Consideration of Internal Board Camera Optics for Triaxial Testing Applications

The application of small board cameras, located within a triaxial cell to determine radial and axial strain, was investigated. Specifically, charge-coupled device (CCD) sensors were utilized in conjunction with precision pinhole apertures to capture images from within the triaxial cell. The cameras were fully immersed in electronics-grade silicone oil and were able to withstand cell pressures that are common to triaxial testing (up to 1034 kPa (150 psi)). The small size of the cameras allowed for implementation within the triaxial cell, thereby avoiding: (1) the cumbersome corrections that are required to account for refraction at the confining fluid–cell wall–air interfaces and magnification due to cell wall curvature, and (2) the amount of space required for outside-of-the-cell monitoring systems that utilize cameras. The final design of the cameras was based on an iterative testing process in which various types of small board cameras, lenses, and finally pinhole apertures were investigated. The advantages of the lensless pinhole aperture camera design included: (1) lack of optical aberrations, such as those encountered in traditional lensed camera systems, (2) practically infinite depth of field, allowing for sharp, close-up images, and (3) wide-angle field of view without the distortions that are associated with the use of wide-angle lenses. As discussed herein, the pinhole cameras were optimized for optical resolution and light entry to minimize the effect of diffraction patterns that are commonly associated with pinhole apertures. The resolution of the cameras was determined to be sufficient for the potential application of the cameras (volume measurements). The instrumentation presented herein provides a novel alternative to the state-of-the-art outside-of-the-cell photogrammetric instrumentation that is currently employed to monitor soil specimens during triaxial tests.