Abstract:
An optical assembly is disclosed including two laterally variable bandpass optical filters stacked at a fixed distance from each other, so that the upstream filter functions as a spatial filter for the downstream filter. The lateral displacement may cause a suppression of the oblique beam when transmission passbands at impinging locations of the oblique beam onto the upstream and downstream filters do not overlap. A photodetector array may be disposed downstream of the downstream filter. The optical assembly may be coupled via a variety of optical conduits or optical fibers for spectroscopic measurements of a flowing sample.

Description:
BACKGROUND 
       [0001]    An optical filter may be used to transmit a spectral band or a spectral component of incoming light. A high pass filter, for example, transmits light at wavelengths longer than an edge wavelength of the filter. Conversely, a low pass filter transmits light at wavelengths shorter than an edge wavelength. A bandpass filter transmits light at wavelengths proximate to a center wavelength of the filter within a bandwidth of the filter. A tunable bandpass filter is an optical filter, the center wavelength of which may be adjusted or tuned. 
         [0002]    A spectrometer may measure an optical spectrum of incoming light. A scanning-type spectrometer may use one or more tunable bandpass filters to select different spectral components of the incoming light. A scanning-type spectrometer operates by scanning the center wavelength of the tunable bandpass filter while measuring optical power levels of light transmitted through the tunable bandpass filter, so as to obtain the optical spectrum. Alternatively, a polychromator-type spectrometer uses a wavelength-dispersing element optically coupled to a photodetector array for parallel detection of the optical spectrum. 
         [0003]    Conventional optical filters and spectrometers are bulky, which limits their usefulness in portable light-sensing devices and applications. Linearly variable filters have been used in spectrometers to provide a wavelength separating function. Referring to  FIG. 1A , a conventional linearly variable filter  10  may be illuminated with white light, which includes top  11 , middle  12 , and bottom  13  multi-wavelength light beams. The top  11 , middle  12 , and bottom  13  multi-wavelength light beams may strike the linearly variable filter  10  at respective top  11 A, middle  12 A, and bottom  13 A locations. The linearly variable filter  10  may have a center wavelength of a passband varying linearly along an x-axis  18 . For instance, the filter  10  may transmit a short wavelength peak  11 B at the top location  11 A; a middle wavelength peak  12 B at the middle location  12 A; and a long wavelength peak  13 B at the bottom location  13 A. 
         [0004]    Referring to  FIG. 1B , a conventional spectrometer  19  may include the linearly variable filter  10  of  FIG. 1A , a tapered light pipe  14  disposed upstream of the linearly variable filter  10 , and a linear array  15  of photodetectors disposed downstream of the linearly variable filter  10 . In operation, non-collimated incoming light  16  may be conditioned by the light pipe  14  to produce a partially collimated light beam  17 . The linearly variable filter  10  may transmit light at different wavelengths as explained above with reference to  FIG. 1A . The tapered light pipe  14  may reduce a solid angle of the incoming light  16 , thereby improving spectral selectivity of the linearly variable filter  10 . The linear array  15  of photodetectors may detect optical power levels of light at different wavelengths, thereby obtaining an optical spectrum, not shown, of the incoming light  16 . 
         [0005]    The tapered light pipe  14  may often be the largest element of the spectrometer  19 . A collimating element, such as tapered light pipe  14 , may be needed because without it, the spectral selectivity of the linearly variable filter is degraded. This may happen because the linearly variable filter  10  includes a stack of thin dielectric films. The wavelength-selective properties of thin film filters are generally dependent on the angle of incidence of incoming light, which may deteriorate spectral selectivity and wavelength accuracy of thin film filters. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1A  is a diagram of a conventional linearly variable filter; 
           [0007]      FIG. 1B  is a diagram of a conventional optical spectrometer that includes the linearly variable filter of  FIG. 1A ; 
           [0008]      FIG. 2A  is a diagram of an optical filter, including a pair of laterally variable bandpass filters spaced apart and fixed relative to each other; 
           [0009]      FIG. 2B  is a diagram of center wavelength dependences of the laterally variable bandpass filters of  FIG. 2A ; 
           [0010]      FIG. 2C  is a diagram of a side view of the optical filter of  FIG. 2A  illustrating a principle of spatial filtering by the optical filter; 
           [0011]      FIG. 3  is a diagram of the optical filter of  FIG. 2A  in a side cross-sectional view showing an acceptance angle of the optical filter; 
           [0012]      FIG. 4A  is a diagram of a top view of a fiber-coupled optical spectrometer assembly including a straight optical conduit; 
           [0013]      FIG. 4B  is a diagram of a side cross-sectional view of the fiber-coupled optical spectrometer assembly of  FIG. 4A ; 
           [0014]      FIG. 4C  is a diagram of a top view of a variant of the fiber-coupled optical spectrometer assembly of  FIG. 4A ; 
           [0015]      FIG. 5  is a diagram of a side view of the optical spectrometer assembly of  FIGS. 4A and 4B  including a slanted relay lightpipe for transmission spectral measurements of fluids or flowing granular materials; 
           [0016]      FIGS. 6A and 6B  are diagrams of top and side cross-sectional views, respectively, of a spectrometer assembly equipped with a flow cuvette having a slab cavity; 
           [0017]      FIGS. 7A and 7B  are diagrams of top and side cross-sectional views, respectively, of a spectrometer assembly equipped with a flow cuvette having a cylindrical cavity; 
           [0018]      FIGS. 8A-8D  are schematic plan views of segmented laterally variable optical filters; 
           [0019]      FIG. 9  is a schematic cross-sectional view of an optical assembly including the segmented first and second optical filters of  FIG. 8A  and a photodetector array; 
           [0020]      FIG. 10A  is a three-dimensional view of an optical assembly comprising the segmented laterally variable optical filter of  FIG. 8B  and a 2D photodetector array; 
           [0021]      FIG. 10B  is a diagram of a schematic three-dimensional view of an optical assembly comprising the segmented laterally variable optical filters of  FIG. 8B  and a plurality of photodetector arrays; 
           [0022]      FIG. 11  is a schematic side view of an optical filter assembly including a circular polarizer; 
           [0023]      FIG. 12A  is a diagram of a side cross-sectional view of an optical assembly comprising an optical objective for multispectral imaging; 
           [0024]      FIG. 12B  is a diagram of a plan view of an image of an object overlaid onto a two-dimensional detector array of the optical assembly of  FIG. 12A ; and 
           [0025]      FIGS. 13, 14, and 15  are flow charts of methods of manufacture of various embodiments of optical spectrometer assemblies of the disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    The following detailed description refers to the accompanying drawings. The same reference numbers in different drawings may identify the same or similar elements. 
         [0027]      FIGS. 2A and 2B  are diagrams of an optical assembly  20  ( FIG. 2A ) for spectral filtering of light according to an example implementation described below. For example, the optical assembly  20  may include sequentially disposed first  21 A and second  21 B laterally variable bandpass optical filters separated by a distance L in an optical path  22  of signal light  23 . The second laterally variable bandpass optical filter  21 B may be fixed relative to the first laterally variable bandpass optical filter  21 A in the optical path  22  downstream of the first laterally variable bandpass optical filter  21 A. In other words, the second laterally variable bandpass optical filter  21 B may be disposed and fixed so that it may not be moved laterally with respect to the first laterally variable bandpass optical filter  21 A. As shown in  FIG. 2B , the first  21 A and second  21 B laterally variable bandpass optical filters each may have a bandpass center wavelength λ T  varying in a mutually coordinated fashion, that is, varying with distance along a common first direction  25  represented by x-axis. The first direction  25  is transversal to the optical path  22 . The term “laterally variable” as used herein is defined to mean that the bandpass center wavelength λ T  varies in any direction transversal to the optical path  22  such as, for example, the first direction  25 . By way of a non-limiting example, the bandpass center wavelength λ T  of both the first  21 A and second  21 B laterally variable bandpass optical filters of  FIG. 2A  may have respective monotonic, e.g. linear dependences  24 A,  24 B, as shown in  FIG. 2B . The center wavelength dependences λ IT (x) and λ 2T (x) of the first  21 A and second  21 B laterally variable bandpass optical filters, respectively, on the distance along the first direction  25 , represented by the x-coordinate, may be identical, or may be shifted with respect to each other. For example, the center wavelength dependences λ IT (x) and λ 2T (x) may be such that λ 2T  (x)=λ IT (x+x 0 ), where x 0  is a constant; or scaled e.g. λ 2T  (x)=cλ IT  (x), where c is a constant e.g. 0.9&lt;c&lt;1.1. The term “coordinated fashion” or “mutually coordinated” as used herein with respect to the bandpass center wavelength λ T  is defined to mean a pre-determined functional relationship between the center wavelength dependences λ IT (x) and λ 2T (x) of the first  21 A and second  21 B laterally variable bandpass optical filters, respectively. 
         [0028]    The configuration of the optical assembly  20  may enable a dependence of spectral selectivity of the optical assembly  20  on a degree of collimation of the signal light  23  to be lessened as compared to a corresponding dependence of spectral selectivity of the second laterally variable bandpass optical filter  21 B on the degree of collimation of the signal light  23 . This performance improvement of the optical assembly  20  may result from a spatial filtering effect illustrated in  FIG. 2C . In monochromatic light at a wavelength λ 0 , the first  21 A and second  21 B laterally variable bandpass optical filters may be approximately represented by slits having “openings”  26  corresponding to locations along the x-axes where the center wavelength λ T =λ 0 . Outside of the “openings”  26 , the first  21 A and second  21 B laterally variable bandpass optical filters may be essentially opaque for the monochromatic light at the wavelength λ 0 . The “openings”  26  define an acceptance cone, or solid angle  27  (2θ), which depends on the inter-filter distance L. Any rays outside of the solid angle  27  may be blocked, thus improving the spectral selectivity of the second laterally variable bandpass optical filter  21 B. 
         [0029]    The operation of the optical assembly  20  of  FIGS. 2A-2C  may be further explained by referring to  FIG. 3  showing the optical assembly  20  in a side cross-sectional view. As shown in  FIG. 3 , the center wavelength λ T  may increase from left to right along the first direction  25 , shown as the x-coordinate, for both the first  21 A and second  21 B laterally variable bandpass optical filters. In  FIG. 3 , the bandpass center wavelengths λ T  of the first  21 A and second  21 B laterally variable bandpass optical filters may be linearly dependent on the x-coordinate: 
         [0000]      λ T =λ 0   +DΔx   (1)
 
         [0030]    where λ 0  represents a reference bandpass center wavelength at a reference point x 0 , D represents the proportionality coefficient, herein termed a “slope” of a laterally variable filter, and Δx represents an offset from the reference point x 0 . The slope D may correspond to the slopes of the linear dependences  24 A and  24 B in  FIG. 2B , which may, but does not have to, be identical to each other. Deviations from identical slopes of the linear dependences  24 A and  24 B may be advantageous in some applications. 
         [0031]    In the example implementation of  FIG. 3 , the first  21 A and second  21 B laterally variable bandpass optical filters may be aligned with each other, so that the reference point x 0  corresponding to the reference bandpass center wavelength λ 0  of the second laterally variable bandpass optical filter  21 B is disposed directly under the reference point x 0  corresponding to the reference bandpass center wavelength λ 0  of the first laterally variable bandpass optical filter  21 A. The first laterally variable bandpass optical filter  21 A may function as a spatial filter for the second laterally variable bandpass optical filter  21 B, defining an angle of acceptance  30  for the second laterally variable bandpass optical filter  21 B. The angle of acceptance  30  may be limited by left  31 L and right  31 R marginal rays at the reference wavelength λ 0 , each propagating at the angle θ to a normal  32  to the first  21 A and second  21 B laterally variable bandpass optical filters and striking second laterally variable bandpass optical filter  21 B at the same reference point x 0 . The angle of acceptance  30  may be derived from a passband  33 A of the first laterally variable bandpass optical filter  21 A as follows. 
         [0032]    In the geometry illustrated in the example implementation of  FIG. 3 , the left marginal ray  31 L may strike the first laterally variable bandpass optical filter  21 A at a location x 0 −λx. Transmission wavelength λ L  at that location may be, according to Eq. (1), λ L =λ 0 −Dλx. Since the left marginal ray  31 L is at the reference wavelength λ 0 , the left marginal ray  31 L may be attenuated depending on the width of the passband  33 A of the first laterally variable bandpass optical filter  21 A; for sake of this example, a 10 dB bandwidth is taken to be 2Dλx. Thus, the left marginal ray  31 L may be attenuated by 10 dB. Similarly, the right marginal ray  31 R may strike the first laterally variable bandpass optical filter  21 A at a location x 0 +λx. Transmission wavelength λ R  at that location may be, according to Eq. (1), λ R =λ 0 +DΔV. The right marginal ray  31 R may also be attenuated by 10 dB. All rays at the reference wavelength λ 0  within the acceptance angle  30  may be attenuated by a value smaller than 10 dB; and all rays at the reference wavelength λ 0  outside the acceptance angle  30  may be attenuated by a value larger than 10 dB. The first laterally variable bandpass optical filter  21 A may function as a spatial filter, effectively limiting the numerical aperture (NA) of incoming light to be separated in individual wavelengths by the second laterally variable bandpass optical filter  21 B. This may result in reduction of the dependence of spectral selectivity of the optical assembly  20  in comparison with the corresponding dependence of the spectral selectivity of the single second laterally variable bandpass optical filter  21 B on the degree of collimation of the signal light  23 . If the first laterally variable bandpass optical filter  21 A were absent in the optical assembly  20 , the spectral selectivity of the optical assembly  20  would be much more dependent on the degree of collimation of the signal light  23 . Typically, the signal light  23  may result from scattering or luminescence of a sample, not shown, so that the signal light  23  is not collimated. The lack of collimation of the signal light  23  in the absence of the first laterally variable bandpass optical filter  21 A would result in worsening of overall spectral selectivity unless a dedicated collimating element, such as a tapered light pipe, is used. Herein, the term “spectral selectivity” includes such parameters as passband width, stray light rejection, in-band and out-of-band blocking, etc. 
         [0033]    For small angles θ, for example θ&lt;5° 
         [0000]      θ≈Δ x/L   (2), or
 
         [0000]        L≈Δ   x /θ  (3)
 
         [0034]    When the space between the first  21 A and second  21 B laterally variable bandpass optical filters is filled with a transparent medium having a refractive index n, Eq. (3) becomes 
         [0000]        L/n≈Δx/θ   (4)
 
         [0035]    Eq. (4) may define an approximate relationship between the inter-filter distance L, the refractive index n of the inter-filter gap, a lateral distance Δx along the first direction  25 , corresponding to a bandwidth of the first laterally variable bandpass optical filter  21 A, and the resulting acceptance half-angle θ. A more precise relationship may take into account the wavelength offset due to non-zero angle of incidence, which typically results in a blue shift (i.e. towards shorter wavelength) of the bandpass center wavelength λ T . For instance, the right marginal ray  31 R at the reference wavelength λ 0  striking the first laterally varying bandpass optical filter  21 A at the position x 0 +Δx may be tilted by the angle θ, which shifts the transmission characteristic of the first laterally varying bandpass optical filter  21 A to shorter wavelengths. If this wavelength dependence is to be accounted for, the shoulders of the passband  33 A may shift to the left i.e. shorter wavelengths: 
         [0000]      λ 1 ≈[(λ 0   +DΔx )( n   eff   2 −θ 2 ) 1/2   ]/n   eff   (5)
 
         [0036]    where n eff  represents an effective refractive index of the first laterally variable bandpass optical filter  21 A. 
         [0037]    Although in  FIG. 2B , the first  21 A and second  21 B laterally variable bandpass filters have linearly variable bandpass center wavelengths λ T  as defined by Eq. (1) above, the center wavelengths λ T  of the first  21 A and second  21 B laterally variable bandpass optical filters may be monotonically non-linearly, for example parabolically or exponentially, increasing or decreasing in the first direction  25 . The bandpass center wavelengths λ T  dependence may also be non-gradual, e.g., stepwise. The dependence of the bandpass center wavelength λ T  on the x-coordinate along the first direction  25  of the first  21 A and second  21 B laterally variable filters may be identical, or may be different to enable optimizing or varying of the acceptance angle and/or wavelength response of the optical assembly  20 . In one embodiment, the bandpass center wavelengths λ T  of the first  21 A and second  21 B laterally variable bandpass optical filters may be aligned with each other, such that a line connecting positions corresponding to a same bandpass center wavelength λ T  of the first  21 A and second  21 B laterally variable bandpass optical filters forms an angle of less than 45 degrees with the normal  32  to the second laterally variable bandpass optical filter  21 B. For non-zero angles with the normal  32 , the acceptance cone  30  may appear tilted. Thus, it may be possible to vary the acceptance cone  30  direction by offsetting the first  21 A and second  21 B laterally variable bandpass optical filters relative to each other in the first direction  25 . Furthermore, the angle may vary along the first direction (x-axis)  25 . 
         [0038]    For a better overall throughput, it may be preferable to have a lateral distance λx i  along the first direction  25 , corresponding to a bandwidth of the first laterally variable bandpass optical filter  21 A larger than a corresponding lateral distance Δx 2  along the first direction  25 , corresponding to a bandwidth of the second laterally variable bandpass optical filter  21 B. In one embodiment, the first  21 A and second  21 B laterally variable bandpass optical filters each may have a 3 dB passband no greater than 10% of a corresponding bandpass center wavelength λ T . 
         [0039]    The first  21 A and/or second  21 B laterally variable bandpass optical filters may include a thin film layer stack including two, three, and more different materials, e.g., high-index and/or absorbing layers may be used to reduce overall thickness of each of the first  21 A and second  21 B laterally variable bandpass optical filters. The first  21 A and/or the second  21 B laterally variable bandpass optical filters may include diffraction gratings e.g. sub-wavelength gratings, dichroic polymers, etc. An additional laterally variable bandpass optical filter may be provided in the optical path, the additional filter having a bandpass center wavelength varying in a coordinated fashion with the bandpass center wavelengths of the first  21 A and second  21 B laterally variable bandpass optical filters. 
         [0040]      FIGS. 4A and 4B  are diagrams of an optical spectrometer assemblies  40  according to an example implementation described below. The optical spectrometer assembly  40  of  FIGS. 4A  and  4 B may include, for example, the optical assembly  20  of  FIG. 2A  and may further include an optical fiber  41  extending between its first  41 A and second  41 B ends for conducting the signal light  23  from the first end  41 A to the second end  41 B. 
         [0041]    An optical conduit  42  may extend between its first  42 A and second  42 B surfaces. The first surface  42 A may be optically coupled, i.e. via an air gap or by a direct physical contact, to the second end  41 B of the optical fiber  41  for receiving the signal light  23  and conducting the signal light  23  in the optical conduit  42  from the first surface  42 A to the second surface  42 B. The second surface  42 B may be optically coupled to the first laterally variable bandpass optical filter  21 A for receiving the signal light  23  for propagation along the optical path  22 . A multi-element sensor  43 , such as a photodetector array, may be optically coupled to the second laterally variable bandpass optical filter  21 B. The sensor  43  may include photodetectors  43 A disposed along the first direction  25  for wavelength selective detection of the signal light  23  propagated through the second laterally variable bandpass optical filter  21 B. 
         [0042]    In the exemplary embodiment shown in  FIGS. 4A and 4B , the optical conduit  42  may include a planar parallel slab of homogeneous transparent material, for example glass or an injection-molded transparent plastic material. The slab may have a plurality of external surfaces, for example the first  42 A and second  42 B surfaces, which may be flat or curved. The slab may be configured for unconstrained propagation of the signal light  23 , e.g. the slab may be continuous or hollow. The slab may be disposed generally parallel to the first direction  25 , and optionally mechanically coupled to the first laterally variable bandpass optical filter  21 A. 
         [0043]    A portion  23 A of the signal light  23  may be reflected from the first laterally variable bandpass optical filter  21 A. The portion  23 A may include light at wavelengths other than the transmission wavelength at a particular reflection location of the first laterally variable bandpass optical filter  21 A. To recycle the portion  23 A, the optical conduit  42  may include a reflective wall or walls  44  for redirecting at least a portion of the reflected light portion  23 A back to the first laterally variable bandpass optical filter  21 A. 
         [0044]    Turning to  FIG. 4C , an optical spectrometer assembly  45  is shown according to an example implementation described below. The optical spectrometer assembly  45  of  FIG. 4C  may further include an elbowed optical conduit  46  instead of the straight optical conduit  42 . The elbowed optical conduit  46  may enable a more compact mechanical configuration. The elbowed optical conduit  46  may have the first surface  42 A, the second surface  42 B, and a third surface  42 C, e.g. a flat or curved surface disposed in the optical path  22  between the first  42 A and second  42 B surfaces, for receiving the signal light  23  from the first surface  42 A and reflecting the signal light  23  towards the second surface  42 B. The third surface  42 C may be optionally mirrored, or left uncoated when the refractive index of the elbowed optical conduit  46  is high enough for the signal light  23  to reflect by total internal reflection (TIR): n&gt;1/sin(α), where n is the refractive index of the conduit  46 , and a is the angle of incidence of the signal light  23  on the third surface  42 C. The straight optical conduit  42  or the elbowed optical conduit  46  may include multiple conduit branches coupled to multiple individual optical fibers, not shown. 
         [0045]    Referring to  FIG. 5 , an optical spectrometer assembly  50  is shown according to an example implementation described below. The optical spectrometer assembly  50  of  FIG. 5  may include an optical probe  52  optically coupled to the first end  41 A of the optical fiber  41 , for collecting the signal light  23  emanating from a fluid or granular sample  51  when the sample  51  is illuminated with illuminating light  53 , and for coupling the signal light  23  to the first end  41 A of the optical fiber  41 . In the example implementation shown in  FIG. 5 , the fluid or granular sample  51  is held in a cuvette  55  having a transparent window  58  at the bottom for transmitting through the illuminating light  53 . For instance, the signal light  23  may represent transmitted illuminating light  53 , or scattered illuminating light  53 , or luminescence, such as fluorescence or phosphorescence. 
         [0046]    Still referring to  FIG. 5 , the optical probe  52  may include a relay lightpipe  59  extending between its first  59 A and second  59 B ends. The first end  59 A, herein termed “distal” end, that is the farthest from the optical fiber  41 , may be configured for contacting or inserting into the sample  51 , thereby collecting the signal light  23  emanating from the sample  51 , and the second end  59 B, herein termed “proximal” end, that is, the closest to the optical fiber  41 , may be configured for optical and mechanical coupling to the first end  41 A of the optical fiber  41 . The relay lightpipe  59  of the optical probe  52  may be configured for unconstrained propagation of the signal light  23  in bulk of the relay lightpipe from the first  59 A to the second  59 B end. For instance, the relay lightpipe  59  may be made of glass or a rigid transparent, chemically inert plastic, so that it can be inserted through a fluid or granular overlayer  57  down to the sample  51 . The relay lightpipe  59  may also be made hollow, with mirrored internal walls. 
         [0047]    In the example implementation shown in  FIG. 5 , the first (distal) end  59 A of the relay lightpipe  59  may include a slanted optical surface  56 , which may cause the sample  51  flowing in a direction  54  to exert a pressure onto the slanted optical surface  56 , which may facilitate the collection of the signal light  23 , especially for granular samples  51  or samples  51  including a fluid suspension of a solid material. 
         [0048]    It is to be understood that the relay lightpipe  59  is only one possible embodiment of the optical probe  52 . Other embodiments of the optical probe  52  may include an irradiance probe, a reflection/backscatter probe, a transmission cuvette, an oxygen probe, a fluorescence or phosphorescence probe, etc. The optical fiber  41  may include a bifurcated fiber including a branch for delivering the illuminating light  53  to the transmission cuvette, for example. 
         [0049]    Referring now to  FIGS. 6A and 6B , an example implementation of a flow spectrometer optical assembly  60  may include a light source  61  for providing the illuminating light  53 , an elongated optical cuvette  62  extending generally parallel to the first direction  25  ( FIG. 6B ), the optical assembly  20  of  FIG. 2A , and the sensor  43 . 
         [0050]    The elongated optical cuvette  62  may include an inlet  63 A for receiving the sample  51  in fluid form, a substantially transparent sidewall  64  defining a cavity  65  in fluid communication with the inlet  63 A, for receiving and containing the sample  51  while transmitting the illuminating light  53  through the sidewall  64  for illuminating the sample  51  received in the cavity  65 . Upon illumination, the sample  51  received by the cavity  65  emits the signal light  23 . The transparent sidewall  64  may be configured for transmitting the signal light  23  through the transparent sidewall  64  for optical coupling the signal light  23  to the first laterally variable bandpass optical filter  21 A for propagation along the optical path  22 . The elongated optical cuvette  62  may further include an outlet  63 B in fluid communication with the cavity  65 , for outputting the sample  51  illuminated with the illuminated light  53 . 
         [0051]    The sensor  43  may be optically coupled to the second laterally variable bandpass optical filter  21 B. The photodetectors  43 A of the sensor  43  may be disposed along the first direction  25  for wavelength selective detection of the signal light  23  propagated through the second laterally variable bandpass optical filter  21 B. For a more uniform illumination of the sample  23  in the cavity  65 , the light source  61  may be elongated as shown in  FIG. 6B , extending generally parallel to the first direction  25 . For example, an incandescent lamp having a tungsten spiral extending along the first direction  25  may be used. The wall  64  of the elongated optical cuvette  62  may function as a lens facilitating refracting or focusing the illuminating light  53  onto the cavity  65  containing the sample  51 , and/or facilitating refracting or focusing the signal light  23  onto the sensor  43  ( FIG. 6A ). 
         [0052]    In the example implementation shown in  FIGS. 6A and 6B , the cavity  65  has a slab portion  65 A extending parallel to the first direction  25 , e.g. a planar parallel slab. This may enable the liquid sample  23  to be thin in the cavity  65 , for example thinner than 1 mm, or thinner than 2 mm if the light source  61  has a high optical power, for instance when the light source  61  includes, or is coupled to, a laser source. Small thickness may be useful for obtaining absorption spectra of aqueous solutions dominated by vibrational frequencies of water. 
         [0053]    Turning to  FIGS. 7A and 7B , an example implementation of a flow spectrometer optical assembly  70  is shown. The flow spectrometer optical assembly  70  of  FIGS. 7A and 7B  includes a flow cuvette  72  having an inlet  73 A, an outlet  73 B, a transparent sidewall  74  defining a cavity  75  having a cylindrical portion  75 A having an optical axis  75 B, which may extend substantially parallel to the first direction  25 . The cylindrical portion  75 A of the cavity  75  allows for a larger volume of the sample  51  to be held therein, which may be more suitable for obtaining absorption spectra of organic solutions. Specific applications may require other path lengths. Similarly to the flow spectrometer optical assembly  60  of  FIGS. 6A and 6B , the transparent sidewall  74  of the flow spectrometer optical assembly  70  of  FIGS. 7A and 7B  may function as a lens facilitating refracting the illuminating light  53  onto the cavity  75  containing the sample  51  and/or facilitating focusing the signal light  23  onto the sensor  43  ( FIG. 7A ). 
         [0054]    In one embodiment, the sensor  43  may include a 2D array of photodetectors, including multiple rows of the photodetectors  43 A. Preferably, each such row may extend parallel to the first direction  25 . The 2D array of photodetectors may be used to simultaneously obtain spectra of the signal light  23  in different wavelength ranges. 
         [0055]    In an example implementation, the first  21 A or second  21 B laterally variable bandpass optical filters, or both  21 A and  21 B laterally variable bandpass optical filters of the optical assembly  20  ( FIG. 2A ) may be segmented.  FIGS. 8A-8D  are diagrams of schematic plan views of optical assemblies according to example implementations described below. Referring specifically to  FIG. 8A , first  221 A and second  221 B segmented laterally variable bandpass optical filters of an optical assembly  80 A may each include an array  85 A of bandpass optical filter segments e.g.  81 A,  82 A,  83 A,  84 A for the first segmented laterally variable bandpass optical filter  221 A, arranged side by side in the first direction  25 ; and an array  85 B of bandpass optical filter segments  81 B,  82 B,  83 B,  84 B for the second segmented laterally variable bandpass optical filter  221 B, arranged side by side in the first direction  25 . 
         [0056]    Each bandpass optical filter segment  81 A- 84 A of the first segmented laterally variable bandpass optical filter  221 A may have a laterally invariable, i.e. constant, transmission center wavelength λ T  different from a transmission center wavelength λ T  of an immediate neighboring bandpass optical filter segment  81 A- 84 A. For example, the transmission center wavelength λ T  of the second bandpass optical filter segment  82 A may be different from the transmission center wavelength λ T  of the first bandpass optical filter segment  81 A and the third bandpass optical filter segment  83 A, and so on. The same rule may hold for the second segmented laterally variable bandpass optical filter  221 B: each bandpass optical filter segment  81 B,  82 B,  8 AB,  84 B of the second segmented laterally variable bandpass optical filter  221 B may have a laterally invariable, i.e. constant, transmission center wavelength λ T  different from a transmission center wavelength λ T  of an immediate neighboring bandpass optical filter segment  81 B- 84 B. As a result, the bandpass center wavelengths of the first  221 A and second  221 B segmented laterally variable bandpass optical filters may laterally vary stepwise from segment to segment, and/or non-monotonically from segment to segment. 
         [0057]    As illustrated by arrows  82  in  FIG. 8A , the transmission center wavelengths λ T  of the bandpass optical filter segments  81 A,  81 B,  81 C, and  81 D of the first  221 A and second  221 B segmented laterally variable bandpass optical filters may be mutually coordinated. By way of a non-limiting example, the transmission center wavelengths λ T  may be equal to each other: the transmission center wavelength λ T  of the first bandpass optical filter segment  81 A may be equal to the transmission center wavelength λ T  of the first second bandpass optical filter segment  81 B, and so on. The transmission bandwidths of the corresponding bandpass optical filter segments of the first  221 A and second  221 B segmented laterally variable bandpass optical filters may be equal to each other, e.g. no greater than 10%, and more preferably no greater than 2% of the corresponding transmission center wavelengths λ T  of the bandpass optical filter segments  81 A- 84 A. For a better overall throughput of the optical assembly  80 A, transmission bandwidths of the bandpass optical filter segments  81 A- 84 A of the first segmented laterally variable bandpass optical filter  221 A may be greater than transmission bandwidths of the corresponding bandpass optical filter segments  81 B- 84 B of the second segmented laterally variable bandpass optical filter  221 B. By way of an illustrative, non-limiting example, the transmission bandwidths of the bandpass optical filter segments  81 A- 84 A of the first segmented laterally variable bandpass optical filter  221 A may be no greater than 2% of the corresponding transmission center wavelengths λ T  of the bandpass optical filter segments  81 A- 84 A, while the transmission bandwidths of the bandpass optical filter segments  81 B- 84 B of the second segmented laterally variable bandpass optical filter  221 B may be no greater than 1% of the corresponding transmission center wavelengths λ T  of the bandpass optical filter segments  81 B- 84 B. 
         [0058]    Turning to  FIG. 8B , an optical assembly  80 B according to an example implementation, may be a two-dimensional (2D) segmented optical filter assembly. The first  221 A and second  221 B segmented laterally variable bandpass optical filters of the optical assembly  80 B may each include 2D arrays of the bandpass optical filter segments  81 A- 84 A and  81 B- 84 B. By way of illustration, the first segmented laterally variable bandpass optical filter  221 A may include four one-dimensional arrays  85 A,  86 A,  87 A,  88 A arranged side by side in the second direction  25 ′ and combined into a two-dimensional array, each such one-dimensional array  85 A- 88 A including the bandpass optical filter segments  81 A- 84 A having transmission center wavelengths λ T  unique to the entire two-dimensional array and arranged side by side in the first direction  25 . Similarly, the second segmented laterally variable bandpass optical filter  221 B may include one-dimensional arrays  85 B,  86 B,  87 B,  88 B arranged side by side in the second direction  25 ′ and combined into a two-dimensional array, each such one-dimensional array  85 B- 88 B including the bandpass optical filter segments  81 B- 84 B having a unique transmission center wavelength λ T  and arranged side by side in the first direction  25 . The transmission center wavelengths λ T  of the bandpass optical filter segments  81 A- 84 A and  81 B- 84 B of the first  221 A and second  221 B segmented laterally variable bandpass optical filters may be mutually coordinated along the first direction  25  and along a second direction  25 ′ perpendicular to the first direction  25  and transversal to the optical path  22  (not shown in  FIGS. 8A, 8B ). In one embodiment, a black grid  89  separating neighboring bandpass optical filter segments  81 A- 84 A or  81 B- 84 B of at least one of the first  221 A and second  221 B segmented laterally variable bandpass optical filters may be provided for suppressing light leakage between neighboring bandpass optical filter segments  81 A- 84 A or  81 B- 84 B. 
         [0059]    According to one aspect of the disclosure, the transmission center wavelengths λ T  of neighboring bandpass optical filter segments  81 A- 84 A and  81 B- 84 B for each array  85 A- 88 A and  85 B- 88 B need not be successive, that is, need not be disposed in an increasing or decreasing order. The stepwise laterally variable bandpass center wavelength of the first  221 A or second  221 B segmented laterally variable bandpass optical filters needs not be monotonically increasing or decreasing. In fact, it may be preferable to “scramble” the transmission center wavelength λ T , so neighboring bandpass optical filter segments  81 A- 84 A and  81 B- 84 B for each array  85 A- 88 A and  85 B- 88 B differ in the transmission center wavelength λ T  by a magnitude larger than a “typical” wavelength increment of the transmission center wavelength λ T . By way of a non-limiting example, referring to  FIG. 8C , transmission center wavelengths λ T  of neighboring bandpass optical filter segments  81 A- 84 A and  81 B- 84 B of a segmented filter  80 C are shown (in nanometers) for each array  85 A- 88 A. In  FIG. 8C , the top left segment  81 A of the top row  88 A has the transmission center wavelength λ T =700 nm, while its immediate neighbor to the right  82 A has the transmission center wavelength λ T =900 nm, and its immediate neighbor below  87 A has the transmission center wavelength λ T =1050 nm. The transmission center wavelengths λ T  of the bandpass optical filter segments  81 A- 84 A and  81 B- 84 B of the first  221 A and second  221 B segmented laterally variable bandpass optical filters may be spread across a wavelength range with a constant or variable wavelength step such that the transmission center wavelengths λ T  of the neighboring bandpass optical filter segments  81 A- 84 A and  81 B- 84 B of the first  221 A and second  221 B segmented laterally variable bandpass optical filters differ at least by an integer multiple of the constant or variable wavelength step. For instance, if the wavelength step is 25 nm, that is, the transmission center wavelength λ T  of the bandpass optical filter segments  81 A- 84 A and/or  81 B- 84 B includes the values of 700 nm; 725 nm; 750 nm; and so on, the transmission center wavelengths λ T  of the neighboring bandpass optical filter segments  81 A- 84 A and  81 B- 84 B of the first  221 A and second  221 B segmented laterally variable bandpass optical filters may differ at least by 125 nm=5*25 nm, that is, five times the wavelength step. For example, the minimum difference between the transmission center wavelengths λ T  of the neighboring bandpass optical filter segments in each individual array  85 A- 88 A, that is, in horizontal direction in  FIG. 8C , is between the leftmost bottom bandpass optical filter segments  81 A (1000 nm) and  82 A (875 nm) in the bottom array  85 A. All the other differences in each individual array  85 A- 88 A in  FIG. 8C , that is, in horizontal direction, are larger. The differences in vertical direction may be somewhat smaller in this example, e.g. at least 75 nm=3*25 nm, that is, three times the wavelength step. Thus, the differences in the transmission center wavelengths λ T  of the horizontal or vertical optical filter segments  81 A- 84 A and/or  81 B- 84 B may be at least three times the wavelength step. The wavelength step may be variable i.e. the transmission center wavelength λ T  of the optical filter segments  81 A- 84 A and/or  81 B- 84 B may include, for example, the values of 700 nm; 711 nm; 722 nm; 733 nm; and so on. The total number of the optical filter segments  81 A- 84 A and/or  81 B- 84 B may of course vary. The bandpass optical filter segments  81 A- 84 A of the first  221 A or second  221 B segmented laterally variable bandpass optical filters may include a colored glass, an absorptive pigment, or a dye, for absorption of light at wavelengths other than wavelengths of corresponding passbands of the bandpass optical filter segments  81 A- 84 A. 
         [0060]    In one embodiment, the first  221 A or second  221 B segmented laterally variable bandpass optical filters may have a segmented portion and a continuously varying portion. For instance, referring to  FIG. 8D , an upstream filter  321 A of an optical assembly  80 D is a continuously varying λ T  filter, and a downstream filter  321 B of the optical assembly  80 D includes a continuously varying portion  21 B′ and a segmented portion  21 B″. Similarly to the optical assembly  20  of  FIG. 2A , the bandpass center wavelengths of these upstream  321 A and downstream  321 B filters of the optical assembly  80 D of  FIG. 8D  may vary in a mutually coordinated fashion along the first direction  25  and/or along the second direction  25 ′. 
         [0061]    Turning to  FIG. 9  with further reference to  FIGS. 2A and 8A , an optical spectrometer assembly  90  may include a sensor  93  optically coupled to the second laterally variable bandpass optical filter  21 B of the optical assembly  20  of  FIG. 2A  or the second segmented laterally variable bandpass optical filter  221 B of the optical assembly  80 A of  FIG. 8A . The sensor  93  may have a one-dimensional array of photodetectors  93 A disposed along the first direction  25  separated by boundaries  93 B between the individual photodetectors  93 A. Thus, the photodetectors  93 A may be disposed for wavelength selective detection of the signal light  23  propagated through the second segmented laterally variable bandpass optical filter  221 B. For embodiments including the optical assembly  80 A of  FIG. 8A , the sensor  93  may have one photodetector corresponding to each segment  81 B- 84 B. In the example implementation shown in  FIG. 9 , the black grid  89  may be disposed between neighboring bandpass optical filter segments  81 B- 82 B,  82 B- 83 B, and  83 B- 84 B of the second segmented laterally variable bandpass optical filter  221 B and along the boundaries  93 B between the photodetectors  93 A. In one embodiment, the black grid  89  may extend between the first  221 A and second  221 B segmented laterally variable bandpass optical filters, as shown. 
         [0062]    Referring to  FIG. 10A , an optical spectrometer assembly  100 A according to an example implementation may include a sensor  103  optically coupled to the second segmented laterally variable bandpass optical filter  221 B of the optical assembly  80 B of  FIG. 8B  or the optical assembly  80 D of  FIG. 8D . The sensor  103  may have a two-dimensional array of photodetectors  103 A optically coupled to the second segmented laterally variable bandpass optical filter  221 B and having the photodetectors  103 A disposed along the first direction  25  and the second direction  25 ′, for wavelength selective detection of the signal light  23  propagated through the second segmented laterally variable bandpass optical filter  221 B. 
         [0063]    Turning to  FIG. 10B , an optical spectrometer assembly  100 B according to an example implementation may include a plurality of sensors  105 ,  106 ,  107 ,  108  disposed side by side along the second direction  25 ′ and optically coupled to the second segmented laterally variable bandpass optical filter  221 B of the optical assembly  80 B of  FIG. 8B  or the optical assembly  80 D of  FIG. 8D . Each of the sensors  105 - 108  may include a photodetector array extending along the first direction  25 . For instance, the first sensor  105  may include an array of photodetectors  105 A extending along the first direction  25 ; the second sensor  106  may include an array of photodetectors  106 A extending along the first direction  25 ; the third sensor  107  may include an array of photodetectors  107 A extending along the first direction  25 ; and the fourth sensor  108  may include an array of photodetectors  108 A extending along the first direction  25 . The sensors  105 - 108  may be spaced apart along the second direction  25 ′, or may be joined. Each sensor  105 - 108  may be optically coupled to the second segmented laterally variable bandpass optical filter  221 B. Each sensor  105 - 108  may have a corresponding operational wavelength range, and a corresponding plurality of the bandpass optical filter segments  85 B- 88 B optically coupled to the sensor  105 - 108 . By way of a non-limiting example, silicon (Si) based sensor arrays may be used in a visible—near infrared range of wavelengths between 200 nm and 1100 nm, and indium gallium arsenide (InGaAs) based sensor arrays may be used in an infrared range of wavelengths between 500 nm and 2600 nm. The transmission center wavelengths λ T  of the pluralities of the bandpass optical filter segments  85 B- 88 B (and, accordingly,  85 A- 88 A) may be selected to be within the operational wavelength ranges of the corresponding photodetector arrays  105 - 108 . In this way, a multi-spectral optical spectrometer assembly may be constructed. It is further noted that the segmented filter configurations of the optical assemblies  80 A- 80 D of  FIGS. 8A-8D , and the sensor configurations of  FIGS. 10A, 10B  may also be used, for example, in the optical spectrometer assemblies  50  of  FIG. 5, 60  of  FIGS. 6A and 6B, and 70  of  FIGS. 7A and 7B . 
         [0064]    Referring to  FIG. 11 , a circular polarizer  110  according to an example implementation may be disposed in the optical path  22  between the first  221 A and second  221 B laterally variable bandpass optical filters, for suppressing light  23 ′ reflected from the second laterally variable bandpass optical filter  221 B. The circular polarizer  110  polarizes the incoming light  23  to be in clockwise circular polarization, for example. The reflected light  23 ′ will be counterclockwise polarized due to reversal of the direction of propagation. The reflected light  23 ′ may be suppressed by the circular polarizer  110 , i.e., an absorbing circular polarizer which removes the energy of the reflected light  23 ′. The circular polarizer  110  may also be disposed between the first  21 A and second  21 B laterally variable bandpass optical filters of the optical assembly  20  of  FIG. 2 , to suppress light reflected from the second laterally variable bandpass optical filter  21 B. 
         [0065]    Turning now to  FIGS. 12A and 12B , an imaging optical assembly  120  according to an example implantation, may include, for example, the optical assembly  80 B of  FIG. 2B  and an objective lens  121  optically coupled to an optional diffuser  122  optically coupled to the first segmented laterally variable bandpass optical filter  221 A for forming an image  123 A of an object  123  on the diffuser  122  or directly on the first segmented laterally variable bandpass optical filter  221 A. The first  221 A and second  221 B segmented laterally variable bandpass optical filters may each have the respective invariable bandpass optical filter segments  81 A- 84 A,  81 B- 84 B (only the segments  81 A- 84 A of the first segmented laterally variable bandpass optical filter  221 A are shown for brevity) grouped into “compound pixels”  124 , each compound pixel  124  including a pre-defined set of laterally invariable bandpass optical filter segments  81 A- 84 A,  81 B- 84 B having pre-defined transmission center wavelengths λ T  common to each compound pixel. This configuration may be similar to one employed in color CMOS sensors used for digital photography, only the number of the filters segments  81 A- 84 A may be at least 5, or even at least 12. Such configurations may enable multi-spectral imaging of the object  123 . 
         [0066]    The sensor  103  ( FIG. 10A ) may be optically coupled to the second segmented laterally variable bandpass optical filter  221 B ( FIGS. 12A, 12B ). The sensor  103  may include photodetectors  103 A disposed along the first direction  25  and the second direction  25 ′, for wavelength selective detection of the signal light  23  propagated through the first segmented laterally variable bandpass optical filter  221 A and the second segmented laterally variable bandpass optical filter  221 B. The diffuser  122 , when used, may spread the image  123 A formed by the objective lens  121  on the first segmented laterally variable bandpass optical filter  221 A. The objective lens  121  may be replaced with another image-forming optical element such as a concave mirror, for example. The 2D sensor  103  may be replaced with the 1D sensor  93  of  FIG. 9  or with the plurality of sensors  105 - 108  of  FIG. 10B . 
         [0067]    Referring to  FIG. 13 , a method  130  of making an optical spectrometer assembly of the disclosure may include a step  131  of providing the first laterally variable bandpass optical filter  21 A and a second laterally variable bandpass optical filter  21 B. In a step  132 , the second laterally variable bandpass optical filter  21 B may be fixed at the distance L from the first laterally variable bandpass optical filter  21 A in the optical path  22  downstream of the first laterally variable bandpass optical filter  21 A. Finally in a step  133 , the sensor  43  may be optically coupled to the second laterally variable bandpass optical filter  21 B. As explained above, the sensor  43  may include the photodetectors  43 A disposed along the first direction  25  for wavelength selective detection of the signal light  23  propagated along the optical path  22  through the second laterally variable bandpass optical filter  21 B. 
         [0068]    Turning to  FIG. 14 , a method  140  of making the optical spectrometer assembly  50  of  FIG. 5  may include a step  141  of providing the optical probe  52  for collecting the signal light  23  emanating from the sample  51  when the sample  51  is illuminated with the illuminating light  53 . In a step  142 , the first end  41 A of the optical fiber  41  may be optically coupled to the probe  52  for receiving the signal light  23  collected by the optical probe  52  and propagating the signal light  23  in the optical fiber  41  towards its second end  41 B. In a next step  143 , the first surface  42 A of the optical conduit  42  may be optically coupled to the second end  41 B of the optical fiber  41  for receiving the signal light  23  propagated to the second end  41 B of the optical fiber  41  for propagating in the optical conduit  42  towards its second surface  42 B. In a next step  144 , the first laterally variable bandpass optical filter  21 A may be optically coupled to the second surface  42 B of the optical conduit  42  for receiving the signal light  23  propagated in the optical conduit  42 . 
         [0069]    In a following step  145 , the second laterally variable bandpass optical filter  21 B may be fixed at the distance L from the first laterally variable bandpass optical filter  21 A in the optical path  22  of the signal light  23  downstream of the first laterally variable bandpass optical filter  21 A. Finally in a step  146 , the sensor  43  may be optically coupled to the second laterally variable bandpass optical filter  21 B. A one-dimensional or two-dimensional detector array may be used in place of the sensor  43 . 
         [0070]    Referring to  FIG. 15 , a method  150  of making the optical spectrometer assembly  60  may include a step  151  of providing the light source  61  for providing the illuminating light  53 . In a step  152 , the optical cuvette  62  may be provided. In a step  153 , the first  21 A and second  21 B laterally variable bandpass optical filters may be provided. In a step  154 , the second laterally variable bandpass optical filter  21 B may be fixed at the distance L from the first laterally variable bandpass optical filter  21 A in the optical path of the signal light  53  downstream of the first laterally variable bandpass optical filter  21 A. In a step  155 , the first laterally variable bandpass optical filter  21 A may be optically coupled to the transparent sidewall  64  for receiving the signal light  53 . Finally in a step  156 , the sensor  43  may be optically coupled to the second laterally variable bandpass optical filter  21 B. A one-dimensional or two-dimensional detector array may be used in place of the sensor  43 . In the methods  130 ,  140 , and  150 , segmented laterally variable bandpass optical filters  221 A and  221 B may be used instead of the laterally variable bandpass optical filters  21 A and  21 B. 
         [0071]    An optical filter and spectrometer may involve the processing of input data and the generation of output data. This input data processing and output data generation may be implemented in hardware and/or software. For example, specific electronic components may be employed in a processor, module, or similar related circuitry for implementing the functions associated with providing an optical filter and/or a spectrometer in accordance with the various example implementations described above. Alternatively, one or more processors operating in accordance with instructions may implement the functions associated with the exemplary implementations described above. Such instructions may be stored on one or more processor readable storage media (e.g., a magnetic disk or other storage medium), or be transmitted to one or more processors via one or more signals embodied in one or more carrier waves. 
         [0072]    The present disclosure is not to be limited in scope by the specific example implementations described herein. Indeed, other implementations and modifications, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other implementation and modifications are intended to fall within the scope of the present disclosure. Further, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein.