A quantitative comparison of an over driven light emitting diode (LED) and a high intensity discharge lamp (HID) as illumination sources for high-speed schlieren imaging is presented. A custom pulser circuit utilizing a new and improved driver circuit was developed to overdrive the LED by a factor of ten while simultaneously reducing pulse widths to sub-microsecond durations. The LED system has been developed as a simple and inexpensive alternative light source to discharge lamps and pulsed laser systems, which are typical for high-speed schlieren imaging. Image quality of a decaying spherical shock wave, produced from the unsteady release of an under-expanded helium jet, is analyzed to assess comparative performance. The effects of framing rate, camera exposure time, and pulse duration on image quality were assessed and compared for the novel LED and a high intensity discharge lamp. Framing rates of 10,000 fps and 50,000 fps and exposure times of 1 µs and 10 µs were tested. Image quality was assessed qualitatively through side-by-side comparisons of fluid dynamic features such as the resolution of shock waves, compression waves, and shear layers. Quantitative analysis was performed through the comparison of the signal-to-noise ratio at the various conditions. LED performance was found to be superior when imaging fast events and inferior when imaging slower events. Results and potential system improvements indicate that the LED system is ideal for low-cost, high-speed flow imaging. Keywords schlieren imaging-light emitting diode (LED)-high intensity discharge (HID) lamp-shock propagation 1 Introduction Recent advances in digital camera technology have allowed for the visualization of high-speed, unsteady flows with high spatial and temporal resolution at a reasonable cost. This has enabled new experimental work to be performed on applications related to explosions and hypersonic aerodynamics using high-speed schlieren photography. For example, Mizukaki et al. (2013) performed background oriented schlieren (BOS) measurements of an open-air detonation of a C-4 explosive [1]. The optical displacement of natural obstacles in the background such as trees and grass were used to extract the shock wave overpressure during the explosion. In these experiments, a Phantom v10 (Vision Research Inc., NJ) camera, providing 800 x 600 pixels at 10,000 fps, was used. In an explosion chamber, Ciccarelli et al., (2013) used schlieren photography to study the flame acceleration process responsible for deflagration-to-detonation transition (DDT) [2]. A FASTCAM SA5 (Photron, CA) camera was used, which provides 1024 x 744 pixels at 10,000 fps. Saravanan et al., (2011) used a high-speed schlieren system in a shock tunnel to study the hypersonic flow around a missile-shaped body [3]. A Phantom v7.2 (Vision Research Inc., NJ) camera was used to provide 450 x 450 pixels at a framing rate of 10,000 fps.
