Inspection system for a straight pipe inner surface by using multiwavelength digital holography

 A pipe or a cylinder is used in many places such as a power plant, semiconductor factory, and a part of machine. It is highly important to maintain and inspect pipes for their safely operating. Several methods such as an eddy current method, an ultrasonic inspection method, a light sectioning method have been used for the pipe inspection. However, the methods cannot achieve both shape measurement and visualization of the pipe inner surface at the same time. To obtain both surface profile and visual information of inner surface, we have proposed multiwavelength digital holographic method.

 Figure 1 shows the experimental configuration for a pipe inspection. One red laser diode with wavelength of 639 nm and two DPSS lasers having different center wavelength of 532 nm and 473 nm are used as multiwavelength light source. For the inner surface profilometry of a pipe, digital holographic contouring using two-wavelength method has been used. To generate a color image of pipe inner surface, two DPSS lasers having different wavelengths are used. To illuminate the pipe inner surface, the cone-shaped mirror (CSM) attached to a metal rod is used. The beam reflected by the CSM illuminates the inner wall of the pipe. Then, the reflected light from the inner surface is reflected back to a CCD plane as the object beam.

Fig. 1 Experimental setup.

 In the experimental arrangement, the lateral information of an inner surface is transferred into the circular image of the CSM. To obtain the three-dimensional shape of the inner wall, the circular image is transformed into the rectangular one using the simple image processing. Figures. 2 and 3 show the height profile of inner wall.


Fig. 2 The original circular distribution of height profile.

Fig. 3 The rectangular distribution transformed from Fig. 2.

All transformed images are combined to represent the developed image of the pipe inner wall. Figure. 4 show the height profile and color image of pipe inner wall.

Fig. 4 Combined images for the development of inner wall of the copper pipe:
(a) color image of the inner wall generated from three reconstructed intensity images of R, G, B and (b) height profile of the inner wall.