Introduction

Integrating spheres, in combina-tion with UV/Vis and UV/Vis/NIRspectrophotometers, are extremelyversatile accessories that are essen-tial for high precision reflectanceand transmission measurementson virtually any solid or liquid.Application areas include themeasurement of the specular,diffuse and total reflectance ofmaterials as well as the transmit-

tance of clear and turbid liquidsand semi-opaque solid materials.Minimal sample preparation isusually required, with samplesoften being measured in theiroriginal form. A range of integrat-ing spheres is available for thePerkinElmer LAMBDA™ 950/850/650 spectrophotometers; for fulldetails of configurations andspecifications, please see appli-cation note 006691_17.

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Transmission Measurements Using Integrating SpheresFor the LAMBDA 950/850/650 UV/Vis/NIR and UV/Vis Spectrophotometers

A choice of 60 mm and 150 mmspheres are available to addressthe widest range of applications in the industry

Modular sampling platform allows quick change of samplingaccessories with no set-up

Dual sample compartments allowvirtually any sample or accessoryto be accommodated

150 mm sphere conforms tointernational measurementguidelines including ASTM, DIN and CIE

Key Features/Benefits

System design and sampling flexibility

In a busy lab, instruments are morevaluable if they can be rapidly re-configured to run different analyses.The LAMBDA 950/850/650 familywas designed with this in mind. The60 mm and 150 mm integratingspheres are mounted in intelligentsampling modules located in thehuge dual sampling area which isan integral part of the design. Thismodular accessory design meansthat it’s easy to switch from trans-mission measurements using thestandard detector to measurementswhich require an integrating sphere.

The large size of the dual samplecompartments (Figure 1) providesmaximum accessibility for accuratesample positioning and allowslarger samples to be easily accom-modated. A wide variety of samplingaccessories, such as variable angletransmission holders, small spotbeam kits and depolarizers/polarizerscan be used in combination withan integrating sphere, increasingthe number of applications which

can be performed on the system. This means that accessories do notneed to be squeezed into a singlecompartment, compromising per-formance and productivity. Forexample, an integrating sphere can be used as a detector for otheraccessories, such as the V-N specu-lar reflectance accessories, or fortransmittance measurements onhighly scattering samples.

Another advantage of the LAMBDAsystem design is that it allows a“straight-through” optical geometry(Figure 2) in which the beams arepassed straight through the samplecompartment to the integratingsphere using all-reflecting optics.This avoids the undesirable use oflenses to steer the beam towardsthe detector, ensuring that thehomogeneity, collimation anduniformity of the beams are main-tained throughout.

Choosing an integrating sphere

Several factors must be taken intoconsideration when choosing anintegrating sphere, the majorconsideration being whether to usea 60 mm or 150 mm design. Smallerintegrating spheres are moreefficient collectors of light becausethere will be fewer reflectionswithin the sphere before photonsreach the detector. This means that a 150 mm sphere attenuates thebeam to a greater extent than a 60 mm sphere. Therefore, for highly

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Figure 1. The LAMBDA series dual sample compartments.

Figure 2. The optical design of the PerkinElmer 150 mm integrating sphere.

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absorbing samples where little lightenters the sphere, the highestsignal-to-noise ratios will beachieved by using a 60 mm sphere.

Another important factor is the portfraction, defined as the ratio of thetotal port area relative to the totalarea of the sphere. It is significantlylower for a 150 mm sphere (around2.5%) than for a 60 mm sphere(around 11%). The port openings,such as the entrance beam port andsample holders are clearly seen inFigure 2. A low port fraction, incombination with a large internalsurface area, ensures that the incidentbeam is distributed evenly aroundthe sphere’s surface before it reachesthe detector, increasing measurementaccuracy. This is important, as manyCIE and ASTM methods specify aminimal port fraction. For example,the CIE color method specifies afigure of less than 10% for colormeasurements and ASTM 1003-95requires a total port fraction of lessthan 4% for haze measurements ontransparent plastics.

Transmission measurements

When using an ordinary transmissionsampling arrangement, it is verydifficult to make accurate measure-ments on solid samples such asglass or plastics. These samplesrefract or distort the beam, causing itto hit different spots on the detectorand increasing the amount of straylight in the system. Therefore, meas-ured values are not reproducibleand do not match predicted valuesor values from other instruments.Contributing factors include roughsample surfaces, non-parallelsurfaces or sample thickness effects.

A solution to this problem is toreplace the standard instrumentdetector with an integrating spherewhich, due to its ability to collectlight from a wide range of inputangles, can correct for inaccuraciesdue to refraction or light scatteringeffects. On the 150 mm sphereshown in Figure 2, the sample islocated in the transmittance sample holder at the entrance port. Spectralon™ blanking platesare fitted to the reflectance sampleholder and reference ports.

A typical example which illustratesthe use of an integrating sphere fortransmission measurements is theanalysis of an automotive paint filmto measure the level of UV blocking.This relates to coat durability whenexposed to UV radiation over time.When measured using the normaltransmission detector, the transmis-sion is very close to 0% due to thehigh level of scattering. Figure 3shows the transmission level in the

UV/Vis range measured using anormal detector (blue spectrum)and also an integrating sphere (redspectrum). The integrating sphere’sability to perform a diffuse trans-mission measurement yields a farmore accurate value.

Thick samples also cause problemsdue to light scatter, refraction andbeam distortion. Again, an integra-ting sphere can compensate for theseeffects and provide extremely ac-curate results. For example, a grey

glass television tube lens, measuring15 mm, was measured to under-stand the transmission propertiesin the UV/Vis region (Figure 4).

Finally, a spectrum of protectivegoggles shows how an integratingsphere can provide very accurateresults even when the sample iscurved (Figure 5). The goggleswere attached directly to thetransmission sample holder of the

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Figure 3. Spectra of a metallic automotive paint layer measured using a standard transmission detector (blue) and an integrating sphere (red).

150 mm sphere so that all trans-mitted light was collected. Notethe high visible light transmission and complete blocking of the UVradiation. The oscillating fringepattern is due to the anti-abrasioncoating applied to the lens.

Conclusion

An integrating sphere provides an ideal solution for accurate andreproducible measurements on avariety of difficult transmissionsamples. Even very thick samplesand those which scatter and refractthe beam can be readily measuredin a controlled and precise way. The150 mm integrating sphere meets allinternational measurement guide-lines such as those set by ASTM,DIN and CIE.

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006991_11 Printed in USA

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Figure 5. Spectrum of protective eyewear measured using a 150 mm integrating sphere.

Figure 4. Transmission spectrum of a television tube lens using an integrating sphere detector.