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Optical Techniques for Studying StrayLight in SpectrophotometersA.W.S. Tarrant aa Department of Home Economics, University of Surrey, Guildford,Surrey, EnglandPublished online: 16 Nov 2010.

To cite this article: A.W.S. Tarrant (1978) Optical Techniques for Studying Stray Light inSpectrophotometers, Optica Acta: International Journal of Optics, 25:12, 1167-1174, DOI:10.1080/713819727

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OPTICA ACTA, 1978, VOL. 25, NO. 12, 1167-1174

Optical techniques for studying stray light in spectrophotometers

A. W. S . TARRANTDepartment of Home Economics, University of Surrey, Guildford,Surrey, England

(Received 10 August 1978)

Abstract. In spectrophotometers the term ' stray light ' is used to refer to theminute amount of unwanted light having wavelengths outside the narrow bandisolated by the optical system . The effect of stray light is to reduce the accuracyof the instrument and in some cases to restrict the wavelength range over whichthe instrument may be used . The paper describes an optical method by whichthe stray light transmission of a monochromator may be determined for variouswavelengths . If the particular instrument studied is found to show stray lightbehaviour of a type which varies continuously with wavelength, a simple tech-nique can be applied for assessing the errors that will occur with any given sample .The relative importance of near-stray and far-stray light is discussed .

1. Stray light and its importanceSpectrophotometers are very widely used in chemical work and in colour

specification . To obtain measurements at a variety of wavelengths, the usualtechnique is to use a ' white ' light source and a monochromator to isolate anarrow band of wavelengths from its spectrum . However, a small amount oflight of unwanted wavelengths always emerges from the monochromator as aresult of optical scattering within it ; spectroscopists refer to this as ' stray light ' .Generally, the intensity of the stray light is minute-often only a few millionthsof that of the main beam-but in some circumstances its presence can lead toquite serious errors of measurement . When transmission measurements areattempted at the peak of a narrow absorption band, for example, the samplemay absorb perhaps 99 per cent of the wanted light, but allow all the stray lightof other wavelengths to be transmitted freely ; consequently it is often impossibleto make accurate transmission measurements on such samples, where theabsorbance (optical density) is greater than 2 .0 . Likewise, when a spectro-photometer is set to the extreme end of its wavelength range so that the sourceoutput or detector response to the wanted light is low, the effects of stray lightbecome relatively large so that significant errors of measurement occur .

Our remarks are mainly applicable to grating instruments because theserepresent the large majority of commercial spectrophotometers.

2. Stray light in practiceThe questions asked by the spectrophotometer user (who is not usually

knowledgeable in optics) are ' Is the stray light going to be significant in anyparticular measurement and, if so, what can I do about it ? ' .

There are three ways of studying the stray light in any particular instrumentby the use of the Beer-Lambert law [1], by the use of short-wavelength cut-off

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filters [2] and optical methods . The Beer-Lambert method will provide in-formation about the errors which will occur when a particular solution ismeasured with a particular instrument, and the filter method will indicate ifthere is significant stray light present in an instrument, but little more . Inorder to gain a better understanding of the problems, a general study of straylight was made by the author at Sira Institute, using an optical technique [3] .Previous optical studies were made by Pritchard [4] in 1955, and by Rossler [5] .Work on scattered light from diffraction gratings is currently in progress .

3 . An optical methodThe principle of the author's original studies was to replace the entrance

slit of the monochromator by a pinhole, illuminate it with incoherent mono-chromatic light and measure the intensity of scattered light at points all over theimage plane of the monochromator (i .e . the plane of the exit slit) with a ` pin-hole detector ' .

By repeating this procedure with light of a variety of wavelengths it wouldhave been possible, theoretically, to compute the stray light transmission of theinstrument when used with real entrance and exit slits . It was decided not touse lasers as the light sources, as it would have been difficult to simulate theillumination conditions which occur in a real instrument, and it is doubtfulwhether the ` coherent scattering ' would be identical with the ` incoherentscattering ' that occurs in practice . In order to produce monochromatic lightof sufficient intensity, a high-pressure, compact-source mercury arc was usedand light of the desired wavelength was isolated by a double monochromatorindependent of the one being tested . The scattered light at the chosen pointof the image plane was isolated with a pinhole, and measured with an EMItype 6256B photomultiplier ; a chopping technique employing a phase sensitivedetector was used . When measurements of the direct beam (i .e . the ' wanted 'light) were made, high-density neutral filters were used to attenuate the beam toa suitable level .

Experiments of this type yielded much useful information on the generalnature of stray light which is reported in an accompanying paper [6] . Afterconsideration of these results it was decided that, rather than compute the straylight performance of a monochromator from data obtained in such studies withpinholes, it would be better to set up the monochromator with real entranceand exit slits and make direct measurements . The apparatus used for studiesof this kind is shown in figure 1 . In this figure the mercury arc is shown at A,and the chopper disc is at C, immediately adjacent to the exit slit of the isolatingdouble monochromator . The high-density filters (D) are introduced herewhen necessary, and a concave mirror E is used to refocus the beam on theentrance slit H of the monochromator under test . The light emerging fromthe exit slit is measured by the photomultiplier F, and the signal obtained fromthe phase-sensitive detector is displayed either on a digital voltmeter, x -yrecorder or data logger .

By moving the wavelength-setting mechanism of the monochromator overits entire range and plotting the intensity of the scattered light emerging fromthe exit slit it is possible to obtain a curve such as that shown in figure 2 . Thecentral peak, when the direct beam is transmitted, is shown on a greatly reduced

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Optical techniques for studying stray light in spectrophotometers

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0 t200

400

DVM

Figure 1 . Basic layout of apparatus .

DoubleMonochromator

0 0JpD

F

PSD

I

600 800

Wavelength setting

Figure 2 . Typical stray light profile obtained with an ' Optica ' monochromator.

scale ; if shown on the same scale as the scattered light it would be about 25 kmhigh . This curve, or' profile', shows how much light of wavelength 546 . 1 nm,relative to the direct beam, would be transmitted at each wavelength setting ofthe instrument in practice . By compiling a series of profiles for different wave-lengths it is possible, by a simple matrix operation, to determine what the truespectral transmission of the monochromator will be when it is set to a particularwavelength setting .

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A . W. S. Tarrant

It is important not to confuse the terms ' wavelength ' and ' wavelengthsetting ' . The ' wavelength setting ' indicates the position to which the move-able optics in a monochromator are set, and that is independent of the wavelengthof light being studied ; the wavelength of the light being studied and the wave-length setting of the instrument are independent variables .

In practice, the light obtained from the mercury arc was not strictly mono-chromatic as the effects of pressure and Doppler broadening in the arc are suchthat the effective line-width was of the order of 1 .5 nm ; moreover, there is aweak continuum discharge in lamps of this type . The essential quality requiredof the test beam is that it be free of light of other wavelengths rather than bestrictly monochromatic . It was found that the combination of line source anddouble monochromator gave a beam of appropriate spectral purity for this work .However, because this light was redispersed in the test monochromator, thefinite line-width produced an image in the image plane that was slightly widerthan the exit slit, making the intensity recorded for the direct beam transmissionslightly too low . Corrections were made for this effect by scanning the wholewidth of the line and using the usual area integration process [3] .

4 . Calculation of stray light errorsAn important object of tests such as these is to determine the magnitude of

the errors due to stray light that will occur in practice, when the chosen mono-chromator is used in conjunction with a particular source, sample and detector .If we use A to denote wavelength, EA the power output of the light source atwavelength A , M. the transmission factor of the monochromator, TA the trans-mission of the sample and S A the sensitivity of the detector, then in the case of asimple spectrophotometer used to measure a sample against air, the detectorresponse on the sample beam is given by

00R' = f EAMA TASA dA,

0

and the response on the reference beam bycc

R" = f EAMaSA d A.0

Thus, the transmission value indicated by the instrument will be given by00f ExMJ~S.~ d A

T=0

00I E.M.S. d A0

So if we know the complete set of values of MA , i .e . the true spectral transmissioncurve of the monochromator, for a given wavelength setting and we have com-plete data for the EA , TA , and SA curves, we can calculate the transmission valuethat the instrument will indicate and thus determine the magnitude of the straylight error . Since in most cases this will not be very large, we do not need toknow TA very precisely . If a number of stray light profiles (such as that infigure 2) are recorded at wavelengths within the whole range over which the

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instrument is to be used, it is a simple matter to determine Ma for any givenwavelength setting by re-arranging the data in a matrix calculation with the aidof a computer .

However, in practice the profiles were obtained only for the wavelengths ofthe principal mercury lines, as a very high source intensity is needed to produceenough stray light for an effective measurement . It has been found that insome monochromators the stray light profile varies in a completely continuousway with wavelength, whereas in others it does not . In the former case thecalculation of M. for a given wavelength setting can be much simplified by the` figurine ' technique, described below .

10

-300 0 +300

Wavelength setting

relative to main beam

Figure 3 . Stray light profiles for three different wavelengths, showing similarity of shape .

In cases such as this it is found that if the stray light profiles are drawnusing the wavelength setting corresponding to the line wavelength as the origin,the shape of the profiles for the different wavelengths are closely similar, butthey are of different heights, as shown in figure 3 . Thus it is possible to repre-sent any one of them by a function which represents the mean shape and a` height factor ' for each wavelength . This factor invariably increases towardsshorter wavelengths, i .e. the shorter the wavelength, the greater the proportionof stray light . If the height factor is plotted against wavelength it is found thatit may be adequately represented by an equation such as h=A+B/ A2 , where Aand B are constants . The mean shape, termed the ` figurine ', may be repre-sented as y = f (x), and the spectral transmission factor for the monochromatorat any wavelength setting x may be written as

M(A, x)=h . f(x) =(A+B/A2)f(x) .

Once the shape of the figurine and the height factor/wavelength relationshiphave been determined for any monochromator which behaves in this way, it isa simple matter to compute the stray light error which will occur with the instru-ment in practice .

The relationship between height factor and wavelength tell us, in effect,how the stray light varies with wavelength, and it is largely dependent on thestate of cleanliness of the optics of the instrument, and of the sort of dirt uponthem . On one occasion an old monochromator was being dismantled by the

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author, when a dead fly (species Musca domestica defuncta) fell from a recess ofthe case and landed in the middle of the collimating mirror . The author,yielding to a natural impulse, blew it off, and when the instrument was re-assembled it was found that the height factor/wavelength relationship haddrastically changed . There was now much less stray light at the longer wave-lengths, whereas at the shorter wavelengths there was only a small reduction .This was consistent with the ` large ' dirt, such as hair or fibres, which gives riseto low-angle scatter, having been blown off, while the ' fine ' dirt, such asminute particles of grease or spores, which gives rise to Tyndall scattering,remained .

Unfortunately, several monochromators do not show this simple type ofbehaviour . The stray light often arises largely from ghost images [7] whichare not suitable for treatment by the figurine method, or it is found that there isno apparent consistency of shape between the profiles recorded at differentmercury wavelengths . In this respect the use of a mercury arc as the source inthe test gear is a distinct limitation. High-pressure xenon arcs have been triedwith the object of obtaining stray light profiles at other wavelengths . At thetime of writing some profiles have been recorded in the visible region in thisway, but further work is needed before reliable results can be quoted . Ifprofiles could be recorded at sufficiently close wavelengths, say every 20 nm,it would be possible to compute the stray light errors occurring in practice fromthem, but it would involve a large mass of input data .

5 . Typical resultsOne of the monochromators studied was of the type supplied with the Optica

CF4 spectrophotometers, and showed the ` figurine ' behaviour very well .The stray light errors occurring when used with typical detectors and sourceswere calculated for various samples. One of these was a chemical solutionshowing a narrow absorption band (see figure 4) and the other was a reflectancesample-a cobalt blue tile of the type commonly used in testing colorimeters .With the solution, it was found that if this monochromator had been used with adeuterium lamp and an S-5 detector, and had been set to a total band-width of3 .5 nm, the transmission indicated at the peak of the absorption band at 300 nmwould have been 1 .464 per cent, whereas the real transmittance would have been1 .420 per cent . In practice this would represent an error in the concentrationof about 0 . 7 per cent of the actual value . In the case of the cobalt tile, if atungsten lamp and S-20 detector had been used the stray light errors would havebeen much less significant and the error in the chromaticity resulting from straylight error considerably less than the `just noticeable colour difference ' .

6 . DiscussionIn the course of calculating stray light errors by this method it is possible

to determine the proportion of the error contributed by stray light of each wave-length . Such a spectral distribution of stray light error is shown in figure 5,which shows the distribution of the error in the case of the solution referred toabove, and in figure 4, when the monochromator is set to the peak absorptionwavelength of 300 nm . This result is a typical one . The hatched area repre-sents 50 per cent of the total, and it will be seen that stray light of wavelength

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within ± 60 nm of the working wavelength is responsible for half the total error .In other words, ` near stray ' light is just as important as ` far stray ' light inproducing stray light errors . This finding may be a little surprising at firstsight, but it arises because in this monochromator (and in all others so far studied)

100

80

40

20

200

0200

400

600

800

Wavelength (nm)

Figure 4. Transmittance of a solution known to commonly cause stray light errors whenmeasurement of the minimum is attempted .

400

600

Wavelength (nm)

Figure 5 . The spectral distribution of stray light error ; the hatched area represents50 per cent of the error .

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the proportion of stray light rises rapidly as the wavelength setting approachesthat of the working wavelength .

The causes of near stray light are discussed in § 2 .7 of the accompanyingpaper [6] ; in practice, low-angle scattering by dust and scratches is its maincause. Earlier workers [1, 8] have also remarked on the high proportion ofnear stray light .

7 . Some results of special interestThis paper has been concerned mainly with techniques, and many of the

results it has led to are described in the accompanying paper [6] . During thecourse of the work some experience of the stray light behaviour of mono-chromators has been gained [7, 9] and there are a few points which may be madeusefully here .

(1) It is possible, by using this technique, to determine the stray lightcharacteristics of a monochromator and thus to determine what errorswill occur when the instrument is used in practice with a given lightsource, detector and sample. This process is greatly simplified if themonochromator shows continuous variation of stray light with wave-length, and the ` figurine ' method may be used .Although the proportion of stray light transmitted invariably increaseswith decreasing wavelength, the increase is not spectacular .The effect of near stray light in producing errors is by no means small,and commonly equals or exceeds that of far stray light . Hence the useof a colour filter as a ' stray light eliminator ' is not likely to be totallyeffective. Even the use of a double monochromator will not entirelyeradicate near stray light .

ACKNOWLEDGMENTS

This work was carried out as a multi-client project of Sira Institute Ltd,and I wish to record my thanks to the sponsors, Beckman RIIC Ltd, EMI Ltd,ICI Plastics Ltd, International Nickel Co Ltd, 3M Research Ltd, Perkin ElmerLtd, Kodak Ltd, Pye Unicam Ltd, Wellcome Research Ltd, and the UVSpectrometry Group . I also wish to thank the University of Surrey for theaward of a study-leave year for this work. Finally, I have to thank Dr . M. A .Cutter and Mr. S . Brown who have carried out most of the experimental worksince my return to Surrey .

REFERENCES

[1] POULSON, R. E ., 1964, Appl . Optics, 3, 99 .[2] ASTM Manual on Recommended Practice in Spectroscopy, p . 94 .[3] TARRANT, A. W . S., 1976, Electro-Optics/Laser International 76 (IPC Science and

Technology Press), p . 40.[4] PRITCHARD, B . S ., 1955, ,7. opt. Soc. Am ., 45, 356 .[5] RoSSLER, F., 1974, Optik, 41, 293 .[6] BROWN, S ., and TARRANT, A. W. S., 1978, Optica Acta, 25, 1175 .[7] TARRANT, A . W . S., and BROWN, S., 1978, A Study of Stray Light in Three Mono-

chromators (Sira Institute Ltd) .[8] TUNNICLIFF, D . D ., 1955, Y. opt. Soc. Am ., 45, 963 .[9] TARRANT, A. W. S., 1976, Stray Light in Spectrophotometers (Sira Institute Ltd) .

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