A schlieren apparatus capable of recording index of refraction gradients in self-luminous discharges such as arcs, sparks, and inductively coupled plasmas is described. Resolution of better than 50 μm is obtained over a clear, bright, uniform field. The geometric design of the system cancels coma, and small off-axis angles are used to keep astigmatism low.
S. E. Mathews, Thomas M. Niemczyk, and John P. Walters, "A Schlieren System for Time and Space Resolved Photography of Self-Luminous Electrical Discharges," Appl. Spectrosc. 34, 200-206 (1980)