# Scintillometer

The magnitude of the refractive index fluctuations is usually measured in terms of ${\displaystyle C_{n}^{2}}$, the structure constant of refractive index fluctuations, which is the spectral amplitude of refractive index fluctuations in the inertial subrange of turbulence. Some types of scintillometers, such as displaced-beam scintillometers, can also measure the inner scale of refractive index fluctuations, which is the smallest size of eddies in the inertial subrange.