Abstract: An optical obstructing sensing system is described. Control signals are optically transmitted across the obstruction sensing area thereby obviating the need for a physical connection between the array of emitters and receivers which define the obstruction sensing area. Optical commands control the scanning functions of the obstruction sensing array. The optical conditions in the obstruction sensing area are analysed by means of optical beams transmitted from one side of the obstruction sensing area to the other. In this manner, corrections for ambient light, obstruction sensing array geometry, and spurious signals may be accounted for and inappropriate compensation made. The invention further describes an obstruction sensing method where the decision as to whether an obstruction is present in the sensing area depends on the optical environment, geometry of the obstruction sensing area and intervening obstructions. These factors are taken into account in deciding whether a valid obstruction event is detected.