An optical element used in an optical scanning apparatus includes a diffracting surface composed of a diffraction grating. A mechanical efficiency of the transmission is determined as a function of whether the power unit is operating in a power generation or regeneration mode, and the torque output of the power unit is estimated from that and other hydrostatic parameters. The low-noise amplifier also includes a feedback circuit in communication with the control terminal and the first terminal. The method is characterized by measuring a voltage between the first and second electron conducting layers during a period of the open circuit and controlling a pulse parameter of the voltage pulses dependent on the measured voltage, where the pulse parameter is one of pulse duration and pulse voltage.