An Optimized Stereo Digital Image Correlation Setup Based on Parfocal Zoom Lenses for Full-Field Measurements at 0.5–2× Magnification

Stereo-digital image correlation (DIC) measurements can be challenging when working with small specimens. Achieving high-precision data requires careful selection of hardware, stereo-rig design, illumination, speckling, calibration, and minimization of noise levels.

This study presents an optimized stereo-DIC setup based on parfocal zoom lenses for full-field measurements at magnifications ranging from 0.5× (14.3 × 16.9 mm²) to 2× (3.5 × 4.2 mm²).

The advantages of using parfocal zoom lenses over fixed-focal-length lenses (and extension tubes) for full-field measurements at small fields of view (FOVs) are demonstrated through quantitative comparisons of the temporal evolution of pseudo-strains and null strain analysis from rigid body translation experiments. The optimal speckling parameters for each magnification are determined by analyzing gray-level histograms, mean intensity gradient ( \(\overline{\nabla \text{G}}\) $\overline{\mathrm{\nabla }\text{G}}$ ), and subset size. The challenges of calibration at high magnifications are discussed, along with strategies for obtaining acceptable results.

The accuracy of the presented stereo-DIC setup is evaluated through the study of localized phase transformation on a 1 mm diameter superelastic NiTi wire under tension, column buckling, and compression deformations. The presented setup provides highly consistent full-field data over the 0.5–2× magnification range.

The results highlight the benefits of using parfocal zoom lenses for stereo-DIC measurements over a range of small FOVs.