How does a Polariscope make defects within PET preforms visible ?

Viewing by our naked eyes under natural light , many PET preforms seems flawless . Is there any better way to observe . Dedicated PET preform Polariscope adopts plane Polarizing light . By comparing the patterns when a Preform is placed Parallel and 45 ° to the polarizing axis with an Optimum PET preform ,it is easy to recognize 12 common defects within a PET preform .
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