Pyrometric Imaging System Captures Explosion Parameters

A technique involving high-speed, high-fidelity imaging with optical filtering and signal processing techniques may make setting off explosives and capturing the data in real time a reasonable alternative to developing a theoretical simulations to test explosives.

Research chemist Kevin McNesby and colleagues at the U.S. Army Research Laboratory, Lawrence Livermore National Laboratory, and Los Alamos National Laboratory have developed an optical technique to image explosions in high resolution at 20,000 to 40,000-fps resolutions and costs approaching computer simulations. The researchers cited increasingly fast cameras and light sensors as key enablers of high-resolution experimental imaging.

A horizontal view measuring 2 m across of a 2 kg TNT cylinder being top-detonated. The detonation products expand into air at a rate of several mm/μsec, with a shock at the surface of the products (the explosive near-field). As air drag slows the detonation products, the shock detaches and travels through the ambient air — the explosive mid-field. Courtesy of McNesby/U.S. Army Research Laboratory.

The Imaging systems produces information about explosive behavior by capturing multiple variables during an explosion — pressure, temperature and chemical species maps — rather than a single point measurement, requiring additional shots for each variable.

Information gathering involved pyrometry, a technique for estimating temperature of incandescent bodies based upon their spectra of emitted thermal radiation. The experimental setup, which is specific to the type of explosive being investigated, employed a two-color imaging pyrometer comprising two monochrome cameras filtered at 700 and 900 nm, and a full-color single pyrometer that achieved wavelength resolution with a Bayer-type mask covering the sensor chip.

For each of their rigs, the framing speeds were 20,000 to 40,000 fps at a resolution of approximately 400 × 500 pixels with an exposure per frame of 1 to tens of μs.

The pyrometers were 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 was similarly captured via measuring the emission spectrum of each targeted molecule. The setup allowed a spatial resolution for a 1-kg explosive charge down to the 1-mm scale.

However the researchers said the mapping technique resulted in wider error bars than those of conventional point measurement techniques. Future work will also include upgrading the imaging system for a tenfold increase in speed at full resolution.

The research was published in Review of Scientific Instruments (doi: 10.1063/1.4949520).