title: Orbital angular momentum light in microscopy.
abstract: Light with a helical phase has had an impact on optical imaging, pushing the limits of resolution or sensitivity. Here, special emphasis will be given to classical light microscopy of phase samples and to Fourier filtering techniques with a helical phase profile, such as the spiral phase contrast technique in its many variants and areas of application.This article is part of the themed issue 'Optical orbital angular momentum'.
authors.affiliation: Division for Biomedical Physics of the Medical University of Innsbruck, Müllerstraße 44, 6020 Innsbruck, Austria monika.ritsch-marte@i-med.ac.at.
authors.forename: Monika
authors.identifier: http://orcid.org/0000-0002-5945-546X
authors.initials: M
authors.lastname: Ritsch-Marte
keywords: Fourier optics; optical orbital angular momentum; spiral phase contrast microscopy