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First polariser at 45 degrees with the main stress axis | Sample which gives difference in phase | Compensator | Analyser at minus 45 degrees with the main stress axis |

The inserted compensator compensates at a defined way for the phase shift in the sample. The phase-shift of the compensator is changed till the intensity behind the analyser is zero, this is when the total phase-shift is zero. Then the phase-shift in the sample is opposite to the phase-shift which is set in the compensator, so can be read from the compensator. The light vectors at the different stages are:

After polariser at +45: EyA = sin( w t) ExA = sin( w t) |
After Sample EyB = sin( w t) ExB = sin( w t + f ) |
After Compensator: EyC = sin( w t) ExC = sin( w t) |
After Analyser at -45: EyD = 0 ExD = 0 |