Touch screen apparatus, including a display unit including a touch surface, a number of light pulse emitting units for emitting light pulses over and across the touch surface, a number of light pulse receiving units for measuring amounts of light received from the light pulse emitting units, circuitry for selectively activating the light pulse emitting units and the light pulse receiving units, and a calculating unit to control the circuitry to provide a low-frequency idle mode and a high-frequency active mode, wherein only one of the light pulse emitting units and one of the light pulse receiving units are activated in the idle mode, and to determine that a light pulse in the idle mode is broken, based on the measured amounts of light received at the one receiving unit.