Camera Captures Explosives in Detail

A frame from a temperature movie of a fireball produced by a 1000 g TNT charge encased in a boron/polymer matrix (Source: AIP)

A frame from a temperature movie of a fireball produced by a 1000 g TNT charge encased in a boron/polymer matrix (Source: AIP)

To study explosives, sans explosives, new techniques involving high-speed, high-fidelity imaging with optical filtering and signal pro­cessing techniques have recently made setting off explosives and capturing the data in real-time a reasonable alter­native to developing a new simu­lation.

“Advances in high speed imaging, especially the recent availa­bility of extremely fast cameras and light sources – approaching hundreds of kHz illu­mination and imaging rates at near megapixel image sizes – have brought experimental imaging closer to the resolution achievable with simu­lations,” said Kevin L. McNesby a Research Chemist at the US Army Research Labo­ratory in Aberdeen, Maryland. The advances in image capturing allow the researchers to lower costs for obtaining infor­mation about explosive behavior by capturing multiple variables like pressure, temperature and chemical species maps for each shot, rather than a single point measurement. This allows them to run one explosion, rather than several.

The researchers’ method of information gathering involves pyrometry, a technique for estimating tempe­rature of incandescent bodies based upon their spectra of emitted thermal radiation. Their setup, which is specific to the type of explosive being investigated, employs a two-color imaging pyrometer, which consists of two mono­chrome cameras filtered at 700 nanometers and 900 nanometers, and a full-color single pyrometer that achieves wavelength resolution with a Bayer-type mask covering the sensor chip. For each of their rigs, described in full in the paper, the framing speeds are 20,000 – 40,000 frames per second, at a resolution of approximately 400×500 pixels with an exposure per frame of one to tens of microseconds.

The pyrometers are also able to capture the air shock structure of the detonation event, allowing for simultaneous measure­ment of tempera­ture and pressure. Infor­mation regarding the chemical species is similarly captured via measuring the emission spectrum of each targeted molecule. Their setup allows them to obtain a spatial resolution for a one-kilogram explosive charge down to the one-milli­meter scale. However, these mapping tech­niques result in wider error bars than those of legacy point measure­ment techniques, an issue McNesby and his colleagues hope to improve on. Future work for the researchers will also include installing a full upgrade of their imaging rig, which will result in a tenfold increase in speed at full resolution. (Source: AIP)

Reference: K. L. McNesby et al.: Quantitative imaging of explosions with high-speed cameras, Rev. Sci. Instrum. 87, 051301 (2016), DOI: 10.1063/1.4949520

Link: Army Research Laboratory, Aberdeen Proving Ground, Aberdeen, Maryland, USA

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