Range Limitation and its Extension by WPA Series

Range extension (1) Rotating Polarizer vs. Rotating Phase Plate

Sensor Structure Comparison
PA Series
Integrated Polarizer array + CCD sensor
→ Same principle as Rotating Polarizer method

 
Rotating Polarizer method

WPA Series
Integrated Phase-Plate array + uniform polarizer + CCD sensor <3-layer unique sensor technology>
→ Same principle as Rotating Phase Plate method

 
Due to the difference of principle, measurement range of WPA is twice bigger.

Range extension (1) Rotating Polarizer vs. Rotating Phase Plate

Principle: compute polarization from the brightness profile obtained when rotating filter
・When filter = polarizer, left-handed and right-handed states look the same
・When filter = phase plate, both left and right-handed states can be measured

Measurement range for Rotating Phase-Plate is bigger

Range extension (2) Multiple Wavelength Measurement

Reason of measurement range extension

In addition to the improvement in the measurement principle, as a modification of the polarization sensor, using multiple wavelengths allows another improvement in the measurement range.

With only one wavelength, phase shifts greater than one wavelength
(modulo wavelength) look the same.
→Comparing data at multiple wavelengths allows making the difference.

Measurement with multiple wavelengths

【See figure above】
Let’s assume that we want to determine the value on the horizontal axis using one value, α, on the vertical axis. It is easy to convince ourselves that there are several solutions, A, A + λ、A + 2λ, … between which we cannot choose.
However, if we have several values for two different curves (for example the red curve and the green curve on the figure), we can now choose one unique correct solution.

System structure
PA System uses one single wavelength selection filter centered at 530nm, while WPA uses a revolving filter allowing switching between multiple measurement wavelengths.

Actual system: PA Series and WPA Series

Wide range measurement with WPA

Measurement of retardation > 1 wavelength is available
Evolution of measured value with increasing applied force.


Line profile

 

Measurement data example: difference between PA and WPA

Plastic lens, birefringence ranging from 0 to approx. 800nm

Data acquired with PA

Data acquired with WPA

・With PA, data is wrapped around 100nm, while WPA allows acquiring correct values.
・Even areas with much greater values on the periphery are captured correctly with WPA.

Category

  • #Birefringence & stress strain measurement solutions
  • #2-D Birefringence Measurement System
  • #Birefringence Measurement System

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