Integrated photoelasticity in a soft material: phase retardation, azimuthal angle, and stress-optic coefficient

•Azimuthal angle which corresponds to stress direction is successfully measured and verified•Despite large deformation of material, the experimental and analytical results reasonably agree•Stress-optic coefficient of gelatin used is measured to be 3.12×10−8 1/Pa•For soft materials, integrated photoelasticity is found to be a key optical technique to measure 3D stress fields