Whereas FCS is a point measurement providing diffusion time at a given observation volume, svFCS is a technique where the observation spot is varied in order to measure diffusion times at different spot sizes. The relationship between the diffusion time and the spot area is linear and could be plotted in order to decipher the major contribution of confinement. The resulting curve is called the diffusion law.
This technique is used in Biology to study the plasma membrane organization on living cells.
where  is the y axis intercept. In case of Brownian diffusion, . In case of a confinement due to isolated domains,  whereas in case of isolated domains, .
svFCS studies on living cells and simulation papers
Sampling-Volume-Controlled Fluorescence Correlation Spectroscopy (SVC-FCS):
z-scan FCS
FCS with Nano-apertures: breaking the diffraction barrier
STED-FCS: