A new method is proposed for reading local deformation of granular material by using reflection of light. In this method, two plastic optical fibers, both having diameter of 1mm, are used to read local disturbance or deformation at a point in granular material, such as sand, soil, or broken rocks. As light is sent into the 1st fiber and reaches its end, it leaves the fiber and hits surfaces of nearby particles. Part of the reflected light then gets into the 2nd fiber, which can be digitally measured by using photo diode. The paper introduces the fundamental mechanism of how the system works, shows several examples in which the proposed method is tested to interpret deformation of granular material, passage of water or air bubble, collapse of mass of granular materials, and so on. The results shows interesting relation between local deformation of granular material red by light and overall behavior of the mass made of those particles.

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