During recent years, the use of fringe projection techniques for generating three-dimensional (3D) surface information has become one of the most active research areas in optical metrology. Its applications range from measuring the 3D shape of MEMS components to the measurement of flatness of large panels (2.5 m × .45 m). The technique has found various applications in diverse fields: biomedical applications such as 3D intra-oral dental measurements [1], non-invasive 3D imaging and monitoring of vascular wall deformations [2], human body shape measurement for shape guided radiotherapy treatment [3, 4] , lower back deformation measurement [5] , detection and monitoring of scoliosis [6] , inspection of wounds [7, 8] and skin topography measurement for use in cosmetology [9, 10, 11]; industrial and scientific applications such as characterization of MEMS components [12, 13] , vibration analysis [14, 15] , refractometry [16] , global measurement of free surface deformations [17, 18] , local wall thickness measurement of forced sheet metals [19] , corrosion analysis [20, 21] , measurement of surface roughness [22, 23] , reverse engineering [24, 25, 26] , quality control of printed circuit board manufacturing [27, 28, 29] and heat-flow visualization [30] ; kinematics applications such as measuring the shape and position of a moving object/creature [31, 32] and the study of kinematical parameters of dragonfly in free flight [33, 34] ; biometric identification applications such as 3D face reconstruction for the development of robust face recognition systems [35, 36] ; cultural heritage and preservation [37, 38, 39] etc. One of the outstanding features of some of the fringe projection techniques is their ability to provide high-resolution, whole-field 3D reconstruction of objects in a non-contact manner at video frame rates. This feature has backed the technique to pervade new areas of applications such as security systems, gaming and virtual reality. To gain insights into the series of contributions that have helped in unfolding the technique to acquire this feature, the reader is referred to the review articles in this special issue by Song Zhang, and Xianyu Su et al. A typical fringe projection profilometry system is shown in Fig 1. It consists of a projection unit, an image acquisition unit and a processing/analysis unit. Measurement of shape through fringe projection techniques involves (1) projecting a structured pattern (usually a sinusoidal fringe pattern) onto the object surface, (2) recording the image of the fringe pattern that is phase modulated by the object height distribution, (3) calculating the phase modulation by analyzing the image with one of the fringe analysis techniques (such as Fourier transform Figure 1 : Fringe projection profilometry system method, phase stepping and spatial phase detection methodsmost of them generate wrapped phase distribution) (4) using a suitable phase unwrapping algorithm to get continuous phase distribution which is proportional to the object height variations, and finally (5) calibrating the system for mapping the unwrapped phase distribution to real world 3-D co-ordinates. Fig. 2 shows the flowchart that depicts different steps involved in the measurement of height distribution of an object using the fringe projection technique and the role of each step. A pictorial representation of the same with more details is shown in Fig. 3 . During the last three decades, fringe projection techniques have developed tremendously due to the contribution of large number of researchers and the developments can be broadly categorized as follows: design or structure of the pattern used for projection [40, 41, 42, 43, 44, 45, 46, 47, 48, 49] , method of generating and projecting the patterns [50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62] , study of errors caused by the equipment used and proposing possible corrections [63, 64, 65, 66] , developing new fringe analysis methods to extract underlying phase distribution
One outstanding feature of this fringe projection method is that it can provide highresolution, whole-field 3D reconstruction of objects REF .
Fringe Projection Techniques : Whither we are ?
ICPR (4)
Physics