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Photoelastic techniques are used to make both qualitative and quantitative measurements of the forces within idealized granular materials. The method is based on placing a birefringent granular material between a pair of polarizing filters, so that each region of the material rotates the polarization of light according to the amount of local stress. In this review paper, we summarize the past work using the technique, describe the optics underlying the technique, and illustrate how it can be used to quantitatively determine the vector contact forces between particles in a 2D granular system. We provide a description of software resources available to perform this task, as well as key techniques and resources for building an experimental apparatus. [ABSTRACT FROM AUTHOR]

An introduction to articles published within the issue is presented on topics including imaging granular particles with electromagnetic waves, the advantages of X-ray tomography, and the measurement of particle displacements and velocities.

A correction to the article "Focus on imaging methods in granular physics," by Axelle Amon and colleagues published in the July 10, 2017 online issue is presented.