Ultrasonic lens cleaning utilizes the vibration of the lens to expel the water droplets on the lens. To atomize the water droplets, the acceleration needs to be above a threshold. The threshold can be estimated by the following equation:

Where $\alpha$ is the critical acceleration —the minimum acceleration at which droplets are ejected from a driven surface, Mm/s² or 10⁶ m/s²

$\sigma$ = 72 x 10^-3 N/m, the surface tension of water

$\rho$ = 1000 kg/m³, the density of water

$\omega$ is the angular frequency, krad/s

Figure 3-1 plots the critical acceleration versus frequency. A low frequency requires a low acceleration to expel the water. Therefore, designing a system that works at a relatively low frequency (20kHz - 100kHz) is preferred . For any LCS design, the goal is to create a design whose lens surface acceleration level is higher than the threshold.

Figure 3-1 The critical acceleration to atomize water versus frequency