A new three-wavelength chromatic-corrected hybrid phase shifting method for single-camera digital photoelasticity

Abstract

Digital photoelasticity is a non-contact inspection technique, that requires new strategies to unwrap the stresses map based on color images. Therefore, this paper presents a new three-wavelength chromatic-corrected hybrid phase-shift method for single-camera digital photoelasticity applications. For the experiments, the intensities of isochromatic images are simulated considering a birefringent sample compressed and a circular polariscope configured to produce a bright field image that avoids the effects caused by isoclinics. To increase the range of the fringe order, three LEDs with peaks of close wavelengths were used. Additionality, for each LED a RGB color image is simulated. The red channel of each image is used to generate a new synthetic chromatic-corrected image (CCI) thereby: the red channel of λ3 is the red channel of the new CCI, and the green and blue channels of the CCI use the respective red channels of λ2 and λ1; additionally, the wavelengths must satisfy the following condition (λ3 > λ2 >λ1). An inverted image of the CCI is computed. Thus, with the CCI and its inverted image, six images are stored, and with these images and some trigonometric relations proposed by Ekman and Nurse a wrapped phase map is extracted. Finally, an unwrapping algorithm is applied to reconstruct the stresses map. The results show that the method improves the detected maximum order and reduces stress map distortions compared to similar color phase shifting approaches. Furthermore, since the algorithm requires only a camera and a circular polariscope setup, it can be implemented in dynamic experimental applications.