Abstract:
Concentric spectrometers are plagued with internal reflections due to inherent nature of more than one optical surface possessing a common center of curvature. Reflections from optical surfaces arise when there is a difference or change in the refractive index of the media in which an optical beam or ray of a given wavelength is propagating. Internal reflections in concentric optical systems can produce a myriad of undesirable optical phenomenon at the image plane such as multiple images of an object, interference fringes, and stray light. As a result a loss in contrast or detection limit arise from such phenomenon in which light or detectable radiation that impinges on the image plane does not add to the formation of the intended image, (stray light). The present invention produces high quality images without the optical phenomenon(s) that arise from internal reflections by removing the reflected radiation from propagating through the optical system.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon, and claims the benefit of, my Provisional Application Ser. No. 60/167,491, filed Dec. 1, 1999. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to spectrometers and spectrographs having optical elements in which the surfaces of the said elements include a common center of curvature. More specifically the invention pertains to a Dyson, “unit magnification optical system without Seidel aberrations”, which includes concentric spectrometers and concentric spectrographs in which the Dyson optical system is applied. 
     BACKGROUND OF THE INVENTION 
     Concentric optical systems such as described by J. Dyson, JSOA, “Unit Magnification . . . Aberrations”, Vol. 49, No. 7, pp.713-716, provide large image fields free of Seidel aberrations and are thus able to form images of high quality and resolution. This optical arrangement has been applied to advantage by L. Mertz, Applied Optics, “Concentric Spectrographs”, Vol. 16, No. 12, pp. 3122-3124, and W. Slutter (EP 0 862 050 A2; 1998) to spectrometers and spectrographs to produce high quality spectral dispersion of optical energy. 
     Internal reflections of light on optical surfaces can degrade the quality of the image formed at the image aperture. The image can be degraded in a myriad of optical phenomenon as a result of internal reflections. On example of degradation is by the formation of multiple images of the object at the image aperture. Another example of degradation caused by internal reflections is the formation of interference fringes at the image aperture. Still another example in which the quality of the image may be degraded is in the loss of contrast or detection limit when reflected light from an optical surface impinges out of focus at the image aperture and does not contribute to the formation of the image of the object. The object in spectrometers is typically the entrance slit or entrance aperture through which the optical energy to be analyzed enters the spectrometer. 
     In a concentric spectrometer of the Dyson optical configuration, there are two specular surface reflections that contribute to, or give rise to, internal reflections. Both reflections originate at the convex surface of the plano-convex lens, which lies concentric to a concave diffraction grating. The first reflection occurs when light transmitted from the object plane falls incident on the convex surface of the lens and the second occurs when the diffracted light from the grating impinges on said convex surface. Either one or both of these reflections can degrade the image quality of the spectrometer. 
     The effects of internal reflections have been reduced in the prior art by the deposition of various antireflection coatings on the optical surfaces within the concentric spectrometer. An antireflective coating(s) may be applied to optical surfaces to reduce the differential change in refractive index when the ray propagates from one optical media such as air, to a second optical media of different refractive index such as glass, thus reducing the magnitude of the specular reflection. Many examples of coatings on optical surfaces exist that reduce the magnitude of reflections on optical surfaces. Indeed a great amount of literature has been devoted to the study of single, and multiple, layer depositions that reduce reflections on optical surfaces. 
     A high performance antireflection coating(s), usually a multi-layer dielectric coating that substantially reduces the reflection at a surface, has inherent disadvantages. The disadvantages include a high cost of production, a narrow range of wavelengths for which the reflection loss is low, limited angles of incidence in which rays may propagate with low reflection, and are fragile requiring special handling, cleaning and environmental considerations. Indeed, these high performance coatings can cause a greater magnitude of reflection than a surface without an antireflective coating when used beyond the wavelength range of design. Regardless of the coating(s) used, the internal reflections are not reduced over a wide wavelength range to a level which degradation of the image quality does not take place by one or more of the aforementioned optical phenomenon within concentric optical systems. 
     Another means in which internal reflections have been reduced in concentric spectrometers is by the reduction of the numerical aperture (NA) of the spectrometer. One way the NA of the spectrometer can be reduced is by the placement of an optical stop within the spectrometer. This is undesirable since the result is a loss in detectable signal, a decrease in the etendue or throughput of the optical system, and can add to or increase the stray light within the spectrophotometer since more energy must be absorbed within the confines of the spectrometer. 
     There is a need, therefore, for the mitigation of internally reflected light within a concentric spectrometer such that no degradation of the image quality or loss in contrast occurs yet high etendue is preserved. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the limitations of internal reflections in concentric spectrometer optics by mitigation of the specular reflected rays from propagating through the optical system. As a result, the quality of image at the detection or image plane is preserved while maintaining high etendue. 
     In accordance with the present invention an object and an image aperture are provided, through which optical radiation, or light, enters the spectrometer through said object aperture and a spatially dispersed image of said object aperture by wavelength is formed at said image aperture. The field or extent of the object and image apertures along with the focal ratio of the optical system defines the limit within which light may propagate the optical system. Rays that propagate the system at the limit of the aperture fields are the marginal rays. 
     A concave diffraction grating is provided that reflects and diffracts incident light from an object(s) from the object plane and participates in the formation of a spatially dispersed image of the object(s) by wavelength at the image plane. A plano-convex lens is provided through which light from the object plane is transmitted to the diffraction grating, where the light it is diffracted and reflected, then transmitted again through the same plano-convex lens to form a spectrally dispersed image of the object within the image plane. The convex surface of the lens and concave surface of the diffraction grating are concentric, or nearly so, about a common center of curvature. An optical axis is also provided that includes the center of curvature of the optical elements and extends through the mechanical axis of both the lens and diffraction grating. A radial distance is defined from the optical axis that includes both object and image apertures. 
     Meridional planes, which include the optical axis, are defined to be perpendicular to the x-y path of a given diffracted ray as that ray propagates from the grating towards the convex surface of the lens. A diffracted ray that falls incident on the convex lens surface of the lens will give rise to specular reflection. A ray that falls incident on the convex lens surface prior to intersection of the rays&#39; meridional plane will cause the specular reflection to be directed to impinge on the grating a second time. This ray can further propagate the optical system by means of zero order diffraction to form a spectra or part of a spectra within the image aperture which does not correlate to the spectra of interest. The result is an increase in stray light. A diffracted ray that intersects the rays&#39; meridional plane prior to impinging on the convex lens surface will cause the specular reflection to be directed away from the diffraction grating and image plane without possibility of secondary reflection or diffraction. A baffle or series of baffles and or surfaces containing light absorbing media can be used accordingly to prevent further propagation within the spectrometer without interference with the optical path of the spectrometer. The present invention excludes the possibility of the diffracted rays from impinging on the surface of the convex lens without prior intersection of the meridional plane, thus eliminating further propagation of internal specular reflections. 
     Another aspect of the invention is the use of baffles which need only prevent propagation of light from one direction since secondary diffraction or reflection does not occur on the grating surface. The baffles can therefore be optimized to more completely mitigate stray light by multiple impingement of the reflected light on the absorbing media or baffles to further reduce the total stray light within the spectrometer. 
     Further in accordance with the present invention, marginal rays extending from the object aperture toward the optical axis are not permitted to intersect the optical axis prior to the convex surface of the lens as internal reflections will result. Furthermore, marginal rays that extend away from the optical axis define the clear aperture of the optical system. 
     Another feature of the present invention is an aperture mask placed in close proximity to the concave surface of the grating. This aperture mask limits the NA of the spectrometer and additionally reduces stray light in the spectrometer. The edge or near periphery of the grating is typically not of optical quality. The edge of a grating is usually chamfered to decrease the frailty of the grating but can have the undesirable effect of scattering light towards image aperture. Also, the optical surface beyond the clear aperture of the grating includes defects from the replication or manufacturing processes. Thus an aperture mask provides a surface that is absorbent to light, serves to limit the NA of the spectrometer, and masks that region beyond the clear aperture of the grating to prevent the scattering of light towards the image aperture. 
     Further in accordance with the current invention, the object and image apertures lie in parallel or coincident plane(s) in close proximity to, or are included within, the planar surface of the plano-convex lens. The thickness of the plano-convex lens and lens radii, separation of the convex surface of the lens and concave surface of the diffraction grating, radii of the diffraction grating, and grating groove density are concurrently adjusted to minimize aberrations within the spectrometer apertures and provide the desired spatial dispersion. Furthermore, the radial distance from the optical axis at which the object and image apertures reside and the diameters of the lens, grating, and aperture mask are adjusted in accordance with the invention to provide the desired numerical aperture for the spectrometer while eliminating internal reflections. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top plan view (x-z plane) of a known modified concentric spectrograph with object and image apertures not included within the dispersion axis. 
     FIG. 2 is a side view (y-z plane) of FIG.  1 . 
     FIG. 3 is an axial view (x-y plane) of FIG. 1 looking through the planar surface of the lens toward the diffraction grating. 
     FIG. 4 is an axial view (x-y plane) of a known modified concentric spectrograph showing two object and image apertures. 
     FIG. 5 is an alternative arrangement, axial view (x-y plane), of a known modified concentric spectrograph with two object and image apertures. 
     FIG. 6 is a top plan view (x-z plane) of FIG. 1 with non-restrictive aperture stop and showing path of internal reflection. 
     FIG. 7 is a top plan view (x-z plane) of FIG. 6 with aperture stop in position that eliminates internal reflections. 
     FIG. 8 is a top plan view (x-z plane) of FIG. 7 with reduced numerical aperture. 
     FIG. 9 is an axial view (x-y plane) of FIG. 7 with reduced numerical aperture. 
     FIG. 10 is a top plan view (x-z plane) of the preferred embodiment of the present invention. 
     FIG. 11 is a side view (y-z plane) of a preferred embodiment. 
     FIG. 12 is an axial view (x-y plane) of FIG. 11 preferred embodiment. 
     FIG. 13 is a top plan view (x-z plane) of apparatus for implementation of the preferred embodiment. 
     FIG. 14 is a section view of FIG. 13 showing details therein of the preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 10-14 there is shown a concentric spectrometer of Dyson optical configuration with mitigation of internal reflections in the preferred embodiment of the present invention. Prior art systems pertaining to spectrometer designs are shown in FIGS. 1-9. 
     Shown in FIGS. 1 and 2 is a modified concentric spectrometer  200  with shown object aperture  205 , and image aperture  211 . A plano-convex lens  215  with planar surface  216  and convex surface  217  are shown. The diffraction gating  220  with dispersion surface  221  is positioned at a distance along the optical axis  203  such that convex surface  217  of lens  215  are concentric or nearly so. Object aperture  205  is coincident with object plane  202  through which light enters spectrometer  200  where upon light is refracted by lens  215  to substantially fill grating surface  221 . Light is diffracted by grating surface  221  per known grating equation knλ=sin α+sin β, where k is the groove density of the grating, n is the order of diffraction, λ is the wavelength of incident light, α is the angle of incident, and β the angle of diffraction. Diffracted light propagates through lens  215  towards image plane  201  where a spectral image is formed within image aperture  211  where said spectra  206  may be detected as a spatial dispersion of light vs. wavelength in FIG.  3 . When the order of diffraction is zero, n=0, the angle of incidence is equal to the angle of dispersion and a reflection takes place. For a given dispersion several orders of diffraction are present, positive, negative, and zero. The zero order  210  image of object aperture  205  is shown relative optical axis  203  as reference to the n=1 order of diffraction. Meridional plane  225  containing optical axis  203  is defined perpendicular to grating grooves of grating surface  221  and separates object aperture  205  from image aperture  211 . Object aperture  205  is disposed at distance  272  from meridional plane  225  so as to prevent a reentrance of diffracted light by image aperture  211 . 
     A modified concentric spectrometer  200  has several undesirable qualities, which degrade the purity of the spectra and limits the detectable range of the spectrometer due to stray light. The cause of this degradation in performance is due to internal reflections between the convex surface  217  of lens  215  and grating surface  221 . An internal reflection known to contribute to stray light within image aperture  211  is illustrated in FIG. 6 showing formation and path of reflection. Light to be analyzed enters the spectrometer through object aperture  205  and propagates along ray path  250  to planar surface  216  of lens  215 . The ray proceeds along path  251 ,  252  to impinge on grating surface  221 . At grating surface  221  the ray is diffracted by angle vs. wavelength. For a given wavelength ray  260  proceeds to lens surface  217  where a portion of the light is reflected along path  265  and the remaining light is transmitted through lens  215  as ray path  268  towards image aperture  211 . Some of the reflected rays  265  impinge on the grating a second time and are reflected in accordance with a zero order of diffraction per diffraction equation as rays  266 . The zero order rays  266  of the second incidence on diffraction grating surface  221  proceed through lens  215  along ray path  267  to impinge within image aperture  211  creating a second partial image of the primary spectra  206 . This partial spectrum is not spatially correlated to primary spectra  206  and prevents detection of specific wavelengths for which gives a spectrometer utility in the field of atomic absorption spectroscopy. Diffracted rays  260  from grating surface  221  follow a x-y path towards convex lens surface  217  and impinge on the lens surface  217  within area  250 . For a given x-y ray path a meridional plane, a plane of reflection  236 , is defined to include optical axis  203  and be perpendicular to x-y line of propagation shown in FIG.  3 . Specular reflections contribute to unwanted partial spectral images when the plane of reflection  236  dissects surface area  250  of convex lens surface  217 , identified as area  251  in FIG.  3 . 
     A modified concentric spectrometer of known prior art, Dyson configuration, may include more than one object and image aperture as shown in FIGS. 4-5. Light enters the spectrometer through object aperture  405 ( 505 ) and  475 ( 575 ), transmitted through lens  415 ( 515 ) towards concave grating surface  421 ( 521 ) of grating  420 ( 520 ). Image apertures  411  and  481  are located on the same side of meridional plane  425  through which primary spectra  406  and  476  of object aperture  405  and  475  are imaged. Image apertures  511  and  581  are on different sides of meridional plane  525  through which primary spectra  506  and  576  of object aperture  505  and  575  are imaged. Zero order images of object apertures  405 ( 505 ) and  475 ( 575 ) are shown relative to the optical axis as  410 ( 510 ) and  480 ( 580 ) respectfully. The axis of dispersion  435  and  445  for spectra  406  and  476  may be coincident so as to make possible that both spectra be incident on a single detector. The dispersion axis  535  of spectra  506  and dispersion axis  545  of spectra  576  are disposed on different sides of meridional plane  525  to provide greater separation of the two spectra so as to provide additional space independent detection means. 
     Regardless of the number of object or image planes or spatial arrangement, internal reflections can occur if surface areas  450 ( 550 ),  460 ( 560 ) of lens  415 ( 515 ) are dissected by planes of reflection  436 ( 536 ),  446 ( 546 ) respectfully. Specular reflections from convex lens surface within sections  451 ( 551 ) and  461 ( 561 ) contribute to the formation of unwanted partial spectral images degrading the performance of the spectrometer. Another disadvantage of the prior art dual arrangements is the object apertures  405 ( 505 ) and  475 ( 575 ) are placed at a different distance from the optical axis  403  than the image apertures  411 ( 511 ) and  481 ( 581 ). The nonsymmetrical arrangement of apertures about the optical axis  403  degrades image quality of spectra  406 ( 506 ) and  476 ( 576 ). 
     Aperture stops have long been used in as a means of controlling aberration and limiting the NA of optical systems. An example of an aperture stop  290  used in prior art modified concentric spectrometer is shown in FIG.  3  and FIG.  6 . The aperture stop  290  in FIG. 6 limits the NA of the spectrometer but does not prevent internal reflections from impinging on grating  220  and resulting in subsequent degradation in quality of primary spectra  206  of FIG.  3 . The size of the aperture stop  390  of FIG.  7  through which light is permitted to pass can be decreased in a modified concentric spectrometer to prevent internal reflections from the convex surface  317  of lens  315  from impinging on grating surface  321  of grating  320 . Light enters the modified spectrometer through object aperture  305  of object plane  302  along ray path  350 . Ray path  350  is transmitted through the planar surface  316  lens  315  along ray path  351  to convex lens surface  317 . Light is refracted by lens  315  and impinges on concave grating surface  321  where the said light is diffracted towards lens surface  317 . A significant portion of the usable light  353  that would contribute to the formation of the primary spectra  306  within image aperture  311  is prevented from doing so by aperture stop  390 . The result of this obscuration creates a situation by which the gain of the detection means must be increased to account for the decrease in intensity of the spectra. 
     Obscuring entrant polychromatic light within a spectrometer also increases the amount of stray light generated by scattering of light off of aperture stop  390 . This results in an undesirable decrease in contrast or increase in the signal to noise ratio, thus limiting the detection range even though the reflection  365  has been eliminated by aperture stop  390 . The reduced NA of the spectrometer is best illustrated in FIG.  8  and FIG.  9 . The grating  320  is obscured by aperture stop  390  permitting light to impinge on grating surface  321  within the opening aperture stop  390 . Diffracted light impinges on the lens surface  317  in area  350  all on side of reflection plane  336  that does not contribute to internal reflections at the expense of a significant reduction in etendue as compared to same area  250  of FIG.  3 . 
     The problems of modified concentric spectrometers (i.e., stray light due to internal reflections, stray light due to obscuration of entrant light, stray light due to defects of the grating surface beyond the clear aperture, partial non-correlated spectra, diminished etendue) have been solved in this invention in a concentric spectrometer that mitigates internal specular reflections. Light enters a specular mitigating concentric spectrometer  100  through object aperture  105  of object plane  102  of FIG.  10  and FIG.  11 . Within the object aperture  105  may be disposed a plurality of entrant ray paths each of which can be dispersed to form individual spectra of each polychromatic entrant ray path within image aperture  111  of image plane  101 . 
     In accordance with the present invention, object plane  102  and image plane  101  need not lie in coincident space and may be displaced from planar surface  116  of plano-convex lens  115 . Further in accordance with the present invention, a Dyson optical configuration including a convex surface  117  of lens  115  and concave surface  121  of diffraction grating  120  are separated along optical axis  103  at a distance which minimized third order aberrations. An aperture mask  190  is introduced close to or included on concave surface  121  of grating  120 . The aperture mask  190  limits the NA of the spectrometer to that which is grater than NA of the entrant rays. So conceived aperture mask  190  impedes the propagation of stay polychromatic entrant rays towards object or image planes  101  and  102  respectfully by means of absorption. Additionally aperture mask  190  prevents said stray polychromatic entrant rays from impinging on concave grating surface  121  beyond the clear aperture of grating  120  where surface defects and edge chamfer reside thus preventing propagation or generation of stray light. Still another aspect of aperture mask  190  is to prevent specular reflected rays  165  of impinging rays  160  on convex surface  117  from propagating toward object and image planes  101  and  102  respectfully. Object aperture  105  and image aperture  111  are placed at radial distance  137  from optical axis  103  shown in FIG. 12 along which the image quality is optimized. There is therefore a desire to maximize the cord length of radii  137  that intersects image aperture  111  to provide the highest quality of image within image aperture  111 . For a given entrant ray path within object aperture  105  the dispersion axis  135  intersects radii  137  of high image quality. By symmetry zero order image  110  of object aperture  105  also falls incident at radial distance  137  from optical axis  103 . Therefore, per this invention zero order image  110 , object and image aperture  105  and  111  are all substantially at same radial distance  137  from optic axis  103 . A unique aspect of this geometry provides a means for polychromatic entrant rays of object aperture  105  to be reflected off the internal concave surface  117  of lens  115  and directed away from image aperture  111  as reflected rays  166  of FIG.  10 . The result of this geometry is a reduction in stray light impinging on image aperture  111 . The radial distance of  137  and diameter of lens  115  are selected so as marginal rays originating from object aperture  105  do not intersect optical axis  103  but can propagate through the convex surface  117 . This dictates a minimum diameter for radial distance  137  at substantially half that of the diameter of lens  115 . Furthermore, the marginal rays determine the required clear aperture of grating  120  and subsequently the inside diameter of aperture mask  190 . Judicial selection of optical parameters, by those skilled in the art, must be made therefore to obtain a desired spectral and spatial dispersion of light and operating numerical aperture. The optical parameters include and but are not limited to the materials, dimensions, and radii of the optical components as well as groove density of grating  120  spatial location of components per the context of this disclosure. Optical parameters so selected provide a concentric spectrometer that eliminates specular reflected rays  165  from propagating the optical system of a specular mitigating concentric spectrometer  100  as disclosed in FIG.  10  and FIG.  11 . 
     One implementation of the preferred embodiment of present invention is shown in FIG.  13  and FIG.  14 . External view FIG. 13 shows light entering spectrometer  100  through optical fiber  118  of integral fiber optic assemble which includes fiber connector  158  and strain relief  159 . Optical fiber  118  is attached to fiber connector  158  by epoxy adhesive  119 . Fiber connector  158  is located by fiber collet  157  and held in place by clamp  191  using screw  192 . Spectrometer body  150  provides a means to mount and align optical components and maintain said alignment over a wide operating temperature range. Metering rods  153  are threaded into body  150  and secured at one end by locking nuts  154 . The other end of metering rods  153  pass through a flange in spectrometer body  150  to which adjustment nuts  155  are secured. Slots  152  permit one end of the spectrometer body  150 , preferentially the grating end of the spectrometer, to flex in three degrees of freedom; pitch about the x-axis, yaw about the y-axis, and translation along the z-axis. This provides means to position the grating  120  relative to the all other fixed elements of the embodiment achieving x-y positioning and focus of the spectral image  106  within the image aperture  111 . Slots  152  provide additional utility as a thermal expansion joint in spectrometer body  150 . Metering rods  153  of sufficiently low coefficient of thermal expansion CTE, such as Invar, maintain the relative position of the lens  115  to grating surface  121  while allowing body  150  to change linear dimension over temperature at slots  152 . By proper selection of the affixation points of the metering rods  153  to the spectrometer body  150  by those skilled in the art, changes in focus are sufficiently eliminated over a wide operating temperature range. Slots  151  in spectrometer body  150  are used in conjunction with grating retaining ring  193  of FIG. 14 to place a controlled load on grating  120  and permit rotational alignment of said grating  120  while maintaining contact with spectrometer body  150 . Flexure  151  also assures that the contact between grating  120  and body  150  will be maintained over the operating temperature range of the spectrometer. 
     Another feature of the preferred embodiment is that all alignment adjustments are accessible from one end of the spectrometer making possible in situ alignment of the spectrometer. Plano-convex lens  115  is held in spectrometer body  150  by housing  156 . Circuit board  195  locates detector  112  relative to fiber collet  157  and provides a means to supply power to and extract signal from detector  112 . Object aperture  105  preferentially takes the form of an optical slit formed by an opaque metal deposition, such as nickel, on optically transparent substrate, such as SCHOTT GLASS TECHNOLOGIES B270 glass,  104  to which optical fiber  118  is abutted. Substrate  104  is attached to end of fiber collet  157  so as to maintain alignment of fiber connector  158  with optical fiber  118  to optical slit  105 . Circuit board  195  with attached detector  112  and fiber collet  157  with slit  105  are held against planar surface of lens  115  by means of retaining ring  194 . Optical cement  107  of index of refraction nearly equal to that of the substrate  104 , detector window of detector  112  and piano-convex lens  115  bonds detector  112  and substrate  104  to lens  115  providing mechanical stability without loss due to specular surface reflections at these optical interfaces. Optical cement  107 , such as EPOXY TECHNOLOGIES EPO-TEK 301-2, or others having the desired optical and physical properties can be used. Surface reflections between optical fiber  118  and substrate  104  are similarly eliminated by use of said same optical cement located at  108 . 
     The baffle structure  180  is comprised of a series of individual baffles such as baffle  181  made of or coated with a light absorbent material and may include aperture mask  190  as an integral part thereof. The spacing between individual baffles and the aperture of each baffle or both may be adjusted such that stray light or light which has been reflected from convex lens surface  117  impinges first on a baffle such as  181  then on baffle wall  182 . A baffle structure so deployed provides no less than two opportunities for the absorption of stray light, effectively doubling the absorptivity of the material or absorbing media of baffle structure  180 . Baffle structure  180  is loosely fit into spectrometer body  150  and held in place by an elastic adhesive such as silicone adhesive  196 . The silicone adhesive  196  permits baffle structure  180  to deflect internal to spectrometer body  150  when adjustments are made in pitch and yaw of grating  120  by means adjustment nuts  155  without becoming detached from body  150 . Baffle structure  180  also prevents ambient light from entering the spectrometer body  150  through slots  152  in body  150  by blocking the light path created by slots  152 . 
     It will be apparent to those skilled in the art that the optical axis may be altered by the introduction of plane reflecting surfaces, such as fold mirrors or prisms, along the optical path to obtain a desired spatial orientation of components. Regardless of the angle or number of reflecting surfaces it can be shown that when the optical axis is rendered as a straight line, as an optical tunnel diagram, the rays path through the spectrometer relative to the optical axis remains unchanged and is within the scope of this invention. 
     A concentric spectrometer that mitigates internal specular reflection is thus realized providing high quality spectral images not limited by optical phenomenon created by the reflection of light by internal optical surfaces. 
     The detailed embodiments disclosed herein may be accomplished in a variety of forms without departing from the scope or intent of this invention by those skilled in the art and is not limited to the disclosed embodiments but should be defined by the claims which follow.