where p i j k ℓ ℓ is the symmetric part of the photoelastic tensor (the photoelastic strain tensor), and s k ℓ ℓ is the linear strain. The antisymmetric part of P i j k ℓ ℓ is known as the roto-optic tensor. From either definition, it is clear that deformations to the body may induce optical anisotropy, which can cause an otherwise optically isotropic material to exhibit birefringence. Although the symmetric photoelastic tensor is most commonly defined with respect to mechanical strain, it is also possible to express photoelasticity in terms of the mechanical stress.