Size metrology of a pure phase object based on the laser probe beam reshaping

•Detection and measurement of the size of a transparent phase object.•The phase object reshapes the probe laser beam.•The experimental method does not need numerical calculations.

We propose a simple method for determining the size of a transparent object based on the reshaping of a laser probe beam which is initially Gaussian in shape. For a given ratio between the sizes of the incident beam and of the phase object, the diffracted laser probe beam is transformed in the far-field region into a hollow beam. The detection of the intensity dip in the beam centre is made with a simple photodiode. The potential of the proposed technique for quantifying the nanoscale change of the phase object could be very useful to study the dynamics of a living cell membrane for instance.