https://phys.org/news/2018-07-triple-highly-pure-polarization-components.html

"Light consists of a mixture of two polarization components," Ang says. "Polarization beam splitters are used to separate a mixed-polarization beam into two channels for each polarization."

When the splitting is imperfect, the two channels can contain small proportions of the opposite polarization, an effect known as crosstalk.