Improved method for isochromatic demodulation by RGB calibration

Abstract

The red-blue-green ͑RGB͒ calibration technique consists in constructing an a priori calibration table of the isochromatic retardation versus the triplet of RGB values obtained with a RGB CCD camera. In this way a lookup table ͑LUT͒ is built in which the entry is the corresponding RGB triplet and the output is the given retardation. This calibration ͑a radiometric quantity͒ depends on the geometric and chromatic parameters of the setup. Once the calibration is performed, the isochromatic retardation at a given point of the sample is computed as the one that minimizes the Euclidean distance between the measured RGB triplet and the triplets stored in the LUT. We present an enhanced RGB calibration algorithm for isochromatic fringe pattern demodulation. We have improved the standard demodulation algorithm used in RGB calibration by changing the Euclidean cost function to a regularized one in which the fidelity term corresponds to the Euclidean distance between RGB triplets; the regularizing term forces piecewise continuity for the isochromatic retardation. Additionally we have implemented a selective search in the RGB calibration LUT. We have tested the algorithm with simulated as well as real photoelastic data with good results.

Review of RGB photoelasticity

“…7; the line representing the colours is wound as a skein (inward) with increasing retardation, as noted in [59].…”

“…The light source that allows to obtain the greater modulation of the RGB signals is a tricolour source, that is, a source whose spectrum is composed of only three wavelengths [57,58]; a very good modulation can be also obtained using commercially available fluorescent lamps with a discrete emission spectrum [41,59,60]. This aspect is detailed in Section 5.2.…”

“…The number of triplets in the LUT can be increased using numerical techniques of interpolation [59,61,62]. In order to minimize the effect of electronic noise and non-uniformities of the light field in determination of the LUT, digital filtering techniques, such as median filtering, are also applied [61].…”

“…use of a light source with three colours (narrow band) [57]; use of a white light source and three narrow band filters (R, G and B) [58]; use of fluorescent lamps with a discrete emission spectrum, [41,59], with such lamps the possibility of determining 20 fringe orders [59] or at least 12 fringe orders [60] is cited; use of a scanner with the ability to determine fringe orders up to five [71].…”

RGB photoelasticity applied to the analysis of membrane residual stress in glass

“…In order to obtain the best results using RGB photoelasticity, a fluorescent white light source with a discrete emission spectrum should be used [14,17]. It is also important that the light source is uniform and has a high intensity in order to minimize the opening of the diaphragm of the optical system.…”