Basic characteristics of visualization process of transparent objects are analyzed and calculated when phase-contrast
technique with photothermal Zernike cell is employed. Edge enhancement effect is observed under the conditions, when
sizes of illuminating beam and investigated object are comparable. It is demonstrated that the visualization process has a
resolution close to the diffraction one. Numerical results of the visualization process in a scheme with photothermal cell
are compared to the results obtained in the same schemes with Zernike cell based on local Kerr nonlinearity and with
traditional linear Zernike plate.