= Degree of polarization
{title2=$\mathcal P=(\mu_1-\mu_2)/(\mu_1+\mu_2)$}
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The <degree of polarization> quantifies how strongly a transverse optical field departs from an unpolarized mixture. Here $\mu_1\geq\mu_2\geq0$ are the <eigenvalues> of its <coherency matrix>. For two mutually uncorrelated orthogonal linear components of intensities $I_1,I_2$, $\mathcal P=|I_1-I_2|/(I_1+I_2)$. It is undefined when the total <light intensity> is zero. A signed difference is a separate convention that also identifies a preferred polarization direction.
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