Performance evaluation of an overdriven LED for high-speed schlieren imaging

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 over‐drive 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.