Photoelatic images: intensity analysis
The fringe pattern, which is due to internal varying stress structure of particles, can be calculated using theory of elasticity. The contact forces can be obtained using this pattern at the points of contacts. In many similar experiments, spatial derivative of image intensity is used to retrieve local stresses. This method requires high resolution imaging of the media of order of 100 pixels per particles diameter in order to distinguish fringes. High resolution imaging is currently possible only by using quasi-static imaging. However, studying fast dynamics such as impact, rheology, or stick-slip requires high speed imaging and limits the resolution. Consequently, the fringes are not clear enough to be used for calculation and the noise may dominate the calculation. As an alternative, image intensity can be used to quantify the internal stress.