By photoelasticity principles, a diffused light or lens type polariscope, generates the data of principle stress difference at various points on the model. But when these values of shear stress are not sufficient for our purpose and when we desire to have separate values of principle stresses at a point, we adopt some more means of separating these stresses.

Shear difference method, oblique incidence method, electric analogy method are various methods used for this purpose

Shear difference method is a theoretical mathematical method of separating the stresses. But this method is very tedious and time consuming and often needs a computer for analysis of complete model. Electric analogy method, which works on laplace equation, needs very costly equipment. Oblique Incidence method is a method of separating the principle stresses in which one more relation between two principle stresses is obtained by directing the light incident on model on oblique angle.

An Equipment –

This is a simple device to be used as on accessory along with a polariscope. It consists of a pair of prisms fitted in two separately rotable prism holders. These prisms change the direction of light incident on it. A prism on light box side deflects the light in 450 A second prism makes this oblique ray of light, parallel to original ray of light. So the model kept between two prisms, receives a light ray at an angle, which is 450 in this case.
A scale reads the angular position of these prisms. A linkage fabricated from square bars is provided to adjust the X , Y and Z co-ordinates of prism so as to reach at the point of interest.