A geometric model of surface motion measurement by objective speckle imaging

摘要

When a specimen with a diffuse surface is illuminated using coherent laser light, an objective speckle pattern is formed within the adjacent space by the scattered light rays. The proposed geometrical model describes how the objective speckle pattern moves as a conceptual rigid-body within the volume of space adjacent to the illuminated area in response to motions of that area. For small motions, the distance between the illuminated area and the camera sensor is approximately constant. Thus, the locus of all possible parts of the adjacent space that may be sampled by the camera forms the surface of a hemisphere. The speckles within that hemisphere move as a rigid-body, so that a measurement of the speckle motion at one part is representative of the motion of the whole hemisphere. This geometrical approach can be interpreted intuitively, so facilitating understanding of how a specific experimental arrangement and type of surface motion affect observed speckle motion. Despite the relative simplicity of the proposed Speckle Hemisphere Model, the speckle motion sensitivity characteristics that it predicts are identical with the findings from the prior more abstract mathematical approaches. Experimental measurements are made to illustrate example practical applications. In addition, guidance is provided on how to tailor measurement sensitivities for desired types of object motion.