Low-coherence optical profilometry with multi-reflection reference mirrors for large-depth object measurement.

A new, to the best of our knowledge, optical configuration for digital holographic profilometry for surface profile measurement of large-depth objects is proposed. Two multi-reflection mirrors were employed to extend the maximum axial measurable range by a factor of 2 without any degradation of the spatial resolution. By adjusting the distance and the position of the two multi-reflection reference mirrors, the system can be made more flexible for measuring different parts of the object. In addition to the axial extension, the two-mirror system also increases the visibility of the interference fringes so that the object profile can be measured with high accuracy.

Area-coding method in frequency comb profilometry fused with optical interferometry for measuring centimeter-depth objects with nanometer accuracy.

Area coding masks in a frequency comb profilometer (FCP) based on a single-pixel imaging architecture are introduced for measuring a practical metal object that has weaker reflection than a specular object does. In such a case, it is important to increase the intensity of the encoded object light on the photodetector area because a photodiode operated at a high frequency of more than 1 GHz is generally small. The area-coding masks can concentrate more light on the focal point compared with random-coding masks that are commonly used. The increased intensity also increases the number of pixels in the FCP, and consequently accurate matching is achieved between the data obtained by optical interferometry and the FCP data. It was demonstrated that the introduction of area-coding masks increased the detected light intensity and allowed us to measure a practical metal object with 16 times more sampling points.