A pre-correction method for oblique projection of grating fringes in a phase measuring profilometry system

In a phase measuring profilometry system, while the camera optical axis is perpendicular to the reference plane and the projector is installed obliquely, the sinusoidal characteristic of the grating fringes is lost (i.e., non-uniform pitch and amplitude), thus affecting the measurement accuracy. In this paper, a universal correction method for grating fringes is proposed, in the case where the projector optical axis is not perpendicular to the reference plane. This method includes pose correction (i.e., uniform pitch) and light intensity correction (i.e., uniform amplitude) of grating fringes. The pose transformation model, which is used to convert the pixel coordinates from the projector coordinates system into the camera coordinates system, is established by projecting a single image. Next, the digital grating fringes pattern with pre-corrected pose can be obtained, according to the established pose transformation model and the projector resolution. Regarding the light intensity correction, after capturing a projected blank pattern with pre-corrected pose, the light intensity correction coefficients on the reference plane are quantified by iteration method, and they can be used to correct the light intensity of grating fringes. The experimental results show that the presented method can effectively realize the sinusoidal correction of grating fringes.