UFR PhITEMPhysics, Engineering, Earth, Environment, Mechanics

Visualization of a quasi-transparent object using spatial filtering. A high-pass filter (small black circular mask) is placed at the center of the object's Fraunhofer diffraction pattern (a tank filled with water into which a drop of glycerol is dropped). The image of the tank is displayed on the screen, with the glycerol clearly visible (against a black background) thanks to filtering. The source used here is a 532 nm double YAG laser, but this experiment can also be carried out using white light (see below).

Phase contrast: visualization of a (transparent) phase object through spatial filtering (same setup as above, but in white light). The top object is a tank filled with water into which a drop of glycerol is dropped. We can see the image of the tank without (left) and with (right) filtering. The phase object is clearly visible against a black background, thanks to high-pass filtering.
Below, a sinusoidal phase grating has been established using an ultrasonic emitter (its horizontal surface can be seen at the top of the image). This emitter generates a standing acoustic wave in the water (pressure modulation, hence refractive index modulation, hence phase modulation for the light wave). Here again, the phase object is made visible by filtering.

Same set-up as above, but using a HeNe laser. These photos show the actual image and diffraction pattern of different objects, with and without filtering. Top images: object = 2 superimposed gratings (1 vertical and 1 horizontal); filter = vertical slit. Only the horizontal grating is transmitted (Abbe experiment). Middle images: object = raster image; the image is detrained by low-pass filtering. Bottom images: object = coarse grid; filter = small black mask (high-pass): only the edges of the object appear bright against a black background (strioscopy).

The same applies to an object with opaque and transparent parts (medal).