Research On Optical Imaging And Color Measurement For Laser Rainbow Holographic Printed Matter

Mechanical vision inspection technology is one of the most active hot spots in today’s inspection research. Recently, in printing industry, using machine vision and image processing technology to inspect print quality has gradually become a trend. Owing to the special optical properties of surface gratings of laser rainbow holographic printed matter, the imaging by white light illumination can generate rainbow-colored interference fringes, which increase the difficulty of color measurement of holographic printed matter. With the trend that holographic technique is widely used in printing industry, the problem of imaging and color measurement for laser rainbow holographic printed matter needs to be solved. Based on the optical properties of the surface diffraction grating on laser rainbow holographic printed matter, this paper designed an optical imaging system, then used the properties of color and the mapping relation between color spaces effectively solved the problem of imaging and color measurement for laser rainbow holographic printed matter.First, according to the diffraction properties of surface grating of laser rainbow holographic printed matter, a special light source-dome light which is special for holographic printed matter is designed to inhibit the diffraction effect well on laser rainbow holographic printings and improve the imaging quality of holographic. And, a line scanning charge coupled device (CCD) imaging system is constructed based on the dome light. The image quality of laser rainbow holographic printed matter which captured under the imaging system increased greatly.Secondly, according to the characteristic that the color of any objects can correctly describe by its reflectance spectra, this paper proposed a color measurement method for holographic printed matter. This method constructed a6-channel line scanning CCD imaging system, then obtained6-channel spectral response curve by imaging holographic printed matter under the imaging system. Based on the spectral response data, principal component analysis technique is used to reconstruct object’s color and reflectance spectra. The experiments show that the proposed method of color measurement has the characteristics of high-precision in spectral reconstruction, and is effective to solve the problem of color measurement of laser rainbow holographic printed matter.Thirdly, this paper focused on the transformation of color space, and a large number of comparing experiments with real data are done. Then, based on the experiment results, distribution pattern of chromatic aberration is studied, and a segmentation method for neural network training is proposed. The measuring accuracy of this method with3-channel CCD imaging system and the measuring accuracy of color reconstruction by reflectance spectra with6-channel CCD optical imaging system are on the same grade. This method need only a batch of typical color mark to build neural network model at the early time of system debugging stage, and has high-precision of color reconstruction and lower requirement of imaging system, which can fit the requirement of online color measurement in the industrial environment.