Another unique and extremely powerful feature of PicoTwist�s magetic tweezers apparatus is its ability to measure the Z position of the bead of interest in real-time. To achieve this goal, the microscope is illuminated with a parallel light beam emitted by a high-brightness LED. Under these conditions, the bead image is decorated with a series of diffraction rings whose shape and number vary regularly as the image is progressively defocussed. Using a lookup table of the bead�s diffraction pattern, the acquisition software has been designed to evaluate the bead z position from the defocussed image of the diffraction rings. LED illumination provides a very stable, easily adjustable light source whose spatial coherence is sufficient to obtain a sharp ring pattern and which is sufficiently low power to avoid spurious interference effects.

The image of the bead is obtained through a fast CCD camera coupled to the microscope. The standard device is proposed with a 768 x 576 pixels 60 Hz camera, we can also provide it with a 1380 x 1040 pixels 31 Hz camera.

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