What Is a Polariscope? A Guide to Viewing Stress in Glass and PET Preforms
A polariscope makes the internal stress of a transparent material visible. The sample is placed between two polarizing filters in front of a light source, and stressed zones appear as colored or dark patterns. Glass makers use polariscopes to check residual stress in bottles, jars and glassware, and PET preform producers use them to detect injection molding defects.
How does a polariscope work?
Ordinary light vibrates in all directions. A first polarizing filter, the polarizer, lets through only the light that vibrates in one direction. A second filter, the analyzer, turned at 90° to the first, blocks that light, so the field of view looks dark.
When a transparent sample is placed between the two filters, stress or molecular orientation inside the material changes the way light travels through it. The material becomes birefringent: it splits the light into two components that travel at different speeds. Part of the light then passes through the analyzer, and stressed zones appear as bright or colored fringes on the dark background.
Two kinds of patterns are read on a PET preform:
Isochromatic fringes: the colored pattern seen when the preform is held at 45° to the polarizing axis. It indicates the amount of birefringence.
Isoclinics: the black band seen when the preform is held parallel to the polarizing axis. It indicates the direction of the molecular orientation.
Main applications
Glass containers and glassware: checking residual stress in bottles, jars, tableware, scientific glassware and ampoules.
PET preforms: detecting defects from injection molding through the pattern of the polymer flow lines.
Process settings: comparing preforms after a change of injection settings or molds.
Glass production: quick visual checks of glass bottles with inspection lighting systems.
Key concepts: birefringence, residual stress and qualitative inspection
Birefringence: the property of a stressed or oriented transparent material to split light in two. The more stress, the more birefringence, and the more fringes or colors appear.
Residual stress: stress that stays in a part after it has been made. In glass, it can be harmful, because it may contribute to failure in service, or useful, because it can increase strength. Either way, quality control needs to know it is there.
Tint plate: a 530 nm tint plate, as on the AT2E PL-G, gives a full color representation of stress. The background appears magenta instead of dark, and small stresses show as changes of color.
Qualitative inspection: a polariscope like the PL-G shows where stress is and how it is distributed, rather than a stress value. For a quantitative result, ASTM C148, Standard Test Methods for Polariscopic Examination of Glass Containers, measures the optical retardation that indicates the quality of the anneal, either by comparison with reference standards or with a polarimeter.
Polarized sunglasses, the everyday polariscope
Look at the rear window of a car through polarized sunglasses, and you may see a pattern of dark spots or bands. The tempered glass of the window carries built-in stress, light from the sky is partly polarized, and the sunglasses act as the analyzer.
A polariscope does the same thing in a controlled way. It has its own polarized light source and its own analyzer, so the pattern depends only on the sample and not on the sky. This is what makes it possible to compare one preform or one bottle with another.
Checking a polariscope
A polariscope gives an image rather than a number, so checking it means checking the image:
With no sample in place, check that the field of view is evenly lit and uniform: dark on a plain polariscope, or an even magenta on a viewer with a tint plate such as the PL-G.
View a reference sample, such as a preform or a bottle kept as a limit sample, and compare its pattern with the reference image.
Clean the filters and replace any lamp that has lost intensity, then repeat the check.
Reference samples and photos also help operators judge patterns in the same way, which matters as much as the instrument itself for a visual inspection.
AT2E polariscopes and inspection lights
AT2E polariscopes cover glass and PET, from bench viewers to automatic rotation:
PL-G: glass stress viewer with 530 nm tint plate and removable magnifier, viewing area 125 x 105 mm, adjustable working space from 250 to 500 mm.
PL-P: PET preform stress viewer with LED lighting, viewing area 330 x 150 mm (13 x 5.9 in), adjustable angle.
APLP - MPLP: preform polariscope with preform supports and automatic or manual rotation for 360° viewing of several samples at once.
LSS: lighting system with white glass and LED light for quick checks of glass bottles during production.
LS-1: graduated lighting system for a quick check of the dome height of glass bottles up to 89 mm in diameter.
Conclusion
Stress cannot be seen with the naked eye, yet it decides how glass will break and how a preform will blow. A polariscope shows it in seconds. With a clean, evenly lit instrument and reference samples that everyone uses the same way, it becomes a fast and reliable routine check for preforms and glass containers.
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AT2E is an ISO 9001 certified company (AFNOR Certification).
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