Internal displacement measurements based on color fluorescent speckle pattern by multispectral stereo-DIC and refractive index correction

Abstract

Fluorescent digital image correlation (DIC) is becoming popular for measuring 3D profiles and deformations in external surfaces. However, the simultaneous monitoring of interior layers is highly challenging due to the penetrability and refraction of light using monochromatic fluorescence. We propose a color fluorescent speckle pattern (CFSP) method for measuring the internal displacement of transparent objects based on multispectral stereo-DIC and refractive index correction. During sample fabrication, fluorescent speckle patterns exciting different colors are fabricated on both the surface and interior layers of objects. A virtual color stereo-DIC system is utilized to capture the CFSP on the surface and interior layers simultaneously from two different perspectives. Different color channels are practically equivalent to synchronized monochrome vision systems, having separate CFSP in external and internal measurements. In multispectral stereo-DIC calculation, the external surface is initially reconstructed through one channel of the system even if the surface is non-planar. Based on Snell’s law and the CFSP method, the internal layer is then reconstructed and corrected by establishing the geometry of the refractive stereo-DIC through another channel. The relative error of the thickness between two planar layers was proved to decrease from 33.4% to 0.7% after refractive index correction. Further experimental results validate the efficacy of this method for correcting the profile of the non-planar arc profile and determining the internal deformations of disc materials.

Measurement of transparent profile using multispectral 3D digital image correlation

“…A 3CCD color camera (JAI, AP-3200T) with a resolution of 2048×1536 pixels in each R\G\B channel, a UV excitation light source and an optical path auxiliary formed of a reflective prism and two planar mirrors were carefully installed to equivalently work as a stereovision 3CCD DIC system. Two different CFSPs (blue or red) on the front and rear surfaces of the specimen were simultaneously imaged to the left and right parts of the R or B sensor [12].…”

“…R1v is the virtual point of the true point R1 without any refractive distortion correction. More details can be find in our previous work [12] . By establishing a curved surface refraction model (Fig.…”

“…Su [11]et al established a plane refraction model using removable markers on the front surface, which is a flexible post-processing method but it only suits for a surface with known analytical equation. We [12] used color fluorescent speckle pattern (CFSP) to measure multiple layers where the reference front surface could be unknown.…”

Three-Dimensional Shape and Deformation Measurements Based on Fringe Projection Profilometry and Fluorescent Digital Image Correlation via a 3 Charge Coupled Device Camera

“…The fluorescent DIC method was originally introduced by Berfield [ 6 ] as a means of achieving nano-scale deformation measurements. In contrast to ordinary speckles, fluorescent speckles [ 7 , 8 , 9 ] possess several unique advantages, including exceptional contrast, elimination of specular reflectance, and the ability to measure multiple surfaces simultaneously. It is important to note that fluorescent speckles are invisible under white light illumination, necessitating their illumination with ultraviolet (UV) lamps of specific wavelengths, which then allows for the capture of light intensity by the camera sensor.…”