High-speed two-camera imaging pyrometerfor mapping fireball temperatures

John M. Densmore,1,* Barrie E. Homan,2 Matthew M. Biss,2 and Kevin L. McNesby2

1Lawrence Livermore National Laboratory, Livermore, California 94550, USA2U.S. Army Research Laboratory, Aberdeen Proving Ground, Maryland 21005, USA

*Corresponding author: densmore3@llnl.gov

Received 22 August 2011; accepted 14 September 2011;posted 4 October 2011 (Doc. ID 151835); published 18 November 2011

A high-speed imaging pyrometer was developed to investigate the behavior of flames and explosiveevents. The instrument consists of twomonochrome high-speed Phantom v7.3m camerasmade by VisionResearch Inc. arranged so that one lens assembly collects light for both cameras. The cameras are filteredat 700 or 900nm with a 10nm bandpass. The high irradiance produced by blackbody emission combinedwith variable shutter time and f-stop produces properly exposed images. The wavelengths were chosenwith the expected temperatures in mind, and also to avoid any molecular or atomic gas phase emission.Temperatures measured using this pyrometer of exploded TNT charges are presented.OCIS codes: 110.6820, 120.6780, 120.1740, 280.6780.

1. Introduction

Temperature measurements of flames and energeticmaterial reactions can be accomplished in a varietyof ways. Direct contact measurements utilizingthermocouples [1–3] have been successfully used inflames but have proven difficult to implement in lar-ger scale explosive events. In order for the thermo-couple to have the required temporal sensitivity, thesize of the device must be reduced to a point at whichit rarely survives the blast duration. The utility of athermocouple is also restricted, in that it can onlymeasure the temperature at a given point in space.

Optical techniques [4,5] that utilize blackbodyemission theory eliminate the fragility concernslimiting the thermocouple measurements, as theinstrument can be placed a safe distance from theevent. High-speed optical pyrometry can be accom-plished by utilizing two or more single elementdetectors, each having a different wavelength band-pass filter [6]. This technique can be fast, with thetemporal resolution being controlled by the rise timeof the detectors, typically on the order of 10ns forsemiconductor devices. This technique is termedthe integrating pyrometer (IP), in that it integratesthe emission over the field of view (FOV) of the de-tector. The spatial extent for measurement of such

systems depends on the collection optics used to cou-ple the radiation to the detector [6]. If a fiber optic isused without other focusing optics, the FOV is con-trolled by the acceptance angle of the fiber and canencompass a large area for multimode fibers andlarge standoff distances [7].

Because the intensity of a blackbody [Eq. (1)] is astrong function of temperature, the reported tem-perature from an IP having a spatial variation inits thermal profile can be misleading. To illustrate,an ideal blackbody with a normally distributed var-iation in temperature having a mean of 1500K and apeak width FWHM of 200K is shown in Fig. 1. Thetemperature calculated from an IP having this distri-bution is 1730K, which is 15% higher than the meantemperature. The thermal distribution has a pro-found effect on the calculated value of the tempera-ture, as can be seen in Fig. 2. By varying the assumedwidth of the distribution while holding the actualmean temperature constant (1500K), the calculatedtemperature from the IP can be as much as 60%higher than the true average.

To mitigate the issues with the IP and to providespatial information, an imaging pyrometer shouldbe used. Previously developed techniques for thistype of instrument that are capable of the spatial

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mapping of temperature tend to fall into three cate-gories. The first uses an IR sensitive camera [8].These cameras tend to be relatively slow and usuallyexpensive, although those caveats are changing [9].In the second technique [10], multiple exposures aretaken with the camera while bandpass filters are ex-changed between frames. This technique may be use-ful for those processes that are static in time. Othertechniques [11–15] also use one camera but split thesensor into multiple regions, each having a differentnarrow bandpass filter in the optical path. Theseone-camera techniques require complicated opticsto direct multiple images from the scene onto differ-ent areas of the chip. Finally, a one-camera technique[16] was developed in our laboratory that utilizes thestandard Bayer filter that is integral to most colorcameras. This system requires no special opticsbeyond standard lenses. However, the technique re-quires that no significant atomic or molecular emis-sion be present within the visible spectrum. In thiswork, we report on a two-camera, high-speed ima-ging pyrometer that is capable of measuring thespatial temperature profile of flames and energeticmaterial combustion. Although the frame rate is alsolimited by the pixel readout frequency, this rate is

double that of one-camera techniques because eachwavelength can be captured simultaneously.

2. Theory

The instrument described here utilizes the sameoperating principle as all other imaging pyrometers,in that they are based on Planck’s law of radiationfrom a blackbody [4], and will only be covered briefly.The spectral irradiance of a blackbody is given byPlanck’s equation,

Iðλ; ϵ;TÞ ¼ C1

λ5ϵðλÞ

eC2=λT− 1

; ð1Þ

where C1 ¼ 37413 � 10−20 ½W=m2�, C2 ¼ 14388 � 10−6½mK�. The intensity also depends on the wavelengthλ, temperature T, and emissivity ϵðλÞ. In theory,Eq. (1) can be used to measure the temperature onlyif the absolute intensity can be measured and theemissivity, being a function of material and wave-length [17], is sufficiently characterized. The formerrestriction is eliminated by measuring the light in-tensity at two or more wavelengths:

Ri;j ¼σiIðλi; ϵðλiÞ;TÞσjIðλj; ϵðλjÞ;TÞ

¼ σiλiϵðλiÞσjλjϵðλjÞ

eC1T

�1λj−

1λi

�; ð2Þ

where the added parameters σi;j represent the opticalefficiencies and detector sensitivities of the realsystem and can be obtained by calibrating the instru-ment using a blackbody source at a known tempera-ture. Equation (2) has been further simplified bymaking use of the fact that the exponential in Eq. (1)is usually large compared to 1. Because the emissiv-ities of the products of an energetic reaction, espe-cially those products that result in the solid phaseresponsible for the blackbody signature, are not wellknown, the gray body assumption [17,18] (ϵ is inde-pendent of wavelength) is used to eliminate the ratioof emissivity in Eq. (2).

The factors that affect the choice of wavelengthsare the expected temperatures of the event, thespectral sensitivities of the camera, and the presenceof any gas phase spectral emission. For this instru-ment, the target temperature is between 1000 and4000K. A plot of the blackbody radiant flux den-sities (flux) as a function of temperature for 700 and900nm are shown in Fig. 3. The intensity favors thelonger wavelengths at these temperatures, with thepeak blackbody emission shifting below 900nm forT > 3000K. Because the silicon detector’s quantumefficiency decreases above 900nm [19], 700 and900nm were chosen, as this allowed for reasonableintensity while still providing sufficient temperaturesensitivity.

3. Apparatus

The frame rate and spatial resolution are trade-offs that are limited by the geometry of the experi-ment as well as the capabilities of the camera. The

Fig. 1. (Color online) Simulated temperature distribution imagedby the integrating pyrometer (blue curve). The integrating pyrom-eter’s calculated temperature is 15% higher than the meantemperature.

Fig. 2. (Color online) Calculated temperature and mean error asa function of width of the temperature distribution σ. The meantemperature remains constant at 1500K.

6268 APPLIED OPTICS / Vol. 50, No. 33 / 20 November 2011

instrument used in this study consisted of twohigh-speed Vision Research Inc. Phantom v7.3mcameras. The two-camera configuration provideshigher throughput than other single camera designsbecause the two cameras can record data simulta-neously. The two-camera solution also simplifiedthe optical design, providing more versatility in ap-plication. A schematic [Fig. 4] and a photo [Fig. 5] ofthe instrument are shown. The light from the eventis collected through a common optical assemblyand directed to each camera with a beam splitter.Between the beam splitter and each camera is abandpass filter at 700 or 900nm (�2nm) centerwavelength and 10nm (�2nm) FWHM. The opticalcollection portion of the instrument consists of asingle negative lens coupled into a commercial off-the-shelf (COTS) zoom lens. For a standard f-mountlens the specification distance between the backflange of the lens and the focal plane is d ¼ 46:5mm.In order to accommodate the beam splitter and fil-ters, the COTS lens had to be placed approximately2–3 × d from the camera’s sensor plane. The subse-quent increase in effective magnification [20] madeimaging impractical. The negative lens upstreamof the COTS lens serves to extend the effective backfocal length, allowing for the imaging of a variety ofevents at distances ranging from 1 to 5m or more.

To combat parallax errors, the two cameras shareequivalent optical paths. However, significant caremust be taken to correct for image registration ef-fects. These effects are minimized using a hardwareand software approach. Camera one, in Figs. 4 and 5,is fixed and is used to adjust the front optics for FOVand instrument focus. While camera two has four de-grees of freedom (DOFs) built into the mount. Twotranslational and one rotational DOF perpendicularto the optical path provide the ability to register theimages from the two cameras. The last DOF, parallelto the optical path, provides for focusing differentialbetween the two cameras.

The camera control and data acquisition softwareis proprietary and has limited ability to finely tunethe registration. An analysis software suite wasdeveloped in-house to perform the needed image re-gistration, instrument calibration, and gray bodytemperature calculations. The software functionalmap, written in MATLAB, is outlined in Fig. 6. Foreach movie analysis, a temperature and an image

500 1000 1500 2000 2500 3000 3500 4000

Temperature(K)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Bla

ckbo

dy r

adia

nce

( W/m

2sr

−1 )

× 1013

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Inte

nsit

yra

tio

ratio

Fig. 3. (Color online) Spectral intensity of a blackbody at 700nm(blue curve) and 900nm (red curve) and the ratio of the intensities(green curve) as a function of temperature.

Camera 2

Cam

era

1

Negative lens

Standard camera lens

700 nm900 nm

Bandpass filter:

beamsplitter

Fig. 4. (Color online) Schematic of the two-camera pyrometer.

C

A

BD

E

F

Fig. 5. (Color online) Photograph of the two-camera pyrometer.A, camera 1; B, camera 2; C, interference filters; D, beam splitter;E, off-the-shelf commercial SLR lens; F, negative lens.

Blackbodyimages

Imagecorrections

Calculateσ1/σ2

Temperaturecalibration

Imagecalibration

MovieAnalysis

Registration

imageregistrationparameters

Pyrometermovies

imagecorrection

registration

Calculatetemperature

maps

Fig. 6. (Color online) Data analysis software flow chart. Dataanalysis is broken down into three parts: temperature calibration,image calibration, and movie analysis.

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calibration are required. For determining the sensi-tivity ratio (σ1=σ2), a blackbody (Omega BB-4A) set at1256K is imaged by the instrument. A backgroundvalue is obtained by averaging the intensity of pixelsnot associated with the image of the blackbody. Thisnumber is then subtracted from the average pixelvalues included in the blackbody image. The ratio ofthe measured blackbody intensity is compared tothat expected from theory, resulting in the requiredcalibration factor (σ1=σ2).

Although the pixel registration is first performedin the hardware setup, fine adjustments can be per-formed in the software suite. For this, an image fromeach camera is obtained having at least two pointlight sources or other features that can be used to cal-culate the translation, rotation, and magnificationcorrection factors. To correct for dark current errors,images are recorded with a lens cap covering thelens and then subtracted from the data. This may bedone through the camera control software or duringanalysis. The parameters from the image and tem-perature calibrations are then used to calculate thetemperature of the event.

4. Results

The two-camera pyrometer has been used to imagethe fireball produced by exploded TNT charges.The charge was an 815 g right circular cylinder witha 165 g Pentolite booster charge positioned at the top.An RP-83 detonator was used to initiate the detona-tion. The cameras’ frame rate was 10,000 frames persecond with an exposure time of 97 μs. Spatial tem-perature maps of the fireball are shown in Fig. 7.If one of the cameras’ signal is saturated or belowthe background value, then the temperature is notcalculated, and that pixel is displayed as white. Inthe first frame, light from the central region of thefireball saturates the 700nm channel camera, anda temperature cannot be calculated. From frame twowe observe that the top region and perimeter of the

fireball has started to cool, while the center remainswarmer. In the last frame the fireball has cooled andis relatively homogeneous.

5. Conclusions

We have developed a two-camera pyrometer usingoff-the-shelf high-speed digital cameras. The mono-chrome cameras are filtered at discreet wavelengthsso that a temperature can be calculated by using theratio method. Currently the instrument is limited byusing a combination of a standard SLR with a nega-tive lens. Further work is being done to develop cus-tom optics to improve the speed and resolution of theinstrument. Additional improvements could be madeby using imaging sensors whose response extendsinto the near IR, such as indium gallium arsenide de-vices. The downside of such sensors currently is theirslow response times compared with silicon devices.

This research was supported in part by an appoint-ment to the U.S. Army Research Laboratory Postdoc-toral Fellowship Program administered by the OakRidge Associated Universities through a contractwith the U.S. Army Research Laboratory. This workwas also supported in part by the Strategic Environ-mental Research and Development Program(SERDP) of the Department of Defense (DoD) andthe Defense Threat Reduction Agency (DTRA).

References1. M. Katsuki, Y. Mizutani, and Y. Matsumoto, “An improved

thermocouple technique for measurement of fluctuating tem-peratures in flames,” Combust. Flame 67, 27–36 (1987).

2. A. Ballantyne and J. B. Moss, “Fine wire thermocouplemeasurements of fluctuating temperature (in turbulent diffu-sion flames),” Combust. Sci. Technol. 17, 63–72 (1977).

3. P. A. Kinze, Thermocouple Temperature Measurement (Wiley,1973).

4. T. R. Harrison, Radiation Pyrometry and its UnderlyingPrinciples of Radiant Heat Transfer (Wiley, 1960).

5. H. J. Kostkowski and R. D. Lee, “Theory and methods ofoptical pyrometry,” NBS Monograph 41 (U.S. Department ofCommerce, National Bureau of Standards, 1962).

6. M. D. Tarasov, A. I. Tolshmyakov, F. O. Kuznetsov, O. N.Petrushin, V. S. Petushkov, Yu. A. Savel'Ev, M. Y. Tarakanov,and V. A. Til'Kunov, “Detonation and shock wave fronttemperature measurement by two-wave pyrometer based onoptical fibers,” Proc. SPIE 2869, 894–899 (1997).

7. K. L. McNesby and B. E. Homan, “Real-time optical mea-surements for improved understanding of enhanced blastmaterials,” ARL-TR-3483 (U.S. Army Research Laboratory,2005).

8. C. Meola and G. M. Carlomagno, “Infrared thermographyof impact-drive thermal effects,” Appl. Phys. 96, 759–762(2009).

9. J. W. Ahn, R. Maingi, D. Mastrovito, and A. L. Roquemore,“High speed infrared camera diagnostics for heat flux mea-surement in NSTX,” Rev. Sci. Instrum. 81, 023501 (2010).

10. D. F. Simmons, C. M. Fortang, and D. B. Holtkamp, “Usingmultispectral imaging to measure temperature profiles andemissivity of large thermionic dispenser cathodes,” Rev. Sci.Instrum. 76, 044901 (2005).

11. S. P. Mates, D. Basak, F. S. Biancaniello, S. D. Ridder, andJ. Geist, “Calibration of a two-color imaging pyrometer and its

Fig. 7. (Color online) Spatial temperature map of an explodedTNT charge. A frame rate of 10,000 fps was used record theimages. The white area inside the perimeter of the fireball at t ¼0:1 and 0:2ms is due to the saturated camera signal. This area isexcluded from the temperature calculation.

6270 APPLIED OPTICS / Vol. 50, No. 33 / 20 November 2011

use for practical measurements in controlled air plasma sprayexperiments,” J. Therm. Spray Technol. 11, 195–205 (2002).

12. R. Parker and R. Allor, “Imaging pyrometer for monitoringthe surface temperature of a spray formed steel billet,” Proc.SPIE 4360, 80–89 (2001).

13. J. E. Craig, D. Lee, R. A. Parker, F. S. Biancaniello, andS. D. Ridder, “Temperature imaging measurements with atwo wavelength imaging pyrometer,” in Proceedings of the18th ASM Heat Treating Society Conference, H. W. Waltonand R. A. Wallis, eds. (ASM International, 1999), pp. 84–92.

14. M. B. Kaplinsky, J. Li, N. J. McCaffrey, V. Patel, E. S. H. Hou,N. M. Ravindra, C. N. Manikopoulos, and W. F. Kosonocky,“Recent advances in the development of a multiwavelengthimaging pyrometer,” Opt. Eng. 36, 3176–3187 (1997).

15. M.F.Hopkins, “Multi spectralmeasurement usinganovel highspeed imaging pyrometer,” Proc. SPIE 3173, 91–98 (1997).

16. J. M. Densmore, M. M. Biss, K. L. McNesby, and B. E. Homan,“High speed digital color imaging pyrometry,” Appl. Opt. 50,2659–2665 (2011).

17. D. Ng and G. Fralick, “Use of a multiwavelength pyrometerin several elevated temperature aerospace applications,”Rev. Sci. Instrum. 72, 1522–1530 (2001).

18. F. Goulay, P. E. Schrader, and H. A. Michelsen, “Effect ofthe wavelength dependence of the emissivity on inferredsoot temperatures measured by spectrally resolved laser-induced incandescence,” Appl. Phys. B 100, 655–663(2010).

19. Vision Research, “v7.3 spectral response,” http://www.visionresearch.com/uploads/Docs/SpectralResponse/V7.3 SensorSpectral Response.pdf.

20. A. Hornberg, ed., Handbook of Machine Vision (Wiley-VCH, 2006).

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