The compensation mechanism may be a glass plate that is disposed in an optical path of a polarization component so that the optical path length of that polarization component can be made substantially equal to {HEAD ~ the optical path length} of {TAIL ~ the other polarization component} that traverses through a half wave plate 
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The compensation mechanism may be a glass plate that is disposed in an optical path of a polarization component so that the optical path length of that polarization component can be made substantially equal to { HEAD ~ the optical path length } of { TAIL ~ the other polarization component } that traverses through a half wave plate
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