A known retarder inserted in a polarization microscope to compare phase delay with an unknown specimen. Parallel slow directions add signed retardations; a slow direction paired with a fast one subtracts them. General relative angles require composing polarization transformations rather than scalar addition.
A quartz retarder used to identify specimen fast and slow directions through changes of interference colour or compensation. Its own retardation and orientation must be specified before unique final colours can be predicted.
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