This paper describes a measuring system which automatically selects and identifies angularly deflected electromagnetic rays of a pencil beam exploring a dispersive medium. The schlieren deflection is dependent on the refractivity gradient transverse to the ray path. This yields fairly localized measurement of material inhomogeneities. Applied for the first time to a tokamak plasma, this schlieren method yields fast time and good spatial resolution measurement of the particle density distribution. Without any need for data inversion this method also delivers monitor signals of the center‐core peak density and flattening of the parabolic particle density distribution. Schlieren measurements of magnetohydrodynamic particle density perturbation give an insight into plasma transport and collapse mechanisms.