Available online at www.sciencedirect.comProceedings

Proceedings of the Combustion Institute 33 (2011) 743–750

www.elsevier.com/locate/proci

of the

CombustionInstitute

Soot and thin-filament pyrometry using a colordigital camera

Peter B. Kuhn, Bin Ma, Blair C. Connelly, Mitchell D. Smooke,Marshall B. Long *

Department of Mechanical Engineering, Yale University, New Haven, CT 06511, USA

Available online 6 August 2010

Abstract

A simple and compact temperature and soot volume fraction diagnostic technique based on ratiopyrometry has been studied. Two different consumer digital single lens reflex cameras were evaluatedfor use as pyrometers. The incandescence from soot and a SiC filament was imaged at the three wave-lengths of each camera’s color filter array (CFA). After characterization of the detector’s signal responsecurves, temperatures were calculated by two-color ratio pyrometry using a lookup table approach. A SiCfilament with known emissivity was shown to provide an absolute light intensity calibration, which furtherallows the soot volume fraction to be determined. Measurements were carried out on four different flameswith varying levels of soot loading. The filament-derived gas temperature and soot temperature measure-ments have been compared with computational results and overall good agreement has been shown. Sootvolume fraction measurements have been compared with previous LII results, with excellent agreement forboth cameras tested.� 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Keywords: Optical diagnostics; Soot pyrometry; Thin-filament pyrometry

1. Introduction

Knowledge of temperatures and soot concentra-tions can prove valuable in understanding the pro-cesses responsible for soot production. Opticalpyrometry has proven to be a practical measurementtechnique that can provide the temperature and con-centration of soot particles in a flame by samplingthe incandescence of soot that has been heated to

1540-7489/$ - see front matter � 2010 The Combustion Institdoi:10.1016/j.proci.2010.05.006

* Corresponding author. Address: Department ofMechanical Engineering, Yale University, P.O. Box208284, New Haven, CT 06520-8284, USA. Fax: +1203 432 6775.

E-mail address: marshall.long@yale.edu (M.B.Long).

flame temperatures [1–6]. In regions where soot isnot present, gas temperatures have been inferredfrom pyrometry of thin filaments inserted into aflame [7–11]. Prior pyrometry methods have incor-porated a variety of detection methods ranging fromsingle photodetectors to scientific-grade imagers.Recently, Maun et al. [7] provided an overview ofimaging systems used for pyrometry measurementsand further showed that a consumer-grade digitalcamera could provide results for thin filamentpyrometry (TFP). In their work, the total intensityof light was measured with the camera and corre-lated to thermocouple temperature measurements.

In the work described here, a color digital cam-era is also utilized, but the ratio of two colors isused to determine the temperature of both sootand a SiC fiber inserted into nonsooting regions

ute. Published by Elsevier Inc. All rights reserved.

744 P.B. Kuhn et al. / Proceedings of the Combustion Institute 33 (2011) 743–750

of a coflow diffusion flame. Incandescence fromthe soot and the fiber is imaged at the three wave-lengths of the camera’s color filter array (CFA)and the temperature is calculated using two-colorratio pyrometry [1–3]. It is further demonstratedthat the image of the filament can provide anabsolute intensity calibration, which allows calcu-lation of the soot volume fraction.

2. Camera characterization

Over the last several years, a number of digitalsingle lens reflex (DSLR) cameras have been intro-duced, with a steady increase in performance fac-tors such as resolution, dynamic range, andsensitivity. The possibility of using these detectorsfor quantitative measurements is attractive fromthe standpoint of cost, but their suitability for suchan application is not a given, since consumer digitalcameras are not designed for use as scientific detec-tors. To establish the reliability of the cameras forquantitative work, the performance of two cameraswas investigated under controlled conditions. It isimportant to avoid some of the built-in image pro-cessing algorithms that are appropriate for snap-shots, but could invalidate the assumption thatthe signal can be related linearly to light intensity.Fortunately, many consumer digital cameras havethe ability to record images in “raw” mode, whichminimizes the internal processing of the informa-tion read from the camera chip.

Two representative cameras from recent genera-tions of consumer DSLRs are considered here – theNikon D70 and Nikon D90. Each offers the neces-sary manual user control of settings and 12-bit rawdata format at a reasonable price. The D70 is basedon a CCD detector (23.7 mm � 15.6 mm) with 6.1million (effective) pixels (2014 � 3040). The D90utilizes a CMOS sensor (23.6 mm � 15.8 mm) with12.9 million pixels (2868 � 4352). All imageenhancement options, such as sharpness, contrast,color, and saturation, were set to either “normal”or “none,” as applicable, in order to ensure shot-to-shot consistency. The ISO number was set to200 (the lowest standard setting). A white balanceof “direct sunlight,” was selected, but this settingsimply specifies a set of color balance multipliersthat were not used in processing the image data.

Files were saved in the camera’s “NEF” format,which is a 12-bit lossless compressed raw format.Although used by many camera manufacturers asa designation for unprocessed images, raw is not astandardized format (unlike TIF, JPG, etc.). Conse-quently, some careful examination of the resultingimage is necessary to ensure consistent reconstruc-tion of the underlying intensity data. To facilitate amore transparent analysis, the open-source image-processing software OMA [12] was used to captureimages, transfer them to the computer, and perform

subsequent processing. Decoding of the Nikon-spe-cific data format is done within the OMA programusing a publicly available software library [13].

It was verified that the signal detected by theD70 in all three channels decreased linearly withattenuation and increased linearly with exposure,up to the point where the channels saturate (seeSupplementary data). The signal is seen toincrease linearly with increasing exposure timeup to the point of saturation. A line fit to the datain the linear region results in an R2 value that isgreater than 0.999 for all three color channels.Within the linear region, the signal ratio betweenthe different color channels exhibits a standarddeviation of 1% across all data triplets. Similarresults were obtained using the D90. In the follow-ing, the cameras were operated in this linearregime.

The pyrometry experiments described belowrequire characterization of the spectral responseof the detector/CFA system, which is not availablefrom the manufacturer. To provide this data, thespectrum of an illumination source was imagedonto a ground glass diffuser (DG20-1500) usingan imaging spectrograph (Jobin Yvon CP200), witha 200 groove/mm grating and a 0.05 mm entranceslit. The dispersed spectrum was imaged onto eachdigital camera through a Nikkor 50 mm f/1.4 lens.The data were separated into three images corre-sponding to the red, green, and blue (RGB) chan-nels of the CFA. Spectral scaling was determinedby imaging the 404.7, 435.8, 546 and 578.2 nm linesfrom a mercury vapor lamp. Normalization for thespectrum of the illumination source and opticalthroughput of the spectrograph was obtained byimaging the spectrum with a scientific camera(Cooke Sensicam with SuperVGA sensor), whosetypical spectral response has been tabulated bythe manufacturer. The illumination source (Fiber-Lite Series 180) was observed to be stable, givingno noticeable variation over time. All componentsused were fixed in position, with the lens apertureset to f/2. Only the camera body was changed fordifferent trials. The spectral response of the NikonD70 and D90 were measured with a BG-7 filter inthe setup in order to provide better balance betweenthe red, green, and blue intensities that are encoun-tered in the pyrometry experiments. Figure 1 showsthe measured spectral response for the D70 andD90. The RGB channels from both cameras showsimilar characteristics – significant spectral overlapbetween channels, with a shape not accurately rep-resented by a single Gaussian curve.

To further characterize the detectors, an inde-pendent blackbody calibration was performed.Both the D70 and D90 recorded images of a black-body source (Pegasus R, Model 970) at tempera-tures ranging from 800 to 1200 �C, in incrementsof 50 �C. Data from these calibrations are pre-sented in the next section.

350 400 450 500 550 600 650 7000

0.005

0.01

0.015

0.02

0.025

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0.035

0.04Solid=D70 responseDashed=D90 response

Wavelength (nm)

Sig

nal (

arb)

Fig. 1. Nikon D70 and D90 CFA response curves,coupled with a BG-7 filter, for the blue, green and redfilters (from left to right). (For interpretation of thereferences to color in this figure legend, the reader isreferred to the web version of this article.)

1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 21000

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G/R BlackbodyB/G BlackbodyB/R BlackbodyG/R CalibrationB/G CalibrationB/R CalibrationG/R sootB/G sootB/R soot

Fig. 2. D90 signal ratios vs. temperature lookup tablefor a blackbody or gray body (solid lines) and soot(dashed lines), validated by an independent blackbodycalibration. Individual dots show the results of theblackbody calibration.

P.B. Kuhn et al. / Proceedings of the Combustion Institute 33 (2011) 743–750 745

3. Ratio pyrometry background

The intensity of radiation emitted by a materialat wavelength k, is dependent on the material’semissivity e(k) and the temperature T accordingto Planck’s law:

Iðk; T Þ ¼ eðkÞ 2phc2

k5 ehc=kkT � 1ð Þð1Þ

where c is the speed of light, h is Planck’s con-stant, and k is the Boltzmann constant. The mea-sured signal, SF, detected through a color filter ina time interval s is the intensity of radiation inte-grated over the detection wavelengths:

SF ¼ ð2phc2ÞsZ k2

k1

eðkÞgðkÞk5 ehc=kkT � 1ð Þ

dk ð2Þ

where g(k) accounts for the efficiency of the detec-tor, the combined lens and filter transmittance, aswell as a geometric factor. k1 and k2 are the lowand high limits of the filter bandwidth, which areapproximately 400 and 700 nm. In conventional ra-tio pyrometry applications, the filter bandwidths attwo central wavelengths are selected to be narrowcompared with the spacing between two filters. Thisapproach, outlined by Levendis et al. [1], takes asignal ratio at two spectral bands, and uses the finitefilter widths and mean multipliers for each filter atthe central wavelengths to simplify Eq. (2). The re-sult is an implicit temperature equation that can besolved iteratively for a measured signal ratio using acalibration that characterizes the detection system.In this approach, the filters are described only by asingle mean wavelength and a spectral width.

The assumption of narrowband detection fil-ters made to derive the implicit temperature equa-tion, however, is not valid for the CFAs on thecolor digital cameras being considered here (seeFig. 1). Clearly the filter functions are broad andrelatively complex and thus the Planck function

varies significantly within each of the three spec-tral detection windows of the CFA. In previouswork [14–16] it was shown that the effect of thevariation in the Planck function over a broadspectral window could be overcome by calculatinga new effective filter wavelength for each iterationof the temperature equation. Alternatively, it ispossible to evaluate Eq. (2) without makingapproximations of the filter profiles. In thisapproach, the signal ratio is derived directly fromEq. (2) to be

SF 1

SF 2

¼R

gF 1ðkÞ eðkÞ

k5 ½expðhc=kkT Þ � 1��1dkRgF 2ðkÞ eðkÞ

k5 ½expðhc=kkT Þ � 1��1dkð3Þ

for filters F1 and F2, where gF1(k) and gF2

(k) are therespective transmission efficiencies for each colorchannel as a function of wavelength. The ratiocan be calculated by evaluating the integrals inEq. (3) numerically based on the characteristics ofthe detection system for a range of input tempera-tures. Assuming the signal ratio is single valuedover the temperature range of interest, this lookuptable can be used to determine temperatures basedon the ratio of two colors. A camera with three sep-arate filters will provide three ratios, all of whichcan be used in determining the most likely finaltemperature.

Figure 2 shows three signal ratios as a function oftemperature for the D90 camera. Solid lines corre-spond to the ratios calculated for a blackbody sourceusing Eq. (3) and the measured camera responseshown in Fig. 1. Individual dots show the results ofthe blackbody calibration, and dashed lines showthe expected signal ratio for soot. The agreementbetween the measured blackbody ratios and theratios calculated from the measured spectral curvesare improved by small modifications (<5%) to theRGB scaling parameters and these are included inthe ratios shown in Fig. 2 as well as in the responseshown in Fig. 1. To generate the soot lookup table,the emissivity of soot is assumed to vary with

746 P.B. Kuhn et al. / Proceedings of the Combustion Institute 33 (2011) 743–750

k�1.38. This is based on the wavelength dependenceof the refractive index measured by Chang andCharalampopoulos [17] together with the expressionfor emissivity as a function of refractive index fromRef. [2]. As the emissivity of the SiC fiber does notvary with wavelength in the visible region of thespectrum (e = 0.88) [7], the emissivity terms in Eq.(3) cancel, and the blackbody lookup table can beused. A similar set of lookup tables was generatedfor the D70 and is shown in the Supplementarydata.

4. Thin filament temperature measurement

Following the detector characterization, datawere taken in sooting, axisymmetric, laminar ethyl-ene diffusion flames, with varying degrees of fueldilution by nitrogen. The burner consists of a0.4 cm inner diameter vertical tube, surrounded bya 7.4 cm coflow. Flames with different fuel mixtureswere studied with ethylene concentrations of 32%,40%, 60%, and 80%, by volume. For all flames, thefuel velocity at the burner surface had a parabolicprofile and the air coflow was plug flow, both withan average velocity of 35 cm/s. This configurationhas been studied previously as part of an ongoing

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Fig. 3. Filament temperature Tf, filament-derived gas temperatemperature Tc from four nitrogen-diluted ethylene flames.

experimental and computational collaboration[18]. A description of the computations used herefor the temperature and soot volume fraction com-parisons is included in the Supplementary data.

The flames were imaged through the BG-7 filterat f/16 with an 85 mm focal length lens, offset fromthe camera body with an extension ring. The lensconfiguration was chosen to approximate parallelray collection, which is a necessary assumptionfor the Abel inversion required for the soot pyrom-etry described in the next section [19,20]. The fila-ment, with a length of �7 cm and diameter of15 lm as measured with an optical microscope,was attached with tape to metal rods and placedhorizontally in the flame. Measurements were madeat 30.0, 39.0, 59.6 and 83.9 mm above the center ofthe burner for the 32%, 40%, 60% and 80% flames,respectively. These locations were well beyond thesooty region to avoid interference from sootemission.

For each filament temperature measurement,two NEF images, with and without the filamentabove the flame, were obtained. The one withoutthe filament served as background and was sub-tracted pixel by pixel. With a perfect imaging sys-tem, the image of the fiber on the detector wouldbe smaller than the width of one pixel (by factors

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P.B. Kuhn et al. / Proceedings of the Combustion Institute 33 (2011) 743–750 747

of 4.4 and 3.2 for the D70 and D90, respectively,after 2 � 2 binning) but aberrations in the imagingsystem resulted in the fiber image being spreadover several pixels. A region surrounding the fila-ment is selected to include all RGB pixels contain-ing filament radiation and the intensities areintegrated along the axial direction in this region.Dividing the RGB integrated intensities by eachother yielded three ratios, and three consistenttemperatures were obtained from the ratios byapplying the appropriate blackbody lookup table(Fig. 2). The maximum variation of the tempera-tures obtained from the three ratios is �20 K,while the average temperature is almost the samewith the temperature lookup by the B/R ratio,

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Fig. 4. Measured (top) and calculated (bottom) soot temperatconcentrations (from left to right) of 32%, 40%, 60% and 80%

with a maximum difference around 3 K. Figure 3shows the filament temperatures above each ofthe four flames as measured by the D90 camera(in all the following results, the D70 measurementsare included in the Supplementary data). The fluc-tuations along the profile come from the imperfec-tion of the filament and variation of pixels.Because of the rapid drop-off in intensity at lowertemperatures, filament temperatures lower than�1400 K became dominated by noise. It shouldbe noted that when a new filament was positionedabove the flame, the RGB intensity and derivedtemperatures were not stable for several minutes.After aging for �3 min, the filament temperaturesand RGB signals reach a stable state.

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ures from four nitrogen-diluted ethylene flames with fuelC2H4.

748 P.B. Kuhn et al. / Proceedings of the Combustion Institute 33 (2011) 743–750

The filament is heated by convective heat trans-fer from the surrounding hot gases and cooled byradiation transfer with the ambient environmentand axial heat conduction along the filament.Therefore the filament temperature is always lowerthan the gas phase temperature and a correctionprocess is needed to obtain gas temperature. Thecorrections are dependent on gas properties, veloc-ity, composition, and temperature. The computa-tions for the four N2 diluted flames showed thatN2 occupies more than 70% volume downstreamat the filament position. Therefore, gas propertiesused here can be based on pure N2, for simplicity,without losing too much accuracy. The correctionprocedure followed the analysis of Maun et al. [7]with details given in the Supplementary data.

As shown in Fig. 3, Tf is the filament tempera-ture, Tg is the derived gas phase temperature whileTc is the computational result. The derived gas

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Fig. 5. Soot volume fraction measured using pyrometry (topflames with fuel concentrations (from left to right) of 32%, 40

temperature is in good agreement with the compu-tational results for the 32% and 40% flames. Forthe higher fuel concentration flames, the com-puted temperature is seen to be higher than themeasured temperatures. This difference may beattributed to less radiation loss in the computa-tions, since the computational results predict alower soot volume fraction than is measured.

5. Soot temperature and volume fraction in foursooting laminar flames

Soot temperature measurements were performedin the same setup as the filament temperature mea-surement. Each flame image was separated intothree RGB images and then converted to radial pro-files using an Abel inversion. Prior to the Abel inver-sion, the image of the full flame must be divided into

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) and LII (bottom) from four nitrogen-diluted ethylene%, 60% and 80% C2H4.

P.B. Kuhn et al. / Proceedings of the Combustion Institute 33 (2011) 743–750 749

a half image, with the symmetry axis on one side ofthe half image. Best results were obtained by consid-ering each radial slice of the image (i.e., one row ofpixels), finding the centroid of the row, and foldingthe intensities about that centroid. This makes useof both halves of the image, resulting in less noisein the final Abel-inverted profile, and eliminatesany need for manual cropping of the image. Divid-ing the radial profiles by each other to determinethe measured signal ratios and applying the sootlookup table gives three soot temperature profiles,with a maximum variation of�30 K. The final tem-perature is an average of the three. Figure 4 (top)shows the measured soot temperatures from eachof the flames obtained with the D90. As might beexpected, the peak temperature decreases as the fuelconcentration increases, due to increased soot load-ing and resultant radiative heat loss. Since sootpyrometry can only measure temperature wheresoot is present, the calculated temperature profilesshown in Fig. 4 (bottom) have been cropped to showonly those regions where soot was calculated to benonzero. While the computational model gives asomewhat different prediction of the sooting region,(which in turn results in a different shape of thecropped temperature profile) the same generaltrends are seen in both measurement and computa-tion, with peak temperatures decreasing as the sootloading increases.

Once the soot temperature is known, the sootvolume fraction, fv, can be determined if an abso-lute intensity calibration is available for the specificoptical configuration used. Following the formula-tion of Cignoli et al. [3] this can be expressed as

fv ¼ �kS

KextLln 1� eLðk; T LÞ

sS

sL

SSk

SLk

�

� exp � hckkS

1

T L� 1

T S

� �� ��ð4Þ

where kS is the effective filter wavelength at the soottemperature, Kext is the dimensionless extinctioncoefficient, L is the absorption length (the dimen-sion of one binned pixel), eðk; T LÞ is the emissivityof the calibration source, and s, Sk, T are detectorparameters, signal intensity and temperature,respectively, with subscripts L and S correspondingto the calibrated light source and soot, respectively.Kext was taken as 8.6 here, which is consistent withvalues given by Krishnan et al. [21], and matchesthe value used in the LII experiment that the sootvolume fraction results are compared against [18].The effective filter wavelength, kS, is the wavelengthat the maximum value of the product of the camerafilter response (see Fig. 1) and the Planck blackbodyintensity evaluated at the particular soottemperature.

A blackbody calibration source or other cali-brated light source can be used for calibration, aslong as the optical configuration is not changed.A more convenient source of absolute intensity cal-

ibration, however, is available from the image ofthe filament discussed above. As long as the emis-sivity is known, this measurement provides a tem-perature (higher than that available fromcommon blackbody sources) and an absoluteRGB light intensity correlation. This has theadvantage of being simple to implement, since thefilament and soot measurements are performed inthe same experimental configurations and the geo-metric factors therefore cancel. The ratio of opticalparameters sS

sLsimply accounts for any difference in

exposure times and the ratio of the length imagedonto a pixel to the filament diameter.

Figure 5 (top) shows the soot volume fractionmeasurements obtained in the four flames usingthe D90 and Fig. 5 (bottom) shows the soot vol-ume fraction measured in the same flames usinglaser-induced incandescence (LII) and reportedpreviously [18]. Agreement between the two mea-surement techniques is very good. It should benoted that, as with the LII results shown inFig. 5, self-absorption is not considered. For themost highly sooting flames, this could affect theresults. Incorporating the absorption into thealgorithm could help improve the measurementaccuracy for flames with higher soot loading.However, this approach is more mathematicallycomplicated and is beyond the scope of this work.

6. Conclusions

Two consumer DSLRs, (one based on a CCDdetector, the other with a CMOS sensor) have beencharacterized and were found to work well for ratiopyrometry of SiC fibers and soot. The spectralresponse of each detector was characterized, whichallows the generation of lookup tables relating thethree color ratios from the detector’s RGB filtersto the temperature of the fiber or soot. Use of aBG-7 color glass filter equalized the intensity inthe three channels. Four C2H4 flames with varyinglevels of N2 dilution were studied and results for thetemperature obtained with the two detectorsshowed good agreement with each other as well asreasonable agreement with recent calculations.The SiC fiber image was shown to be useful as anabsolute intensity calibration, which then allowsthe determination of the soot volume fraction fromthe ratio pyrometry measurements. Results fromboth detectors were in excellent agreement withthose obtained in previous studies using LII.

Acknowledgements

The research was supported by the DOE Officeof Basic Energy Sciences (Dr. Wade Sisk, contractmonitor), the National Science Foundation (Dr.Phil Westmoreland, contract monitor), NASA

750 P.B. Kuhn et al. / Proceedings of the Combustion Institute 33 (2011) 743–750

(Dr. Dennis Stocker, contract monitor) and theAFOSR (Dr. Fariba Fahroo, contract monitor)under contracts DE-FG02-88ER13966, CTS-0328296, NNC04AA03A, and AFOSR FA9550-06-1-0164, respectively.

Appendix A. Supplementary data

Supplementary data associated with this articlecan be found, in the online version, at doi:10.1016/j.proci.2010.05.006.

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