Abstract: A laser transmitter emits a beam which scans about a scanning axis to effect a curtain of light which defines a boundary of a working space. Photosensors mounted on the outboard portion of a crane or other construction equipment generate warning signals to the construction equipment operator and other nearby personnel whenever these photosensors move into the light curtain. Additional curtain sensing equipment positioned to intercept the rotating laser beam with each cycle of rotation develops an alarm signal which activates an alarm horn and/or alarm light whenever the laser transmitter fails to provide the light curtain.