Weisberger, J.M., Bathel, B.F. Single source/cutoff grid, self-aligned focusing schlieren system

Abstract

A compact focusing schlieren system is presented that eliminates the need for a separate source and cutoff grid. Light is projected through a grid ruling and onto a background, where it forms an image of the ruling. The light from this projected image is then reflected back onto the original grid ruling, with a polarizing prism imparting a small offset between the two. Translation of this prism along the instrument axis provides an adjustment of sensitivity of the resulting schlieren images, equivalent to the knife-edge insertion percentage of a conventional schlieren system. The manipulation of the polarization state of the light through the system allows the projected and reflected light to be coincident, maintaining a small footprint for the system, which can effectively be mounted to the front of an imaging camera. Both small-scale and large-scale systems are demonstrated, with fields-of-view ranging from tens of millimeters to approximately 500 mm-square. Because retroreflective material is used, the system is ideal for wind tunnel facilities without optical through-access, and it is demonstrated that windows have little effect on the resulting images, with only a slight reduction in image contrast observed when imaging at normal incidence to a window. The ease of construction and alignment of the system, as well as its conventional-schlieren-comparable image quality, indicates that the system could be used as a replacement of conventional systems where a focusing ability is desired.