Contrast reversal and enhancement of phase spot array/grid with an intensity-dependent refractive index medium as phase filter

Observation of transparent objects by the method of nonlinear phase contrast

“…In the general case for low-contrast observed objects, any selective manipulations in the spectral region lead to the change of the structure of their image, and in the case of transparent objects lead to their visualization. The possible use of nonlinear-optic processes to produce the necessary phase shift in phase contrast systems was demonstrated [8][9][10][11][12][13][14][15] . For realization of this process, one can use media with any type of cubic nonlinearity.…”