# Polarization of Light from Basics to InstrumentsPage 7

## Polarimeters

Polaroid-type polarimeters

Dual-beam polarimeters

Slide 61

Polaroid-type polarimeter for linear polarimetry (I)

Use a linear polarizer (polaroid) to measure linear polarization . [another cool applet] Location: http://www.colorado.edu/physics/2000/applets/lens.html

Polarization percentage and position angle:

Part IV: Polarimeters, polaroid-type

Slide 62

Polaroid-type polarimeter for linear polarimetry (II)

Disadvantage: half of the light is cut out

Other disadvantages: non-simultaneous measurements, cross-talk .

Move the polaroid to 2 positions, 0º and 45º (to measure Q, then U)

Part IV: Polarimeters, polaroid-type

Slide 63

Polaroid-type polarimeter for circular polarimetry

Polaroids are not sensitive to circular polarization, so convert circular polarization to linear first, by using a quarter-wave plate

Polarimeter now uses a quarter-wave plate and a polaroid

Part IV: Polarimeters, polaroid-type

Slide 64

Dual-beam polarimeters Principle

Instead of cutting out one polarization and keeping the other one (polaroid), split the 2 polarization states and keep them both

Use a Wollaston prism as an analyzer

Disadvantages: need 2 detectors (PMTs, APDs) or an array; end up with 2 ‘pixels’ with different gain

Solution: rotate the Wollaston or keep it fixed and use a half-wave plate to switch the 2 beams

Part IV: Polarimeters, dual-beam type

Slide 65

Dual-beam polarimeters Switching beams

Part IV: Polarimeters, dual-beam type

Unpolarized light: two beams have identical intensities whatever the prism’s position if the 2 pixels have the same gain

To compensate different gains, switch the 2 beams and average the 2 measurements

Slide 66

Dual-beam polarimeters Switching beams by rotating the prism

Part IV: Polarimeters, dual-beam type

Slide 67

Dual-beam polarimeters Switching beams using a ½ wave plate

Rotated by 45º

Part IV: Polarimeters, dual-beam type

Slide 68

Dual-beam polarimeter for circular polarization - Wollaston and quarter-wave plate

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