use of a 2d to 1d dimension reduction fiber-optic array for multiwavelength imaging sensors

Volume 55, Number 2, 2001 APPLIED SPECTROSCOPY 2170003-7028 / 01 / 5502-0217$2.00 / 0q 2001 Society for Applied Spectroscopy

Use of a 2D to 1D Dimension Reduction Fiber-OpticArray for Multiwavelength Imaging Sensors

MARIA V. SCHIZA, MATTHEW P. NELSON, M. L. MYRICK, andS. MICHAEL ANGEL*Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208 (M.V.S., M.L.M.,S.M.A.); and ChemIcon Inc., Pittsburgh, Pennsylvania 15208 (M.P.N.)

A dimension reduction � ber-optic array is used to measure the re-sponse of a stacked-layer, image-guide CO2/O2 sensor, simulta-neously at several different wavelengths. Two different image-guideCO2/O2 sensor con� gurations are described: a stacked-layer sensor,where luminescence indicators for CO2 and O 2 are uniformly coatedon the tip of the sensor; and a side-by-side coated sensor where thetwo indicators are coated on different halves of the � ber tip. It isshown that a single image-guide measurement, made by using thedimension reduction array, can be used to generate response plots,intensity pro� les, and reconstructed images at different lumines-cence wavelengths. The spatial resolution of an image guide sensoris limited by the number of � bers used to construct the dimensionreduction array.

Index Headings: Dimension reduction; Image guide; Fiber-optic ar-ray; Luminescence imaging; pH sensor; O2 sensor; Imaging sensor;Fiber-optic sensor.

INTRODUCTION

Most � ber-optic sensors use a single optical � ber or� ber bundle as a simple ‘‘light pipe’’ for the transmissionof optical signals from a single measurement point. Thus,they provide no spatial information. Recently, a numberof reports have described the use of small-diameter imageguides to make sensors that allow both chemical and spa-tial information to be measured remotely or in situ. Im-aging probes have been described for spectral measure-ments including remote Raman imaging1 and laser-in-duced breakdown spectroscopy.2 A number of imagingchemical sensors have been described, including multi-analyte sensors with multiple sensing chemistries on thesame � ber, as well as applications involving the mea-surement of concentration gradients over a very smallspatial range.3–9

Spatial information in an image-guide sensor is ob-tained by the use of a two-dimensional (2D) array detec-tor such as a CCD or ICCD (charge-coupled device orintensi� ed charge-coupled device). An image of the distaltip of the � ber, or the spatial region immediately adjacentto the tip, is transmitted through the � ber and imagedonto the array detector at the proximal end. This is in-herently a single-wavelength approach, and separate im-ages must be acquired at each wavelength of interest.Thus, measuring two different molecular probes at twodifferent luminescence wavelengths requires two separateimages to be acquired, as well as images at nonlumi-nescence wavelengths to establish the background (e.g.,up to six separate images).

Received 15 August 2000; accepted 24 October 2000.* Author to whom correspondence should be sent.

The use of � ber-optic arrays for simultaneously mea-suring emission spectra of multiple objects (e.g., multi-object spectroscopy) within an image was reported in1986.10 For this application, the image is formed on thetip of a � ber-optic bundle and the individual � bers arealigned vertically (1D array) at the slit of a spectrograph.In this way, a separate high-resolution spectrum can beacquired simultaneously for each object of interest in the� eld of view. This technique is now commonly used forastronomical spectral imaging.10–15 The idea has recentlybeen extended for analytical spectral measurements andis known as dimension reduction. The dimension reduc-tion technique is most useful when images need to beobtained simultaneously at more than a single wave-length, such as when spectral changes occur rapidly. Ap-plications have been described for micro-Raman imaging,near-infrared (NIR)-Raman, visible and infrared absorp-tion, and � uorescence imaging, confocal � uorescence im-aging, and imaging of laser plumes.16–23 In this paper, wereport the use of dimension reduction by using a 2D to1D � ber array containing 608 � bers, in combination withimage guides for imaging chemical sensors. The advan-tages of this technique include very high spectral reso-lution; simultaneous imaging at many different wave-lengths; and direct measurement of the development ofchemical concentration gradients, in situ. The disadvan-tages include increased complexity of the system; limiteduse of the multiwavelength advantage for most lumines-cence indicators; and some loss of spatial resolution de-termined ultimately by the number of vertical pixels onthe detector.

The feasibility of this approach is demonstrated by si-multaneously measuring O2 and CO2. The sensor is madeby using methods similar to those previously describedin the literature, 24–28 by coating the tip of the image guidewith a sol-gel membrane containing 2,29-bipyridylruth-enium(II) chloride [Ru(bpy)3

21], and 8-hydroxy-1,3,6-pyrenetrisulfonic acid, trisodium salt (HPTS) for O2 andCO2 measurements, respectively.

EXPERIMENTAL

Imaging System Design. A schematic diagram of thesystem is shown in Fig. 1. The illumination system con-sists of a xenon lamp (Xe-150 W) and power supply (Ori-el, Models 6255, 68805), and a 0.22 m f /4 monochro-mator (Spex, Model 1681) with a 1200 grooves mm21

holographic grating. The imaging system consists of asingle-stage f /2 spectrograph (Laser Raman Systems)with a 600 grooves mm21 grating and a thermoelectrically

218 Volume 55, Number 2, 2001

FIG. 1. Schematic diagram of imaging setup with the � ber-optic array.FA, � ber-optic array; O, microscope objective; S , spectrograph; D, de-tector; L, lens; F, � lter-470 BP � lter; DC, dichroic mirror; IG , imageguide. Also shown inside the circled area, the different coating schemes:(a) stacked-layer and (b) side-by-side coated. L1, Ru(bpy)3

21 entrappedin TEOS sol-gel; L2, hydrophobic sol-gel; L3, HPTS entrapped in or-ganically modi� ed sol-gel.

FIG. 2. Detailed illustration of the square 2D side of the � ber array. The 2D side consists of 19 ribbons with 32 square � bers, 25 mm wide, 2 mmthick clad. The distance between ribbons of � bers, separated by the epoxy layer, is 2–3 mm.

cooled 576 3 384 ICCD detector (Princeton InstrumentsInc., Model ITE/ICCD) thermostated at 245 8C.

The construction of the dimension reduction array hasbeen previously described by Nelson et al.22 Brie� y, the2D to 1D � ber-optic array is constructed of 19 ribbons(Collimated Holes Inc.), each containing 32 � bers 25 mmsquare, for a total of 608 � bers. Each � ber has a 0.66numerical aperture (NA), with a 2 mm thick cladding. Thetwo-dimensional end of the � ber bundle is arranged as 19rows by 32 columns of � bers, as shown in Fig. 2, withthe ribbons separated by an epoxy layer ; 2–3 mm thick.The area not covered by � bers is about 15–20% on the2D side of the � ber array. The � bers are aligned verticallyon the 1D side of the array by placing the � ber ribbons

end to end. The length of the 1D side of the array is about17–20 mm. A 203, 0.40 NA (Newport, Inc.) microscopeobjective is used to project the image of the tip of theimage guide sensor onto the 2D side of the array. The 1Dside of the array is appended to a � xed slit. Because the1D side of the array is taller than the slit, only 17 of the19 ribbons, 544 � bers, were seen on the ICCD chip atonce. Re-imaging the image-guide sensor onto the arrayallows � exibility in terms of using a single array with avariety of image-guide sensors. Also, the alignment be-tween the sensor and the array is not as dif� cult as thealignment of the array with the spectrometer.

Reconstruction of the original image that is focused onthe 2D side of the array is a matter of determining theorientation of all of the � bers in the array. This procedurehas been discussed previously.19,22,23 Brie� y, the positionof all 19 � ber ribbons is already known for both the 2Dand 1D sides of the array from its construction. The realissue becomes one of determining the x,y position of theICCD pixels corresponding to each � ber ribbon. Thisprocess is accomplished with the use of a target or ref-erence image with sharp, well-de� ned edges.22

Sensor Preparation. Sensors were prepared by usingimage guides (Sumitomo Electric IGN-10/13) that con-sisted of 13 000 single ; 9 mm � bers in a 1 mm diameterbundle. The two coating con� gurations are shown in Fig.1. Sensor membranes were prepared with the use of com-binations of tetraethylorthosilicate (TEOS), 3-aminopro-pyltriethoxylsilane (APTES, Aldrich, WI), and polydi-methylsiloxane (PDMS) (Gelest, PA) precursors, as re-ported by Nivens et al.29 An O2-sensitive membrane,made by trapping Ru(bpy)3

21 in a pure TEOS gel, wascoated on the tip of the image guide. The TEOS mem-brane was made by acid catalysis. In the case of thestacked-layer sensor con� guration, a separator hydropho-bic membrane, containing mostly PDMS, was coatedover the O2-sensitive membrane to keep out moisture. ForCO2 measurements, a pH-sensitive dye, HPTS (Aldrich,WI/Sigma, MO), was entrapped in a mixed TEOS/AP-TES membrane, buffered by addition of pH 8 buffer so-lution. The CO2 indicator membrane either was coated

APPLIED SPECTROSCOPY 219

FIG. 3. Luminescence of single-layer coated image-guide sensors. (A) CO2 sensor response before (0%) and after exposure to 29% CO2 andcorresponding � rst-order images. (B) O2 sensor response before (0%) and after exposure to 24% O2 and corresponding � rst-order images.

over the other two in a stacked-layer con� guration or wascoated on only one side of the tip of the image guide ina side-by-side con� guration. In the case of the side-by-side con� guration, no hydrophobic membrane was need-ed. The CO2 membrane side of the sensor was buffered.Sensor membranes were applied by dip coating, followedby drying for at least 24 h at 50–65 8C between coatings.The � bers were kept under dry N2 until used. An exci-tation wavelength of 470 nm was used for all measure-ments. The luminescence maxima were 525 and 600 nmfor HPTS and Ru(bpy)3

21, respectively.CO 2 and O2 Measurements. The � bers were tested at

different CO2 and O2 concentrations by exposing them tothe in� ow gas concentration for at least 1 min—maxi-mum response was reached in one minute or less for allconcentrations tested. The CO2 and O2 gas concentrationswere achieved by diluting the pure gases with N2. Allconcentrations are reported as %, on a volume/volumebasis (% v/v). CO2 was dried by � owing it through adesiccant. Images of the � ber-guide sensors were ac-quired by using 1.5 s exposures. Fiber images at a se-lected wavelength were reconstructed from 1D spectralimages with the use of an in-house program that waspreviously described.22


FIG. 4. Luminescence of stacked-layer, image guide CO 2 /O2 sensor. (A) Sensor response before (0%) and after exposure to 40% CO2, andcorresponding � rst-order images. (B) Sensor response before (0%) and after exposure to 24% O2, and corresponding � rst-order images.

RESULTS AND DISCUSSION

The purpose of this paper is to demonstrate the use ofthe dimension reduction 2D to 1D � ber array for multi-wavelength measurements with an image-guide sensor.No attempt was made to optimize sensor response or todetermine sensitivity. Image-guide sensors were preparedand initially tested for response in a simple � uorimeter.Sensors that showed large responses to CO2 and O2 weretested further by using the imaging apparatus shown inFig. 1. Initial tests using a single indicator on an imageguide for either CO2 or O2 were performed to determineresponse and signal levels with the 2D to 1D dimensionreduction system.

Figure 3 shows the response of single-indicator, image-guide sensors to (A) 29% CO2 and (B) 24% O2, comparedto 0% (pure N2 gas). The images (right side of Fig. 3)show how the luminescence signal of each � ber, or � berribbon, from the 1D side of the array decreases in re-sponse to (A) 29% CO2 and (B) 24% O2. Wavelengthseparation of the luminescence intensity from the two in-dicators is clear from the images. The spectral plots (left)were made by summing the luminescence intensity fromthe 1D array images in the vertical direction and plottingthe intensity vs. wavelength. The luminescence intensityshown in the 1D array images is most intense for � berslocated in the center part of the 1D array and decreases


FIG. 5. Response of stacked-layer, image-guide CO2 /O2 sensor to CO2

(A) and O2 (B).

®

FIG. 6. First-order (top) and reconstructed (lower) images showing response of stacked-layer, image-guide CO2 /O2 sensor to 0% and 40% CO2

(A) and 0% and 24% O 2 (B), respectively. The images were reconstructed at 525 and 600 nm, to show the responses to CO2 and O2, respectively.

FIG. 7. Contour plots (right) showing the response of the stacked-layer, image-guide CO2 /O 2 sensor to 0% and 40% CO2 using (A) the imagereconstructed at 525 nm as shown in Fig. 6A, and (B) the image reconstructed at 600 nm. As expected, there is no signi� cant CO2 response at the600 nm wavelength. The grayscale-coded key below the contour plots indicates the luminescence intensity across the tip of the image-guide sensor.The intensity cross sections in A and B (left) show an approximately Gaussian intensity distribution of luminescence at the tip of the image guidesensor (region selected by white box in contour plots). (A) Closed circles (v) 0% CO2, open circles (V) 40% CO 2; and (B) closed triangles ( m )0% CO2, open triangles ( n ) 40% CO2.

towards the top and bottom of the array. This � ndingmight result from a combination of vignetting, becauseof the large height of the 1D array compared to the en-trance slit, uneven illumination of the image guide withthe lamp, and uneven coating of the indicator on the � bertip. Some attempt was made to correct for vignetting inthe system and to ensure even illumination of the imageguide.

In general, vignetting by the spectrometer slit will leadto decreased luminescence being observed for the top andbottom parts of the 1D side of the array. Because of theconstruction of the array, this will lead to decreased sig-nal levels from areas located on opposite sides of the � bertip. It is also possible to reduce the signal slightly morefrom one side of the � ber than the other. A quantitativedescription of the effect is beyond the scope of this work,and additional information can be found in Ref. 22.

Figure 4 shows the response of a stacked-layer, image-guide CO2 /O2 sensor to (A) 40% CO2 and (B) 24% O2,

respectively, compared to 0% (pure N2 gas). The images(right side of Fig. 4) show how the luminescence signalof each � ber, or � ber ribbon, on the 1D side of the arraydecreases in response to (A) 40% CO2 and (B) 24% O2.Again, wavelength separation of the luminescence inten-sity from the two indicators is clear from the images.Upon exposure to CO2, the intensity of the luminescenceimage decreases at 525 nm (left side of images), whilethe luminescence intensity around 600 nm (right side ofimages) is relatively unaffected. However, when the im-age guide sensor is exposed to O2, the intensity decreasesaround 600 nm, while the luminescence intensity on theleft side of the image is relatively unaffected. These ef-fects are also shown by the spectral plots in Fig. 4 for(A) CO2 and (B) O2. Although there is spectral overlapbetween the two indicators, this factor did not affect thepurpose of this study, which was to demonstrate the di-mension reduction technique. High concentrations of CO2

and O2 were used to minimize this effect.Figure 5 shows the response of the stacked-layer, im-

age-guide CO2 /O2 sensor to different concentrations of(A) CO2 and (B) O2. Note that the responses are shownas percent decrease since the signal level is maximizedat 0% concentration. Triplicate measurements at eachconcentration were made, and 62s is indicated by theerror bars. Relative percent decrease of the luminescencesignal for the stacked-layer sensor is ; 80% for 40% CO2

and ; 85% for 24% O2. The relative sensor response wasdetermined by summing the intensity at the wavelengthcorresponding to the luminescence maximum, using lu-minescence images of the 1D side of the array, like thoseshown in Figs. 4A and 4B. For the data shown, the in-tensity of a � ber ribbon in the middle of the image wasused, and the measurements were made at 525 and 600nm for CO2 and O2, respectively. The ribbon selected alsocorresponds to the region of the most intense lumines-cence on the � ber tip.

The dimension reduction technique is most usefulwhen chemical images or concentration gradients need tobe obtained simultaneously at more than a single wave-length. In the above examples, this capability allows cal-ibrations to be made for both indicators from a singleimage. Although, in this case only two indicators areused, calibration calculations still require measurementsat up to six different wavelengths. For example, two in-tensity measurements at the wavelength of each lumines-cence maximum, and four additional measurements, arerequired to characterize the background around each lu-minescence maximum. Obviously, this can be done byusing bandpass � lters that are selected for the regions ofinterest. However, in cases where the signal is changingrapidly, the process of changing � lters might be too slow.


FIG. 9. Reconstructed images showing the response of the side-by-side coated CO2 /O2 image guide sensor to CO2 (top half of each image) andO2 (lower half of each image). The top and lower half of the images were reconstructed at 525 and 600 nm, to show the responses to CO2 and O2

respectively. The entire sensor tip is � uorescent in dry nitrogen (A). The luminescence intensity of the lower half decreases in O 2 with little changein the top image (B), while the luminescence intensity of the top half decreases in CO 2 with little change in the lower image (C ).

¬

FIG. 8. Contour plots (right) showing the response of the stacked-layer, image-guide CO 2 /O2 sensor to 0% and 24% O2 using (A) the imagereconstructed at 600 nm as shown in Fig. 6B, and (B) the image reconstructed at 525 nm. There is some O2 response at the 525 nm wavelength.The grayscale-coded key below the contour plots indicates the luminescence intensity across the tip of the image guide sensor. The intensity crosssections in A and B (left) show an approximately Gaussian intensity distribution of luminescence at the tip of the image guide sensor (regionselected by white box in contour plots). (A) Closed triangles ( m ) 0% O2, open triangles ( n ) 24% O2; and (B) closed circles (v) 0% O2, open circles(V) 24% O2.

With the dimension reduction technique, it is possibleto reconstruct images at any number of different wave-lengths, from the measurement of a single array image.Figure 6 shows images that are reconstructed at (A) 525nm and (B) 600 nm, to show the response of the stacked-layer image-guide sensor to CO2 and O2, respectively, asa function of position on the � ber tip. The 1D array im-ages showing the response of the sensor to (A) 40% CO2

and (B) 24% O2 as a function of wavelength, are shownat the top in this � gure. The images in the lower part ofFig. 6 are reconstructed from the 1D array data, to showhow the luminescence intensity decreases at (A) 525 nmand at (B) 600 nm, in response to CO2 and O2. In thelower images, the circle represents the tip of the imageguide sensor. These images show the distribution of lu-minescence across the tip of the � ber at the wavelengthused to reconstruct the image.

Figures 7 and 8 show the sensor response to CO2 andO2 in a different way, but using the same data shown inthe 1D array images (upper images) in Fig. 6. Figure 7Ashows how the sensor responds at 525 nm upon exposureto 40% CO2, as a function of position on the � ber tip.Figure 7B shows that there is little or no response acrossthe � ber tip at 600 nm, upon exposure to CO2. The in-tensity pro� les in this � gure show the Gaussian-like dis-tribution of luminescence across the � ber tip (region se-lected by white box in contour plots), and also show thatthe relative change in sensor response is independent ofposition on the � ber, as expected. In a similar fashion,Fig. 8 shows how the sensor responds to O2 at (A) 600nm and (B) 525 nm. In this case however, there is someresponse at 525 nm upon exposure to O2, because of therelatively high spectral overlap between HPTS andRu(bpy)3

21.The images shown in Figs. 7 and 8 also clearly show

that the sensor tip is not uniformly luminescent, as wasdiscussed for Fig. 3. The intensity is higher near the mid-

dle of the sensor tip in the images that are reconstructedto show the HPTS luminescence at 525 nm. However,the intensity is higher near the edge of the � ber tip in theimages that are reconstructed to show the Ru(bpy)3

21 lu-minescence at 600 nm. This result seems consistent withuneven tip coating.

The stacked-layer con� guration is most useful for pro-viding the highest possible spatial resolution chemicalimages. Thus, it is useful for measuring rapidly changingchemical concentration gradients over a very small spa-tial scale. However, the dimension reduction technique isalso useful for sensors where the indicator regions arephysically separated on the � ber. This concept is dem-onstrated by the images in Fig. 9.

Figure 9 shows the response of a side-by-side coatedimage-guide CO2 /O2 sensor. In these images, the HPTSindicator layer (e.g., CO2 sensitive layer) is at the top andthe Ru(bpy)3

21 indicator layer (e.g., O2 sensitive layer) isat the bottom of the circled region. The luminescenceimages shown in Fig. 9 were all reconstructed at both525 and 600 nm, in order to show response to CO2 andO2, in the same image. This � nding also illustrates an-other signi� cant advantage of the dimension reductiontechnique: the ability to construct chemical images at anycombination of wavelengths. The entire sensor tip is lu-minescent in dry nitrogen (A). The luminescence inten-sity of the lower half decreases in O2, with little changeof intensity of the top image (B), while the luminescenceintensity in the top half decreases in CO2 with littlechange of intensity in the lower image (C ).

CONCLUSION

The dimension reduction technique provides the abilityto measure several regions on the sensor tip at differentwavelengths or at any combination of wavelengths, si-multaneously. However, it should be pointed out that, in


this case, the number of different indicators that can beclearly separated in the reconstructed images depends onthe resolution of the 2D to 1D array. At the resolutionused in this study, about 50 mm, it should be possible tosimultaneously measure approximately 300 indicatorspots on a 1 mm image guide sensor. The spatial reso-lution is ultimately determined by the number of verticalpixels on the detector. Moreover, it is theoretically pos-sible to measure each spot at many different wavelengthssimultaneously; however, in the case of the stacked-layercon� guration, this approach is limited by overlap of in-dicator emission. Spectral overlap is not as important forthe side-by-side coated sensor, or for any sensor wherethe indicators are physically separated on the � ber tip.The dimension reduction technique is most useful for rap-idly changing chemical concentration gradients. It is lessuseful for chemical systems that change slowly, andwhere discrete sensors can be used. The number of wave-lengths that can be used in a practical sensor is limitedby the broad luminescence of most indicators. However,in situations where chemical concentration gradientschange rapidly over time or where background signalschange rapidly, this technique can be very useful. Obvi-ously, it will also be useful for indicators that have nar-rower emission bands such as certain inorganic indicatorsof temperature, or for the use of less common probessuch as Raman indicators.

ACKNOWLEDGMENTS

This work was supported by NSF EPSCoR (Grant No. EPS-9630167)and the Of� ce of Naval Research (Grant No. N00014-97-1-0806). Sup-port was also provided by the U.S. Department of Energy under GrantNumber DOE/EPSCoR Cooperative Agreement Number DE-FC02-91ER75666, Amendment Number A004. The authors would like to ex-press their appreciation to Karyn M. Coates for her help with dataacquisition.

1. H. T. Skinner, T. F. Cooney, S. K. Sharma, and S. M. Angel, Appl.Spectrosc. 50, 1007 (1996).

2. B. J. Marquardt, D. N. Stratis, D. A. Cremers, and S. M. Angel,Appl. Spectrosc. 52, 1148 (1998).

3. P. Pantano and D. R. Walt, Anal. Chem. 67, 481A (1995).

4. D. R. Walt, Acc. Chem. Res. 31, 267 (1998).5. K. S. Bronk, K. L. Michael, P. Pantano, and D. R. Walt, Anal.

Chem. 67, 2750 (1995).6. K. L. Michael, L. C. Taylor, S. L. Schultz, and D. R. Walt, Anal.

Chem. 70, 1242 (1998).7. K. S. Bronk and D. R. Walt, Anal. Chem. 66, 3519 (1994).8. A. A. Panova, P. Pantano, and D. R. Walt, Anal. Chem. 69, 1635

(1997).9. G. Liebsch, I. Klimant, B. Frank, G. Holst, and O. S. Wolfbeis,

Appl. Spectrosc. 54, 548 (2000).10. S. C. Barden, and K. Scott, 169th A.A.S. Meeting, Pasadena, Cal-

ifornia (1986), Abstract #30.12, Vol. 18, p. 951.11. S. C. Barden and A. R. Wade, ‘‘Densepak and Spectral Imaging

with Fiber Optics’’, in Fiber Optics in Astronomy, S. C. Barden,Ed. (Astronomical Society of the Paci� c, San Francisco, California,1988), p. 113.

12. S. C. Barden and P. Massey, ‘‘Nessie-A Versatile Multi-Fiber Feedon the KPNO Mayall 4-Meter Telescope’’, in Fiber Optics in As-tronomy, S. C. Barden, Ed. (Astronomical Society of the Paci� c,San Francisco, California, 1988), p. 140.

13. I. Parry, M. Kenworthy, and K. Taylor, ‘‘SPIRAL Phase A: A Pro-totype Integral Field Spectrograph for the AAT’’, in SPIE Proceed-ings for Optical Telescopes of Today and Tomorrow, A. L. Arde-berg Ed. (SPIE, Bellingham, Washington, 1997), Vol. 2871, p.1325.

14. R. Haynes, D. Lee, J. Allington-Smith, R. Content, G. Dodsworth,I. Lewis, R. Sharples, J. Turner, J. Webster, C. Done, R. Peletier, I.Parry, and S. Chapman, Publ. Astron. Soc. Pac. 111, 1451 (1999).

15. D. M. Meyer and J. T. Lauroesch, Astrophys. J. 520, L103 (1999).16. J. Ma and D. Ben-Amotz, Appl. Spectrosc. 51, 1845 (1997).17. A. D. Gift, J. Ma, K. S. Haber, B. L. McClain, and D. Ben-Amotz,

J. Raman Spectrosc. 30, 757 (1999).18. B. L. McClain, J. Ma, and D. Ben-Amotz, Appl. Spectrosc. 53,

1118 (1999).19. M. P. Nelson, W. C. Bell, M. L. McLester, and M. L. Myrick, Appl.

Spectrosc. 52, 179 (1998).20. Y. Q. Li, S. Sasaki, T. Inoue, A. Harata, and T. Ogawa, Appl. Spec-

trosc. 52, 1111 (1998).21. H. Suto, Infrared Phys. Technol. 38, 93 (1997).22. M. P. Nelson and M. L. Myrick, Rev. Sci. Instrum. 70, 2836 (1999).23. M. P. Nelson and M. L. Myrick, Appl. Spectrosc. 53, 751 (1999).24. J. Brinker and G. Scherer, Sol Gel Science (Academic Press, San

Diego, California, 1989).25. L. L. Hench and J. K. West, Chem. Rev. 90, 33 (1990).26. R. Reinsfeld and C. K. Jorgenson, Chemistry Spectroscopy and

Applications to Sol-Gel Glasses (Springer-Verlag, Berlin, 1992).27. B. C. Dave, B. Dunn, J. S. Valentine, and J. I. Zink, Anal. Chem.

66, 1120A (1994).28. G. W. Scherer, J. Non-Cryst. Solids 100, 77 (1988).29. D. A. Nivens, Y. Zhang, and S. M. Angel, Anal. Chim. Acta 376,

235 (1998).

use of a 2d to 1d dimension reduction fiber-optic array for multiwavelength imaging sensors

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