80 H. Tang et al. Fig. 10.1 (a) Holographic setup for shape measurement. (b) Timeline of the event for shape measurement accurate shape measurement on the semi-transparent TM surface. Accuracy of the new method is estimated by a measure of a stepwise gauge provided by the National Institute of Standards and Technology (NIST). We successfully applied the new shape measurement method on a fresh postmortem human TM without any surface preparation. 10.2 Methods Digital holography applies optical sensors (cameras) to record the optical path difference of the reference beam and object beam from a coherent light source. It can reconstruct the optical wavefield (amplitude and phase) scattered by the measured object. If a controlled change of illumination angledθ is introduced to the holographic setup shown in Fig. 10.1a, the contour of the sample’s shape (in thez-direction) has a relationship with the wavelengthλand the illumination angleθ as follows [7]: dP = 2π λ sinθz dθ, (10.1) where dPis the optical phase difference due to the change of the illumination angle. To increase the robustness of the measurement and avoid spatial phase unwrapping errors, we introduced a continuous change of the illumination angle and capture serials of high-speed images at discrete timings during the change. The timelines of changing the illumination and camera capture are shown in Fig. 10.1b. Each black line represents one serial of 200 frames of high-speed image acquisition at 67200 frame per second, and the blue line is the illumination angle. Because each serial of high-speed imaging is much faster (0.03 s) than the motion of the illumination change (1 s), the θ is assumed to be constant during the individual serial of high-speed image acquisition. Temporal phase unwrapping is performed by calculating the cumulative sum of the consecutive optical phase difference to solve the total optical phase dPm difference between the first and last serials of high-speed image acquisitions [7]: dPm = m m=1 2π λ sinθz dθm (10.2) where dPm is the total optical phase difference due to the mstep changes of the illumination angle. 10.3 Results A NIST traceable gauge is used to determine the measurement resolution. An image of the gauge is shown in Fig. 10.2, and the measured area is shown in the zoomed view. The minimum height change of the target is 0.25 mm (1.5–1.25 mm). A 3D mesh plot of the NIST gauge and the measured shape of the gauge along the red line are shown in Fig. 10.3a and b. The shape result suggests the shape measurement can distinguish height difference as small as 0.25 mm and surface jump as large as 1.5 mm. The histogram of the red line in Fig. 10.3b and the standard deviation at each height are shown in Fig. 10.4. The measured results are normally distributed around three significant peaks corresponding to individual steps of the NIST gauge with a maximum standard deviation of 0.044 mm.
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