Off-axis tilt elimination in digital holographic microscopy by self-hologram rotation: applications in surface metrology

Off-axis tilt in digital holographic microscopy is an obstacle to perfect measurement. The tilt is induced by the off-axis angle geometry between the object wave and the reference wave of the interferometer. This study uses a compensation method based on self-hologram rotation to remove the off-axis tilt in the reconstructed phase-contrast image. The Twyman-Green interferometer is employed with a microscope objective to investigate two objects: the first is a micro-lens array and the second is a step height. A hologram of the object is captured and then reconstructed to extract the wrapped phase map. The same hologram is rotated 180o and then reconstructed to extract the wrapped phase map. The wrapped phase maps extracted from the original hologram and its rotation 180o are unwrapped by the graph cuts algorithm and subtracted to produce a continuous phase-contrast image free from the off-axis tilt. The windowed Fourier filtering (WFF) method is applied to reduce the speckle noise in the continuous phase-contrast image to the slightest measure. Experimental results show that the off-axis tilt in the reconstructed phase-contrast image is completely removed. Moreover, the WFF’s application significantly enhances the measurement profile.