The brewster (B) is a non-SI unit used to measure the susceptibility of a material to photoelasticity, or the value of the stress-optic coefficient of the material.
The unit has dimensions reciprocal to those of stress.
square metres per newton (m2/N or 1/Pa) or
square centimetres per dyne (cm2/dyn).
[1] The unit is named after David Brewster, who discovered stress-induced birefringence in 1816.