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We present a digital holographic method to increase height range measurement with a reduced phase ambiguity using a dual-directional illumination. Small changes in the angle of incident illumination introduce phase differences between the recorded complex fields. We decrease relative phase difference between the recorded complex fields

Noninvasive characterization of surface shape and deformation is very important in many industrial applications such as on-line quality control and reverse engineering [

In this study, we combined a reduced phase dual-directional illumination and analysis of speckle displacement methods to be used together for a large stepped sample without phase wrapping in the reconstructed data. A combination of two methods can make a robust system since the analysis is not only based on phase differences but is also based on speckle displacements.

For measurement of the shape we take the difference between the reconstructed phases which are recorded before and after a small tilt

Schematic of simplified set-up for dual-direction illumination shape measurement (a). The measured coin edge (b). The squared area has been recorded and used for measurement.

The sensitivity of the set-up (height range measurement) for having unwrapped phase can be then calculated as

Therefore the shape of the object will be measured by

We choose a 2 cent Euro coin to measure the groove depth of its edge. A laser beam with the wavelength of 532.35 nm was used to illuminate the test object. The interference pattern was recorded by a high resolution CCD camera (Vistek, Eco655) with 2448×2050 pixels of the size 3.45×3.45

Reduced phase distribution as a result of small change in the angle of incident. Scaling (phase reduction) factor is

Using

A vertical cross section over the measured shape of the groove in coin edge.

We have also used speckle displacements to measure the height (depth), independent of phase information which leads to a solution without being in a need for unwrapped phase information; see [

Figure

The speckle displacement map 10 mm behind the object, on the object plane, and 10 mm in front of the object in (a), (b), and (c), respectively.

Depth measurement using speckle displacement calculations. (a) is the height profile and (b) is an inverted cross section over a horizontal line.

We have shown that, by introducing a small change on the angle of incident, it is possible to scale the phase difference between two recorded complex fields. Therefore, adjustment of phase reduction factor will give a possibility to keep the phase difference in the range of

The data used to support the findings of this study are available from the corresponding author upon request.

The author declares that there are no conflicts of interest regarding the publication of this paper.

The research was performed as part of the employment of the author at Luleå University of Technology, during his visiting research at Universität Stuttgart. The work is also presented as an abstract in 2018 Progress in Electromagnetics Research Symposium (PIERS-Toyama). The author would also like to thank Dr. Alok Kumar Singh from the Institute für Technische Optik, Universität Stuttgart, for valuable discussions.