We present a new method for enhancing images of blood vessels in skin tissues by using tri-band illumination. Transmitted-light vein images captured by a camera contain an image blur due to light scattering in skin. The blur can be described by a point spread function (PSF) that is a function of the thickness of skin layers in front of a vein and the extinction coefficients of skin tissues. The PSFs cannot be directly observed because the depth of a vein in skin tissue is unknown and the thickness of the skin tissues in different parts of the human body vary. Moreover, skin wrinkle patterns are observed as dark lines and need to be eliminated for clear vein imaging. We propose a method for removing image blur and skin wrinkle patterns from transmitted-light images of veins by using tri-band illumination. First, wrinkle patterns are separated from vein patterns by using a difference between the light absorbances of blood at two wavelengths. Subsequently, image blurs caused by light scattering at skin layers are removed by using a PSF estimated from two vein images. The key observations in this work are that at one of the three wavelengths to obtain the vein images the extinction coefficient of skin tissues must be twice as large as that at another of the wavelengths, and that at the third wavelength the extinction coefficient of blood must be smaller than it is at either of the other two wavelengths. This allows us to estimate true vein patterns without knowing the depth of a vein and to eliminate skin wrinkle patterns from a vein image. Our experiments show that our method can separate skin wrinkle patterns form vein patterns and that it reduces blur and improves the contrast of vein images better than a conventional method does. The results indicate that our method will contribute to the development of highly accurate personal authentication technology based on vein patterns.
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