Shape measurement of microscopic structures using digital holograms

An optical system based on digital holographic interferometry for shape mea surement of microscopic objects will be described. Two separate lensless di gital holograms of an object under test ate recorded on a CCD faceplate. Be tween the first and the second recording, the illumination beam is tilted. The phases of the two wavefronts are obtained from the complex amplitudes o f the digitally reconstructed wavefronts. with the shape calculated from th e phase difference. For the reconstruction, a digital 'quasi Fourier' holog ram is simulated. With the CCD faceplate close to the object, recording wit h high numerical aperture (NA), and thus a short depth of field, is achieve d. Digital focusing of the object is necessary for larger depths of field. Experimental results are presented for microscopic objects with small and l arge depths of field. (C) 1999 Published by Elsevier Science B.V. All right s reserved.