Diffraction limitations for plasma electron density measurements with schlieren methods

Restrictions imposed by diffraction on schlieren electron density measurements are discussed. Applicability criteria are derived. It is shown that diffraction partly compensates for the advantage of longer wavelengths: while the deflection angles increase with lambda(2), the gain in measuring precision increases only with radicallambda. The criteria are not restricted to a particular spectral range. However, they were derived with special regard to extending schlieren methods to the far infrared (FIR).