Tech Briefs

Using high-speed camera pyrometers to measure and map fireball/shock expansion velocities.

An effort has been made within the US Army Research Laboratory (ARL) to extract quantitative information on explosive performance from high-speed imaging of explosions. Explosive fireball surface temperatures are measured using imaging pyrometry (2-color 2-camera imaging pyrometer; full-color single-camera imaging pyrometer). Framing cameras are synchronized with pulsed laser illumination to measure fireball/shock expansion velocities, enabling calculation of peak air-shock pressures. Multicamera filtering at different wavelengths enables visualization of light emission by some reactant species participating in energy release during an explosion. Measurement of incident and reflected shock velocities is used to calculate shock energy on a target.

A schematic of the ARL imaging pyrometer rig as employed for temperature measurements of exploding spheres of the explosive formulation C-4.

Results of these measurements are used to construct maps of temperature, pressure, reactant species, and shock energy on a target. This information is valuable to evaluate explosive performance, models of performance, and barriers designed to enhance protection and survivability. These techniques and instruments were developed, in part, to improve productivity by lowering testing costs, allowing a single event to yield temperature, pressure, chemical species, and performance data.

Pyrometry is the method of estimating temperature of incandescent bodies from standoff, or noncontact methods. In this project, time-resolved temperature maps of detonations of explosives are made using a full-color single-camera pyrometer where wavelength resolution is achieved using the Bayer-type mask covering the sensor chip and a 2-color imaging pyrometer employing 2 monochrome cameras filtered at wavelengths of 700 and 900 nm, respectively, (wavelength regions adjustable). Each rig operates on the assumption of gray body behavior, but each is specific to the type of explosive being investigated.

For many CHNO-based explosives (e.g., TNT [C7H5N3O6], the formulation C-4 [92% RDX, C3H6N6O6]), hot detonation products are mainly soot and permanent gases, presenting an approximation of a gray body emitter. For these systems, the single-camera rig may be appropriate. For metalized explosives, narrow-band light emission from gas phase molecular and atomic species (e.g., AlO near 484 nm, BO 2 near 560 nm, and K near 760 nm) necessitates the use of the 2-camera rig to make measurements in spectral regions free of discrete features.

Additionally, strong C2 or CH emission from nonsooting explosive fireballs may present a significant source of error. For each measurement approach, it is mandatory to measure a time-resolved emission spectrum during the event to ensure the absence of discrete emission in the spectral window used for temperature measurement.

For each pyrometer rig, framing speeds are 20,000–40,000 frames per second (fps) at a resolution of approximately 400 × 500 pixels with an exposure per frame of one to tens of microseconds. Each system is temperature calibrated using a standard blackbody source (Omega Engineering), and checked for accuracy using an air/acetylene diffusion flame.

A schematic of the ARL imaging pyrometer rig (showing the 2-camera rig and the full-color rig as described) is shown, as employed for temperature measurements of exploding 227-gram (g) spheres of the explosive formulation C-4. Also shown in this figure are a spectrograph capable of measuring time-resolved emission spectra and a 3- color spatially integrating pyrometer used as a check of the imaging devices. The 3-color spatially integrating pyrometer (700, 850, and 1,000 nm; 10-nm bandpass) can provide submicrosecond time resolution but is biased toward measuring the hottest portion of an emitting medium because of the T4 dependence of intensity.

This work was done by Kevin L. McNesby, Matthew M. Biss, Barrie E. Homan, Richard A. Benjamin, Vincent M. Boyle Sr, and John M. Densmore of the Army Research Laboratory. ARL-0199

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Imaging Detonations of Explosives (reference ARL-0199) is currently available for download from the TSP library.

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