Abstract:
Well plates adaptable for specimen sampling in the biological, chemical and pharmaceutical sciences are fabricated by dissolving fusedly-retained cores from the cladding material of a fused fiber plate to define a capillary plate including first and second faces and a plurality of through-voids into which fluidic samples may be deposited for analysis. Closed-bottom wells are defined by bonding one of the first and second faces to a base plate or by securing into well-sealing positions over the open ends of selected through-voids optical elements, each of which optical elements exhibits a predetermined optical property. Cladding material including reducible ions is exposed to a reduction atmosphere in order to blacken selected regions of a well plate, thereby enhancing sample analysis by reducing such disadvantageous phenomena as autofluorescence.

Description:
PROVISIONAL PRIORITY CLAIM 
   Priority based on Provisional Application Ser. No. 60/500,806, filed Friday, Sep. 5, 2003, and entitled “MICRO-WELL PLATES AND METHODS OF FABRICATING THE SAME,” and Provisional Application 60/510,621, filed Friday, Oct. 10, 2003, and entitled “MICRO-WELL PLATES AND METHODS OF FABRICATING AND SELECTIVELY BLACKENING THE SAME” is claimed. 

   BACKGROUND 
   Micro-well and nano-well plates, which are, in relevant industries, collectively referred to as “microtiter plates,” are used widely in biological, chemical and pharmaceutical research applications for the purposes of selectively retaining and analyzing small quantities of biological and chemical agents. Currently, microtiter plates are predominantly fabricated from injection-molded and/or machined plates of plastic is formed to included multiple, well-ordered well sites. For various reasons, including analytical efficiency and conservation of valuable reagents, the pharmaceutical industry, for example, has placed increasing emphasis on miniaturized sample-screening formats. Accordingly, well-density on microtiter plates has continually increased and, for instance, a standard 3×5-inch microtiter plate currently includes 1536 holes (i.e., wells). It will be appreciated, however, that the degree to which well sites can be miniaturized in an effort to accommodate more numerous well sites on a plate of standardized dimensions is limited by conventional fabrication methods. 
   In response to the call for well-site miniaturization, the fiber optics industry has undertaken limited efforts to fabricate well plates by selectively intagliating optical fiber faceplates. More specifically, the state of the art in this regard is represented by a process in fabrication of a fiber optic faceplate including a plurality of cores fusedly retained by fused cladding material in accordance with standard processes well-known to those of ordinary skill in the optical-fiber-component fabrication arts. The cores are fabricated from a core material that is soluble in a predetermined solvent in which the cladding material is relatively insoluble. The faceplate is then chemically etched from one side to partially dissolve selected cores to define a set of closed-bottom wells in the fused cladding material. As illustrated by well plate of FIG. A, one limitation of the current faceplate intagliation process is the prohibitive difficulty of defining wells of uniform depth that exhibit “intended” bottom profiles. The well plate of FIG. A exhibits non-uniform well depth and irregular well bottoms, characteristics that are exaggerated for illustrative purposes. 
   Accordingly, in light of the limitations of traditional well-plate fabrication processes and the more nascent efforts to intagliate optical fiber faceplates for adaptation as well plates, there exists a need for improved methods of fabricating well plates exhibiting large numbers of small, well-defined and uniform well sites. 
   SUMMARY 
   Implementations of the present invention are directed to methods of fabricating micro-well plates useable for the containment and analysis of small volumes of chemical and biological materials and, in various embodiments, to micro-well plates made in accordance with the methods. 
   Various aspects employ techniques analogous to those applied in the fabrication of optical fiber faceplates. For instance, various implementations include the formation of a fused fiber bundle including a plurality of fused fibers extending generally along a longitudinal axis between first and second ends. Each fiber includes a core and a cladding, the core being soluble in a first solvent (e.g., an acid or base) and the cladding being relatively insoluble in the first solvent. In various aspects, each of the core and the cladding comprises glass. When individual fiber preforms, each of which comprises a cladding tube and a core bar inserted therein, are bound, heated and drawn, each cladding tube collapses and fuses around the core positioned therein and the claddings of adjacent fibers become fused to one another resulting in a unitary structure (i.e., a “fused bundle”). The formation of such structures is generally known among fabricators of fused optical fiber components. 
   The fused fiber bundle is cut along, but not necessarily parallel to, a plane that extends perpendicularly to its longitudinal axis to form a plurality of fused fiber plates, each of which fused fiber plates includes first and second faces. In various implementations, the first and second faces of a fused fiber plate are ground and polished to create smooth faces and, if desired, a fused plate of uniform thickness or alternative profile. The plate is exposed to the first solvent (e.g., immersed) to etch out (i.e., dissolve) the cores, thereby forming a “capillary plate” comprising the fused cladding material including a plurality of voids corresponding in position and cross-section to the pre-etch positions and cross-sectional geometries of the dissolved cores. In a typical version, the capillary plates are exposed to the first solvent for a period of time sufficient to produce a selected set of voids including “through-voids” that extend through the capillary plate between the first and second faces thereof. 
   In accordance with one set of implementations, a base plate of material (e.g., glass or plastic) including first and second sides is bonded to the first face of a capillary plate including through-voids to define a unitary well plate including a plurality of wells, each of which wells has an open top end, a closed bottom end and a well wall extending between the open top and closed bottom ends. Representative bonding techniques and agents for bonding the capillary and base plates include, by way of non-limiting example, (i) heat fusing, with or without frit, (ii) epoxy or other polymeric adhesive bonding agent, (iii) sol gel, (iv) laser tacking and (v) anodic bonding. Although the aforementioned bonding is performed, in some implementations, subsequent to the formation of a capillary plate, in an alternative fabrication method, the capillary plate and the base plate are bonded together prior to etching core material from the capillary plate. Alternatively configured well plates fabricated in general accordance with the foregoing methods include at least one of (i) various well sizes in the same capillary plate, (ii) clear, translucent or opaque capillary plate material, and (iii) a base plate including plurality of adjacently-bonded image conduits such as fused optical fibers (e.g., an optical fiber faceplate) including, in some versions, graded-refractive-index (i.e., GRIN) optical fibers. In addition, it will be appreciated that the wells can be randomly arranged or organized into well-ordered arrays, depending on the application for which a particular embodiment is to be used. Moreover, well size, cross-sectional geometry and diameter are variable within the same well plates by fusing into the initial fiber bundle cores of correspondingly various geometries and diameters/r dii. Although “diameter” is frequently thought of narrowly as the longest chord that can be fitted within the curve defining a circle, the more general definition of that term is applicable to this description and the appended claims. For instance, chords within squares, rectangles, hexagons, and even, irregular shapes are also diameters. A radius is a line segment extending from the geometric center of a shape to the boundary of the shape. Nothing in the preceding explanation should be construed to attribute to the terms “diameter” and “radius” a meaning more narrow than common usage and technical mathematical usage would attribute to them. 
   Various alternative embodiments include wells having integrated optical-focusing bottoms. Illustrative versions include a capillary plate in which each well of a selected plurality of wells is “plugged” or “capped” by a focusing element such as a ball lens, an aspheric lens or a GRIN optical fiber, by way of non-limiting example. In various such versions, the lens element serves the dual functions of providing a closed bottom for the well to which it is applied and facilitating empirical study of contents deposited in the well. Alternative versions integrate fiber segments formed from cores that are fused into the surrounding cladding material during fabrication of a fused bundle. In such a version, one of the first and second faces and of a fused fiber plate cut from the fused bundle is exposed to a core solvent for a period of time sufficient to etch away a portion, but not the entire length, of each core of a selected set of cores with the remaining, non-etched segment of each core serving as the closed bottom end of a well and, as applicable, a focusing element or a light-filtering element, for example. 
   In still further embodiments, the clad material defining an interior wall of each void of a selected set of voids in a capillary plate includes reducible ions (i.e., ions that can be caused to accept electrons). In various aspects, such embodiments are subjected to conditions (i.e., a reducing atmosphere) that cause reducible ions within the clad material to accept electrons (i.e., be reduced), thereby “blackening” the material. As discussed in greater detail in the detailed description, and illustrated in the drawings, the blackening of the void/well walls in various embodiments yields desirable characteristics including, but not limited to, the maintenance of a relatively low and constant level of undesirable autofluorescence over a range of light intensities and wavelengths. It should be noted that not all ions will result in blackening when such ions are reduced and, therefore, ions that result in blackening when reduced are to be selected in various implementations. Moreover, it should be noted that “blackening” is used throughout the specification and claims in a broad, informal sense and includes, for example, darkening other than strictly blackening and that may manifest itself in various shades of brown or gray by way of non-limiting example. More specifically, “blacken,” “blackened” and “blackening” should be read and interpreted as broadly as “darken,” “darkened,” and “darkening” regardless of actual color and shade characteristics. 
   It will be appreciated that the cladding material can be blackened by design in any portion desired and that selective blackening is not limited to void or well walls. For example, either face of a capillary plate may be blackened in addition to, or to the exclusive of, well or void walls. Moreover, the blackening is, in various versions, performed prior or subsequent to other fabrication steps, depending on the desired result. For instance, according to three alternative methods of fabricating a well plate including a capillary plate that is at least partially blackened, a capillary plate including reducible ions is exposed to a reducing atmosphere to at least partially blacken the cladding material prior to bonding with a base plate; (ii) subsequent to bonding with a base plate and (iii) both prior and subsequent to bonding with a base plate. In other versions in which it is desired to produce a capillary plate in which just at least one of the two faces is blackened, a fused fiber plate including cladding with reducible ions is exposed to a reducing atmosphere prior to dissolving the cores therefrom. 
   Representative embodiments are more completely described and depicted in the following detailed description and the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. A depicts a selectively intagliated optic fiber faceplate exhibiting a plurality of well sites; 
     FIG. B depicts a fused fiber bundle including a plurality of cores surrounded, and retained in position, by fused cladding material; 
     FIG. C shows fused fiber plates cut from the fused fiber bundle of FIG. B; 
       FIG. 1A  shows a capillary plate, which includes a plurality of through-voids created by etching out cores from a fused fiber plate, being brought into contacting engagement with a base plate; 
       FIG. 1B  shows the capillary and base plates of  FIG. 1A  bonded together to form a well plate; 
       FIG. 1C  is a cross-sectional view of a well plate formed by bonding a capillary plate to a base plate; 
       FIG. 1D  is a cross-sectional view of a well plate, such as the well plate in  FIG. 1C , in which the base plate is formed from adjacently fused optical fibers; 
       FIG. 2A  depicts an alternative well plate formed by plugging through-voids in a capillary plate with various illustrative optical-focusing elements; 
       FIG. 2B  shows a well plate formed by the selective controlled partial etching of cores from one side of a capillary plate; 
       FIG. 3A  shows a large-well/small-well plate having open wells in each of two adjacently bonded capillary plates wherein the wells in one capillary plate are larger in cross-sectional area than the wells of the adjacent plate; 
     FIGS.  3 Bi and  3 Bii depict two stages in the assembly of a large-well/small-well capillary plate; 
     FIGS.  3 Biii and  3 Biv depict two stages in an alternative method of fabricating a large-well/small-well capillary plate; 
       FIG. 4  illustrates the insertion of a glass core rod into a glass tube cladding in accordance with the “rod in tube” method of optical fiber fabrication known to those of ordinary skill in the art of the optical fiber fabrication; 
       FIG. 4A  is an end view of an illustrative cylindrical core rod in a cylindrical tube cladding in which the cladding material includes reducible ions adjacent its inner surface; 
       FIG. 5A  shows a fused capillary plate, including a plurality of through voids, created by etching out cores from a fused fiber plate; 
       FIG. 5B  is a cross sectional view of a representative segment of the capillary plate of  FIG. 5A  showing reducible ions included in the fused fiber cladding material along the void walls; 
       FIG. 5C  shows an illustrative capillary plate like the capillary plate of  FIGS. 5A and 5B  inside a hydrogen-firing furnace where it is exposed to heated gas including diatomic hydrogen to reduce reducible ions in the cladding material; 
       FIG. 5D  shows the capillary plate of  FIGS. 5A through 5C  after reduction of reducible ions and the resultant void-wall blackening; 
       FIG. 5E  shows an alternatively blackened capillary plate including blackening on the faces thereof; and 
       FIG. 6  shows comparative autofluorescence data for (i) a blackened capillary plate, (ii) a clear (i.e., non-blackened) capillary plate and plain clear (i.e., non-blackened) glass. 
   

   DETAILED DESCRIPTION 
   The following description of methods of fabricating micro-well plates, and of micro-well plates fabricated in accordance therewith, is demonstrative in nature and is not intended to limit the invention or its application of uses. The various implementations, aspects, versions and embodiments described in the summary and detailed description are in the nature of non-limiting examples falling within the scope of the appended claims and do not serve to define the maximum scope of the claims. 
   Referring to FIGS. B and C, various implementations include one of (i) fabricating and (ii) providing a fused fiber bundle  10  including a plurality of cores  12  extending through fused cladding material  14  along a longitudinal axis A L  between first and second ends  16  and  18  of the fiber bundle  10 . As is generally known by those of ordinary skill in the art of optical-fiber component fabrication, a fused bundle such as the illustrative bundle  10  of FIG. B is formed by temporarily binding, and then heating and drawing, a plurality of constituent fiber preforms each of which fiber preforms includes a core bar and a cladding tube disposed around the core bar (not shown). When the bound assembly of fiber preforms is heated and drawn, each cladding tube collapses around, and fuses to, the core bar inserted therein and the cladding tubes of adjacent fiber preforms fuse to one another resulting in a unitary structure (i.e., a fused bundle  10 ) including a plurality of cores  12  fusedly retained within fused cladding material  14 . 
   Referring to FIG. C, fused fiber plates  20  are formed by cutting the fused bundle  10  perpendicularly to the longitudinal axis A L  thereof. Each fused fiber plate  20  has a first face  22  and a second face  24 . In a typical implementation, the first and second faces  22  and  24  are ground and polished to create smooth, planar faces. However, cutting, grinding and polishing to create other-than-planar faces and plate profiles that are of other-than-uniform thickness is within the scope and contemplation of the invention. 
   In various implementations, the cores  12  are made from a material that is more soluble in a predetermined solvent than the fused cladding material  14  to facilitate selective chemical etching of cores  12  from the fused cladding material  14 . When at least a portion of the fused fiber plate  20  is exposed to the predetermined solvent for a sufficient length of time, cores  12  are etched out of the fused cladding material  14  resulting in a capillary plate  30  such as the illustrative capillary plate shown in  FIG. 1A . The capillary plate  30  of  FIG. 1A  has first and second faces  32  and  34  and comprises the fused cladding material  14  and a plurality of voids  40  corresponding in position and cross-sectional geometries to the pre-etch positions and cross-sections of the cores  12  dissolved out of the fused fiber plate  20 . The voids  40  in the illustrative capillary plate  30  of  FIG. 1A  are referred to as “through-voids” because they extend through the capillary plate  30  between the first and second faces  32  and  34 . 
     FIGS. 1A through 1C  depict the fabrication of a well plate  60  incorporating the illustrative capillary plate  30  and a base plate  50 . The base plate  50  is, for example, a planar plate of glass including first and second sides  52  and  54 . The first face  32  of the capillary plate  30  is bonded to the first side  52  of the base plate  50  to form a well plate  60  including an upper side  62 , a lower side  64  and a plurality of wells  70 , each of which wells  70  has a closed bottom end  72 , an open top end  74  and a well wall  76  extending between the open closed bottom and open top ends  72  and  74 . Depending on the types of analyses to be performed on materials deposited into the wells  70 , the base plate  50  can alternatively exhibit at least one optical attribute of a selected set of optical properties including, for example, (i) transparency, (ii) translucency, (iii) selective electromagnetic wavelength filtration, (iv) electromagnetic wavelength polarization, (v) dispersion, (vi) image focus, (vii) image magnification and (viii) image reduction. In various alternative embodiments, a base plate  50  fabricated from fused optical fibers facilitates one or more of the aforementioned attributes. For instance, in some versions, selective light filtration is achieved by the use of doped or tinted core glass in a fused fiber base plate  50 . In another instance, base-plate focusing properties are achieved by fusing GRIN optical fibers to form the base plate  50 .  FIG. 1D  depicts a version of a well plate  60  in which the base plate  50  is comprised of adjacently fused optical fibers that may, in alternative versions, include at least one of tinted cores  12  and cores  12   g  exhibiting graded refractive index profiles retained by fused cladding material  14 . For the sake of simplicity, and in accordance with conventional industry terminology, cores  12   g  that are fusedly retained in a faceplate, and that exhibit graded-refractive-index profiles, are alternatively referred to as GRIN fibers or, where appropriate, GRIN fiber segments. The incorporation of GRIN fibers in the base plate  50 , in alternative implementations, facilitates at least one of (i) the focused illumination of fluidic samples (not shown) contained in the wells  70  from the second side  54  of the base plate  50  and (ii) the observation of samples in the wells  70  through the second side  54  of the base plate  50 . 
   Referring to  FIG. 2A , an alternatively configured well plate  60  includes wells  70  having integrated optical-focusing closed bottom ends  72 . The well plate  60  of  FIG. 2A  comprises a capillary plate  30  wherein each void  40  of a selected plurality of voids  40  is closed by a focusing element  80  to form a well  70 . In the illustrative example of  FIG. 2A , one example of each of a ball lens  82 , an aspheric lens  84  and a GRIN optical fiber segment  86  is shown defining the closed bottom end  72  of a well  70 . In alternative embodiments, focusing elements  80  are secured into a well-sealing position by, for example, at least one of press fitting, fusing, epoxy or other adhesive bonding agent, laser tacking and anodic bonding. In addition to sealing the void  60  over which it is applied, a focusing element  80  facilitates empirical study of contents deposited in the well  70  for analysis. 
   Each illustrative focusing element  80  of  FIG. 2A  is positioned over one end of a through-void  40  after fabrication of a capillary plate  30  and then secured in place. Distinguishably, the well plate  60  of  FIG. 2B  includes integrated GRIN fiber segments  86  formed from cores  12  that are fused into the surrounding cladding material  14  during fabrication of a fused bundle  10 . One of the first and second faces  22  and  24  of a fused plate  20  cut from the bundle  10  is exposed to a core solvent for a period of time sufficient to etch away (dissolve) a portion, but not the entire length, of each core  12  of a selected set of cores  12  such that the remaining, non-etched segment  86  of each core  12  does not extend the full distance by which the first and second faces  22  and  24  are separated and is, therefore, recessed with respect to the etched face  22  or  24  and serves as the closed bottom end  72  of a well  70  and as a focusing element  80 . The depth and volume of a well  70  is controllable by regulating the exposure time of the core  12  to the solvent. It will be appreciated that a well plate  60  having wells  70  of various predetermined depths can be formed by, for example, selective masking and exposure of cores  12  and that the fused plate  20  can, in various implementations, be etched from either or both of the first and second faces  22  and  24 . Moreover, well volume is a function of cross-sectional geometry and diameter of the cores  12  initially present in a fused plate  20  and, in various embodiments, cores  12  of various geometries and diameters are incorporated into the same fused plate  20 . 
   Referring to  FIG. 3A , a large-well/small-well plate  90  includes a first capillary plate  30 A bonded to a second capillary plate  30 B. Each of the first and second capillary plates  30 A and  30 B includes wells  70  that are open at either end. That is, they are through-voids  40  extend through the capillary plate  30 . Moreover, in the illustrative example, each well  70 A of a selected set of wells  70 A within the first capillary plate  30 A is in fluid communication with a plurality (at least two) smaller wells  70 B within the second capillary plate  30 B. In one alternative version, a large-well/small-well plate  90  is fabricated by bonding a first capillary plate  30 A to a second capillary plate  30 B after the capillary plates  30 A and  30 B have been independently fabricated. There exists a statistical probability that such a version will include small wells  70 B that are not aligned (not in fluid communication) with a larger well  70 A when the two plates  30 A and  30 B are brought together, as shown in the large-well/small well plate  90  of  FIG. 3A . Accordingly, referring to FIGS.  3 Bi and  3 Bii, when it is desired that each small well  70 B align with a larger well  70 A, an alternative implementation calls for etching the cores  12  from a first fused plate  20 , such as those shown in FIG. C, to form a first capillary plate  30 A and then bonding the resulting capillary plate  30 A to a non-etched second fused plate  20 B including cores  12  smaller in cross-sectional area than the through-voids  40  in the first capillary plate  30 A. A core solvent (not shown) is then introduced into the larger wells  70 A of the first capillary plate  30 A to etch out cores  12  in the second fused plate  20 B only where cores  12  in the second fused plate  20 B are aligned with large wells  70 A thereby forming the second capillary plate  30 B including wells  70 B. Cores  12  in the second capillary plate  30 B that are not aligned with a larger well  70 A in the first capillary plate  30 A remain fused within the second capillary plate  30 B. In still another implementation, a large-well/small-well plate  90  is fabricated by bonding a first fused fiber plate  20 A to a second fused fiber plate  20 B and then exposing the bonded plates  20 A and  20 B to a core solvent (not shown) in order to etch out the cores  12 A and  12 B from, respectively, fused fiber plates  20 A and  20 B and form the capillary plates  30 A and  30 B as shown in FIGS.  3 Biii and  3 Biv. In the particular version of FIGS.  3 Biii and  3 Biv, the first fused fiber plate  20 A includes cores  12 A that are larger in cross-section than cores  12 B in fused fiber plate  20 B. Accordingly, subsequent to core dissolution, the wells  70 A in capillary plate  30 A are larger in cross-section than the wells  70 B in the capillary plate  30 B. 
   Large-well/small-well plates  90  are alternatively useable as filters elements. They are also adaptable for use in the study of materials in liquid form that are retained by capillary forces in the small wells  70 B of the second capillary plate  30 B. The large wells  70 A facilitate cleaning of the small wells  70 B by the introduction of, for example, water or a cleaning solution through the large wells  70 A to displace material from the small wells  70 B. 
   As stated in the summary, various versions include voids  40 /wells  70  defined by blackened cladding material  14 . An illustrative method of fabricating a capillary plate  30  including voids  40  defined by blackened cladding material  14  is explained in general terms in conjunction with  FIGS. 4 through 5D . It is to be understood that such capillary plates  30  can be used in the fabrication of any of the various illustrative well plates described above through the execution of variously combined blackening steps described herein and fabrication steps described above. Moreover, as with the non-blackened embodiments described above, some of the details of the standard rod-in-tube, and bundling and fusing, methods of fabricating fused bundles and faceplates are not provided because they are established methods known to those of ordinary skill in the optical fiber fabrication arts. 
   Referring to  FIGS. 4 and 4A , various implementations include insertion of a core  12  into a tube of cladding material  14  having inner and outer surfaces  14   i  and  14   o  that includes a plurality of reducible ions I R  at least adjacent the inner surface  14   i  of the cladding material  14 . A fused fiber bundle  10  is then formed using multiple, adjacently bound fibers in accordance with the method previously described for non-blackened versions and shown in FIGS. B and C. The bundle  10  is then cut to form one or more fused plates  20 , as previously discussed, and cores  12  are selectively etched therefrom to form a capillary plate  30  having a plurality of voids  40  defined by walls of cladding material  14  including reducible ions I R  as shown in  FIG. 5A  and the cross-sectional view of  FIG. 5B . 
   Referring to  FIG. 5C , the capillary plate  30  is then placed into a reducing atmosphere such as hydrogen-firing furnace  500  where it is exposed to heated hydrogen gas H 2  for a predetermined length of time. At a predetermined temperature falling within a predetermined temperature range, the di-atomic hydrogen H 2  reduces reducible ions I R  in the cladding material  14  defining voids  40 . The reduction of the reducible ions I R  results in the blackening of the cladding material  14 , as shown in  FIG. 5D , which, as discussed briefly in the summary, yields advantageous autofluorescence-negating characteristics to the capillary plate  30 . The length of time and temperature ranges required to achieve desired results depend on such factors as (i) the type of glass used as cladding material  14 , (ii) the nature of the reducible ions I R , and (iii) the concentration of diatomic hydrogen H 2  present in the furnace  500 . One illustrative successful method executed by the inventors exposed a capillary plate  30  to hydrogen H 2  for 12 hours at 450° C. Also variable are the inner diameters and cross-sectional geometries of the voids  40 , the thickness of the capillary plate  30  and the surface area and configuration of the capillary plate  30 . For instance, depending on intended usage, voids  40  having diameters of anywhere from 10 microns to 1000 microns are advantageous, although this range by no means constitutes a limit on the scope of the invention. 
   In the illustrative example of  FIGS. 4 through 5D , tubes of cladding material  14  having reducible ions I R  only in the vicinity of the inner surface  14   i  thereof were used. Accordingly, blackening was restricted primarily to those portions of cladding material  14  defining the walls of voids  40 . It will be appreciated that when tubes of cladding material  14  having reducible ions I R  present throughout the tube are used, the first and second faces  32  and  34  of the capillary plate  30  may also be subject to blackening, as in the illustrative version of  FIG. 5E . More generally, wherever the cladding material  14  includes reducible ions I R  that are exposed to heated hydrogen gas during hydrogen firing, that portion of the cladding material  14  can be caused to blacken. Accordingly, if, for example, tubes of cladding material  14  having reducible ions I R  throughout are used, and it is not desired that the faces  32  and  34  be blackened, the capillary plate  30  can be hydrogen fired and then the plate  30  can be ground and/or polished to remove blackened face material. 
     FIG. 6  shows illustrative comparative autofluorescence data for (i) a blackened capillary plate, (ii) a clear (i.e., non-blackened) capillary plate and a plain clear (i.e., non-blackened) glass plate. The chart includes autofluorescence intensity data at three different output wavelengths (i.e., 400, 600 and 800 nanometers) for each of two input wavelengths (i.e., 635 and 532 nanometers). For instance, the intensities of autofluorescence at 400 nm for the 532 nm input light is relatively even for the blackened capillary plate, the plate glass and the non-blackened (i.e., “clear”) capillary plate. However, the intensities of autofluorescence at 800 nm corresponding to the blackened capillary plate, the plate glass and the non-blackened (i.e., “clear”) capillary plate are dramatically disparate for 532 nm input light. More specifically, while the autofluorescence intensity for the blackened capillary plate  30  is relatively unchanged over all three output wavelengths (i.e., 400, 600 and 800 nm), the autofluorescence intensity at 800 nm of the clear capillary plate is over 4× the value it is at 400 nm. The steady, ascertainable value of autofluorescence intensity over a large wavelength range is a desirable attribute of blackened capillary plates  30  in part because it provides a relative “noise” constant that can be made known to users of the blackened capillary plates  30  and because it provides a much lower signal to noise ratio at higher wavelengths than the dramatically increased autofluorescence characteristic of the non-blackened capillary plates. 
   The foregoing is considered to be illustrative of the principles of the invention. Furthermore, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired that the foregoing limit the invention to the exact construction and operation shown and described. Accordingly, all suitable modifications and equivalents may be resorted to that appropriately fall within the scope of the invention as expressed in the appended claims.