The three-dimensional (3-D) shape measurement using defocused Ronchi grating is advantageous for the high contrast of fringe. This paper presents a method for measuring spatially isolated objects using defocused binary patterns. Two Ronchi grating with horizontal position difference of one-third of a period and an encoded pattern are adopted. The phase distribution of fringe pattern is obtained by Fourier analysis method. The measurement depth and range is enlarged because the third harmonic component and background illumination is eliminated with proposed method. The fringe order is identified by the encoded pattern. Three gray levels are used and the pattern is converted to binary image with error diffusion algorithm. The tolerance of encoded pattern is large. It is suited for defocused optical system. We also present a measurement system with a modified DLP projector and a high-speed camera. The 3-D surface acquisition speed of 60 frames per second (fps), with resolution of 640 × 480 points and that of 120 fps, with resolution of 320 × 240 points are archived. If the control logic of DMD was modified and a camera with higher speed was employed, the measurement speed would reach thousands fps. This makes it possible to analyze dynamic objects.
A method for high-speed measurement of the three-dimensional (3D) shape of spatially
isolated objects is proposed. Two sinusoidal fringe patterns with phase difference
π
and an encoded pattern are used to measure the 3D shape. A modified Fourier transform
profilometry (FTP) method is used for phase retrieval and obtaining high-quality texture.
The measurable slope of the height variation is larger than for methods based on
traditional FTP and the same as that for methods based on phase measurement
profilometry (PMP). The number of patterns is less than for the high-speed methods
based on PMP, using which isolated objects can be measured. Consequently,
this approach is less sensitive to object motion. In the proposed method, the
encoded pattern consists of vertical stripes with width the same as the period of the
sinusoidal fringe. Three gray levels are used to form the stripes. Six symbols are
encoded with these three gray levels. Then, a pseudorandom sequence is constructed
with an alphabet of these six symbols. The stripes are arranged according to the
sequence to form the pattern. In the procedure of phase unwrapping, the strings
(subsequences) are constructed with symbols corresponding to three neighbor periods of
the deformed fringe. The position of the subsequence is worked out by string
matching in the pseudorandom sequence. The ranking number of the fringe is
identified and then the absolute phase of the deformed fringe is obtained. The 3D
shape of the objects is reconstructed with triangulation. A system consisting of a
specially designed digital light processing projector and a high-speed camera is
presented. The 3D capture speed of 60 frames per second (fps), with a resolution of
640 × 480
points, and that of 120 fps, with a resolution of
320 × 240
points, were achieved. Preliminary experimental results are given. If the control logic of the
digital micromirror device was modified and a camera with higher speed was employed, the
measurement speed would reach thousands of fps. This makes it possible to analyze
dynamic objects.
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