Ultraviolet characterization of integrating spheres

Ultraviolet characterization of integrating spheres

Ping-Shine Shaw,* Zhigang Li, Uwe Arp, and Keith R. LykkePhysics Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA

*Corresponding author: [email protected]

Received 20 February 2007; accepted 26 March 2007;posted 10 April 2007 (Doc. ID 80231); published 9 July 2007

We have studied the performance of polytetrafluoroethylene integrating spheres in the ultraviolet (UV)region with wavelengths as short as 200 nm. Two techniques were used for this study; first, the spectralthroughput of an integrating sphere irradiated by a deuterium lamp was analyzed by a monochromator.Second, a UV laser beam was directed into an integrating sphere, and spectrally dispersed laser inducedfluorescence was studied. Significant absorption and fluorescence features were observed in the UVregion and attributed to the contamination in the integrating sphere. We demonstrate that integratingspheres are easily contaminated by environmental pollutants such as polycyclic aromatic hydrocarbonsemitted from engine exhaust. Baking of the contaminated integrating sphere can reverse some but notall of the effects caused by contaminants. The implications for using integrating spheres for UV mea-surement are discussed.

OCIS codes: 120.3150, 140.3610, 300.2530, 120.5630.

1. Introduction

Integrating spheres play an indispensable role when itcomes to diffusing and depolarizing radiation. Theyare widely used in radiometric applications because oftheir superior performance in transforming radiationto a nearly ideal Lambertian distribution despite vari-ations in the conditions of the incident radiation. In thevisible and near ultraviolet (UV) region, polytetrafluo-roethylene (PTFE) has been proven to be the bestmaterial for integrating spheres because of high reflec-tivity and chemical inertness. Integrating spheresmade with sintered PTFE are commercially availablefrom many manufacturers.

For UV applications, however, concerns over theperformance of diffusing materials, such as PTFE,were first raised three decades ago [1] when inte-grating spheres were used for spectral irradiancemeasurements of deuterium lamps. When integratingspheres were irradiated by UV radiation from deute-rium lamps, spurious effects in the measured spectralirradiance were observed and attributed to fluores-cence from diffusing materials induced by UV irradi-ation. A later study [2] confirmed the fluorescenceeffect from PTFE powder and suggested that contam-inants in the powder were responsible for fluorescence.

Since then only a few studies on the UV reflectanceand fluorescence of diffusing materials have been re-ported [3–7] and, to the best knowledge of the authors,no UV work on integrating spheres has been docu-mented. In one study of PTFE as a diffuse reflectancematerial for space-borne remote sensing application[3,4], contaminants were again shown to be responsi-ble for the degradation in the reflectance of PTFE sam-ples when irradiated with radiation from a Xe lamp.By using several chemical analysis techniques, the na-ture of the contaminants was identified only as volatilehydrocarbon impurities intrinsic to the commercialmaterial. Furthermore, it was also shown that contam-inants in PTFE samples can be cleaned by baking invacuum although a clean PTFE sample can be contam-inated again when exposed to aromatic hydrocarbons.Later, more studies reported measurements of thedegradation in the hemispherical reflectance of PTFEmaterial irradiated by Xe or Hg lamps [5–7] with nomention of its implication for PTFE integratingspheres.

The correlation between the hemispherical reflec-tance of PTFE material and the throughput of aPTFE integrating sphere is given by the followingexpression [8]

1 August 2007 � Vol. 46, No. 22 � APPLIED OPTICS 5119

�out � �in

r2A2�1 � �A1 � A2��1 � r�1 � �A1 � A2��

,

where �in and �out are the flux entering and exitingan integrating sphere, A1 and A2 are the ratio of thearea of the entrance and exit port to the total innersurface area of the sphere, and r is the hemisphericalreflectance of the diffusing material. At high reflec-tance and small port area, as is the case for a typicalintegrating sphere, the throughput is highly sensitiveto small variations in r. Given the reported vulnera-bility to degradation in the UV hemispherical reflec-tance of PTFE, it is expected that the degradation inthe throughput of a PTFE integrating sphere will bemuch more pronounced. The instability of the UVthroughput of an integrating sphere could result inlarge errors in long-term measurements.

In addition to throughput, fluorescence from anintegrating sphere is even more challenging to UVmeasurement. Fluorescence, generally induced by in-cident radiation from a wavelength region to the blue,introduces spurious spectral effects and leads to mea-surement errors that are difficult to identify. A studyof the UV properties of integrating spheres in boththroughput and florescence will have important im-plications for radiometric measurements in the UV,such as source calibration, reflectance standards, andremote sensing [9].

Integrating spheres made with PTFE are formedby two major techniques. Most commercially avail-able products are made with sintered PTFE andmany laboratories, including the National Instituteof Standards and Technology (NIST), also constructtheir own integrating spheres by pressing PTFE pow-der onto a spherical surface. Sintered PTFE integrat-ing spheres are more structurally sound but concernshave been raised about the resin used to form thespherical shape [3]. Here, we report the characteriza-tion of both sintered and pressed PTFE integratingspheres in the UV to identify the nature of an inte-grating sphere’s absorption and fluorescence proper-ties using two techniques. First, the integratingsphere under study is irradiated by a deuteriumlamp, and the relative spectral throughput of theintegrating sphere is measured using a monochroma-tor system. Second, the integrating sphere is irradi-ated with a beam from a tunable UV laser, and thelaser induced fluorescence (LIF) is analyzed by aspectrometer. The former technique measures the ab-sorption spectra and total fluorescence while the lat-ter provides fluorescence spectra excited by a fixedwavelength. The combination of absorption and flu-orescence spectra provides insight into the chemicalnature of any contaminants responsible for fluores-cence. Many commercially available sintered PTFEintegrating spheres, along with home-made pressedPTFE integrating spheres, were measured using bothtechniques. We studied the role of contaminants forthe observed absorption and fluorescence as well asthe degradation of integrating spheres under UV ir-radiation. We also studied integrating spheres with

controlled exposure to environmental pollutants,such as gasoline and diesel exhaust from engines.The effect of baking integrating spheres in both vac-uum and air on contaminants is discussed. Detailedresults and analysis as well as some recommenda-tions on using integrating spheres for UV work aregiven.

2. Experimental Setup

In characterizing integrating spheres, two UV sourceswere used to probe the integrating sphere under study;a deuterium lamp and a tunable laser system. Thedeuterium lamp emits continuous radiation in the UVfor the study of spectral throughput while the lasersystem provides several discrete wavelengths for LIFstudy. By using these two sources, we investigatedboth pressed and sintered PTFE integrating spheres,with the former made at NIST using PTFE powdersand the latter from several different manufacturers.For better comparison, all of the spheres investigatedwere chosen with an inner diameter �3.8 cm and�1 cm diameter for both entrance and exit apertures.Pristine integrating spheres were also subject to dif-ferent treatments and studied using both techniques.These treatments include baking of the integratingsphere, prolonged exposure to a deuterium lamp forstability study, and exposure to engine exhaust toassess the effect of environmental pollutions.

A. Spectral Throughput Measurements

The spectral throughput of an integrating sphere wasmeasured by utilizing part of a UV irradiance cali-bration system at the Facility for Irradiance Calibra-tion Using Synchrotrons (FICUS) at NIST [10,11]. Asshown in Fig. 1, a deuterium lamp was placed at atypical distance of 30 cm from the entrance port of theintegrating sphere under study. The spectral distri-bution of the exiting radiation from the integratingsphere was measured with a 0.25 m Czerny–Turnermonochromator coupled to a temperature-stabilizedphotomultiplier tube. The bandwidth of the mono-chromator was set at 5 nm and the wavelength rangewas scanned between 200 and 400 nm for this work.Order-sorting filters were not necessary for this

Fig. 1. Schematic diagram of the experimental setup for mea-surement of integrating sphere spectral throughput.

5120 APPLIED OPTICS � Vol. 46, No. 22 � 1 August 2007

range since wavelengths shorter than 200 nm wereeliminated by air and window absorption.

The measured spectral distribution using the abovesetup consists of several spectral effects, such as theoutput of the deuterium lamp, the throughput of theintegrating sphere, the throughput of the monochro-mator, and the responsivity of the photomultipliertube. To isolate the throughput of the integratingsphere, a separate measurement was performed byremoving the integrating sphere and placing the deu-terium lamp directly in front of the entrance slit of themonochromator. The resulting spectral output, asshown in Fig. 2, was used to normalize all spectraldistributions measured with an integrating sphere.While this normalization process does not yield abso-lute throughput of the integrating sphere under studyat a specific wavelength, the normalized spectral dis-tribution is proportional to the spectral throughput ofthe integrating sphere by a constant throughout thewavelength region of measurements. Absorption struc-tures in the integrating spheres can be easily identifiedfrom normalized spectral distributions.

B. Laser Induced Fluorescence Measurements

Tunable lasers at the NIST facility for Spectral Irra-diance and Radiance Responsivity Calibrations usingUniform Sources [12] were used for the study of LIFfrom integrating spheres. The tunable laser, with arange of wavelength from 200 to 400 nm for thiswork, was generated by frequency doubling, tripling,and quadrupling of a quasi-continuous wave (cw)mode-locked Ti:sapphire laser. The quasi-cw output,at 76 MHz and 2 ps duration, was in the mW powerrange. The wavelength uncertainty was less than0.01 nm.

The setup used to measure fluorescence from anintegrating sphere is very similar to the setup used tomeasure the spectral throughput using a deuteriumlamp discussed previously except that the output ra-diation from the integrating sphere was measured by a

commercial charge coupled device (CCD)-array spec-trograph instead of the monochromator as shown inFig. 3. Because of the high input laser power irradiat-ing the integrating sphere, the CCD-array spectro-graph was used to achieve a wider spectral range (from200 to 800 nm), higher resolution (�3 nm in band-width), and shorter acquisition time. An optical fiberdirected the radiation from the exit port of the inte-grating sphere to the entrance port of the spectro-graph. Inside the spectrograph, dispersed radiationwas detected by a two-dimensional cooled ��10 °C�CCD array with a total of 1024 � 128 pixels. Theoutput of the array detector was binned alongthe column with 128 pixels, and the resolution ofthe instrument was 15 bits.

The measured spectral distribution using the LIFsetup with fixed wavelength laser excitation consistsof fluorescence from the integrating sphere and theprimary exciting laser line on top of two slowly de-caying wings around the primary laser peak (i.e., linespread function) contributed by scattered stray lightinside the spectrograph [11,13,14]. Shown in Fig. 4is a typical line spread function measured with a360 nm laser beam. For better comparison betweendifferent measurements, all spectral distributionsare normalized to the peak at the wavelength of theexcitation laser.

Fig. 2. Spectral output with the deuterium lamp irradiating themonochromator entrance directly. This signal was used to normal-ize spectral outputs measured with integrating spheres to obtainthe relative throughput of the integrating spheres as explained inthe text.

Fig. 3. Schematic diagram of the experimental setup for the LIFmeasurements on integrating spheres.

Fig. 4. Typical line spread function of the spectrograph with thelaser wavelength of 360 nm.


3. Results

To have a general evaluation on the impact of absorp-tion and fluorescent effects of integrating spheres onthe UV radiometry, we characterized a number ofnewly acquired commercial integrating spheres fromseveral manufacturers. For comparison, we also stud-ied some commercial integrating spheres that havebeen extensively used in our laboratory. Pressed in-tegrating spheres were not commercially availableand were packed at NIST using PTFE powders ofHalon [15]. The effect of baking integrating sphereswas studied at length. We also measured integratingspheres that were exposed to environmental contam-ination to shed light on the causes of UV absorptionand fluorescence. Finally, we discuss the temporalstability of integrating spheres.

A. Pristine Integrating Spheres

Shown in Fig. 5 are the results of spectral throughputmeasurements performed on several sintered andpressed integrating spheres. Note that all throughputcurves were normalized with the throughput at400 nm to account for the variation in the integratingsphere throughput owing to differences in sphere ge-ometry. All curves in Fig. 5 display a general trend ofgradual decrease in throughput toward shorter wave-lengths in accordance with the reported decrease inreflectivity from PTFE plaques [2,9]. In addition to thegradual decay, all integrating spheres show the sametwo-peak-absorption structure below 300 nm. Notethat this structure is very similar to the measuredreflectance factor of sintered PTFE samples reportedpreviously [9]. For wavelengths above 300 nm, thethroughput is generally smooth, but some integrat-ing spheres exhibited a broad peak. Of all integrat-ing spheres shown in Fig. 5, the pressed integratingsphere has the smoothest spectral throughput withthe absorption structure barely distinguishable, in-

dicating that the absorption structure is contrib-uted to by contaminants and not an intrinsicproperty of the PTFE material. In general, we foundthat pressed PTFE integrating spheres are muchless infected with these absorption bands as well asfluorescence, as will be discussed later. This isagain in accordance with previous observations [9].

To find the fluorescence properties of the integrat-ing spheres under this study, we conducted LIF mea-surements for several integrating spheres used inFig. 5. The measured fluorescence spectra with theexcitation laser wavelength set at 220 nm are shownin Fig. 6. As in the case of the throughput spectra, thefluorescence spectra resemble each other in the wave-length region from 250 to 400 nm. In addition, twosintered PTFE integrating spheres exhibit a broadfluorescence band between 400 and 500 nm. In terms

Fig. 5. Relative throughput measured with (a) a freshly pressedPTFE integrating sphere, and (b)–(d) commercial sintered PTFEintegrating spheres with (b) and (c) new from manufacturers whenmeasured, while (d) had been used for UV radiometry prior to thismeasurement. All curves are normalized at 400 nm to 1. For easyviewing, there is an offset to each curve. The offsets are 0 for (a), 0.6for (b), 0.8 for (c), and 1.1 for (d).

Fig. 7. LIF measurements of a commercial sintered PTFE inte-grating sphere with the excitation laser tuned at (a) 210, (b) 220,(c) 240, (d) 266, (e) 293, and (f) 360 nm. For easy viewing, the peakof each curve at the wavelength of the excitation laser is normal-ized to a different value. The values are 10�7 for (a), 10�5 for (b),10�3 for (c), 10�2 for (d), 10�1 for (e), and 1 for (f).

Fig. 6. LIF measurements of (a) a pressed PTFE integratingsphere and (b)–(d) commercial sintered PTFE integrating sphereswith the excitation laser tuned at 220 nm. All curves are normal-ized to 1 at the laser excitation wavelength of 220 nm. For easyviewing, there is an offset to each curve. The offsets are 0 for (a),0.004 for (b), 0.008 for (c), and 0.012 for (d).


of fluorescence yield, we again found that the pressedPTFE integrating sphere with the least visible ab-sorption band in throughput has the smallest fluo-rescent yield over other sintered PTFE integratingspheres.

The fluorescence spectra of integrating spheres arealso strongly dependent on the wavelength of theexciting laser beam as depicted in Fig. 7 for a sinteredPTFE integrating sphere. The fluorescence between250 and 400 nm decreases as the wavelength of thelaser increases. On the other hand, the fluorescenceband between 400 and 500 nm shows a distinct struc-ture with laser wavelengths of 240, 266, and 360 nm.

These fluorescence peaks have a nearly equal spacingof 1700 cm�1, indicating vibrational progression fromdifferent vibrational transitions.

B. Effects of Baking

Baking of integrating spheres in vacuum has beenpreviously suggested as a way to remove contami-nants [3]. With the characterization techniques usedfor this study, we were able to determine the effec-tiveness of baking integrating spheres both in vac-uum and in air. We found that the effects of bakingare easier to quantify using the LIF measurements.Shown in Fig. 8 is the LIF measurement using sev-eral exciting laser wavelengths performed on a sin-tered PTFE integrating sphere before and afterbaking in vacuum for 40 h at 90 °C. It is clear fromFig. 8 that the fluorescence yield is much reducedafter baking, but not eliminated for all wavelengths.Notice also that the reduction of fluorescence between250 and 400 nm is much less than that between 400and 500 nm. Overall, the remnants of fluorescenceafter baking still resemble the spectra before baking.Our tests show that longer baking time in vacuumdoes not completely eliminate the fluorescence. Thisfact suggests that the contaminants in an integratingsphere are somewhat volatile and baking can removesome, but not all, contaminants.

For even longer laser wavelengths than thoseshown in Fig. 8, the fluorescence is relatively smalland, by itself, cannot be easily separated from the linespread function of the monochromator. Nevertheless,the fluorescence can be readily identified by compar-ing the fluorescence spectra of integrating spheresbefore and after baking. Figure 9 shows such a com-parison for a laser wavelength of 360 nm. The fluo-rescence between 400 and 500 nm before the bake isevident.

Similar results, shown in Fig. 10, were obtained forbaking a pressed PTFE integrating sphere. While weobserved no discernible fluorescence between 400 and500 nm, the reduction of fluorescence between 250

Fig. 8. LIF measurements of a sintered PTFE integrating spherebefore and after baking the integrating sphere 40 h in vacuum. Foreach graph, the top curves are before baking and the bottom curvesare after baking. All curves are normalized to 1 at the correspond-ing laser excitation wavelengths.

Fig. 9. LIF measurements of a sintered PTFE integrating spherebefore and after baking the integrating sphere 40 h in vacuum. Thetop curve is before baking and the bottom curve is after baking.


and 400 nm due to baking is very similar to that of asintered integrating sphere shown in Fig. 8.

It is also of interest to investigate the throughputchange of an integrating sphere after baking. Shownin Fig. 11 is the throughput of two sintered PTFEintegrating spheres and a pressed PTFE integratingsphere before and after baking in vacuum and in air.It is evident that baking tends to smooth the through-put spectra from the absorption structure below300 nm. Baking also improves the throughput in theshort wavelength range; while in some regions on the

longer wavelength side, throughput decreases. Inview of the changes of the fluorescence spectra frombaking, we can interpret this phenomenon as follows:the absorption structure below 300 nm is caused bythe absorption of UV radiation from contaminantsinside the integrating sphere. As the integratingsphere is baked, some of the contaminants leave theintegrating sphere, which leads to improved UVthroughput with reduced absorption structure. Onthe other hand, less contaminants cause decreasedfluorescence, which predominantly fluoresce withwavelengths longer than 300 nm.

Finally, we studied and compared the effects ofbaking an integrating sphere in vacuum and in air,which as a practical matter could be done much eas-ier and less costly. For this comparison, we packedtwo integrating spheres with the same batch of HalonPTFE powder under the same procedure. One inte-

Fig. 10. LIF measurements of a pressed PTFE integrating spherebefore and after baking the integrating sphere 40 h in vacuum. Foreach graph, the top curves are before baking and the bottom curvesare after baking. All curves are normalized to 1 at the correspond-ing laser excitation wavelengths.

Fig. 11. Relative throughput measurements of (a) a sinteredPTFE integrating sphere before and after 65 h vacuum bake, (b) asintered PTFE integrating sphere before and after 24 h air bake,and (c) a pressed PTFE integrating sphere before and after 40 hvacuum bake. Solid lines are before the bake, and open circles areafter the bake.


grating sphere was baked under vacuum at 90 °Cwith a base pressure of 10�3 Pa; while the other wasbaked in an oven in air, also at 90 °C. The changes inthe spectral throughput for both integrating spheresare shown in Fig. 12 for 24 h vacuum baking and105 h baking in air. Both spectra show increasedthroughput for the short wavelength side and de-creased throughput for the longer wavelength side. Itis clear from Fig. 12 that baking in vacuum is muchmore effective than in air. In Fig. 13, we show thechange for throughput at two wavelengths, one long�396 nm� and one short �206 nm�, as a function of thebaking time in air. Even after more than 4 days ofbaking, a plateau was not reached.

C. Effects of Exposure to Environmental Pollutants

The stability of an integrating sphere is critical whenused in a calibration facility, such as FICUS. In cal-ibrating deuterium lamps at FICUS [11], we ob-served that an integrating sphere can have dramaticchanges in its throughput over a few days. The causeof such a change was traced to diesel truck fumesleaking into the room where the measurement was

performed. Since exhaust from vehicles is a commonsource of aromatic compounds and it was reportedpreviously that PTFE integrating spheres are suscep-tible to contamination by hydrocarbon aromatic com-pounds [3], it is of interest to examine the behavior ofintegrating spheres when exposed to such environ-mental pollutants.

This simple but effective experiment started with afresh well-baked sintered PTFE integrating spherewith its UV fluorescence and absorption measured.The integrating sphere was then placed near the tailpipe of a car or a truck with its engine running. Afterseveral seconds of exposure, the integrating spherewas removed and taken into our laboratories for boththroughput and LIF measurements again. We re-peated the experiment for both a gasoline-powered carand a diesel-powered truck, and the results are shownin Figs. 14 and 15. Note that these measurements werenot performed under well controlled exposures and theresults provide a clear qualitative characterization onthe behavior of integrating spheres when exposed topollutants.

For the LIF measurements in Fig. 14, exposure togasoline and diesel exhaust increases the fluores-cence, remarkably, in exactly the same spectral re-gions where the integrating spheres fluoresce beforethe exposure to exhaust. In particular, the increase influorescence spectra from diesel exhaust exposure be-tween 250 and 400 nm closely resembles that ob-served for pristine integrating spheres (shown in

Fig. 14. LIF measurements of a sintered PTFE integratingsphere exposed to exhaust from a gasoline engine and from a dieselengine. Thin solid lines are before the exposure, thick solid linesare after the exposure. For gasoline exposure, dotted line is 19 h ofbaking in vacuum after exposure. For diesel exposure, dotted linesare baking for both 19 and 68 h.

Fig. 12. Relative changes in throughput for pressed integratingspheres after 24 h vacuum bake and after 105 h air bake.

Fig. 13. Relative change in throughput for pressed integratingspheres as a function of baking time in air for 206 and 396 nm.


Fig. 10). As one might expect, the diesel exhaust ex-posure caused a much stronger increase in fluores-cence than the gasoline exhaust.

Figure 14 also shows the results of baking the ex-posed integrating sphere to remove the contami-nants. For gasoline exhaust, after 19 h of baking invacuum at 90 °C, the fluorescence spectrum was al-most fully restored to its original spectra before ex-posure to the exhaust. For diesel exhaust, thefluorescence was only partially reduced even aftermore than 80 h of baking in vacuum. Other cleaningtechniques we tried, such as the use of various sol-vents, did not restore the integrating sphere to itsstate before exposure. Currently, we are still experi-menting with other options to rid the integratingspheres of contaminants.

For the throughput measurements on the exposedintegrating spheres shown in Fig. 15, the diesel ex-posure caused a substantial decrease in throughputover the whole wavelength region of our measure-ment, indicating the presence of a large amount ofcontaminants. As in the case of the LIF measure-ment, baking partially restored the throughput tothat before the exposure. Lastly, exposure to gasolineexhaust resulted in the buildup of the two-peak-absorption structure in the shorter wavelengths wediscussed previously. For the longer wavelengths,there is a slight increase in the throughput. This is incontrast to the observed decrease in throughput afterbaking an integrating sphere discussed previously.

D. Ultraviolet Stability

In radiometric applications aiming for the highestmeasurement accuracy, the stability of an optical el-ement, such as an integrating sphere, is an essentialproperty [5]. Given the vulnerability of PTFE inte-grating spheres to contamination as discussed above,it is not surprising that the stability of an integratingsphere is primarily influenced by the level of contam-ination. We have performed several preliminary teststo check the stability in the throughput of an inte-grating sphere under UV irradiation from a deute-rium lamp.

Our preliminary results show that, in general, thethroughput of an integrating sphere with relativelysmall absorption and fluorescence features, such as abaked integrating sphere, is stable to within a fewpercent in the UV after a few days of continuousirradiation from a deuterium lamp. On the otherhand, the throughput of an integrating sphere withlarge absorption and fluorescence features varies asmuch as 10% under the same UV irradiation even fora few hours. The exact change in throughput variesfrom one sphere to another. At this time, it is notclear if the instability is caused by UV radiationinteracting with the contaminants or a continuedbuildup of contaminants from the surrounding envi-ronment. Further experiments are under way to shedlight on the mechanism for such an instability.

4. Discussion and Conclusions

As discussed previously, all PTFE integratingspheres studied for this work showed a consistent butvarying degree of absorption and fluorescence fea-tures in the UV, and moderate baking can reducethese features. This indicates that the observed flu-orescence and the associated absorption in the UVare not intrinsic to the PTFE material. Contamina-tion of integrating spheres—by somewhat volatileimpurities either during the manufacturing processor exposure to contaminating sources afterward—ismostly responsible for the observed effects. This issupported by a previous study where the existence ofimpurities, identified as organic hydrocarbons, wasconfirmed [3]. Furthermore, it was shown that PTFEcan absorb aromatic hydrocarbons of toluene andnaphthalene after exposure to these compounds [3].Toluene and naphthalene belong to a class of thepolycyclic aromatic hydrocarbons (PAHs), and it isworthwhile to compare our results with that of PAHsin general.

Known for chemical stability from their conjugated�-electron systems, PAHs typically exhibit strong ab-sorption and fluorescence spectra in the UV [16] asa result of the transition of an electron between�-bonding to �*-antibonding orbitals [17,18]. The tran-sition normally involves a ground vibrational level of aground singlet electronic state to a vibrationally ex-cited level of an excited singlet electronic state. Theresulting fluorescence is typically redshifted from theabsorption because of transitions to higher vibrationallevels of the ground electronic state. Because the vi-brational level spacings in ground and excited elec-tronic states are usually similar, the fluorescencespectrum is often virtually the “mirror image” ofthe absorption spectrum [19]. In our observation ofintegrating spheres, the persistent absorption bandbetween 200 and 300 nm and the fluorescence bandbetween 250 and 400 nm fits very well with that ofPAH impurities. For some sintered PTFE integratingspheres, the additional fluorescence band observedbetween 400 and 500 nm (see Fig. 8) contains a clearvibrational structure (�1700 cm�1 spacing) and iscaused by different PAH impurities. Further support

Fig. 15. Relative throughput measurements of a sintered PTFEintegrating sphere before exposure, after exposure to gasoline en-gine exhaust, and after exposure to diesel engine exhaust.


for PAH impurities is given by the close resemblanceof the fluorescence spectra from 250 to 400 nm be-tween pristine integrating spheres and integratingspheres exposed to engine exhaust, which is known tocontain a large number of PAHs.

While the exact chemical compounds of the PAHimpurities cannot be easily identified by absorptionand fluorescence spectra alone, one can still obtaininformation about the chemical nature of the impu-rities from the measured spectra using propertiesthat are generally true for PAHs. In general, as thenumber of conjugated aromatic rings increases, theUV absorption maxima are shifted to longer wave-lengths and exhibit a greater complexity of its absorp-tion spectrum [20]. This explains the low response—forexample, at 254 nm—for PAHs with six or more rings[21]. Similarly for fluorescence, the spectrum alsoredshifts as the size of the conjugated aromatic ringsystem increases. The redshifting of the absorptionand fluorescence spectrum can be explained as a par-ticle (electron) in a box (�-conjugated rings) model—the larger the box, the more closely spaced the levels,and the longer the wavelength of absorption andemission. In the case of “linear” PAHs, such as ben-zene (one ring), naphthalene (two ring), anthracene(three ring), tetracene (four ring), and pentacene (fivering), a comparison between published fluorescencespectra [22] and our results shows that the measuredfluorescence spectrum below 400 nm is caused byPAHs with one to three rings, while fluorescence be-tween 400 and 500 nm is caused by three or four ringPAHs. The lack of fluorescence in the longer wave-length up to 800 nm indicates that only PAHs with asmall number of conjugated aromatic rings werepresent in the integrating spheres we studied as im-purities.

The measurements on integrating spheres ex-posed to engine exhaust also shed light on the na-ture and the origin of the impurities. Emission fromvehicle exhaust contains a variety of PAH com-pounds and their derivatives depending on suchconditions as the fuel type and the degree of incom-plete combustion of fuel [17]. The emitted PAHs canbe in the form of gas phase, or the PAH can beadsorbed or encapsulated in particulates. In gen-eral, diesel exhaust contains more particulates thangasoline exhaust. These particulates—diesel par-ticulate matter (DPM)—include diesel soot andaerosols, such as ash particulates, metallic abrasionparticles, sulfates, and silicates. Our LIF measure-ments on integrating spheres with exhaust expo-sure show that both gasoline and diesel exhaustdeposit PAHs with a low number of aromatic rings,giving the similar fluorescence spectra from thepristine spheres that was identified above as PAHwith one to three aromatic rings. The fact that die-sel exhaust exposure induced a fluorescence peak inthe slightly longer wavelength than gasoline ex-haust exposure suggests that contaminants withhigher ring-number PAHs resulted from diesel ex-haust exposure. In the throughput measurements,as in LIF measurements, exposure to gasoline ex-

haust caused similar absorption features for thepristine integrating spheres. However, diesel expo-sure caused a broad structureless absorption. Thisis most likely due to the deposition of DPM in thesphere that degraded the overall reflectance ofPTFE. It is of interest to note that, in commonenvironmental studies, PTFE membrane filters arepreferred for the collection of particulates becauseof their high collection efficiency and inertness [17].Finally, the fact that baking can restore an inte-grating sphere exposed to gasoline exhaust is inaccordance with the volatility of some lighter PAHcompounds, such as phenanthrene or anthracene,due to thermal desorption. On the other hand, it ispossible that the difficulty in removing contami-nants from a diesel exposed integrating sphere bybaking is because either the PAHs involved have amuch lower vapor pressure or the PAHs weretrapped in the particulates, which are impossible tobake out at these low temperatures �90 °C�.

Given the important role of PAHs as contaminants,there are two possible sources for PAH contamina-tion: (1) the process of manufacturing the integratingsphere, such as the use of resin, and (2) the exposureof integrating spheres to an environment that con-tains PAHs. As shown above, engine exhaust in theenvironment will cause significant contaminationthat affects the performance of an integrating spherein the UV. Even in normal air, suspended atmo-spheric particulate matter that in many cases con-tains a large number of different PAHs could alsocause contamination. This clearly suggests that inte-grating spheres should be protected from contami-nated air where possible to ensure longer stabilityin the UV. Once contaminated with PAH compounds,the PAHs can react in the air or undergo photoreac-tion with UV irradiation causing more instability.This was observed in our stability measurementswhere a contaminated integrating sphere tends tohave much lower UV stability than a baked integrat-ing sphere with less contamination.

From the standpoint of practical UV applicationsof integrating spheres, the absorption, fluorescence,and stability are of great concern for accurate mea-surements. The absorption in the UV causes lowerthroughput and to some extent affects the uniformityof the exiting radiation from an integrating sphere.Fluorescence is more challenging because it altersthe spectral correlation between the input and outputradiation. One way to correct this effect for an inte-grating sphere and monochromator system is to treatthe fluorescence as part of the monochromator linespread function. With the combined system com-pletely characterized using a tunable laser, contri-butions from fluorescence and scattering can bemathematically alleviated from the output spectra byusing a stray light reduction algorithm as was previ-ously demonstrated [11]. While the absorption andfluorescence problems can be solved in principle, theissue of instability caused by contamination fromboth the laboratory and field environment is moredifficult to resolve. Baking and limiting exposure to


environmental contaminants can help with stability.More research on the UV behavior of integratingspheres is currently underway to identify the mole-cules responsible for the contamination, find waysto clean the contaminants, and protect integratingspheres from further contamination.

In summary, we have used two techniques to char-acterize PTFE integrating spheres in the UV: mea-suring both the UV throughput spectrum using adeuterium lamp and the fluorescence spectrum in-duced by a tunable UV laser. All of the integratingspheres studied show significant absorption and flu-orescence features. These features, more pronouncedfor sintered integrating spheres than pressed ones,are remarkably similar for all of the integratingspheres we studied and are consistent with PAH con-taminants. Baking for long periods of time in vacuumcan reduce the level of contaminants but does notcompletely eliminate them. In addition, fluorescenceinduced by the UV laser provided important clues foridentifying the chemical nature of contaminants.PAH contaminants from the environment are themain suspect that deteriorate the performance ofPTFE integrating spheres.

The authors thank Yuqin Zong for lending us thefiber optic spectrograph, and Robert Saunders forpacking integrating spheres. We are also grateful toHoward Yoon and Albert Parr for their discussionsand support of this project.

References1. R. D. Saunders and W. R. Ott, “Spectral irradiance measure-

ments: effect of UV-produced fluorescence in integratingspheres,” Appl. Opt. 15, 827–827 (1976).

2. V. R. Weidner and J. J. Hsia, “Reflection properties of pressedpolytetrafluoroethylene powder,” J. Opt. Soc. Am. 71, 856–861(1981).

3. A. E. Stiegman, C. J. Bruegge, and A. W. Springsteen, “Ultra-violet stability and contamination analysis of Spectralon dif-fuse reflectance material,” Opt. Eng. 32, 799–804 (1993).

4. C. J. Bruegge, A. E. Stiegman, R. A. Rainen, and A. W. Spring-steen, “Use of Spectralon as a diffuse reflectance standard forin-flight calibration of earth-orbiting sensors,” Opt. Eng. 32,805–814 (1993).

5. D. R. Gibbs, F. J. Duncan, R. P. Lambe, and T. M. Goodman,“Ageing of materials under intense UV radiation,” Metrologia32, 601–607 (1995�1996).

6. P. R. Spyak and C. Lansard, “Reflectance properties of pressed

Algoflon F6: a replacement reflectance-standard material forHalon,” Appl. Opt. 36, 2963–2970 (1997).

7. W. Möller, K.-P. Nikolaus, and A. Höpe, “Degradation of thediffuse reflectance of Spectralon under low-level irradiation,”Metrologia 40, S212–S215 (2003).

8. H. J. Kostkowski, Reliable Spectroradiometry (Spectroradiom-etry Consulting, 1997), Appendix B.

9. P. Y. Barnes, M. E. Nadal, and E. A. Early, “Reflectance stan-dards at ultraviolet wavelength,” Proc. SPIE 3818, 9–14(1999).

10. P. S. Shaw, U. Arp, H. Y. Yoon, R. D. Saunders, A. C. Parr, andK. R. Lykke, “A SURF beamline for synchrotron source-basedabsolute radiometry,” Metrologia 40, S124–S127 (2003).

11. P. S. Shaw, U. Arp, R. D. Saunders, D. J. Shin, H. W. Yoon,C. E. Gibson, Z. Li, A. C. Parr, and K. R. Lykke, “Synchrotronradiation based irradiance calibration from 200 nm to 400 nmat SURF III,” Appl. Opt. 46, 25–35 (2007).

12. S. W. Brown, G. P. Eppeldauer, and K. R. Lykke, “Facility forspectral irradiance and radiance responsivity calibrations us-ing uniform sources,” Appl. Opt. 45, 8218–8237 (2006).

13. S. W. Brown, B. C. Johnson, M. E. Feinholz, M. A. Yarbrough,S. J. Flora, K. R. Lykke, and D. K. Clark, “Stray-light correc-tion algorithm for spectrographs,” Metrologia 40, S81–S84(2003).

14. Y. Zong, S. W. Brown, B. C. Johnson, K. R. Lykke, and Y. Ohno,“Simple spectral stray light correction method for array spectro-radiometers,” Appl. Opt. 45, 1111–1119 (2006).

15. Certain commercial equipment, instruments, or materials areidentified in this paper to foster understanding. Such identi-fication does not imply recommendation or endorsement by theNational Institute of Standards and Technology, nor does itimply that the materials or equipment identified are necessar-ily the best available for the purpose.

16. See, for example, D. Cox, J. M. Key, M. S. Lai, J. R. Sutherland,M. R. Zahn, and J. M. Arrington, “On-line supercritical fluidextraction synchronous luminescence spectroscopy: a screen-ing method for polycyclic aromatic hydrocarbons,” J. Under.Chem. Res. 2, 39–42 (2005) and references therein.

17. See, for example, M. L. Lee, M. V. Novotny, and K. D. Bartle,Analytical Chemistry of Polycyclic Aromatic Compounds(Academic, 1981).

18. M. L. Lee, M. V. Novotny, and K. D. Bartle, Handbook ofPolycyclic Aromatic Hydrocarbons, A. Bjørseth, ed. (MarcelDekker, 1983).

19. N. J. Turro, Molecular Photochemistry (Benjamin�Cummings,1965).

20. E. Clar, Polycyclic Hydrocarbons (Academic, 1964).21. R. A. Friedel and M. Orchin, Ultraviolet Spectra of Aromatic

Compounds (Wiley, 1951).22. J. B. Birks, Photophysics of Aromatic Molecules (Wiley,

1970).


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Ultraviolet characterization of integrating spheres