Proceedings Article | 15 November 2007
Yanjun Fu, Wendong Zou, Huirong Xiao, Minggang Chai
KEYWORDS: Fourier transforms, 3D metrology, Image processing, Electronic filtering, Modulation, Digital filtering, Charge-coupled devices, Phase shifts, Optical filters, Cameras
In the industry, the three dimension of the object is often measured. But the method is usually the contact measurement.
And the speed is slowly. The measurement is needed both high precision and fast speed, so the non-contact measurement
is required. The grating projecting is the non-contact measurement with prospects. But there are some difficulties in the
method. Firstly, when the object has the steps shape or there are shadows in the grating stripes, the disconnected phase
can't be correctly unwrapped. Secondly, it is very difficulty to realize the real time digital filter. Now the digital filter is
man-machine conversation, so the speed is slowly. Thirdly, in order to measurement the different object, the adaptive
grating is needed.
In order to resolve the above problems, the grating program is created on the computer. The program has many
functions, including the phase shift, the two-frequency grating and the grating frequency is easy to adjust. So the
adaptive grating is realized. The two-frequency grating is programmed by the computer. And it is projected to the
measured object. The measurement object is placed on the exact rotary platform. The deformed grating is collected in the
Charge Coupled Device (CCD). After getting two images, the two images are mosaiced. Then the clear object image
modulated by the grating is got. The problem of the steps shape or there are shadows in the grating stripe is worked out.
Then the fourier transform is used to process the image. In the traditional fourier transform profilometry, the phase is
worked out as follows: After fourier transform, the zero frequency spectra is shifted to the origin of frequency, then filter
the needed signal. Then the needed signal is shifted to the center of frequency, and then the zero frequency is shifted to
both sides. After inverse fourier transform, the imaginary part is getting, so the phase is getting. But it has a difficult in
the above method, because of three times frequency shift, and the center frequency is difficult to confirm, the frequency
shift can't be correct and the filter can't be designed correctly, and the error can be transferred, so the result of filter is
not well, it has bad effect to the later measurement. The result of measurement is also not well. In order to conquer the
difficult, after the fourier transform, filtering the needed signal without frequency shifting, then inverse fourier
transform. So the phase relational with the frequency and coordinate is getting. The phase of the reference surface is
getting by the same method. Then the difference phase is getting. The real difference phase of low frequency is easy to
got, then the real difference phase of high-frequency is work out based on it. At last, according to the relation of
difference phase and the height, the three dimensional profilometry of the object is reconstructed.
An example of step shape object is done. The three dimensional profilometry is reconstructed successfully. It takes 3
second to reconstruct the three dimensional profilometry. The precision is 0.5mm. The result indicates that the method
has conquered the above problems.
The result indicates that the method is simple, with fast speed and high precision. Three dimension profilometry
measurement of the objected that have the step shape or the shadow in the projecting can be successfully resolved.
