Using lens aperture as a knife-edge in schlieren imaging

A knife-edge is recognized as a critical component of a schlieren imaging system. This knife-edge serves as a cutoff element, which is required to achieve high sensitivity in schlieren imaging. This paper describes a method for transferring the role of the cutoff from an external discrete knife-edge component to the camera lens’s internal aperture. Thus, our approach completely removes an external knife-edge from the schlieren imaging system while maintaining the same sensitivity. We demonstrate this Lens Aperture as a Knife-Edge (LAKE) schlieren imaging in a single-mirror, near-coincident geometry that uses a reduced number of optomechanical elements. We show flow visualization results using LAKE schlieren that demonstrate easy switchability between schlieren versus shadowgraph imaging and horizontal versus vertical schlieren cutoff.