When it comes to explosives, it is difficult to study them without actual explosives (or explosions) on hand. Therefore the U.S. Army Research Laboratory has developed new techniques that involve high-speed, high-fidelity imaging with optical filtering and signal processing.
These new techniques have recently made setting off explosives and capturing the data in real-time a sensible 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 U.S. Army Research Laboratory in Aberdeen, Maryland.
These image-capturing advances reduce the costs associated with obtaining information about explosive behavior by capturing multiple variables: pressure, temperature and chemical species maps, for each shot, instead of a single point measurement. This allows them to run one explosion instead of many.
How do they obtain the information needed?
The researchers' method involves pyrometry, a technique used to estimate the temperature of incandescent bodies based on their spectra of emitted thermal radiation. Their setup is specific to the type of explosive being investigated and employs a two-color imaging pyrometer consisting of two monochrome cameras filtered at 700 nanometers and 900 nanometers, as well as a full-color single pyrometer that achieves wavelength resolution with a mask covering the sensor chip.
Each of the cameras set up employs speeds that are 20,000 to 40,000 frames per second, at a resolution of approximately 400 X 500 pixels with an exposure per frame of one to tens of microseconds.
The pyrometers can also capture the air shock structure of the detonation event, which means the researchers can record simultaneous measurements of temperature and pressure. Information about chemical type is captured in a similar way by measuring the emission spectrum of each targeted molecule. This new setup allows researchers to obtain a spatial resolution for a 35-ounce explosive charge down to the one-millimeter scale.
However beneficial these techniques are, they do result in wider error margins than those of older measurement techniques, which is why McNesby and his colleagues now need to improve on it. Their future work will include installing a full upgrade of their imaging rig, resulting in a tenfold increase in speed at full resolution.