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Fluorescence detection, either involving propagating or near-field emission, is widely being used in spectroscopy, sensing and microscopy. Total internal reflection fluorescence (TIRF) confines fluorescence excitation by an evanescent (near-) field and it is a popular contrast generator for surface-selective fluorescence assays. Its emission equivalent, supercritical angle fluorescence (SAF) is comparably less established although it achieves a similar optical sectioning as does TIRF. SAF emerges when a fluorescing molecule is located very close to an interface and its near-field emission couples to the higher-refractive index medium (n2 > n1) and becomes propagative. Then, most fluorescence is detectable on the side of the higher-index substrate and a large fraction of this fluorescence is emitted into angles forbidden by Snell's law. SAF as well as the undercritical angle fluorescence (UAF) (far-field emission) components can be collected with microscope objectives having a high-enough detection aperture NA > n2 and be separated in the back-focal plane (BFP) by Fourier filtering. The BFP image encodes information about the fluorophore radiation pattern, and it can be analysed to yield precise information about the refractive index in which the emitters are embedded, their nanometric distance from the interface and their orientation. A SAF microscope can retrieve this near-field information through wide-field optics in a spatially resolved manner, and this functionality can be added to any existing inverted microscope., Comment: 3 figures

Roughly half of a cells proteins are located at or near the plasma membrane. In this restricted space the cell senses its environment, signals to its neighbors and ex-changes cargo through exo- and endocytotic mechanisms. Ligands bind to receptors, ions flow across channel pores, and transmitters and metabolites are transported against con-centration gradients. Receptors, ion channels, pumps and transporters are the molecular substrates of these biological processes and they constitute important targets for drug discovery. Total internal reflection fluorescence microscopy suppresses background from cell deeper layers and provides contrast for selectively imaging dynamic processes near the basal membrane of live-cells. The optical sectioning of total internal reflection fluorescence is based on the excitation confinement of the evanescent wave generated at the glass-cell interface. How deep the excitation light actually penetrates the sample is difficult to know, making the quantitative interpretation of total internal reflection fluorescence data problematic. Nevertheless, many applications like super-resolution microscopy, colocalization, fluorescence recovery after photobleaching, near-membrane fluorescence recovery after photobleaching, uncaging or photo-activation-switching, as well as single-particle tracking require the quantitative interpretation of evanescent-wave excited images. Here, we review existing techniques for characterizing evanescent fields and we provide a roadmap for comparing total internal reflection fluorescence data across images, experiments, and laboratories., Comment: 18 text pages, 7 figures and one supplemental figure