Optical path difference
= Optical path difference
{title2=$h=\operatorname{OPL}_2-\operatorname{OPL}_1$}
= OPD
{c}
{synonym}
An <optical path difference> subtracts two <optical path lengths> at the same <frequency> and with specified endpoints and sign. Its <phase difference> contribution is $k_0h$, in addition to any apparatus reference <wave phase>. For <wavefront error>, the comparison is usually between an actual <wavefront> and an ideal reference <wavefront>.