FRESNEL Quick Guide

Example 7. Zernike Phase Contrast Visualization Method

If in the optical scheme that is given in section Spatial Frequency Filtering to replace the mask by a quarter-wave retarding plate one will get a scheme demonstrating Zernike phase contrast method. It makes possible the observation of phase objects otherwise invisible.
If you load zernike.scm you will see the Scheme Window filled with elements that are required to model the operation of the Zernike method.

First three elements after Source are used to compose phase object representing the rotated letter "F". To see it: 1) Move Target to the position after R3. 2) Double click Source, choose Load from file | zernike.pls. 3) Run.
When calculations are over you will see nothing except for the zernike.pls intensity distribution. But if you press button (wave front retrieval) in the Beam Widow, you will get phase object in false colors.

Phase object

Now you can return to the initial scheme and start Run again. After finishing simulations you will see the picture labeled below as "Positive contrast" (constructive interference takes place). In our case this is for positive retardance sign of the quarter-wave plate (scheme element "Lmb/4"). Open the element dialog box and change retardance sign to negative (-0.25). When doing so, notice that the retarder is very small. It has Gaussian profile with the size 510-3 cm.
When running scheme with negative retardance sign of the quarter-wave plate you will get the picture labeled below as "Negative contrast" (destructive interference takes place).
If you move the retarding plate to the position behind the Target and run scheme again you will see only the intensity distribution of the input beam. Pressing button in the Beam Widow, you can get image of the phase object in false colors.

Positive contrast  Negative contrast
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