Measurement of surface topography by RGB Shadow-Moiré with direct phase demodulation

Abstract

In this paper we present the application of a direct demodulation method for the measurement of surface topography by means of Shadow-Moire´. In our set-up, we use three LEDs (with green, red and blue peak wavelengths) to illuminate the grating. Due to the different position of these light sources, a polychromatic Shadow-Moire´fringe pattern is produced, which can be described as the superposition of three monochromatic (red, green and blue) fringe patterns. Taking the image of this polychromatic fringe pattern with a RGB CCD camera, we get a monochromatic fringe pattern stored at each RGB channel of the CCD. The direct demodulation algorithm employed uses these fringe patterns to calculate the wrapped phase map. After unwrapping the phase map using a standard multi-grid technique, we implemented an automatic procedure to detect the area of interest of the phase map by removing low modulation zones and to calculate the absolute value of the phase. In this way it is possible to determine the topography of a surface with a single RGB snapshot maintaining a simple experimental set-up, which is an important feature, especially for the study of dynamic

Review of RGB photoelasticity

“…After the introduction of RGB photoelasticity, RGB applications in the fields of differential interferometry [47] and moiré-contouring [48][49][50] have been developed. The method called Colourimetry-based retardation measurement (CBRM) is similar to RGB photoelasticity.…”

Multiplicative phase-shifting interferometry using optical flow

“…In the past there have been reported works about multiplicative PSI [4,5]. References [4,5] present techniques for demodulating multiplicative phaseshifting interferograms obtained from a photoelastic sample in a load-stepping experiment [4] and from a surface topography measurement by means of an RGB shadow moiré technique [5].…”

“…References [4,5] present techniques for demodulating multiplicative phaseshifting interferograms obtained from a photoelastic sample in a load-stepping experiment [4] and from a surface topography measurement by means of an RGB shadow moiré technique [5]. In both cases, the method is based on determining the sign of the quadrature signal I Q defined as I Q x; y − sinαΦx; y, where α is the multiplicative phase shift between two patterns.…”

“…Finally, the modulating phase can be obtained from the spiral phase transform (SPT) [7]. Note that this method is not affected by the monotonicity restriction as the method used in [4,5] and it is also robust against noise because its regularizating character imposes continuity in the reconstructed phase. However, it is important to mention that the proposed method requires the phase difference between successive interferograms not to be higher than π rad.…”

Optical 3D Deformation Measurement Utilizing Non-planar Surface for the Development of an “Intelligent Tire”

“…This technique uses the phenomenon that apparent strain, when calculated from the raw images, varies according to the distance from the camera. The method does not require laser equipment or patterned light but a single camera; thus it reduces the installation costs compared with conventional optical 3D measurement methods (14)(15)(16) . Moreover the method only mounts the CCD image sensor on the rims; thus it is superior in terms of high maintainability.…”