Glass stress test
The glass item is placed between a polarised backlight and a camera with an analyser. In the crossed-polariser image, zones of residual stress light up; retardation is derived from the analyser angle and the high-stress zone is flagged. A sample above the limit is pushed off the rotary table.
Extinction angle, retardation and stress are written to the sample record; a jar above the limit is pushed into the chute after the table indexes, and a new sample is taken from the infeed conveyor in its place. The result is shown on the stack light.
- 01 Positioning
- 02 Dark field
- 03 Retardation
- 04 Decide
The rotary table brings the sample between the polarised backlight and the camera.
- Analyser angle
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- Retardation
- —
- Stress
- —
Scenes are illustrative simulations; they are not customer references or performance guarantees.
Need
Residual stress in glass is invisible to the eye and to ordinary imaging; it only appears as birefringence in polarised light. Stress concentrates at thick-to-thin transitions such as the heel and the shoulder; the curvature of the jar wall also deflects light and can give misleading brightness.
Solution approach
The sample is held between a polarised backlight and a camera with a rotating analyser. The analyser is first set to the crossed position: unstressed glass stays dark, stressed zones glow. The analyser is then rotated until the measuring point goes dark; the angle on the extinction curve gives the retardation, and the retardation the stress.
Process steps
- 01
Positioning
The rotary table brings the sample between the polarised backlight and the camera.
- 02
Dark field
The analyser turns to the crossed position; the background darkens and stressed zones stay bright.
- 03
Retardation
The analyser is rotated until the measuring point goes dark; the angle on the extinction curve gives the retardation.
- 04
Decide
Peak retardation and the corresponding stress are compared with the limit; a non-conforming sample is pushed into the chute after the table indexes.
Related applications
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