavelengths of spatially homogenized EMR. In some aspects, polarization filter111is not spectrally selective and is configured to be panchromatic and pass all or substantially all of spatially homogenized EMR109. In some embodiments, one or more optical filters in optical filter array110may filter spatially homogenized EMR109both spectrally and polarimetrically (i.e., spectropolarimetrically) and can produce a spectropolarimetric measurement. In some aspects, a spectral filter105and a polarization filter111may be configured as separate filters in optical filter array110and may both be in-register with the same pixel or pixels104.

Many different types of spectral filters105and filter mosaics may be useful for optical filter array110, some of which may be commercially available. In some embodiments of spectral imager100, it may be useful that spectral filters105include bandpass filters and/or notch filters. By way of example, one common type of spectral filter105mosaic is a square or rectangular pattern of bandpass filters that is repeated across an image sensor101.

In many embodiments, the quantum efficiency (QE) of a pixel104is determined primarily by the configuration of the spectral filter105that is in-register with the pixel104. In some aspects, the QE of a given pixel104may be different for different wavelengths of the spectrally filtered, spatially homogenized EMR109passed by the in-register spectral filter105and incident on the given pixel104, and the relationship can be viewed as a spectral line shape201.

FIGS.2A-2Billustrate exemplary embodiments of line shapes imparted by spectral filters. Spectral line shapes201shown inFIGS.2A-2Bindicate the relative QE of three different pixels104for generating an electrical signal from image sensor101.FIG.2Ashows line shapes201for the three different pixels104, each in-register with a differently configured spectral filter105. By way of example, line shape201aindicates the QE (y-axis) of pixel104aat position231of superpixel103as a function of wavelength (x-axis); line shape2016indicates the QE of pixel104cat position232of superpixel103as a function of wavelength; and line shape201cindicates the QE of pixel104eat position233of superpixel103as a function of wavelength. In some embodiments, a bandpass spectral filter105that passes spatially homogenized EMR109, centered primarily at a single wavelength, to pixel104, may result in a line shape201similar to those shown inFIG.2A. In many aspects, image sensor101is configured and manufactured such that all pixels104in a superpixel103will have substantially the same intrinsic electrical response to EMR of a given wavelength that is in the operational spectral band of the spectral imager100, and any differences in QE among the different pixels104would likely result almost entirely from differences in EMR filtering by the spectral filter105in-register with a given pixel104. However in some aspects, image sensor101may include variations in QE that are caused by the pixel104itself. In general, a pixel's line shape201may be regarded as an indication of that pixel's configured sensitivity to various wavelengths of EMR, which is primarily a result of the spectral filter105in-register with that pixel104.

In some embodiments, a spectral line shape201for a given pixel104may be shaped differently from a line shape201imparted by a traditional bandpass spectral filter105. For example, some types of spectral filters105, including metasurface spectral filters, may impart a more complex line shape201for a given pixel104, such as the examples shown inFIG.2B. Line shapes201d,201e, and201fare exemplary complex line shapes201. Each of these line shapes201exhibits variable QEs for a given pixel204, over a wide range of wavelengths. In some aspects, the use of spectral filters105that impart complex line shapes201can provide increased EMR throughput.

In some embodiments, spectral imager100may be useful for spectral analysis, reconstruction, imaging, and/or deconvolution of EMR107that is incident on an optical homogenizer106in-register with a corresponding superpixel103. Numerous computational spectroscopy methods, currently available to a person having ordinary skill in the art, are capable of recovering useful spectra and spectral information from relatively simple line shapes and from complex line shapes. In some aspects, such as for example when different spectral filters105impart simple overlapping line shapes201on in-register pixels104(e.g., as inFIG.2A) within a single superpixel, simple deconvolution methods known in the art may be employed. However, in some embodiments, complex spectral line shapes201such as those inFIG.2Bmay necessitate the use of one or more additional steps during spectral reconstruction. Spectral reconstruction methods such as for example minimization (least squares, regularization, non-negative least squares, and other similar methods) may be useful with complex line shapes. Some useful spectral reconstruction methods may be found in Kohlgraf-Owens et al., Optics Lett 35:2236-2237, 2010; Huang et al., Nature Sci Reports 7:40693, 2017; Redding et al., Nature Photonics 7:746-751, 2013; and U.S. Pat. No. 10,254,164 each of which is incorporated by reference herein in its entirety.

In many embodiments, spectral imager100comprises a plurality113of optical homogenizers106, each optical homogenizer106being in-register with a corresponding different superpixel103in superpixel array102, being positioned to receive incident EMR107, and being configured to spatially homogenize the received EMR107and to pass the spatially homogenized EMR109to the in-register, corresponding different superpixel103.

As used herein, “spatially homogenize EMR” means to homogenize the spatial intensity distribution of received incident EMR107without regard for the wavelength of the EMR. Optical homogenizers106are typically configured to produce equivalent results for EMR of any wavelength within an operational spectral band. In many aspects, the goal of spatial homogenization is to minimize the importance of the spatial location and/or wavelength of EMR on the subsequent illumination of superpixel103. In other words, in many aspects, a key objective of optical homogenizer106is to make EMR that is incident on the superpixel substantially invariant to spatial incident location or spectral content.

In many embodiments, optical homogenizers106are separated from one another by baffles108, each baffle being configured and positioned to maximize confinement of the spatially homogenized EMR109passed by a single optical homogenizer106to the in-register, corresponding different superpixel103. Maximizing confinement of the spatially homogenized EMR109to the in-register, corresponding different superpixel103may serve to prevent spreading of spatially homogenized EMR109to one or more neighboring superpixels thereby suppressing or eliminating crosstalk with neighboring superpixels103. In many embodiments, optical homogenizers106and baffles108are configured to produce a spatially uniform pattern of homogenized EMR109, regardless of where incident EMR107is received on optical homogenizer106.

Optical homogenizers106effect homogenization of the spatial intensity distribution of received EMR107. In some embodiments, optical homogenizers106cause EMR to spread diffusively during spatial homogenization. Optical homogenizers106may be configured to employ any of a variety of means for spatially homogenizing incident EMR107. The terms “optical homogenizer” and “homogenizer” may be used interchangeably herein to refer to optical homogenizer106. In many embodiments, optical homogenizers106may comprise “diffusive media”, also referred to herein as “optical diffusers” and/or “diffusers”301. In some aspects, optical homogenizer106may be configured as a “volume homogenizer” or as a “surface homogenizer”, and diffusers301for use with such homogenizers may be referred to as “volume diffusers” or as “surface diffusers”, respectively. In some aspects, optical homogenizer106may comprise both a volume homogenizer and a surface homogenizer. An optical homogenizer106that is a “surface homogenizer” may have some thickness, but in many aspects, the optical homogenizer thickness309of a surface homogenizer is typically of comparable dimension to the center wavelength of EMR in the operational spectral band of spectral imager100. By way of example, in some aspects a useful thickness309for a surface homogenizer may be about ten times (10×) the center wavelength of the operational spectral band or shorter. In some embodiments, a useful thickness309for optical homogenizer106that is a volume homogenizer is typically larger than the center wavelength of the operational spectral band of spectral imager100, (FIGS.3B-3C), such as about ten times (10×) the center wavelength of the operational spectral band or larger.

As used herein, the “operational spectral band” of a spectral imager is the range of wavelengths, including a minimum wavelength and a maximum wavelength, over which spectral imager100is configured to operate. As described herein, image sensor101comprises a superpixel array102having a plurality of superpixels103, each superpixel103comprising at least four pixels104. Each of the at least four pixels104is in-register with a different one of at least four differently configured spectral filters105, that are configured to spectrally filter spatially homogenized electromagnetic radiation109differently from one another. The differently configured spectral filters105are selected to spectrally filter different wavelengths or wavelength ranges of EMR within the operational spectral band of spectral imager100. The center wavelength of EMR in the operational spectral band is the wavelength that is halfway between the minimum wavelength and the maximum wavelength of EMR in the operational spectral band. In some aspects, the selected operational spectral band of spectral imager100may be determined by the filtering characteristics of the at least four spectral filters105. In some aspects, the selected operational spectral band of spectral imager100may be determined by band-cutoff filters that may be positioned in fore-optics for example, or by insufficient detector quantum efficiency outside of the selected operational spectral band, or by any combination of these factors.

In some embodiments, optical homogenizer106comprising diffusers301may be positioned within void303defined by baffle108, as depicted inFIG.3Aand inFIGS.1A-1B.FIGS.3A-3Care schematic, cross-sectional side views of exemplary embodiments in which optical homogenizer106comprises diffusers301and is positioned within void303defined by baffle108. (To clearly depict void303, optical homogenizer106is not shown at the left side inFIG.3A.) Selected dimensions of baffle108are indicated inFIG.3C. In some embodiments, as used herein baffle wall height305refers to the distance between lower edge306and upper edge307of baffle108, and baffle wall width304refers to the side-to-side thickness of a baffle wall. In many embodiments, baffle wall width304is typically measured at baffle lower edge306, near or at the surface of image sensor101or optical filter array110. As used herein, baffle upper edge307refers to the edge of baffle108that is distal to optical filter array110, and baffle lower edge306refers to the edge that is proximal to optical filter array110.

In some aspects, optical homogenizers106having the exemplary configurations shown inFIGS.3A-3Cmay be useful for effecting a relatively high degree of spatial homogenization of received incident EMR107. In some embodiments, it may be preferable to homogenize EMR over a relatively shorter distance. The exemplary embodiments of spectral imager100shown inFIGS.3A-3Bhave a relatively shorter baffle wall height305. In these aspects, optical homogenizer106is configured as a volume homogenizer and is relatively thinner (i.e., has a smaller thickness309) than in the embodiment shown inFIG.3C. In many aspects, optical homogenizer106with a smaller thickness309may benefit from a reduction in mean free path of EMR during spatial homogenization when compared to an optical homogenizer106having a larger thickness309and may be useful for spatially homogenizing EMR over relatively shorter distances. In these aspects, more strongly scattering diffusers301(i.e., diffusers301causing strong, forward diffusion of EMR rays302) may effectively homogenize EMR prior to EMR rays302reaching optical filter array110. In some aspects, strongly scattering diffusers301may produce undesirable, backscattered EMR (upward pointing EMR rays302inFIG.3A). Undesirable backscatter may be reduced by using an optical homogenizer106that comprises more weakly scattering diffusers301(FIG.3B). In some aspects however, weakly scattering diffusers301may not provide adequate spatial homogenization of received incident radiation107. In some embodiments, an optical homogenizer106having a relatively larger thickness309and comprising more weakly scattering diffusers301may be a useful configuration for providing adequate spatial homogenization of EMR with the additional benefit of increasing EMR throughput. In general, the mean free path, the composition, scattering strength, and density of diffuser301, the optical homogenizer thickness309, the optical homogenizer configuration (e.g., volume vs. surface homogenizers), the dimensions of baffle108(e.g., baffle height305and baffle width304), and other element parameters discussed below in more detail may be adjusted to meet the requirements of a particular spectral imager100.

Diffusers301that may be useful for spatially homogenizing EMR are known to those having ordinary skill in the art. In some embodiments, diffusers301may be particles loaded into a polymer, porous structures, lithographically defined structures, inhomogeneous polymers or bulk materials, or other structures known in the art to be useful for causing diffusive spread of EMR. In some embodiments, it may be preferred that diffusive spread of EMR rays302be wavelength-independent across a spectral band of interest and/or be non-attenuating. However, in some aspects, the aforementioned criteria are not all required.

The exemplary embodiments of optical homogenizers106shown inFIGS.3A-3Care volume homogenizers and the diffusers301used in those embodiments may be referred to as volume diffusers. Exemplary methods for making an optical homogenizer106that is a volume homogenizer include depositing diffusers301in void303to at least partially fill void303. In some aspects, such a method may include depositing dielectric particles in void303, and in some embodiments subsequently infiltrating the dielectric particle matrix with another dielectric material, such as by way of example only, infiltrating SiO2into an Al2O3particle matrix using atomic layer deposition. In some aspects, one or more methods may include producing an inhomogeneous gel and depositing the gel in void303to serve as diffusers301; depositing colloidal solutions of dielectrics for use as diffusers301that can cure or solidify in void303; and/or forming polymer diffusers301by suspending colloidal polymers in a second polymeric media, that may be integrated into void303, wherein the colloid may subsequently be removed by a gas process, liquid process, or plasma process to produce an optically inhomogeneous diffuser material. In some aspects, one or more methods may include depositing diffusers301present in liquid crystal/polymer composites wherein the liquid crystals are dispersed in a polymer matrix. Exemplary methods that may be useful include those found in Zhou et al., RSC Adv. 8:40347, 2018 and in Zhou et al., Liquid Crystals 47:(5) 785-798, 2020, each of which is incorporated by reference herein in its entirety. In one exemplary method, void303may be overfilled with diffusers301followed by subtractive removal of the diffusers301to a desired level using a polish, mechanical wipe, doctor blade, ablation, etch, or similar process. In some aspects, diffusers301may be removed to a desired level that is substantially at baffle upper edge307. In many embodiments, void303need not be completely filled with diffusers301, and in some embodiments may be partially filled with diffusers301.

In some embodiments, void303defined by baffle108is not an enclosed volume. That is, void303is defined by baffle108, but baffle108may remain open at a location distal to optical filter array110by way of baffle opening308at baffle upper edge307and/or baffle108may remain open at baffle lower edge306(FIG.3C) at a location that is proximal to optical filter array110. The spectral imager100embodiments shown inFIGS.3A-3Care examples in which baffle108is open at baffle upper edge307via baffle opening308and at baffle lower edge306. In some embodiments, baffle108may be closed at one or both of baffle upper edge307and baffle lower edge306. By way of example, optical homogenizer106and/or another optically transparent structure may be configured and positioned so as to effectively close baffle opening308. In some aspects, an optically transparent structure may be positioned at the bottom of void303that serves to essentially close baffle108at baffle lower edge306. Such a structure may be fabricated using, by way of example only, a method that comprises performing deep reactive ion etching to an etch stop as is shown inFIG.9. In some aspects, positioning baffle108to be in contact with optical filter array110or image sensor101may be used to close baffle108at bottom edge306.

In some embodiments, spatial homogenization of received incident EMR107may be tailored to limit the angles of incidence of spatially homogenized EMR109rays302at optical filter array110. However, useful limitations on the angles of incidence of EMR rays302at optical array110may vary among spectral imager100embodiments and may depend at least partially on manufacturing strategies and spectral purity requirements for the spectral imager application. To adjust and control the angles of incidence at optical filter array110of spatially homogenized EMR rays302, one or more of several strategies may be useful, and may include for example, providing an external optic such as a field lens; employing one or more than one external optic such as an array1002of microlenses1003that are in registration with the plurality of optical homogenizers for converting a ray bundle incident from a primary external optic to a configuration more conducive to spectroscopy (e.g., converting the ray bundle to a more telecentric configuration); incorporating one or more field stops to limit the effective f-number, such as for example only, by incorporating an array of micro-field stops at the position where incident EMR107enters optical homogenizer106; adjusting baffle wall height305; adjusting homogenizer thickness309; and/or adjusting scattering strength of diffusers301, among other methods.

Exemplary materials useful in forming diffusers301for use with some selected spectral bands of EMR include, but are not limited to, most optical polymers, glasses, and oxides for use with EMR in the visible (VIS) band; most optical glasses and oxides for use with EMR in the shortwave infrared (SWIR) band; silicon, germanium, chalcogenide glasses, most optical-grade salts and fluorides, oxides including for example TiO2, and standard MWIR optics for use with EMR in the midwave infrared (MWIR) band; and germanium, chalcogenide glasses, salts such as for example KBr, NaCl, and fluorides including BaF2, and other standard LWIR optical materials for use with EMR in the longwave infrared (LWIR) band. Diffusers301for diffusing EMR107may be prepared on a substrate or purchased as substrates and may be subsequently textured or otherwise prepared according to known methods.

In some embodiments, diffusers301may be in contact with baffle inner surface112, such as for example when optical homogenizer106completely fills void303. In some aspects, optical homogenizer106may be or may comprise at least part of inner surface112of a baffle108. By way of example, referring toFIG.1A, optical homogenizers106cand106dcomprise baffle inner surface112having deposited diffusers301in the case of106cand being a textured surface112as in the case of106d. In some aspects, some, all, or substantially all of baffle inner surface112may be configured to cause or enhance spatial homogenization of received EMR107. For example only, some, all, or substantially all of baffle inner surface112may be smooth metal, may have surface imperfections, may be textured or roughened, and/or may have diffusers301deposited thereon (e.g., coating904that may include particles or other diffusive media). In some embodiments, optical homogenizer106may comprise at least one of a surface homogenizer, a volume homogenizer, or baffle inner surface112configured to cause or enhance spatial homogenization of received EMR107. In some aspects, optical homogenizer106may comprise any combination of a surface homogenizer, a volume homogenizer, or baffle inner surface112configured to cause or enhance spatial homogenization of received EMR107.

Baffles108may be manufactured using any of a variety of methods known to a person having ordinary skill in the art including, by way of example only, electroplating into a resist template, etching trenches into a substrate and subsequently filling the trenches with baffle media, thermoforming a polymer, imprinting, and/or deep reactive ion etching through a substrate, such as for example a silicon substrate. Some exemplary methods are shown inFIGS.9,11, and13. In many embodiments baffles108are configured and positioned to maximize confinement of homogenized EMR109to the in-register, corresponding different superpixel103, while remaining compatible with requirements of the spectral imager100application. In some embodiments, baffle lower edge306may be positioned at any of a variety of locations in relation to the surface of image sensor101or the surface of optical filter array110. By way of example, in some embodiments, baffle lower edge306may be positioned at the surface of image sensor101, at the surface of optical filter array110(FIG.7C), or at a “standoff” position located at a selected “standoff” distance from the surface of image sensor101or from the surface of optical filter array110(FIGS.3A-3C). In some spectral imager100embodiments, one or more than one baffles108may have baffle lower edge306positioned at the surface of image sensor101, one or more than one baffles108may have baffle lower edge306positioned at the surface of optical filter array110, and one or more than one baffles may have baffle lower edge306located at a selected standoff distance from the surface of image sensor101or from the surface of optical filter array110. In some aspects, the positions of baffles108and baffle lower edge306may be selected based on manufacturing limitations, manufacturing considerations (e.g., pixel acceptance angles, sensor substrate thickness), and/or the likely effectiveness of the positioning for maximizing confinement of homogenized EMR109to the in-register, corresponding different superpixel103. In many embodiments, the structure and composition of optical homogenizers106, the types and structures of diffusers301, baffle wall height305, baffle wall width304, and/or image sensor pixel utilization requirements may be considered in determining placement of baffles108and positioning of baffle lower edges306.

In some embodiments, spectral imager100operability and/or manufacturing requirements may necessitate that one or more walls of baffle108be positioned such that one or more pixels104in superpixel103are partially or completely blocked by a baffle wall from receiving homogenized EMR109. In some aspects, one or more baffle108wall may block one or more rows or columns of pixels104in one or more superpixel103from receiving homogenized EMR109. In some aspects, this may be an acceptable loss of pixel utilization, and a spectral imager100may be configured with a pixel readout strategy that can ignore sensor data from the one or more blocked rows and/or columns of pixels so as to limit data bandwidth and power consumption. In general, the ability to implement this strategy will be dependent on spectral imager100design and programmability and flexibility of the readout circuitry (e.g., the modifiability of image sensor101by a field-programmable gate array, FPGA).

In some embodiments, one or more baffle108walls may partially block one or more pixels104, located at an edge of superpixel103and/or at a corner of superpixel103, from receiving homogenized EMR109.FIG.4is a top-down, schematic view of an exemplary image sensor101having a 2×2 array of superpixels103, each superpixel103having a 4×4 array of pixels104. In this example, each pixel104in each superpixel103is in-register with a single corresponding spectral filter105positioned over the in-register pixel104. For superpixel103a, corner pixels104g,104h,104i, and104jare positioned beneath a corresponding, in-register spectral filter,105g,105h,105i, and105jrespectively, where each combination of pixel104and in-register corresponding spectral filter105is represented, for ease of viewing, by a patterned square labeled with the number of the pixel, e.g.104g,104h,104i,104j, that is positioned beneath the in-register corresponding spectral filter105. In this exemplary embodiment, pixels104kand104mlocated at an edge of superpixel103are blocked along one edge of pixel104by baffle108wall. Corner pixels104g,104h,104i, and104jare each partially blocked by baffle108walls along two edges of the pixel104. The footprint of baffle bottom edges306is represented as thick black lines. It is to be noted that in many embodiments baffle bottom edges306form a footprint that is rectangular or square in shape. However, in some aspects, baffle bottom edges306may form a footprint that is circular or elliptical, randomly shaped, or having another geometrical shape.

In some embodiments, baffle108wall configuration and pixel utilization strategies may be applied to mitigate possible negative effects associated with baffle108walls that partially block pixels104. For example in some aspects, a spectral filter105for use with a corner pixel104or with a pixel104at an edge of superpixel103, can be selected and configured so that the filter passes spatially homogenized EMR109having the highest anticipated irradiance. In many embodiments, an entire superpixel103is operated at the same gain or exposure, and such a configuration may be useful for reducing or balancing the dynamic range of the electrical signals produced in image sensor101in each superpixel103. By way of example, a selected spectral filter105may pass spatially homogenized EMR109that is substantially panchromatic or that substantially spans the solar maximum, whereas an interior spectral filter105(i.e., a spectral filter105in-register with a pixel104that is not occluded by a baffle108, such as pixel104that is not a corner pixel104or a pixel104at an edge of superpixel103) may substantially pass spatially homogenized EMR109that spans a region with lower anticipated irradiance or detector quantum efficiency, such as a near-infrared band for a CMOS sensor. In some embodiments, selected spectral filters105may be duplicated at some pixel104locations on superpixel103to provide a duplicate measurement of received EMR109having the same line shape201.

In some aspects for mitigating negative effects of baffle108walls blocking pixels104, spectral filters105positioned over one or more than one partially blocked pixels (e.g.,104gand104h;104iand104j) may be configured to impart the same spectral line shapes201on those pixels. As shown inFIG.4, pixels104gand104hare represented with the same pattern to indicate that the in-register corresponding spectral filters105(i.e.,105gand105hrespectively) are configured to have the same filtering capability and impart the same line shape. In a similar manner, the patterned squares representing pixels104iand104jindicate that the in-register corresponding spectral filters105(i.e.,105iand105jrespectively) are configured to have the same filtering capability and impart the same line shape. As such, for the example shown inFIG.4, the 16 pixels104in superpixel103aprovide spectral measurements of EMR passed by 14 different types of spectral filters. In some aspects, duplicating spectral filters105at corner pixel positions in a superpixel103may be useful when irradiance of corner pixels104is reduced by caustics or other light non-uniformities that may arise from homogenization of EMR107. Duplication of spectral filters105at corner pixels104in superpixel103results in a reduced percentage of pixel utilization when compared with pixel utilization for a superpixel103that does not have duplicated spectral filters105. By way of example, for a superpixel103having a 3×3 array of pixels104with no duplicated filters105, 100% of pixels will receive spatially homogenized EMR109that has been filtered differently. Whereas, for a superpixel103having a 3×3 array of pixels104with four corner pixels partially blocked and two pairs of duplicated spectral filters (e.g.,105g/105hand105i/105j), ˜77.8% of pixels104will receive spatially homogenized EMR109that has been filtered differently. For a superpixel103having a 5×5 array of pixels104with four corner pixels partially blocked and two pairs of duplicated spectral filters, ˜92% of pixels104will receive spatially homogenized EMR109that has been filtered differently.

Various strategies may be implemented for increasing pixel utilization when one or more pixels104in superpixel103are partially or completely blocked by a baffle108wall from receiving homogenized EMR109. Useful embodiments for increasing pixel utilization may often depend on pixel size, baffle material and fabrication strategy, and/or baffle integration strategy to name some factors. In some embodiments, to prevent baffle108walls from blocking pixels104, image sensor100may be configured to have ROIC processing elements, readout electronics501, or other non-photodetecting electronic circuit elements located in a region that may be blocked by one or more baffle108walls.FIG.5shows a schematic view of an exemplary embodiment of a superpixel and baffle configuration. In this embodiment, baffle108walls may be positioned above readout electronics501to prevent blocking of sensor pixels104. In some aspects, readout electronics501may also be configured to enable specialized operations such as superpixel-level gain or exposure control.

In some embodiments, baffle wall width304may be considerably smaller than a pixel104. By way of example, baffle wall width304may be less than or equal to about ¼ of the pixel pitch1402. In these embodiments, baffle108walls may be positioned between superpixels103and may block less spatially homogenized, filtered EMR109from being received by a pixel104when compared with a configuration having a larger baffle wall width304. In some aspects, by way of example only, baffle108walls that are positioned between superpixels103may block less than or equal to about ⅛thof spatially homogenized EMR109passed to edge pixels104and less than or equal to about ¼thof spatially homogenized EMR109passed to corner pixels104. If deemed useful, the aforementioned mitigation and dynamic range balancing strategies may also be applied in these situations.

In some embodiments, spectral imager100may be configured such that one or more superpixel103has independent gain and/or exposure controls. For example, an electrical circuit may be configured for reducing the gain of or shortening the exposure time of superpixel103that may become saturated before other superpixels103in image sensor101.

In some embodiments, baffle wall width304may be relatively larger. For example, baffle wall width304may be about the same size as or larger than pixel104.FIG.6is a cross sectional, schematic side view of an exemplary embodiment of spectral imager100that may be useful for increasing pixel104utilization when baffle wall width304is relatively large. In this embodiment, baffles108may be positioned such that baffle lower edges306are located at a standoff distance601from spectral filters105. This embodiment may be useful for pixels104configured to have an acceptance angle commensurate with the angles of EMR rays302arriving from the corners of the homogenizer106. For these aspects, the geometry of EMR ray302propagation, baffle108position, and baffle scattering properties can be readily modeled and elements designed using standard ray tracing programs available to a person having ordinary skill in the art. In these embodiments, the aforementioned strategy of employing duplicated spectral filters105at corner pixels104and/or modifying the shape of baffle opening308may be useful for increasing pixel utilization. The geometry of baffle opening308may also be modeled and designed using standard ray tracing tools. In the exemplary embodiment shown inFIG.6, baffles108are positioned to suppress crosstalk with neighboring superpixels103by functioning as a field stop at baffle lower edges306. In some embodiments, positioning baffle lower edges306at a standoff distance601from image sensor101and/or spectral filters105may be useful for maximizing pixel utilization with configurations having pixels104that are positioned behind a substrate or cover glass or having pixels104that cannot be physically contacted by baffle108due to surface treatments or fragility concerns.

In some embodiments, volume optical homogenizers106for use with some spectral bands of EMR (e.g., the UV or IR bands) may be more challenging to manufacture than surface optical homogenizers106, and surface optical homogenizers106may be a more practical option for manufacturing some spectral imagers100. In some aspects then, optical homogenizer106may be or may comprise a surface homogenizer rather than a volume homogenizer.FIGS.6,7A-7C, and8A-8B show exemplary embodiments of spectral imager100comprising optical homogenizers106that are surface homogenizers. In many embodiments, optical homogenizer106that is a surface homogenizer may be positioned at or near baffle upper edge307as in the exemplary embodiments shown inFIGS.6,7A-7C, and8A-8B. Surface optical homogenizers106can be useful for reducing large ray angles at the image sensor101surface. Surface optical homogenizer106may be prepared by methods known in the art such as for example surface etching additive methods and/or deposition methods and may comprise diffusers301that are particle films (e.g., a film comprising Al2O3or TiO2particles), various lithographically defined patterns including metasurface structures, and/or other diffusive media. In many aspects a surface optical homogenizer106may comprise the same types of diffusers301as does a volume optical homogenizer106and may be manufactured in the same manner as a volume optical homogenizer106. In some aspects, a surface optical homogenizer106can be prepared separately from baffles108. For example, in some embodiments a contiguous layer of diffusive media may be prepared separately from baffles108and may be located or positioned over a plurality of superpixels103, as shown for the exemplary embodiment inFIG.8B. In some embodiments then, such as inFIG.8B, optical homogenizer106that is in-register with a corresponding different superpixel103may be or may comprise a region of a larger contiguous layer of diffusive media. As such, optical homogenizer106that is a region of a contiguous layer of diffusive media is the region of that contiguous layer that is configured to spatially homogenize incident EMR107and positioned to pass spatially homogenized EMR109to optical filter array110. There may be regions of the same contiguous layer of diffusive media that do not substantially pass spatially homogenized EMR109to optical array110, such as the regions that are located above baffle108walls that are not positioned over void303, and therefore these regions are not optical homogenizers.

In some embodiments, baffle108having a relatively taller baffle wall height305and having a surface homogenizer106positioned at or near baffle upper edge307may function as a light pipe homogenizer (FIGS.7B and7C). In some embodiments, a surface homogenizer106may comprise a diffractive element, a micro-optical element such as for example a vortex plate, an optical metasurface, and/or a thin film of particles. In some aspects it may be preferable that spectral imager100be configured to direct incident EMR107to arrive at a skew angle resulting in rays302that are skew rays, which are directed by optical homogenizer106toward image sensor101. In some aspects, this can be achieved by configuring spectral imager100to have additional micro-optical elements or an off-axis field lens in the imaging system.

As used herein the term “baffle inner surface”112refers to the “interior” surface of a baffle108wall, that is, the surface of baffle108that faces void303. In some embodiments, as used herein, the term “baffle inner surface”112may include any region of one or more than one baffle inner surface112, and a region of baffle inner surface112may take any shape or have any dimensions up to substantially all or all of the interior surface of baffle108. In some aspects, a baffle cross section that is taken parallel to the plane of image sensor101may be any size, shape, or configuration that is useful for confining spatially homogenized radiation109passed by each optical homogenizer106to the in-register, corresponding different superpixel103or any shape or configuration that is useful for spatially homogenizing EMR107or contributing to the spatial homogenization of EMR107. By way of example useful cross-sectional shapes of baffle108include square, rectangular, circular, elliptical, or a random geometrical shape. In some aspects, a baffle cross section taken at a first distance from image sensor101may have a different shape and/or size when compared with a baffle cross section taken at a second distance from image sensor101or when compared with a baffle footprint at baffle bottom edge306. By way of example only, a dome-shaped or hemispherical baffle108may have a rectangular cross section near or at baffle bottom edge306and an elliptical or circular cross section at a position more distal to the surface of image sensor101, as for the exemplary embodiment inFIGS.12A and12B.

In various embodiments, the term “baffle inner surface”112may refer to any region of an interior surface up to and including all interior surfaces of baffle108. For example, for a baffle cross section that is rectangular, “baffle inner surface”112may refer to one interior side of baffle108up to all four interior sides of baffle108, or any portions thereof. In some aspects baffle bottom edges306may form any useful shape such as for example, square, rectangular, circular, elliptical, or random geometrical shape, and a shape formed by baffle bottom edges306may differ from the shape of a baffle cross section taken at any selected distance from baffle bottom edges306distal to image sensor101.

In some embodiments, at least a part of baffle inner surface112may be configured to be textured, i.e., one or more regions of baffle inner surface112, or all of baffle inner surface112may have surface texture such as the example shown inFIG.7C. For example, baffle inner surface112may be configured as a roughened, faceted, or otherwise-textured surface, such as for example only a wavy surface, so as to enhance spatial homogenization of incident EMR107. In some aspects, baffle108may have surface texture along or substantially along the entire length of baffle inner surface112, i.e., from baffle upper edge307to baffle lower edge306. In some aspects, baffle108may have surface texture positioned at discrete locations of baffle inner surface112. Methods for texturing and roughening light-pipe homogenizers are known to a person having ordinary skill in the art and can be useful here with baffles108. A baffle108having a roughened inner surface112may cause ray302angles to increase as EMR is spatially homogenized and passed to image sensor101. In these aspects, it may be preferred that baffle lower edge306be positioned as closely as possible to the in-register, corresponding superpixel103as is the case inFIG.7C.

In some embodiments, all or part of baffle inner surface112may be reflective, which may promote EMR throughput. However in some aspects, baffle inner surface112may be configured to absorb at least some EMR during spatial homogenization of incident EMR107, which in some aspects can reduce caustics that may result from reflection of EMR rays302.FIGS.8A-8Bshow exemplary embodiments of spectral imager100comprising baffles108configured to have absorbing baffle inner surface112that absorbs EMR rays302.

In some embodiments, spectral filters105may be integrated with optical homogenizers106and with image sensor101. In some aspects, optical filter array110comprising metasurface spectral filters105may be fabricated on a transmissive substrate that is integrated with baffles108having reflective baffle inner surfaces112. The resulting structure may then be added to an image sensor101using for example wafer bonding, die bonding, an external support frame, or other techniques known in the art for integrating micro-optical elements. One exemplary embodiment for fabricating a spectral imager100with an integrated optical filter array110is shown inFIG.9. In this embodiment, at (a) substrate901, such as a silicon substrate, is provided. In some aspects and in accord with the application of spectral imager100, an electroplating step may be required during fabrication (in this example, step (i) could be an electroplating step), and as such it may be preferable that the silicon substrate be heavily doped. Optionally at (b) a KOH etch may be performed where each etch pit905defines the location of a superpixel103. At (c) an optically transparent layer902with an etch stop is produced. For example, optically transparent layer902may comprise an oxide that can be grown (wet or dry based on a required thickness) that may function as a transmissive support membrane. For use as a structural transmissive support membrane, the thickness of optically transparent layer902may be selected to account for optical transmission requirements. In general, it is preferable that the thickness of a transmissive support membrane is selected so as to prevent destructive interference modes that can reduce EMR throughput within the operational spectral band. Structural properties may also be considered during fabrication, such as for example the ability of optically transparent layer902functioning as a transmissive support membrane to withstand physical stresses so as to prevent failure during release or other processing steps. In some aspects, optically transparent layer902may comprise, by way of example, silicon dioxide, non-silicon oxides, nitrides, oxynitrides, and other materials having useful optical, structural, and material properties for a specific spectral imager application. In some aspects, it may be preferred that material selected for optically transparent layer902be optimized for use with a selected spectral band of interest. For example, for a spectral imager100designed for use in the MWIR region of the EM spectrum, silicon may be preferred for use as optically transparent layer902, whereas silicon dioxide would function as an EMR-absorbing material. In these aspects, a Silicon on Insulator (SOI) wafer may be useful, where the oxide (e.g., SiO2) serves as an etch stop, and is subsequently removed with an HF cleaning step. In some aspects, although SiO2is transparent when used with EMR in the VIS and SWIR bands, it may be useful to perform an oxide strip step so as to remove residue from the deep reactive ion etching (DRIE) process. In such aspects, materials such as silicon nitride and silicon oxynitride may be preferred as optically transparent layer902functioning as a transmissive support membrane. In some aspects, such as with a spectral imager100for use in the LWIR band of the EM spectrum, germanium may be preferred as optically transparent layer902that will serve as a transmissive support membrane and can be processed using a Germanium on Insulator (GOI) wafer. In some aspects, such as with a spectral imager100for use in the UV through MWIR bands of the EM spectrum, a Sapphire on Silicon (SOS) wafer may be useful for preparing optically transparent layer902that will serve as a transmissive support membrane.

In some embodiments, optically transparent layer902produced in (c) may optionally be polished as in (d), which in some aspects may expose a portion of the underlying substrate901. Optional polishing (d) may be useful, for example when subsequent steps require a planar surface (such as for nanoimprint lithography), and a silicon substrate901can act as an optical barrier to prevent unwanted light from scattering within the oxide layer between adjacent superpixels103. In some embodiments, it may be useful to have support structure protrude beyond the surfaces of the optical filter so as to prevent optical filter material (in optical filter array110) including spectral filter105material from being in contact with image sensor101during integration step (j). The inclusion of polishing step (d) will typically depend on specific requirements for integration.

Step (e) in this exemplary method, patterning a spectral filter105mosaic, can vary depending on the composition of spectral filters105. By way of example only, in addition to spectral filters105, optical filter array110may further comprise one or more than one of interference filters, plasmonic filters, dielectric metasurfaces, dyes, and/or bulk deposited materials, any of which may require specific considerations for patterning or for different patterning methods. At step (f) in this exemplary fabrication process, the spectral filter105mosaic and optical filter array110are protected and the wafer is mounted onto a handle wafer903. Typically, substrate901, here the silicon wafer, is then ground and polished as in (g) to the desired final thickness. At (h), using anisotropic etching, such as for example DRIE, substrate901(silicon wafer) is then patterned and etched down to the oxide etch stop produced in (c), forming baffles108. Coatings904may then optionally be applied, as desired (i). Exemplary coating904may include metallization of the silicon side walls, that serve as baffle inner surfaces112, so as to promote or suppress reflection of EMR during spatial homogenization. By way of example only, electroplating baffle inner surfaces112with silver (Ag) or gold (Au) may be useful for making baffle inner surface112that promote reflection. In some embodiments for making baffle inner surface112that promote reflection, other useful materials may comprise multilayer dielectric coatings and/or aluminum (Al). Electroplating baffle inner surface112with Cu followed by an oxidizing step to blacken the Cu coating may be useful for making a baffle inner surface112that is EMR-absorbing. In some aspects, an EMR-absorbing baffle inner surface112comprises CuO. Various other carbon-containing treatments (e.g., porous graphitic and carbon nanotube structures), motheye treatments, and chrome may be useful as EMR-absorbing coatings. Other coatings and modifications for promoting reflection of EMR or for enhancing EMR absorption by baffle inner surfaces112are commercially available or otherwise known to one of skill in the art.

In some aspects, high-angle deposition may be useful for coating a baffle inner surface112, to prevent inadvertently coating a transmissive support membrane (optically transparent layer902). In some embodiments, antireflective material layers, such as by way of example a quarter-wave MgF2layer, may be applied to the transmissive support membrane (optically transparent layer902). Following the application of optional coatings904to baffle inner surfaces112, at (j) the formed structure shown in (i) may then be transferred to another support structure, such as a second handle wafer, and in some aspects, may be diced prior to this step. At this point, the formed structure may then be mounted near or integrated with image sensor101. In some embodiments step (j) may be performed after step (k) depending on processing and tooling requirements. At (k) diffusive media such as volume diffusers301are added to enhance the spatial homogenization of incident EMR107. In some aspects, additional optional diffusive media301may include scattering membranes and/or more complex micro-optical structures such as external optics like microlenses1003in a plenoptic configuration that may be designed to re-image the entrance pupil onto the integrated optical homogenizer106. In general, embodiments of spectral imager100will require a calibration step after manufacture, as is typical for other spectral filter mosaic technologies.

The steps inFIG.9illustrate one exemplary method for making an exemplary embodiment of spectral imager100having an integrated optical filter array110. It is not a requirement that all steps be performed in the order shown. One skilled in the art of micro-fabrication of spectral imaging components will understand that some intermediate steps, such as for example lithographic processing, are not explicitly shown. Some embodiments of spectral imager100may have alternative configurations that require different or additional methods of fabrication. By way of example only, in some embodiments, optical homogenizers106may comprise one or more than one of a surface homogenizer, a volume homogenizer, or a textured baffle inner surface112, or any combination of these.

In some embodiments, spectral imager100may comprise external optical elements, such as by way of example only, one or more objective lens1001and/or one or more microlenses1003, which may be present in a microlens array1002. In some aspects, spectral imager100may comprise a camera or an imaging optic. In some aspects, additional optical elements may be external to baffle108and positioned between baffle108and incident EMR107. In some aspects, one or more microlenses1003may be useful for contributing to the spatial homogenization of incident EMR107in addition to the spatial homogenization effected by optical homogenizer106.FIG.10is a schematic cross-sectional, side view of an exemplary embodiment of a spectral imager100that comprises external objective lens1001and microlens array1002having a plurality of microlenses1003, wherein objective lens1001is positioned to receive incident EMR107and to pass incident EMR107to microlens array1002from which the EMR is passed to optical homogenizer106. In some embodiments, each microlens1003in microlens array1002may be positioned to be in-register with a corresponding different optical homogenizer106, such that the radiation passed to the corresponding different optical homogenizer106is then spatially homogenized and passed to the in-register, corresponding different superpixel103. In some aspects, to reduce crosstalk between superpixels103, it may be beneficial that the one or more microlens1003be faster than objective lens1001. Baffles108are typically configured to also reduce crosstalk (i.e., to maximize confinement of spatially homogenized EMR109to corresponding different superpixel103) and in some aspects, may be configured to provide additional spatial homogenization function. In this exemplary embodiment, optical homogenizers106are depicted as surface homogenizers, but may also be volume homogenizers. Useful types of surface and volume homogenizers106have been described previously herein.

FIG.11depicts one exemplary method for manufacturing spectral imager100that comprises external microlens array1002integrated with optical homogenizers106. In this exemplary embodiment, at (a) microlens array1002is provided and may be manufactured to have selected specifications. In some aspects, commercially available microlens arrays1002may also be used. At (b), optical homogenizers106, here surface homogenizers, are added to the side of microlens array1002facing imager sensor101. In some aspects, additional diffusive media301may be added to optical homogenizers106as is useful according to the application of spectral imager100. At step (c), baffles108are added. Baffles108may be produced by electroplating into a template or may be added by integrating a prefabricated array of baffles108, or by other means known to one of skill in the art. One or more baffle inner surfaces112of selected baffles108may be treated so as to have a coating904, such as a reflective coating. The optional treatment of baffles108to apply coating904may depend on the treatment and/or baffle108manufacturing strategy and may be performed before or after the addition of baffles108to the structure. In this exemplary embodiment at (d), the microlens array1002/optical homogenizer106/baffle108assembly is integrated with an image sensor101, which has already been configured to include an optical filter array110.

In some embodiments, spectral imager100may be useful for spatially homogenizing incident EMR107from a variety of spectral bands including EMR having wavelengths in the UV band through the LWIR band. In many aspects, optical materials for use with a given spectral imager100embodiment may be selected to accommodate the spectral band of interest. By way of example, in some aspects transmissive optical materials, reflective materials, and absorbing materials may be selected so as to be transmissive, reflecting, or absorbing, respectively, over the spectral band of interest for a given spectral imager100embodiment. Specific materials useful for transmission, refraction, reflection, and absorption in various spectral sub-bands from the UV through LWIR spectral band are well-described in the art and are known to persons having ordinary skill in the art. It is to be noted that for some image sensor101embodiments designed for use with longer EMR wavelengths, the wavelengths of incident EMR107may be dimensionally similar to one or more dimensions of the micro-optical elements of spectral imager100. In such cases, it may be useful that some structural elements be modeled with more comprehensive electromagnetic modeling schemes, rather than with simple ray tracing.

In some embodiments, for example when baffle108defines a dome-shaped void303as inFIGS.12A-12B, structural elements (e.g., baffle108) may be best modeled as a leaky cavity if the structural element's characteristic dimensions approach less than or equal to about two times (2X) the longest wavelength of incident EMR107in a spectral sub-band of interest. In these aspects, it may also be important to consider the impedance matching conditions of the structural elements. For example, the minimum width of any apertures or constrictions included in the structural element should not be less than half the wavelength of the longest wavelength of EMR to be detected (λmax/2), or else EMR rejection may be unacceptably high.

In some configurations, such as the exemplary embodiment shown inFIGS.12A-12B, spectral imager100may be configured to cause backscatter of at least some EMR rays302during the spatial homogenization process, which may function to “recycle” at least some EMR through the homogenization process one or more times and in some aspects may improve the efficiency of spatial homogenization of incident EMR107.FIG.12Ashows a schematic cross sectional, side view of an exemplary embodiment of spectral imager100, that may be useful for enhancing specular reflection of EMR rays302(e.g.,302a) so as to recycle EMR through the spatial homogenization process. In some aspects, spectral filters105may be configured to reject all out-of-band EMR, reflecting the EMR specularly. In many aspects, specularly reflected EMR rays (302a) may be directed to a different region of superpixel103, where reflected EMR rays302may interact with a spectral filter105that can pass the reflected EMR to an underlying pixel104on image sensor101. The embodiment shown inFIG.12Acomprises baffles108configured to be dome-shaped and configured to form void303that has a dome-like shape (i.e., a dome-shaped void303). Baffle inner surfaces112may be coated with inner surface coating904, which in some aspects, may be a specular coating, such as a reflective metallized coating or a coating comprising scattering diffusers301such as rough particles.

FIG.12Bis a schematic, perspective view of baffle108configured to be dome-shaped that defines dome-shaped void303. Some spectral imager100embodiments comprising baffles108that define dome-shaped voids303and that are configured for enhancing specular reflection may comprise one or more EMR concentrating elements, which may be one or more external optic such as for example a microlens1003positioned and configured to receive incident EMR107and pass it to optical homogenizer106that is positioned at baffle opening308.

As used herein, the terms “dome-like shape”, “dome-shape”, “dome-shaped”, and variations thereof, when describing baffle108or a void303defined by baffle108, may refer in some aspects to a shape substantially similar to the hollow upper half of a sphere (i.e., a hollow hemisphere). However, a dome-like shape of void303or baffle108need not be a complete hemisphere. In some aspects, “dome-shape” or “dome-like shape” may refer to any of numerous other dome-like shapes such as for example any fraction of a hemisphere, e.g., an upper quarter of a sphere or other such portion of a hemisphere (e.g., a segmental dome), a cloister vault (also referred to sometimes as a pavilion vault or domical vault, such as that shown inFIG.12Bin which baffle lower edges303form a rectangular shape), a conical dome, a pointed dome, a faceted dome, or any other dome-shaped structure compatible with the manufacturing and functional requirements of spectral imager100. It is to be noted that a “dome-shaped” baffle108or void303defined by baffle108may also refer to a baffle or void in which a “dome-shape” or “dome-like shape” is a portion of baffle108or void303. By way of example only, a dome-shaped baffle108may comprise a structure in which a dome-shaped region is surmounted on a rectangular cuboid, such that an upper region of baffle108is dome-shaped and a lower region is cuboidal having baffle lower edge306that forms a footprint having a square or a rectangular shape.

FIG.13is an exemplary embodiment for fabricating a spectral imager100comprising baffles108that define dome-shaped voids303and that function in a manner similar to an integrating sphere. At (a), a silicon substrate901is provided and an oxide layer1301is grown on the silicon wafer. At (b), the silicon wafer substrate901is bonded to a handle wafer (not shown), back-thinned, polished, and flipped. At (c), domed-shaped baffles108are formed by isotropic etching, such as for example etching with XeF2or isotropic SF6plasma. When the etching process reaches oxide layer1301, baffle opening308may be formed. In some aspects, it may be useful to utilize the etching process for intentionally introducing surface texture or imperfections on baffle inner surface112, such as by way of example only baffle inner surface112asymmetry or a wavy baffle inner surface112, for reducing caustic effects and promoting spatial homogenization of incident EMR107. In some aspects, a baffle inner surface112may be faceted, i.e., inner surface112texturing may produce facets. At (d), coatings904, which may function as diffusers301for improving spatial homogenization of incident EMR107, may optionally be applied. By way of example only, coatings904may comprise one or more antireflective layers on oxide layer1301, or a reflective film on baffle inner surface112e.g., an aluminum, gold, or silver film for reflection. Additional diffusers301such as disordered dielectric microparticles (e.g., titania) may be positioned in void303, at interface1303between oxide layer1301and void303, or as a coating904on baffle inner surface112. Following the optional application of coatings904to baffle inner surfaces112, at (e) the formed structure shown in (d) may then be integrated with image sensor101and debonded from any handle wafer that may have been used in processing. Optionally, at (f) one or more external optics such as for example microlenses1003may be integrated on the exterior surface of oxide layer1301to promote transmission of received EMR107through baffle opening308. It is not a requirement that all steps be performed in the order shown. One skilled in the art of micro-fabrication of spectral imaging components will understand that some intermediate steps, such as for example lithographic processing, are not explicitly shown. Some embodiments of spectral imager100may have alternative configurations that require different or additional methods of fabrication.

EXAMPLES

FIGS.14A-14Fare top-down schematic views of exemplary superpixel arrays102and baffle lower edge306footprints, including exemplary configurations and dimensions of selected elements for use with a spectral imager100. In some exemplary embodiments, one or more baffle lower edge306is shaped as a square or a rectangle. The exemplary superpixels103and superpixel arrays102shown inFIGS.14A-14Fmay be used to configure selected spectral imager100embodiments for operation in one or more specific regions of the EMR spectrum. In many embodiments of spectral imager100, the exemplary configurations and dimensions of superpixel pitches1404, pixel pitches1402, and associated distances1302between opposing baffle108walls may be useful with a variety of baffle108and void303configurations.

In many embodiments, spectral imager100may be configured to further comprise one or more “structural street” and/or “non-structural street”. The exemplary embodiments shown inFIGS.14A-Fare illustrated as having “structural streets” or “non-structural streets”. As used herein, in some aspects a “structural street” refers to a mechanical structure (e.g., baffle108wall) whose thickness (e.g., baffle wall width304) and position are such that the structure largely blocks a row and/or a column of pixels from exposure to spatially homogenized EMR109. Generally, a structure is defined as a “structural street” if it blocks exposure to spatially homogenized EMR109of an underlying row or column of pixels104by more than about 50% as compared to the exposure of an unblocked row or column of pixels104. In other aspects, a structure (e.g., baffle108wall) may be considered a “non-structural street” if it does not block exposure of a row or column of pixels104to spatially homogenized EMR109by more than about 50%. By way of example only, for the embodiment shown inFIG.14A, the walls of baffles108function as structural streets because they block exposure of the underlying row of surrounding pixels104(the “pixel street” as taught below in Example 1) to spatially homogenized EMR109by more than about 50% as compared to the exposure of an unblocked row or column of pixels104.

It should be understood that, although the exemplary embodiments depicted inFIGS.14A-Fare shown as having either structural streets or non-structural streets, these are exemplary for purposes of explanation. In some embodiments, spectral imager100may be made with baffles108whose walls function only as non-structural streets or only as structural streets. In some embodiments, spectral imager100may be made such that some baffle108walls (or other structures) function as structural streets and some baffle108walls (or other structures) function as non-structural streets. In many embodiments, the utilization of a “structural street” in a spectral imager100may be based on engineering decisions and functional goals for a spectral imager embodiment. Some exemplary factors to be considered when deciding on whether to include or not include a structural street include the number of available pixels104on image sensor101, the degree of optical isolation required between superpixels103, the amount or availability of EMR in a spectral region of interest, and the structural integrity of the micro-optical elements of spectral imager100.

In some embodiments, a spectral imager100configured as described here and as depicted inFIG.14Amay be useful in applications with EMR in the visible and near infrared regions (i.e., the VNIR region) of the EMR spectrum.FIG.14Adepicts an exemplary configuration of a superpixel array102, comprising four superpixels103positioned on a superpixel pitch1404of about 5 μm, each superpixel103having a 16 band (4×4) arrangement of pixels104, the pixels being on a pitch1402of about 1 μm and positioned to be exposed to spatially homogenized EMR109. The footprint of baffle bottom edges306is represented as thick black lines surrounding each 4×4 pixel group. At baffle bottom edges306, distance1302between opposing baffle108walls is about 4 μm. Each 4×4 pixel group is surrounded by a row of pixels104on each side, the surrounding pixels104also being on a pitch1401of about 1 μm and positioned beneath baffle bottom edges306and as such are not visible in the drawing. The row of surrounding pixels104is referred to as a “pixel street”. In this exemplary embodiment, the pixel street is blocked from exposure to spatially homogenized EMR109by baffle108walls. For use in the VNIR region, a spectral imager100comprising the embodiment shown inFIG.14Amay comprise a wafer substrate901that is thinned (as inFIG.9at step (g) or inFIG.13at step (b)) to a thickness of from about 3 μm to about 10 μm. The spectral imager100for use in the VNIR region may be further configured to comprise baffle inner surfaces112having an ˜50 nm thick coating904of ALD-protected Al or Ag if a reflective coating is desired. To produce a version of this configuration comprising dome-shaped baffles108, substrate901may be etched (as inFIG.13at step (c)) such that each baffle bottom edge306forms a substantially square or a square shape. In addition, spectral imager100may comprise microlenses1003integrated on the exterior of oxide layer1301, for enhancing the transmission of EMR through baffle opening308.

In some embodiments, spectral imager100configured as described here and as depicted inFIG.14Bmay be useful in applications with EMR in the UV and/or in the VNIR regions of the EMR spectrum.FIG.14Bdepicts an exemplary configuration of a superpixel array102, comprising four superpixels103positioned on a superpixel pitch1404of about 20 μm, each superpixel having a 16 band (4×4) arrangement of pixels104, the pixels being on a pitch1402of about 4 μm and positioned to be exposed to spatially homogenized EMR109. The footprint of baffle bottom edges306is represented as thick black lines surrounding each 4×4 pixel group. At baffle bottom edges306, distance1302between opposing baffle108walls is about 16 μm. Each 4×4 pixel group is surrounded by a pixel street comprising pixels104on each side, the surrounding pixels also being on a pitch1401of about 4 μm and positioned beneath baffle bottom edges306. In this exemplary embodiment, the pixel street is blocked from exposure to spatially homogenized EMR109by baffle108walls. The thick walls of baffles108function as structural streets in this exemplary embodiment, but this is not a requirement for the indicated application. A spectral imager100comprising the embodiment shown inFIG.14Bmay comprise a wafer substrate901that is thinned (as inFIG.9at step (g) orFIG.13at step (b)) to a thickness of from about 10 μm to about 20 μm, which will depend on the degree of anisotropy of the etch. In this embodiment, spectral imager100may be further configured to comprise baffle inner surfaces112having an ˜50 nm thick coating904of ALD-protected Al or Ag. To produce a version of this configuration using dome-shaped baffles108, the substrate901may be etched (as inFIG.13at step (c)) such that each baffle bottom edge306forms a square or substantially square shape. In addition, this embodiment may comprise microlenses integrated on the exterior of oxide layer1301, for enhancing the transmission of EMR through baffle opening308.

In some embodiments, spectral imager100configured as described here and as depicted inFIG.14Cmay be useful in applications with EMR in the UV and/or in the VNIR regions of the EMR spectrum.FIG.14Cdepicts an exemplary configuration of a superpixel array102, comprising four superpixels103positioned on a superpixel pitch1404of about 20 μm, each superpixel having a 25 band (5×5) arrangement of pixels104, the pixels being on a pitch1402of about 4 μm and positioned to be exposed to spatially homogenized EMR109. The footprint of baffle bottom edges306is represented as thick black lines surrounding each 5×5 pixel group. At baffle bottom edges306, distance1302between opposing baffle108walls is about 19 μm. Each 5×5 pixel group is surrounded by an ˜1 μm non-structural street on each side, the non-structural street being formed by baffle108walls. In this exemplary embodiment, the non-structural street only blocks a small region of each pixel104, at the edge of superpixels103, from exposure to spatially homogenized EMR109. A spectral imager100comprising the embodiment shown inFIG.14Cmay comprise a wafer substrate901that is thinned (as inFIG.9at step (g) orFIG.13at step (b)) to a thickness of from about 10 μm to about 20 μm. In this embodiment, spectral imager100may be further configured to comprise baffle inner surfaces112having an ˜50 nm thick coating904of ALD-protected Al or Ag. To produce a version of this configuration using dome-shaped baffles108, substrate901may be etched (as inFIG.13at step (c)) such that each baffle bottom edge306forms a square or substantially square shape. In addition, this embodiment may comprise microlenses integrated on the exterior of oxide layer1301, for enhancing the transmission of EMR through baffle opening308.

In some embodiments, spectral imager100configured as described here and as depicted inFIG.14Dmay be useful in applications with EMR in the SWIR region of the EMR spectrum.FIG.14Ddepicts an exemplary configuration of a superpixel array102, comprising four superpixels103positioned on a superpixel pitch1404of about 20 μm, each superpixel having a 16 band (4×4) arrangement of pixels104, the pixels being on a pitch1402of about 5 μm and positioned to be exposed to spatially homogenized EMR109. The footprint of baffle bottom edges306is represented as thick black lines surrounding each 4×4 pixel group. At baffle bottom edges306, distance1302between opposing baffle108walls is about 19 μm. Each 4×4 pixel group is surrounded by an ˜1 μm non-structural street on each side, the non-structural street being formed by baffle108walls. In this exemplary embodiment, the non-structural street only blocks a small region of each pixel104, at the edge of superpixels103, from exposure to spatially homogenized EMR109. By way of example only, a spectral imager100comprising the embodiment shown inFIG.14Dmay comprise a wafer substrate901that is thinned (as inFIG.9(g)orFIG.13(b)) to a thickness of from about 10 μm to about 20 μm, which will depend on the degree of anisotropy of the etch. In this embodiment, spectral imager100may be further configured to comprise baffle inner surfaces112having an ˜50 nm thick coating904of Au. To produce a version of this configuration using dome-shaped baffles108, substrate901may be etched (as inFIG.13at step (c)) such that each baffle bottom edge306forms a square or substantially square shape.

In some embodiments, spectral imager100configured as described here and as depicted inFIG.14Emay be useful in applications with EMR in the MWIR or LWIR regions of the EMR spectrum.FIG.14Edepicts an exemplary configuration of a superpixel array102, comprising four superpixels103positioned on a superpixel pitch1404of about 45 μm, each superpixel having a 9 band (3×3) arrangement of pixels104, the pixels being on a pitch1402of about 15 μm and positioned to be exposed to spatially homogenized EMR109. The footprint of baffle bottom edges306is represented as thick black lines surrounding each 3×3 pixel group. At baffle bottom edges306, distance1302between opposing baffle108walls is about 42 μm. Each 3×3 pixel group is surrounded by an approximately 3 μm non-structural street on each side, the non-structural street being formed by baffle108walls. In this exemplary embodiment, the non-structural street only blocks a small region of each pixel104, at the edge of superpixels103, from exposure to spatially homogenized EMR109. By way of example only, a spectral imager100comprising the embodiment shown inFIG.14Emay comprise a wafer substrate901that is thinned (as inFIG.9at step (g) orFIG.13at step (b)) to a thickness of from about 22 μm to about 45 μm, which will depend on the degree of anisotropy of the etch. For use with EMR in the LWIR region it may be preferable to remove oxide layer1301if one was added during fabrication, which may be done with an HF cleaning step for example. In this embodiment, spectral imager100may be further configured to comprise baffle inner surfaces112having an ˜50 nm thick coating904of Au. To produce a version of this configuration using dome-shaped baffles108, substrate901may be etched (as inFIG.13at step (c)) such that each baffle bottom edge306forms a square or substantially square shape.

In some embodiments, spectral imager100configured as described here and as depicted inFIG.14Fmay be useful in applications with EMR in the LWIR region of the EMR spectrum.FIG.14Fdepicts an exemplary configuration of a superpixel array102comprising four superpixels103, each superpixel having a 4×2 arrangement of pixels104, the pixels being on a pitch1402of about 15 μm and positioned to be exposed to spatially homogenized EMR109. On the longer edge of superpixel array102, superpixels103are positioned on a superpixel pitch1404aof about 60 μm. On the shorter edge of superpixel array102, superpixels103are positioned on a superpixel pitch1404bof about 30 μm. The footprint of baffle bottom edges306is represented as thick black lines surrounding each 4×2 pixel group. At baffle bottom edges306, distance1302abetween a first pair of opposing baffle108walls is about 55 μm. At baffle bottom edges306, distance1302bbetween a second pair of opposing baffle108walls is about 25 μm. Each 4×2 pixel group is surrounded by an approximately 5 μm non-structural street on each side, the non-structural street being formed by baffle108walls. In this exemplary embodiment, the non-structural street only blocks a small region of each pixel104, at the edge of superpixels103, from exposure to spatially homogenized EMR109. By way of example only, a spectral imager100comprising the embodiment shown inFIG.14Fmay comprise a wafer substrate901that is thinned (as inFIG.13at step (b)) to a thickness of from about 30 μm to about 60 μm, which will depend on the degree of anisotropy of the etch. For use with EMR in the LWIR region it may be preferable to remove oxide layer1301if one was added during fabrication, which may be done with an HF cleaning step for example. In this embodiment, spectral imager100may be further configured to comprise baffle inner surfaces112having a 50 nm thick coating904of Au. To produce a version of this configuration using dome-shaped baffles108, substrate901may be etched (as inFIG.13at step (c)) such that each baffle bottom edge306forms a rectangular or substantially rectangular shape.

It should be understood that the detailed description and the specific examples, while indicating specific embodiments of the invention, are given by way of illustration only, since various changes, alternatives, variations, and modifications within the spirit and scope of the invention are possible and would be apparent to those skilled in the art from this detailed description. Other objects, features and advantages of the present invention will be apparent from the detailed description.