ASTM-F218:13(R2020) pdf free download – Standard Test Method for Measuring Optical Retardation and Analyzing Stress in Glass.
8.3.2 The reverse sequence Yellow-Brown- BlueGrayYellow, or for larger retardation (approximately >3(X) nm) Yellow-Orange-Red-Blue-Yellow-Orange-Red, indicates compressive stress (S < 0 or — Si,> 0).
8.4 Measure Retardation:
8.4.1 To measure the retardation at any given point, remove the tint plate, place the monochromatic filter in the field of view, and rotate the analyzer with respect to its initial position until maximum extinction (darkness) occurs at the P01. The angle a through which the analyzer must he rotated to the left or the right is a measure of the retardation at the point.
8.4.1.1 In white light, the color of the fringe moving toward the PU! will keep changing. To eliminate possible errors and to increase the contrast, the monochromatic filter, B, must bc inserted for this operation. or the monochromatic lamp must bc used.
8.4.2 The rotation of the Analyzer must be clockwise, if the stress is tensile (S or S — Si.> 0), the measured angle a is indicated directly on the dial, in degrees. When a fractional graduation of the dial is used, the fractionf= a/I SO is indicated on the dial.
8.4.3.3 Instruments equipped with a dual scale. 0 to 180° CW and 0 to 1800 CCW, the angle a is indicated directly when the analyzer is rotated CCW.
8.4.4 When the retardation is required to be measured in a given area or section where several extinction points may exist, rotate the analyzer (CW or CCW) until the maximum extinclion is achieved at each selected point. Use the procedure previously described in this section to measure retardation at those points, and the sequence of the observed colors described in 8.3 to differentiate between tensile or compressive stress.
8.5 When a maximum value is specified and the specimens are of a uniform thickness it is necessary only to set the analyzer at the angle specified and then observe whether any unclosed loop-shaped fringes are present in the stress pattern. if not, it may be concluded that the maximum retardation that is present is less than the specified maximum. If any are present, then the retardation is greater than the specified maximum. To determine the exact magnitude of the retardation, use the method outlined in 8.2 and 8.4.
8.6 When the full-wave plate (also called the tint plate”) is introduced, the polarimeter can be used to reveal a color pattern. White light must be used for this observation, and the analyzer must be set in standard position (perpendicular to the polarizer). Table 2 shows the color distribution that may be expected together with the associated magnitude of the retardation and tension-compression indicated.