1、Basis of Analysis

Due to the uneven stress distribution in the glass during its tempering process, it shows obvious anisotropic properties.  The existence of uneven stress will cause double refraction.  This phenomenon can be observed under polariscope.  Apply polarized light on a piece of tempered glass.  When the two beams of light from one spot interfere with the light from another spot, you will observe the change of colors and brightness in different areas on the surface. It's known as stress mark. Tempered glass always has stress marks.

When the visible light band of the natural light applied on tempered glass is reflected in a certain angle, part of the light gets polarized.  Part of the light transmitted through the glass is also polarized. Both reflected and refracted light are partially polarized.  According to Brewster’s Law, the reflected light is polarized only when the light of incidence is at a specific angle or Brewster’s angle. The stress marks are the most visible when the angle of incidence is at approximately 56 degree.

2、Process of Analysis

Put the glass to be tested under certain light condition.  Observe at an incident angle of 56 degree or with polarized glasses to see irregular bands or spots of stress marks on the glass surface.  Regular glass doesn’t have this phenomenon.

3、Applicability and Limitations

(1)Applicability: Observing stress mark is a non-destructive testing method.  It has a wide range of practical applications.

(2)Limitations: In the absence of polarizer or polarized glasses, the intensity of the light depends on the weather and the incident angle of natural light, making it difficult to see the marks.