## 1 Introduction  
Measuring the 3-D profile of an object surface has become increasingly important in industrial automation, particularly in in-situ product inspection. 3-D surface measurement could be generally classified into two major categories: tactile and non-contact measuring conditions. Coordinate Measuring Machine (CMM) is a general system using both contact and non-contact techniques to measure 3-D profiles. Tactile sensing cannot accurately measure soft object or thin-film surfaces as the probe may scratch or deform the measured surface. In contrast, non-contact techniques can effectively measure surfaces with high spatial resolution and are useful for measuring soft or flexible objects. For non-contact measurement, optical triangulation techniques, such as stereo vision, laser scanning, and structured light techniques, are widely used in industries. However, optical techniques often require uniform surface light reflection to ensure reliable sensing. The absence of such condition would pose difficulty for using optical triangulation techniques to measure a surface with both diffuse and specular reflectivities. One of the significant challenges in optical surface measurement lies in dealing with reflectivity variations of the object surfaces to be tested, such as a semiconductor wafer surface being engraved or marked by laser etching. Measuring objects with high reflectivity variations, such as highly shiny or scattering surfaces, is a critical issue to ensuring measurement accuracy and product quality assurance under automated optical inspection (AOI). Poor 3-D profiling results are common when encountering high reflectance variances on the wafer surface to be tested or reconstructed for 3-D printing or other purposes. One of the popular 3-D optical surface measurement techniques is structured light projection, which is widely employed in industrial testing applications. The techniques basically work according to the triangulation measurement principle with the assumption of uniform light reflectivity on the tested object for ensuring capture of the deformed fringe reflecting from the tested surface. However, in reality, such assumption is not applicable in face of high reflectivity variations on the surface. For example, most industrial IC chips are fabricated using a silicon wafer substrate with an extremely shiny surface and are also embedded with metal bumps having a scattering surface reflectance. Under such condition, the reflected lights from the tested surface are rather complex and emitting in different directions. Consequently, the detecting sensor cannot receive accurate phase information for reconstructing surface morphology. Structured fringe projection is a full-field optical technique for measuring 3-D object surfaces. Owing to its advantages of fast speed, high accuracy, being non-destructive, and full-field testing, fringe projection technique (FPP) has become one of the most promising 3-D surface profiling techniques in 3-D AOI. For obtaining the object shape, the common 3-D measurement techniques include mainly phase shifting profilometry (PSP) [1] and Fourier transform profilometry (FTP) [2]. In general, these techniques project structured light patterns, also called fringes, onto the object surface. Owing to optical path difference (OPD) between inspecting beams, the fringe is deformed according to the 3-D shape of the object surface. To reconstruct the 3-D shape of the object surface, one phase map is computed using a wrapping algorithm, and object height can be extracted using the phase unwrapping algorithm [3] and phase-to-height transformation. PSP is widely employed in industrial inspection because of its promising accuracy and high spatial resolution [4]. It uses a time-resolved wrapping strategy to extract phase information for reconstruction of object profiles. Different from PSP, FTP records distorted fringe images and analyzes object height information using phase modulation and demodulation to acquire a phase map from distorted fringes.