Q2. What are the contributions mentioned in the paper "Fringe projection techniques: whither we are?" ?
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. 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.
Q3. What are some other methods used to generate and project structured patterns onto the objects?
Grating projection systems [53, 54], spatial light modulators [55], diffractive optical elements [56], superluminiscent diode in conjunction with an acousto-optic tunable filter [57], programmable LCDs [45], multi-core optical fibers [58, 59] etc. are among the other methods that are in practice used to generate and project structured patterns onto the objects.
Q4. What is the reason for the lack of a phase-shifting analysis method?
The presence of non-sinusoidal waveforms in the recorded fringe patterns is known to cause significant phase measurement errors (when phase-shifting analysis method is employed) and thereby resulting in non-negligible errors in the measurement of 3D shapes of objects.
Q5. What is the advantage of using a fiber optic interferometer system with the laser input?
Fiber optic interferometer system with the laser diode input enables one to have greater flexibility and compactness of the measurement system [52].
Q6. What is the common method of phase unwrapping?
In the case of dynamic/real-time 3D shape measurement, a stack of 2D wrapped phase maps obtained at different time instants needs to be unwrapped, for which a threedimensional phase unwrapping algorithm is required.
Q7. What is the definition of phase unwrapping?
The process of determining the unknown integral multiple of 2π to be added at each pixel of the wrapped phase map to make it continuous by removing the artificial 2π discontinuities is referred to as phase unwrapping.
Q8. What is the last step in the process of measuring the 3D height distribution using fringe projection technique?
The last important step in the process of measuring the 3D height distribution using fringe projection technique is system calibration.
Q9. What is the simplest way to reconstruct the 3D shape?
color isolation process is the inevitable precursor to the phase-shifting algorithm to effectively reconstruct the 3D shape in the multichannel approach.
Q10. What is the purpose of the phase unwrapping algorithm?
It facilitates the conversion of image coordinates (pixels) to the real-world coordinates and the mapping of unwrapped phase distribution to the absolute height distribution.
Q11. Why is the fringe projection technique used in micro-scale measurements so limited?
It is largely due to the nonlinearity of the projector’s gamma [160] and the nonlinearity of the CCD camera that the recorded fringe patterns are observed to have harmonics (non-sinusoidal waveforms).
Q12. What is the main reason why researchers have not used fringe projection techniques in large scale measurements?
Fringe projection techniques that employ phase-stepping method for the fringe analysis are known to provide highresolution 3D reconstruction with minimal computational investment.
Q13. What are the common methods used for generating the fringe patterns?
Most of the present day automated 3D measurement sys-tems employ the commercially available digital micromirror device (DMD) and liquid crystal display (LCD) projectors for projecting the computer generated fringe patterns onto the object.
Q14. What are the main reasons for the limited use of fringe projection in micro-scale measurements?
The use of multiple projectors in fringe projection profilometry has also been reported to solve some of the baffling problems like the accurate 3D estimation in presence of local shadows, invalid regions and surface isolations [155, 156, 157, 158].
Q15. What is the main reason for the limited use of fringe projection in micro-scale measurements?
it has opened up new avenues for in-situ quality testing of industrial components/products by supporting techniques like inverse projected-fringes [159].
Q16. What is the effect of a fringe pattern on the image?
If the application allows the recording of an additional image of the object without projecting the fringe pattern (see Fig. 4a), then this recorded real-texture information can be mapped onto the estimated 3D shape distribution as shown in Fig. 4d.
Q17. What is the main reason why the fringe analysis technique is not used in large scale measurements?
Spatial fringe analysis methods on the other hand allow the technique to be applicable for real-time 3D measurements but at the cost of resolution.