Shape measurement of microscopic structures using digital holograms

An optical system based on digital holographic interferometry for shape measurement of microscopic objects will be described. Two separate lensless digital holograms of an object under test ate recorded on a CCD faceplate. Between the first and the second recording, the illumination beam is tilted. The phases of the two wavefronts are obtained from the complex amplitudes of the digitally reconstructed wavefronts. with the shape calculated from the phase difference. For the reconstruction, a digital 'quasi Fourier' hologram is simulated. With the CCD faceplate close to the object, recording with high numerical aperture (NA), and thus a short depth of field, is achieved. Digital focusing of the object is necessary for larger depths of field. Experimental results are presented for microscopic objects with small and large depths of field. (C) 1999 Published by Elsevier Science B.V. All rights reserved.