We can then define the scintillation index as a normalized variance of the intensity of the signal (laser beam) according to the following formula [1],[2]: \begin{align} \sigma^2_I = \frac{\langle\, I^2\rangle-\langle\, I\space\rangle^2}{\langle\, I\space\rangle^2} = \frac{\langle\, I^2\rangle}{\langle\, I\space\rangle^2}-1   [\mathrm{unitless}] \end{align} where $\langle\,\cdot\space\rangle$ describes the mean operator.