Accelerated UV weathering device based on integrating sphere technologyJoannie Chin, Eric Byrd, Ned Embree, Jason Garver, Brian Dickens, Tom Finn, and Jonathan Martin Citation: Review of Scientific Instruments 75, 4951 (2004); doi: 10.1063/1.1808916 View online: http://dx.doi.org/10.1063/1.1808916 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/75/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in An accelerated exposure and testing apparatus for building joint sealants Rev. Sci. Instrum. 84, 095113 (2013); 10.1063/1.4821880 Environmental chamber for in situ dynamic control of temperature and relative humidity during x-ray scattering Rev. Sci. Instrum. 83, 025112 (2012); 10.1063/1.3685753 PM 10 Concentrations Related to Meteorology in Volos, Greece AIP Conf. Proc. 1203, 1091 (2010); 10.1063/1.3322316 Characterization of Weathering Degradation in Aircraft Polymeric Coatings Using NDE Imaging Techniques AIP Conf. Proc. 657, 1111 (2003); 10.1063/1.1570257 Capabilities of the Commercial Plant Biotechnology Facility to support plant activities on the International SpaceStation AIP Conf. Proc. 420, 593 (1998); 10.1063/1.54851

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Accelerated UV weathering device based on integrating sphere technologyJoannie Chin,a) Eric Byrd, Ned Embree, Jason Garver, Brian Dickens,Tom Finn, and Jonathan MartinPolymeric Materials Group, Building and Fire Research Laboratory, National Institute of Standardsand Technology, Gaithersburg, Maryland 20899

(Received 23 March 2004; accepted 27 July 2004; published 2 November 2004)

An ultraviolet (UV) weathering device based on integrating sphere technology has been designed,fabricated, and implemented for studying the accelerated weathering of polymers. This device hasthe capability of irradiating multiple test specimens with uniform, high intensity UV radiation whilesimultaneously subjecting them to a wide range of precisely and independently controlledtemperature and relative humidity environments. This article describes the integrating sphere-basedweathering system, its ability to precisely control temperature and relative humidity, and its abilityto produce a highly uniform UV irradiance. ©2004 American Institute of Physics.[DOI: 10.1063/1.1808916]

I. INTRODUCTION

Laboratory devices for artificial ultraviolet(UV) weath-ering play an important role in comparing and/or predictingthe weathering performance of polymeric construction mate-rials and determining the effect of different weathering fac-tors on the performance of a construction material. In orderto definitively establish the necessary correlations betweenindoor and outdoor weathering studies, exposure experi-ments carried out in laboratory devices must be highly repro-ducible and repeatable. Over the years, numerous technicalimprovements have been made in the design, construction,and control of these devices.1–7 However, the reproducibilityand repeatability of exposure results obtained using thesechambers have remained questionable.

The performance of a polymeric material in an UVweathering test is dependent on a large number of factors.Differences in performance between nominally identicalspecimens can generally be attributed to testing conditionsand/or material variability. Some random variability in mate-rial components, formulation, and processing is to be ex-pected, and can be compensated through the use of appropri-ate experimental designs and replication. However, anysystematic variability in testing conditions can seriously biasexperimental results, and result in greatly decreased repeat-ability and reproducibility.

Common sources of systematic errors associated withexisting UV weathering devices include unnatural exposureconditions (e.g., excessively high specimen temperatures,nonterrestrial UV wavelengths), nonuniform spatial irradi-ance over the surface of a specimen, and unwanted temporalchanges in exposure conditions. All of these errors can bemitigated by the use of an integrating sphere-based UV test-ing device. Precise control and monitoring of temperature

and relative humidity in weathering devices can also elimi-nate additional systematic errors in UV weathering experi-ments.

This article describes the major components that makeup the National Institute of Standards and Technology(NIST) integrating sphere-based weathering device, herein-after referred to as the NIST SPHERE(Simulated Photodeg-radation via High Energy Radiant Exposure); namely the in-tegrating sphere, the UV lamp system, the nonimagingoptics, and the environmental chambers. Data on the unifor-mity and temporal stability of UV irradiance, temperature,and relative humidity within this device are also presented.

II. INTEGRATING SPHERE THEORY

The theory of integrating spheres and their use in a widevariety of applications is well established in the literature.8–10

In general, an integrating sphere is a hollow spherical cham-ber with an inner surface that is Lambertian, or diffuselyreflecting. Light entering an integrating sphere undergoesmultiple diffuse reflections at the interior surface, resulting inthe creation of a uniform field of light within the sphere. Thiscollected light can then serve as a quantity to be measured(such as in power measurement of lasers or lamps) or as asource of uniform illumination. The latter capability is uti-lized in the SPHERE.

Integrating sphere theory has its origin in the theory ofradiation exchange between diffuse surfaces.11 Consider theexchange of radiation between two diffuse Lambertian sur-face elementsA1 andA2, with differential areasdA1 anddA2,and separated by a distanceS, as shown in Fig. 1(a). Thefraction of the total flux leavingA1 and arriving atA2 isgiven by

dF1-2 =cosu1 cosu2

pS2 dA2, s1d

wheredF1-2 is known as the exchange factor. If these twosurface elements are contained inside a diffuse sphere sur-

a)Author to whom correspondence should be addressed; electronic mail:joannie.chin@nist.gov

REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 75, NUMBER 11 NOVEMBER 2004

0034-6748/2004/75(11)/4951/9/$22.00 4951 © 2004 American Institute of Physics

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face, andS=2Rcosu1=2Rcosu2, as shown in Fig. 1(b),then Eq.(1) becomes

dF1-2 =dA2

4pR2 . s2d

Equation(2) is independent of the locations of the two ele-ments within the sphere as well as the distance between theelements. This result is significant because it states that thefraction of flux received byA2 is a function only of sphereradius and is the same as for every other point on the spheresurface.

If the differential areasdA1 anddA2 become finite areasA1 andA2, then Eq.(2) becomes

dF1-2 =1

4pR2EA2

dA2 =A2

4pR2 =A2

As, s3d

where As is the total surface area of the sphere. Thus, thefraction of energy received byA2 is proportional to the frac-tion of sphere surface area that it occupies.

The physical significance of the previous analysis is thatevery point on the interior surface of a sphere is equally anduniformly illuminated by multiple reflections from everyother point on the interior surface. Theoretically, it then fol-lows that not only should the output intensity be uniformacross the plane of an exit port, but also that the outputintensity from any port be the same regardless of its position.This condition can be achieved as long as the input flux hasexperienced at least one reflection and does not strike theexit ports directly. This property of integrating spheres canbe exploited to produce an artificial UV weathering device

with greatly improved spatial irradiance uniformity. Thus,the use of an integrating sphere as a uniform UV radiationsource can potentially resolve one of the major sources ofsystematic errors in current UV chamber design; that is, non-uniform irradiance across the dimensions of a specimen andfrom specimen to specimen.

The throughput at a given port of an integrating spherewith multiple ports is given by

Fo

Fi=

rfe

1 − rs1 − f totd, s4d

whereFi is the input flux,Fo is the output flux at the port ofinterest,r is the average sphere wall reflectance,fe is thefraction of the sphere surface area taken up by the exit portof interest, andf tot is the total fraction of the sphere surfacearea taken up by ports.12 If the input flux is known, theoutput flux of an actual sphere can be calculated via Eq.(4).This equation assumes that no portion of the input flux isdirectly incident on any of the exit ports. It is generally rec-ommended thatf tot be less than 5% in order to maintain theuniformity of the output.

In order for the abovementioned equations to be correctand for an integrating sphere to be used successfully as anoptical device, it is critical that the interior sphere surfacescatter light in a Lambertian fashion over the wavelengthregion of interest. If the scattering is non-Lambertian, thenthe basic assumptions for the standard integrating sphereanalysis are violated. The effects of non-Lambertian reflec-tance on the output of integrating spheres have been pre-sented by Hanssen.13

III. OVERVIEW OF INTEGRATING SPHERE-BASEDWEATHERING DEVICE

A. Integrating sphere

The NIST SPHERE is based on a 2-m-diam integratingsphere equipped with a high intensity UV light source and

FIG. 2. NIST 2 m integrating sphere, shown with UV light source andenvironmental chambers.

FIG. 1. (a) Exchange of radiation between two diffuse surface elements, and(b) exchange of radiation in a spherical enclosure.

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environmental chambers, as shown in Fig. 2.14 The integrat-ing sphere is constructed from modular panels, allowing in-dividual panels to be removed as needed for modification orrepair. The exterior shell of the sphere is aluminum, and theinterior surface is lined with poly(tetrafluoroethylene)(PTFE). The sphere currently contains 3211.2-cm-s4.4 in.d-diam ports, and a 61-cms24 in.d-diam topport to accommodate the UV light source. A schematic rep-resentation of the sphere is shown in Fig. 3. To ensure thatdirect strikes on the exit ports from the UV light system donot occur, PTFE baffles are installed above each exit port onthe interior of the sphere, as shown in Fig. 4.

The reflectance properties of the interior PTFE coatingare critical to the throughput of the SPHERE. PTFE has thehighest Lambertian reflectance of any known material in therange of wavelengths known to be photolytically actively

toward organic materials; that is, between 290 and 400 nm.The reflectance of pressed PTFE powder has been studied byWeidner and Hsia,15 and was measured to be.0.98 in theregion from 250 to 2000 nm. In recent years, a solid, ma-chinable PTFE material has been developed for use in inte-grating sphere technology, having the high reflectance of thepowder but with greater durability and high thermal, mois-ture and chemical resistance. PTFE maintains its reflectanceindefinitely under normal laboratory conditions if not con-taminated with organic substances and does not need to berepacked or re-coated.16 In the event that reflectance doesdecrease over time due to surface contamination, the materialcan be sanded or vacuum baked to regenerate its originalreflectance.17 The spectral reflectance of the PTFE materialused in the interior coating of the SPHERE is shown inFig. 5.

B. Ultraviolet (UV) light system

The lamp system that serves as the source of UV radia-tion is a microwave-powered lamp system with an outputthat is rich in the region between 290 and 400 nm. Six VPS/I600-60 lamp modules(Fusion UV, Inc.)18 are incorporatedinto a custom-engineered light shield and are symmetricallyarranged around the top port of the sphere as shown in Fig.6. The total output flux of the six lamps operating at 100%power is approximately 8400 W in the spectral range be-tween 290 and 400 nm.

The bulbs used in the lamp system consist of sealedquartz tubes that are filled with mercury vapor, argon, andtrace amounts of metal halides, which are used to tailor thelamp output. The bulbs do not contain metal electrodes and

FIG. 3. Schematic representation of NIST 2 m integrating sphere.

FIG. 4. Placement of baffles in the interior of the integrating sphere.

FIG. 5. Reflectance spectrum of PTFE material used in sphere interior.

FIG. 6. Top view of integrating sphere, showing arrangement of UV lamps.

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are instead powered by microwave excitation. Each bulb islocated inside an elliptical reflector assembly coated with aproprietary dichroic coating that removes 80%–90% of theinfrared and visible emissions from the lamps. A borosilicateglass window is also installed in the optical path between theUV lamp system and the integrating sphere to remove radia-tion below 290 nm, the value generally accepted as the ter-restrial cutoff wavelength.19 In the event that wavelengths.400 nm are necessary in a given photodegradation study,the dichroic reflectors can be modified to retain a greaterproportion of the visible and infrared spectrum.

The spectrum of the UV light source emitted from asphere exit port, measured with a UV-visible spectrometerequipped with a cosine collection sphere, is shown in Figs.7(a) and 7(b). Superimposed on the SPHERE output spec-trum is the spectrum of natural sunlight, taken in Edgewater,Maryland, on May 14, 2002 at noon. Figure 7(a) shows thetwo spectra plotted on the same scale; Fig. 7(b) shows thesunlight spectrum multiplied by a factor of 30 for a moredirect comparison with the SPHERE output spectrum. At thistime, no attempt has been made to match the spectral irradi-

ance of the UV light source with that of the sun. Instead, it isanticipated that the SPHERE will be utilized in the contextof a reliability-based approach to UV weathering proposedby Martin et al.20 In this approach, the spectral irradiance isbroken down into discrete narrow wavebands to assess thespectral sensitivity of a test material, and once broken downinto narrow wavebands, the spectral irradiance of the originallight source is no longer critical. However, the design of theUV light system does allow for the use of alternative UVsources, such as xenon arc or fluorescent UV lamps. Alter-natively, additives in the bulbs can be modified to produceoutput that more closely simulates natural sunlight.

The high current densities required to sustain the plas-mas in many arc lamps cause a substantial amount of thermalenergy in the visible and infrared regions to be emitted.21,22

In a typical UV chamber, the light source is situated in closeproximity to the specimens, causing specimen temperaturesto approach 60 °C in some instances.23,24 Because the lightsource is located outside the sphere, and the infrared andvisible portions of the lamp output are removed prior to ir-radiating the sphere interior, the sphere temperature is easily

FIG. 7. (a) Spectra of natural sunlight and UV light source emitted from SPHERE exit port, plotted on the same scale, and(b) spectra of natural sunlight andUV light source emitted from SPHERE exit port, with sunlight spectrum multiplied by a factor of 30.

4954 Rev. Sci. Instrum., Vol. 75, No. 11, November 2004 Chin et al.

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maintained at 35 °C or lower. As a safety precaution, thesphere itself is heavily instrumented with temperature sen-sors and cooled with conditioned air to ensure that overheat-ing does not occur. The exterior location of the light systemalso makes it easily accessible for adjustments and repair,and minimizes interruptions to ongoing experiments.

C. Compound parabolic concentrators

Collimation and conveyance of the highly uniform radia-tion emitted from the sphere port to the specimen chambersis accomplished with minimal loss of uniformity and inten-sity using compound parabolic concentrators(CPCs). CPCs(also referred to as nonimaging optical devices, Winstoncones, and cone concentrators) are more efficient than con-ventional image-forming optics in concentrating and collect-ing light. These devices date back to the 1960s and wereonce used for solar collection. A detailed treatment of thissubject is given by Welford and Winston.25

The CPCs used on the 2 m integrating sphere are customdesigned, and are used to collimate the output from thesphere exit port(which is diffuse over 180°) to a divergenceangle of 20°. A schematic representation of the CPC isshown in Fig. 8. The narrow end of the CPC is integral withthe inside surface of the sphere; the wide end is equippedwith angle brackets to support specimen holders.

The CPCs are fabricated from electroformed nickel, andhave an interior base coat of rhodium for corrosion protec-tion, a vapor-deposited layer of aluminum for specular re-flectance, and a top coat of vapor-deposited magnesium fluo-ride for protection against oxidation. The reflectance of thiscoating is approximately 87% in the wavelength range be-tween 200 and 450 nm.

D. Specimen holders

Anodized aluminum specimen holders having a diameterof 15.2 cms6 in.d are secured to the wide end of each CPC.They can be designed to hold a single specimen or multiplespecimens, as shown in Fig. 9(a). Figure 9(b) shows howspecimen holders can be coupled with filter holders in orderto carry out experiments in which UV intensity or spectralwavelengths are varied. Both neutral density filters and band-pass filters are used to modify the UV intensity and spectralUV incident upon the specimens, respectively. A quartz platebetween the specimen holder and filter holder protects thefilters from moisture in the specimen holder.

The specimen holders are engineered to maintain both atemperature and relative humidity environment within tighttolerances. Each specimen holder is equipped with its owncontact heating pads, temperature and relative humidity sen-sors. A pneumatic shutter assembly between the specimenholders and the end of the CPC prevents the operator frombeing exposed to high intensity UV when removing thespecimen holders for analysis. Figure 10 shows the locationand relative orientation of the integrating sphere, the speci-men holder, and the CPC.

E. Environmental chambers

Characterization of UV effects on materials requires thatspecimens be irradiated over a range of exposure conditions.This has been accomplished by equipping each port with anenvironmental chamber in which temperature, relative hu-

FIG. 8. Schematic representation ofcompound parabolic concentrator usedwith integrating sphere.

FIG. 9. (a) Front view of a specimen holder, showing slots for 17 speci-mens.(b) Cutaway view of specimen holder, showing filter and specimenorientation.

Rev. Sci. Instrum., Vol. 75, No. 11, November 2004 UV weathering device 4955

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midity, and UV-visible irradiance can be precisely and inde-pendently controlled. The spectral irradiance of the UVsource is modified by inserting bandpass filters and/or neu-tral density filters in front of the specimen holders to allowthe effect of wavelength and intensity, respectively, to bestudied. With 32 such ports and the capability of exposing 16or more specimens in each port, a multiplicity of environ-mental conditions can be evaluated simultaneously. For in-stance, specimens can be subjected to 60 °C, 75% relativehumidity (RH) at one exit port and 35 °C, 10% relative hu-midity at another exit port. Within each port, the use of dif-ferent bandpass and neutral density filters allows many com-binations of irradiance and wavelength to be studiedsimultaneously. The exposure at a given port can be stoppedor started without disturbing the specimens at the other ports.

The environmental chambers are shown in Fig. 11. Thechamber housing consists of a commercial fiber-reinforcedplastic enclosure, insulated with fiberglass and lined withsheet aluminum. Figure 12 shows a schematic diagram of thetemperature and relative humidity generation system. The

specimen holder and the water tank are heated using contactheaters. The chamber is heated by blowing air over finnedaluminum plates attached to contact heaters inside the enclo-sure. The temperatures of the chamber air, the water tank,and the specimen holders are monitored using crystal-controlled oscillator temperature sensors. Temperature can becontrolled between 25° and 75 °C with a precision of±0.1 °C.

Relative humidity is controlled by the controlled mixingof saturated and dry air streams. House air is passed throughan air drier and split into two streams. One stream is moist-ened by routing it through a saturated cotton filter wick in acommercial stainless steel water tank partly filled with dis-tilled, de-ionized water. The water level in the stainless steeltank is maintained by an automated feedback controlled fill-ing system. The flow rates and hence proportions of the twostreams are controlled by a proportional integral differential(PID) control loop, which is driven by voltages generated by

FIG. 11. Environmental chamber, showing stainless steel water reservoir onthe left and the opening to the CPC inside the chamber on the right.

FIG. 10. Location and orientation of CPC and specimen holder.

FIG. 12. Schematic diagram of heating and humiditygeneration system.

4956 Rev. Sci. Instrum., Vol. 75, No. 11, November 2004 Chin et al.

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the microprocessor board in response to the difference be-tween the set point and the measured RH. On leaving thewater tank, the saturated air stream is immediately mixedwith the dry air stream; the combined stream is then passedthrough the specimen holder. Variable capacitance analogrelative humidity sensors are used to monitor the relativehumidity levels in the specimen holder. The relative humid-ity can be varied between<0% RH and 95% RH with aprecision of ±0.2% RH. Figure 13 shows the stability of thetemperature and relative humidity in the specimen holder asa function of time.

In establishing equilibrium temperature and relative hu-midity conditions, the specimen holder temperature is firststabilized to within tolerance, typically ±1.5 °C. During thisprocess, only dry airs<0% RHd from the humidity genera-tor is pumped into the specimen holder. It is critical thatsamples not be exposed to relative humidity when the tem-perature is varying rapidly because water will condense onthe specimen holder and the test specimens if the dew pointis reached. When the temperature readings have been withintolerance for about 10 min, the relative humidity is adjustedby adding moist air to the dry air stream until the measuredrelative humidity is within tolerance, typically defined as±1% RH. Only when the desired temperature and relativehumidity conditions are established will a pneumatic shutterbetween the integrating sphere and specimen holder be al-lowed to open, thus allowing irradiation of the test speci-mens. The temperature set point can be reached from roomtemperature(nominally 24 °C) in approximately 20 min.Once the temperature set point is reached, the relative hu-midity set point is achieved in an additional 20 min.

If the relative humidity goes out of tolerance during thecourse of the experiment, the shutter will close and the con-troller will attempt to reestablish equilibrium. If the tempera-ture goes out of tolerance, the shutter will close and thehumidity generator will switch to all dry air. Temperature isthen re-equilibrated, followed by relative humidity, accord-ing to the process described above. Opening the chamberdoor before the temperature has cooled down could invokecondensation of the moist air stream, so a latch on the cham-

ber door prevents the chamber door from being opened untilthe relative humidity has dropped to 5%. To prevent exces-sively high temperature excursions, the microprocessor shutsdown the power to the heaters if the temperature reaches75 °C. As a secondary, independent safety feature, thermalfuses placed directly on the heaters also shut down heaterpower if temperatures above 75 °C are reached.

F. Microprocessor control system

Temperature and relative humidity are monitored andcontrolled via a custom software package implemented usingan AVR Mega32 microprocessor(Atmel Corporation) con-tained in a microcontroller assembly. The microcontrollerhas the capability of reading and generating voltages be-tween 0 and 5 V dc, switching relays and reading inputs. Thevoltages read are those representing the relative humidity inthe specimen holders. The voltages generated are thosespecifying the flow rates in the saturated and dry air streams.The chamber and specimen holder temperatures are read ev-ery 8 s by the temperature sensors, which operate on theirown 1 wire bus. The heaters are driven by a variable lengthpulse (of maximum duration 8 s) calculated to 10 ms reso-lution by a PID control loop. Input switches, the chamberdoor status, and an alarm are wired to an interrupt on themicroprocessor so that immediate response can occur. Thestatus of the system is shown on the microcontroller display.Input switches on the microcontroller can be used to manu-ally switch control on and off and to change the temperatureand relative humidity settings locally, although such controlis usually carried out using a personal computer.

Each microcontroller has an address and is on a RS485communications network. The microcontroller with address1 is also used as a hub to mediate traffic between the RS485network and a personal computer(PC). Thus, the PC canmonitor and control the status of each environmental cham-ber on the RS485 network. Temperature, relative humidity,and ultraviolet radiation irradiance measurements are re-corded every 2 min and archived in the PC. The time that theUV-visible radiation is incident on the specimens is also re-corded so that a cumulative exposure time can be calculated.Alarms such as indeterminate status of the shutter, an over-temperature condition or failure of a component arepromptly reported to the PC and displayed in a message box.

IV. INTEGRATING SPHERE IRRADIANCEMEASUREMENTS

A. Throughput

As previously described, the total energy throughput forthe integrating sphere can be determined using Eq.(4). Inthis calculation, an input flux of 4500 W was calculated bymultiplying the total UV lamp output between 290 and300 nm(8400 W, as stated above, for all six lamps runningat 100% power) by the attenuation factors for the dichroicreflectors, heat shield reflectors, and borosilicate window inthe optical path between the UV lamps and the integratingsphere. When the CPCs are used on the integrating sphere,the sphere exit port diameter is 5.08 cms2 in.d, with eachindividual port having a port fractionsfed of 0.000 161. The

FIG. 13. Representative data showing responsivity and stability of tempera-ture and relative humidity in specimen chamber as a function of time. Rela-tive humidity readings are stabilized only after the temperature readingshave equilibrated.

Rev. Sci. Instrum., Vol. 75, No. 11, November 2004 UV weathering device 4957

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percentage of the total sphere surface area occupied by 32ports and the 61-cm-diam top port accommodating the UVlight source is 2.8%, which is well under the 5% “rule ofthumb” used to ensure integrating sphere output uniformity.Using these inputs, the total output flux at a given sphere exitport is calculated to be 10.38 W. At the end of the CPCwhere the specimens reside, taking into account the averageCPC coating reflectance of about 0.8, the output intensity iscalculated to be approximately 479.9 W/m2. Measurementsof the output intensity at the end of the CPC using two in-dependently calibrated spectroradiometers weres479.0±0.2d W/m2 and s512±1.1d W/m2, showing excel-lent agreement with the calculated value.

B. Temporal stability

The temporal stability of the UV lamp system was as-sessed by measuring the spectral output from a sphere exitport as a function of time, using a UV-visible spectrometerequipped with a cosine collection sphere. A UV-visible spec-trum was taken every 30–60 min over a period of approxi-mately 500 h. Seven hundred spectra from this time serieswere essentially identical; the coefficient of variation for thisset of spectra was found to be on the order of 2% in theregion from 300 to 400 nm. In the future, UV irradiancewill be continuously monitored with calibrated UV spectro-radiometers equipped with diffusing probes that will be stra-tegically positioned at several points in the inner surface ofthe integrating sphere and in the specimen holders. Thesemeasurements will be archived and used to quantitativelymonitor the quality of the UV output, which is not onlyaffected by changes in the lamps themselves, but also in thetransmittance or reflectance of any material in the optical

path. This includes the reflectance of the dichroic elements,the sphere wall, the interior of the cone concentrator, and thetransmittance of the filters and other window materials.

C. Uniformity

Ensuring spatial irradiance uniformity over the dimen-sions of a specimen and between specimens is a prime con-

FIG. 14. Representative uniformity map of sphere out-put, taken at the end of the CPC. All measurements arenormalized to the highest intensity measured at the cen-ter point.

FIG. 15. (a) Sphere output taken at the center points of 14 nonequatorialports, all spectra superimposed.(b) Integrated intensity at the center pointsof 14 nonequatorial ports.

4958 Rev. Sci. Instrum., Vol. 75, No. 11, November 2004 Chin et al.

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sideration in designing any weathering device, and is neededto ensure the repeatability and reproducibility of exposureresults. Spectral UV radiation dosage must be known in or-der to compare the performance of materials exposed in thelaboratory and those exposed in the field.20,26 Spatial irradi-ance uniformity is difficult to attain in current UV chambersdesigns due, in part, to the large surface area over whichuniformity must be controlled. Factors affecting irradianceuniformity include nonuniform emittance from the lamps,reflectance from the specimen and walls of the chamber, andphysical limitations imposed by the optical system(e.g., thegeometry of the light source, dimensions of the specimens,and distance between the light source and the specimens).

UV intensity measurements were taken at 98 points atthe output end of the cone concentrators with all six lampsrunning at 100% power. Measurements were carried out us-ing a UV-visible spectrophotometer and a cosine collectionsphere with a 1.27 cms0.5 in.d aperture. The collectionsphere is positioned at the wide end of the CPC and ismounted on a computer-controlled positioning table so thatthe entire output end of the CPC can be scanned in an auto-mated fashion. All measurements are normalized to the in-tensity measured at the center point, which is generally thehighest intensity point.

A representative uniformity profile is shown in Fig. 14,revealing that the average uniformity over all 98 pointswithin the exposure area is 94.0% ±3.4%. Figure 15(a)shows that the UV-visible spectra taken at the center pointsof 14 different CPCs are virtually nondistinguishable. Theintegrated intensities at the center points of these ports fallwithin an extremely narrow range, as seen in Fig. 15(b). Thedata from this series of measurements lead to the conclusionthat both the intraport uniformity and inter-port irradianceuniformity of the integrating sphere output are extremelyhigh.

The ability of the integrating sphere-based UV weather-ing device to produce a high level of UV flux is also usefulfor carrying studies of the law of reciprocity, as well as togreatly accelerate the testing of polymeric materials for use

in outdoor applications. Future capabilities include the appli-cation of mechanical stresses—either cyclic or static—to thespecimens while they are undergoing UV exposure. Thismay be achieved by integrating a mechanical test frame,grips for securing specimens and a servohydraulic or screw-driven loading device, as shown in Fig. 16. Other uniqueexposure environments can also be created, including tem-perature cycling(including freeze/thaw), humidity cycling,and acid rain.

ACKNOWLEDGMENTS

This project was conducted under the auspices of theNIST Coatings Service Life Prediction Consortium, whichincludes as its industrial members Atlas Electric Devices,Dow Chemical, PPG, Akzo Nobel, Millennium, Atofina andSherwin Williams. Federal agency participants include theFederal Highway Administration, Forest Products Labora-tory (Madison, WI), National Renewable Energy Laboratory,the Air Force Research Laboratory, and the Smithsonian En-vironmental Research Center.

1H. R. Hirst, J. Soc. Dyers Colour.41, 347 (1925).2H. E. Weightman, Rubber Age23, 75 (1928).3R. C. Hirt, R. G. Schmitt, N. D. Searle, and P. Sullivan, J. Opt. Soc. Am.

50, 706 (1960).4N. Searle, inAccelerated and Outdoor Durability Testing of Organic Ma-terials, ASTM STP 1202, edited by W. D. Ketola and D. Grossman(American Society for Testing and Materials, Philadelphia, 1994), p. 52.

5M. M. Caldwell, W. G. Gold, G. Harris, and C. W. Ashurst, Photochem.Photobiol. 37, 479 (1983).

6D. Kockott, Die Angewandte Macromolekulare Chemie137, 1 (1985).7G. R. Fedor and P. J. Brennan in Ref. 4, p. 199.8D. K. Edwards, J. T. Gier, K. E. Nelson, and R. D. Ruddick, J. Opt. Soc.Am. 51, 1279(1961).

9D. G. Goebel, Appl. Opt.6, 125 (1967).10K. F. Carr, Surf. Coat. International10, 490 (1997).11K. F. Carr, Surf. Coat. International8, 380 (1997).12A Guide to Integrating Sphere Photometry and Radiometry(Labsphere,

1994).13L. F. Hanssen, Appl. Opt.35, 3597(1996).14U. S. Patent No. 6,626,052(30 September, 2003).15V. R. Weidner and J. J. Hsia, J. Opt. Soc. Am.71, 856 (1981).16S. L. Storm and A. Springsteen, Spectroscopy13, 13 (1998).17NASA Tech Briefs January 1996, p. 64.18Certain trade names and company products are mentioned in the text or

identified in an illustration in order to adequately specify the experimentalprocedure and equipment used. In no case does such an identificationimply recommendation or endorsement by the National Institute of Stan-dards and Technology, nor does it imply that the products are necessarilythe best for the purpose.

19Solar Spectral Irradiance, Commision Internationale de L’Eclairage(CIE)Publication No. 85, 1989.

20J. W. Martin, S. C. Saunders, F. L. Floyd, and J. P. Wineburg,FederationSeries on Coatings Technology: Methodologies for Predicting ServiceLives of Coating Systems(Federation of Societies for Coatings Technol-ogy, Blue Bell, PA, 1996).

21W. E. Thouret, Illum. Eng.(N.Y.) 55, 295 (1960).22V. Schäfer, Appl. Polymer Symposia4, 111 (1967).23K. G. Martin, P. G. Campbell, and J. R. Wright, Proc. Am. Soc. Testing

Mater. 65, 809 (1965).24J. E. Clark and C. W. Harrison, Appl. Polymer Symposia4, 97 (1967).25W. T. Welford and R. Winston,High Collection Non-Imaging Optics

(Academic, New York, 1989).26J. W. Martin, Prog. Org. Coat.49, 23 (1993).

FIG. 16. Proposed specimen chamber with capability for specimen loading.

Rev. Sci. Instrum., Vol. 75, No. 11, November 2004 UV weathering device 4959

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