A device and method to be used in the calibration process for a linear accelerator (LINAC). The optical water surface detector device allows the accurate determine of the height of the water surface in a tank. The device housing includes a light source and a light receiver. The housing can also include a circuit board. The device is mounted on an arm attached to a vertical position mechanism mounted within the tank. The light source within the device is controlled by a main control unit which also receives signals from the receiver and determines the amount of light striking the receiver. The control unit also controls the position of the vertical position mechanism and thus the arm on which the device is mounted.