74 J. Heikkinen and G. S. Schajer in conventional imaging caused by perspective change. This gives the opportunity to use this characteristic to compensate for perspective effects. Furthermore, defocus in conventional imaging leads to blur that washes out surface texture, thus complicating the DIC analysis. Laser speckles, on the other hand, retain sharp contrast independent of defocus because the laser speckles have a 3D structure [3]. Therefore, the motivation for this project is to enhance conventional 2D-DIC analysis by using defocused speckle imaging to compensate for perspective changes and to maintain robust feature tracking. 9.2 Basics of Speckle Imaging The speckle imaging method comprises two distinct types that have functional characteristics that can usefully be exploited in different specific applications. When a speckle pattern is captured using a focused camera, each point in the image corresponds to a specific point on the object (Fig. 9.1a). The associated speckle pattern, called “subjective,” behaves as if it were attached to the surface. Subjective Speckle Imaging (SSI) is thus closely related to 2D-DIC and is useful for measuring surface deformation fields on objects that do not naturally possess strong surface texture. In the case of the other variant, Objective Speckle Imaging (OSI), the speckle pattern is recorded by a bare, lensless, digital sensor. Because there is no lens that would focus light onto the sensor, each pixel receives light from all points within the illuminated surface, and any spatial information about the object surface is lost. OSI is thus applicable for measuring mainly rigid-body motions (RBM). However, unlike SSI, OSI can not only indicate surface displacements but can also measure surface rotations [2]. Furthermore, OSI measurement sensitivity increases with increasing recording distance, which makes it attractive for remote measurements. Figure 9.1b shows the image formation principle of a defocused camera. When a conventionally imaged object is shifted further away from the focal plane, the magnification of the resulting image reduces. The image also becomes blurred, as Fig. 9.1 Image formation in a simple optical system. (a) Object in-focus. (b) Defocused object. do, Do =focused object distance, di, Di =focused image distance, f =lens focal length, ΔL=defocus distance, dlens =lens aperture diameter, and dblur =blur circle diameter
RkJQdWJsaXNoZXIy MTMzNzEzMQ==