1.1 ,This test method covers measurements of direction ofprincipal strains, ε,₁ and ε,₂, and the photoelastic retardation, δ,, using a compensator, for the purpose of analyzing strains in transparent or translucent plastic materials. This test method can be used to measure birefringence and to determine the difference of principal strains or normal strains when the principal directions do not change substantially within the light path.