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In the picture above the material has a high Strain Optical Coefficient (SOC), so many fringes can be seen. Glass has a low SOC, so we have to interpolate between fringes. By eye this is not possible.

After polariser at 45 degree: Vector Ep is equal to Ex + Ey Ex = Ey = sin( w t ) |
Afer the sample, difference is Ey = sin( w t ) Ex = sin( w t + f ) |
Only the vectors normal to Ep pass the crossed polariser Ec = ( sin(wt+f ) - sin(wt) ) /sqrt2 |

Ic | = | 1/2 Io ( sin(wt+f) - sin(wt) ) ^{ 2 } = 1/2 Io ( 1 - cos( f ) ) | (3) |

Measuring the intensity is with the basic set-up as given above. This intensity is measured with a camera and a computer. To calculate the phase-shift we use formula (3). In this formula we have a constant Io which we have to eliminate. This is done with a second measurement, for instance done with the Analyser rotated over 90 degrees, so the two polarisers are parrallel. From the two measurements we can compute the phase-shift f :

Crossed polarisers | Ic = 1/2 Io ( 1 - cos( f ) ) | (3) |

Parrallel polarisers | Ip = 1/2 Io ( 1 + cos( f ) ) | (4) |

Elimination of Io | Icn = ( Ip - Ic ) / ( Ip + Ic ) = cos( f ) | (5) |

Calculation of phase-shift | f = arccos( Icn ) | (6) |

With SAMS we use this principle to measure stress in glass.