Using the Strainoptics ® Laser GASP ® Polarimeter to Measure Surface Stress in Heat-Treated Glass

1. Using the Strainoptics® Laser GASP® Polarimeter to Measure Surface Stress in Heat-Treated Glass

2. Prisms Input Head Output Head Protractor Dial Input Head Tilt-Adjust Knob Vernier Indicator Battery Pack Mirror Pivot Lever (not shown) Laser GASP Components Compensator (Wedge) Assembly

3. Operating PrinciplesLight from a Laser Passes through a Polarizing Filter in the Input Head… Polarizer Analyzer Tin side of float glass Compensator Field of View Entrance Prism ExitPrism

4. Becoming Polarized and Entering the First (Entrance) Prism.

5. Some of the Light that is Not Blocked or Scattered Enters the Glass and Propagates Parallel to the Surface.

6. At a Critical Angle, Some of the Light Emerges from the Glass and Can Be Seen Through the Eyepiece. Light emergesfrom the output tube into the eyepiece after passing through a polarizer and entering a first (entrance) prism, then propagating along the surface of the glass and entering a second (exit) prism, before reflecting off a mirror, and through a special device known as a wedge compensator.

7. The GASP Measures on the Tin Side of Float Glass To use the GASP, the instrument must be placed on the “tin” side of the glass. If not known, an ultraviolet lamp may be used to detect the tin side. Strainoptics sells two different UV lamps for this purpose, one is a double-tube model that operates on rechargeable batteries and the other is a small, battery-operated “mini” version. 12 1259 89 8.68

8. How to Use the UV Lamp to Detect the Tin Side There are two methods of identifying the tin side of the glass using a UV light. In Method A, position the UV lamp at a slight angle above the glass surface. The side that exhibits a hazy appearance is the tin side. In Method B, the UV light is pressed flush against the surface of the glass and viewed from the other side. When a haze appears through the glass, the tin side can be identified as the side against which the UV lamp was placed. Method A Method B 12 1259 89 8.68

9. Measuring on the Practice Sample Place a drop (about 1/2” (12 mm) in diameter) of index matching fluid (Strainoptics MIF-154) on the tin side of the glass at the location where you will be making a measurement (usually marked with an X on the practice sample. The refractive index of this fluid is designed for soda-lime float glass. Do not use other types of fluids with the instrument; doing so may void the warranty and cause damage. 12 1259 89 8.68

10. Reflection of Laser Light Measuring on the Practice Sample With the power switch on the battery pack in the OFF position, insert the small silver plug attached to the Input Head into the small jack on the side of the battery pack. Turn the power switch to the ON position. Verify that the light is on by lifting the instrument slightly and noticing the reflection on the surface of the glass or table (see below). DO NOT LOOK DIRECTLY INTO THE LASER LIGHT. Note: When the Laser GASP is not in use, always turn the power switch OFF and unplug the laser to save battery power. 12 1259 89 8.68

11. Measuring on the Practice Sample Place the GASP body onto its side, with the underside adjacent to the point to be measured. Slowly raise the instrument to the upright position and lower it on the glass so that the index fluid makes contact with both prism surfaces. Make sure that these surfaces are coated with index fluid 12 1259 89 8.68

12. Measuring on the Practice Sample Be careful when picking up or handling the GASP to avoid damaging or misaligning the components. Grasp the vernier support with one hand to move the instrument, support the battery pack with the other hand, and do not allow the prisms to touch any surface except when measuring. Between measurements, return the GASP to its case or allow it to rest on its side. NEVER MOVE THE INSTRUMENT OVER THE SURFACE OF THE GLASS 12 1259 89 8.68

13. Measuring on the Practice Sample With the power switch in the ON position, look through the instrument’s eyepiece. You should see a bright image consisting of diagonal black lines on a red background. The instrument is now set up properly for measurement. The alternating diagonal lines are due to the presence of a wedge compensator. The inclination angle and direction of tilt in these lines (called fringes) indicates the magnitude and type of stress present (left = compression, right = tension). 12 1259 89 8.68

14. Measuring on the Practice Sample NOTE: If the image is poor, not visible, or if the black lines are vertical instead of diagonal, it could be due to misadjustment of the optics, you may not be measuring on the tin side of the glass, or the index fluid is not covering the prism surfaces. If you still cannot obtain a good image, refer to the adjustments and troubleshooting section of this presentation or to the complete Laser GASP user manual for further instructions. For assistance at any time, contact Strainoptics Technical Support at (215) 661-0100, ext. 104 or email stress2@strainoptics.com.

15. Obtaining a Measurement on Your Glass • Using a soft tissue moistened with a commercial glass cleaner, gently clean both the sample and prism surfaces, taking care to remove all abrasive particles that could scratch the prisms. • Determine the tin side of your sample using a UV light. • 3. Place a drop (approx. ½” or 12 mm) of matching index fluidon the measurement location

16. Obtaining a Measurement on Your Glass 4. Insert the silver plug into the small jack of battery pack BEFORE turning the power switch to the ON position to prevent possible damage. 5. Place the GASP on the sample at the measurement location, ensuring that the index fluid is covering the prism surfaces. 6. Look through the instrument’s eyepiece. You should see a bright image consisting of diagonal black lines on a red background. TIP: The image quality may be further improved by slowly turning the mirror pivot thumbscrew. DO NOT OVERADJUST.

17. Obtaining a Measurement on Your Glass 7. While looking through the eyepiece, rotate the protractor dial until the two black reticle lines are parallel to the observed diagonal stress fringes. Turn protractor dial until reticle lines are parallel to stresslines

18. Obtaining a Measurement on Your Glass 8. Note the angle reading at the vernier mark. Vernier Mark

19. Obtaining a Measurement on Your Glass Using the calibration chart supplied with your instrument, find the angle and read across for the surface stress value in the desired units, MPa, psi, or Kgf/cm2. IMPORTANT: The supplied calibration chart is unique to your instrument and is not interchangeable. If this chart is lost or destroyed, contact Strainoptics for a replacement. Specify the serial number of the instrument.

20. Where and How Many Points to Measure The number and location of points measured is determined by each manufacturer; however, ASTM C1048 recommends two surface stress readings in each of five locations, oriented in two directions, 90o relative to each other, for a total of ten readings per sample as shown below. Using the GASP to measure surface stress in heat-treated flat glass in this way complies with ASTM Standard Test Method C1279 and the European EN-12150 directives. 1/4 W 1/4 W W 1/4 W 1/4 W 3/8 L 3/8 L 1/8 L 1/8 L L

21. Primary Mechanical Adjustments

22. Primary Mechanical Adjustments To maximize light intensity – If possible, position the sample horizontally on a table so that you can look into it from its edge. Place the GASP on the sample with its output head facing you. With your eyes level to the sample, you should be able to see the light exiting the glass edge. Loosen the input head locking knob and, while observing the light intensity, slowly move the input head backward or forward until maximum light intensity is seen. Tighten the input head locking knob. Input Head Output Head Input Head LockingKnob

23. Primary Mechanical Adjustments To optimize the light path – Slowly turn the input head tilt-adjust knob while observing the light exiting the sample at the edge. Adjust the light path until it is as close to the top surface of the sample as possible, without losing light intensity. Input Head Tilt Adjust Knob

24. Primary Mechanical Adjustments After optimizing the light path, look down into the eyepiece. If you DO NOT see fringes, slowly move the mirror pivot lever until fringes are visible. Then, slowly rotate the tilt-adjust knob until the fringe lines are well defined. If the image appears cut off at either end, slowly slide the wedge compensator assembly left or right to center the image and maximize the viewing area. Tilt Adjust Knob Input Head Output Head Wedge Compensator Assembly Input Head LockingKnob Mirror Pivot Lever

25. Primary Mechanical Adjustments If you lose or cannot obtain a readable image, the instrument adjustments may have fallen out of range. To reset, move all adjusting controls to the mid-point of the scribed white hash marks. Hash Marks

26. Troubleshooting

27. SYMPTOM: Light source is on, but cannot obtain good image. Possible causes and solutions: (1) GASP is not on tin side of glass (Use UV light to confirm) (2) Insufficient index fluid coverage (Ensure that prisms are coated) (3) Poor prism contact (Prisms are chipped or dirty) (4) Instrument optics not adjusted properly (Reset and readjust) (5) (5) Laser needs alignment (See user manual) (6) Laser needs recharging or replacement (Recharge or replace) (7) Exit prism installed incorrectly (See user manual)

28. SYMPTOM: Instrument does not appear to be measure correctly. Possible causes and solutions: (1) Instrument may be out of calibration (a) Verify with (or order) Strainoptics Cal-Plate. (b) Adjust zero and protractor following instructions in GASP user manual. (c) Contact Strainoptics for recalibration and/or repair.

29. SYMPTOM: Power switch is on, but laser is not. Possible causes and solutions: (1) Batteries need recharging or replacement (Replace or recharge) (2) Laser module needs replacement (Contact Strainoptics) NOTE: The laser module is polarity-sensitive.  Applying the incorrect polarity may damage it. Verify that the tip of the adapter is configured to be positive. There is  a "+" on the plastic just beneath the tip. If the tip has been configured as negative, this may have damaged the laser and/or the battery pack. It is possible to verify laser function by connecting it to a battery pack of another Laser GASP that is functioning properly.

30. Using the Cal-Plate to Verify Measurement Accuracy The Strainoptics GASP CAL-PLATE is a portable, easy to use accessory for verifying the performance of your GASP polarimeter or for ISO self-certification. It is made from soda-lime glass with a known retardation value in a defined area and supplied with a calibration certificate attesting to a factory-certified retardation measurement in nanometers. (Retardation is proportional to stress.) To use the CAL-PLATE, simply take a measurement in the “clear” area with your GASP and compare the reading with the certified value on the calibration sticker. If the results are within +1 degree, the instrument is measuring properly. If not, then refer to the GASP user manual for instructions on adjusting the zero and protractor, and try again. If still not measuring correctly, contact Strainoptics.

31. Thank you for Choosing Strainoptics While it is not meant to replace the complete Laser GASP user manual, we hope that you have found this brief presentation helpful. If you have any questions concerning the operation of your instrument, please contact Strainoptics Technical Support at (215) 661-0100 or email to stress2@strainoptics.com. Strainoptics is a world leader in glass stress and distortion measurement, featuring a complete product line for inspection and quality control of heat-treated glass. In addition to the industry-standard GASP series polarimeters, we offer GES-100 edge-stress meters, RWG roller-wave gages, both visual and automated instrumentation for measuring stress and distortion in architectural and automotive glass, strain viewers, and laboratory polarimeters. Strainoptics, Inc.108 W. Montgomery Ave.North Wales, PA 19454 USAwww.strainoptics.com GASP® is a registered trademark of Strainoptics, Inc.