multilayer material photoelasticity measurement

Hi,

i'm in an internship, and i want to study the stress destribtuion with photoelasticity method, but my problem is that the material that i have is not homogenious but there ares two layers. I can do the measurement for the two material separated, but i dont know how to do that for my layared material.

 anyone have an idea about how to do photoelasticity measurement for multilayere materials ?

Thanks.