Abstract
Fringe projection profilometry is a well-known technique to digitize 3-dimensional (3D) objects and it is widely used in robotic vision and industrial inspection. Probably the single most important problem in single-camera, single-projection profilometry are the shadows and specular reflections generated by the 3D object under analysis. Here a single-camera along with N-fringe-projections is (digital) coherent demodulated in a single-step, solving the shadows and specular reflections problem. Co-phased profilometry coherently phase-demodulates a whole set of N-fringe-pattern perspectives in a single demodulation and unwrapping process. The mathematical theory behind digital co-phasing N-fringe-patterns is mathematically similar to co-phasing a segmented N-mirror telescope.
© 2013 Optical Society of America
1. Introduction
Digital fringe projection 3D profilometry continues to be an active research field as can be seen from the references on this paper [1–9]. In 2010 Gorthi and Rastogi [3] review the major techniques used in 3D shape measurement. Also Wang et al. [4] published practical considerations to take into account when measuring 3D shapes using digital profilometry. Su et al. [5], and Liu et al. [6] explore how to integrate N-camera-projector systems which generates N-fringe-patterns from different positions to obtain a single 3D shape measurement.
In previous non-coherent integration of N-camera-projector profilometry systems, one must first estimate the fringe-boundaries of the N-digitized fringe-patterns [3–7]. This N-times fringe-boundary estimation is followed by N-phase-demodulations, which in turn is followed by N-phase-unwrappings. Doing N-times this boundary, demodulation and unwrapping tasks (for each fringe-pattern) multiplies N-times possible error sources in 3D-shape profilometry [3–7]. Finally we need sum these real-valued, N-unwrapped phases coming from the N-perspectives [3–7]. In contrast co-phase profilometry “blindly” adds-up N-complex signals coming from frequency-shifting the set of N-fringe patterns. In conclusion, previous N-repetition of the whole profilometry processes applied to N-fringe-patterns [3–7], is reduced in co-phased profilometry to a single coherent phase-demodulation and a single unwrapping process of the whole set of N-fringe-patterns.
In this paper we propose to use a single-camera and N-projections directions illuminating the 3D-object. These N-fringe-projections minimize the shadows and specular reflecting zones from the 3D-object. After coherently summing N-analytical-signals, just a single unwrapping process is required..
2. Standard single-camera single-projector profilometry
The typical experimental set-up of single fringe-projection profilometry [1] is shown in Fig. 1.
The linear fringe-pattern projected towards the 3D object may be written as,
Whereis the spatial frequency in radians per pixel. Assuming parallel illumination, the CCD camera sees the following phase-modulated fringe pattern [1] (see Fig. 1),The functionis the background andthe contrast. The phase-modulated fringe-pattern in Eq. (2), has a spatial carrier, and sensitivity [1]; being the polar-angle. The contrasthas information about shadows and specular properties of the 3D object; it drops to zero at shadows and very-high reflections.The spatial (Fourier) phase-demodulation process is given by the following formula [1,2],
The operator is a low-pass filter (gray-disk in Fig. 2) which isolates the required object-phase information at the spectral origin [1,2,8,9].3. Co-phased profilometry: Experimental set-up
For the sake of clarity assume a single camera and 4 collimated projectors at azimuthal phase-step angles, all having the same sensitivity polar-angle (see Fig. 3). Therefore the data consist of the following 4 phase-modulated fringe-patterns,
Note that allremain invariant because the sensitivity polar-angleremains fixed.The 4 projections have the same measuring sensitivity but different azimuthal angles. Note that all contrasts are different due to the shadows and reflections from the 3D objectilluminated form 4 different azimuthal angles.
4. Spatial (Fourier) co-phased demodulation with 4-projector directions
Let us now coherently demodulate our 4-fringe-patterns in Eq. (4). This digital co-phased profilometry algorithm is a generalization of the standard Fourier method, and it is given by
After low-pass filtering one obtains the following co-phased analytical sum,All these complex signals are co-phased and added coherently. At placeswhere a given fringe-contrast is zero (, due to shadows from); their sum may still be much greater than zero, therefore obtaining a well-defined demodulated phase.5. Co-phased profilometry with 4-projection perspectives and 3-temporal phase-steps
Of course we can use the Fourier phase demodulation procedure just seen. But phase-shifting profilometry is more accurate; so next we give some hints and an experiment on how to do it.
To avoid confusion, let us assume just 4-projection perspectives at azimuthal angles, and 3-tempoaral phase-steps; totaling 4x3 = 12 spatial-temporal fringe-patterns,
The vector is the projector position. The co-phased demodulation algorithm is,Some were omitted for clarity. Of course this spatial-temporal algorithm may be generalized to N-projection perspectives, and M-temporal steps.6.- Experimental result with 2 projection perspectives and 4 temporal phase-steps
Here we present an experiment based on just 2-projections and 4-temporal phase-steps. Two projected fringes over the computer-mouseare shown in Fig. 5. Our experimental data is formed by 2 projection-steps at azimuthal-angles, and 4-temporal phase-steps of; totaling 2x4 = 8 spatial-temporal phase-modulated fringe-patterns,
The 8-spatial-temporal carriers are: and(see Fig. 5).The projectors have divergent geometric distortion. Therefore the 2 spatial-carriers and must be estimated first by temporarily removing the object, i.e.) as,
Of course one may also use Fourier phase-demodulation to estimate and, but with temporal phase-shifting interferometry one obtains a higher bandwidth estimation [9].Once the spatial-carriers are estimated, the 3D-mouse is co-phased demodulated as,
The co-phased profilometry of this computer mouse is shown in Fig. 6.In Figs. 6(a) and 6(b) one can see the shadow phase-noise regions where the contrast of the right and left projections drops, , and. Finally Fig. 6(c) shows the unwrapped mouse-phase from our co-phased demodulation algorithm. Figure 6(c) shows the whole mouse without the shadow-noise seen in the two individual projected fringe-patterns in Figs. 5(b), 5(c) (Figs. 6(a), 6(b)). Finally due to the low phase-noise in digital profilometry we used simple line-integration phase-unwrapping to obtain Fig. 6(c).
7. Alternative experimental set-up for co-phase profilometry
Here we give another (we think) more robust, co-phased experimental set-up.
Using the set-up in Fig. 7 one avoids the easy-but-tedious-task of matching all the experimental projector’s distances within millimeter tolerances. In any case, no optical-wavelength tolerances as in co-phasing large segmented telescopes are required.
8. Demodulated phase-noise considerations in co-phase profilometry
Let us rewrite Eq. (6) assuming phase-additive, statistically-independent measuring-noise,
All these phase-noise are independent, so this is the same case (from the phase-noise perspective) as having 4-temporal phase-steps in a single-projector () configuration. In this later case one would obtain,At places with good fringes (in Eq. (12)), the demodulated phase-noise power is reduced by 4 (four) [8,9]. In Eq. (13) we have single-projection and 4 temporal phase-steps, while in Eq. (12) we have 4-projections at azimuthal phase-steps. Therefore Eq. (12) and Eq. (13) are equally noisy at places with no shadows or high reflections, i.e. .9. Conclusions
Here we have presented a single-camera, N-projections co-phased profilometry technique for shape measurement of 3D objects. The set of digitized N-fringe-patterns is coherently demodulated in a single phase-estimation process. The single-step co-phased demodulation of N-fringe-patterns taken from N-perspectives minimizes the shadows and specular reflections from the 3D object being digitized. We have presented 2 alternative experimental set-ups, both easily implemented within millimeter tolerances. We have carried out a simple profilometry experiment based on 2-projection perspectives combined with 4-temporal phase-steps. Using 2 projection directions we were capable of eliminating the shadows and specular reflections from a computer-mouse under analysis. Co-phasing N-fringe-patterns is mathematically similar to co-phasing N-mirrors in a large segmented telescope.
References and links
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