Senarmont Method for measuring of stress in glass

Home Methods Compensator Lambda Plate Fringes

Set-up Senarmont
First polariser at 45 degrees with the main stress axis Sample which gives difference in phase Quarter-wave plate Rotating Analyser at angle set to f / 2 radians relative to -45 degree

The inserted quarter-wave plate adds a phase shift of 90 degrees to the parallel direction. As a result, the light becomes linear polarised at angle of f / 2 radians relative to the original position of the Analyser. So a crossed linear polariser should give a resulting intensity of zero. When the Analyser is rotated till the intensity in a certain point is zero, the phase-shift in that point is equal to 2 times the rotation angle.
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 Quarter-wave plate:

After Analyser at -45+f/2:

EyD = 0
ExD = 0


This method can be automated. For this, the light through the sample is sampled with a camera connected to a computer. Measurements are done for different values (angles) of the rotating Analyser. Out of these measurements the value for the angle (which should give the minimum light intensity of zero) is computed.