optical path length (OPL)

The distance that light would travel through a vacuum to maintain an equivalent phase with light traveling through some medium. For example, light passing through a cell experiences a longer optical path length than light that does not.

Related MicroscopyU Article

Specimen Optical Path Length Variations

Phase contrast microscopy interprets differences in specimen optical path length as fluctuations in light intensity, which are readily observed as variations in contrast through the microscope. This interactive tutorial explores the effects of refractive index and thickness changes on the apparent overall optical path length, and demonstrates how two specimens can have different combinations of these variables but still display the same path length.