The polarizing optical behaviors and the stress-optical behaviors of optically heterogeneous fiber reinforced composites were studied. It was discovered that the light intensity vs azimuth curves (I-θ curves) consist of no more than three main variables, the zero, 2nd, and 4th variable, and can be represented in terms of these three variables with 5ma11 error. The more important point of this paper is the correlation of the 4th variable of I∥-θ curves(// here stands for the testing condition of the plane polariscopes with dark field) with stress. It was discovered that the value of the 4th modulus changes sinusoidally with the change in principal stress difference, (σ1-σ2∥), and the initial phase angle of the 4th variable coincides with the principal birefringent direction. Based on these experimental results, an equivalent birefringence theory was proposed as a substitution for conventional birefringence in the heterogeneous field.