Overview of the technique
STORM is a single molecule localisation microscopy (SMLM) technique. It localises individual molecules and allows achieving resolutions below the Abbe limit. This is achieved by labelling the molecules of interest with a fluorophore (such as GFP) and activate the fluorophores by laser illumination. After being initially fluorescent, they quickly go into a dark state. Over time, they stochastically reappear in small numbers at a time, and their image can be fitted with a Gaussian model. A localisation accuracy of down to 10nm can be achieved. The fitted observations are used to reconstruct a super-resolved image.
The axial position is measured by modifying the point spread function (PSF) to a double helix. The image of the PSF on the camera depends then on the position of the fluorophore relative to the focal plane.