The internal stress of glass and plastic products has a birefringent characteristic. In other words, if there is stress inside of these types of products, light from he polarized lamp will be divided into different units, which produce optical interference phenomenon(ripple patterns) in turn.
Under white-light lamp, you can see aubergine ripples, which is caused by the presence of stress.
From the observation of colored patterns, you can see stress distribution directly, then evaluate and improve the annealing process.
The inspection of plastic molds is more complex as the colored patterns cased by stress is irregular. A similar situation may occur with glass products, which is called stress stretching. For example, a qualified mold will have a large amount of stress released by tension.
The role of full-wave plate :
When testing glass samples without the full-wave plate, usually only black or gray shadow patterns can be seen. The full-wave plate increases or decreases the full wavelength of a phase difference, thereby forming a colored pattern, which is easier to analyze.