Polariscope Strain Viewer – NXG The importance of testing defects in PET & preform products cannot be understated. By ensuring that these products are free of defects, manufacturers can help to ensure the safety of consumers and avoid costly recalls. In addition, testing can also help to improve the quality of the products and identify potential issues early on. Thus, we at Presto have designed this high-quality lab testing instrument known as the Polariscope Strain Viewer- NXG model that has been designed to test the quality and transparency of PET preforms. It is based on the principle that any change in refractive index is due to different strain distributions and this leads to lower transparency of the material.