A flexible three-directional shearography module attachable to a high-resolution digital still camera

The spatial resolution of digital still cameras has been evolving very rapidly. Mobile phone cameras up to 200 megapixels are commercially available today at affordable prices. High resolution creates opportunities to explore optical measurement methods involving carrier fringes. This paper presents a modular accessory that can be attached to the objective lens of a high-resolution digital still camera to transform it into a high-quality and flexible shearography system. With this attachable accessory, and no other modifications, it is possible to acquire shearography images simultaneously in three adjustable shearing directions. The accessory contains three circular apertures with dimensions and spatial arrangement designed to result in three carrier fringe systems with distinct angles, which are separable by the Fourier transform. The diameters of the apertures control the size of the speckle on the sensor and the frequency content present in the signal. Additionally, the device contains two wedges that can be freely rotated to produce shearography images with adjustable shearing directions. Several combinations of shearing directions favor the analysis of defects and the quantification of parameters of the measured deformation field. Although high resolution sensors are necessary for high-quality measurements with carrier fringes, the phase difference fringes obtained do not always require such high resolutions. The paper presents and discusses algorithms for reducing image resolution by excluding poorly modulated pixels, which maximizes the quality of the resulting fringe map. Results of controlled experiments performed in the laboratory with the developed module applied to a fixed 50 mm lens and a 24-megapixel Canon EOS 3 camera are presented and discussed. The developed module can be easily adapted to other models of commercial cameras. The principle of this modular device can be the basis for the development of a family of high-quality and affordable shearography systems. The results obtained were phase maps in three distinct displacement directions, with angles of 0º, 45º, and 135º. Additionally, the phase maps exhibited a quality considered satisfactory for analysis.