Bidirectional Schlieren imaging using dual-LED illumination and a lens aperture as a knife-edge

In this Letter, we demonstrate that employing the Lens Aperture as a Knife-Edge (LAKE) with two identical LEDs has the distinct advantage of obtaining Schlieren images along two cutoff orientations. It is possible to control LED brightness to obtain a desirable weighted sum of both horizontal and vertical Schlieren cutoff images on each camera frame. Powering the LED sources by an affordable and programmable Arduino board enables a rapid switch between the horizontal and vertical Schlieren cutoffs that is exactly synced to the camera frames, resulting in a crosstalk-free Schlieren image sequence. Furthermore, with a little postprocessing, this monochrome image series can be easily transformed into a colorful sequence of bidirectional Schlieren images.