A system to detect obstacles, comprising: a power beam transmission circuit; a power beam reception circuit arranged to receive a power beam from the power beam transmission circuit; an emitter module, the emitter module including: a first emitter arranged to emit a first signal having a first characteristic, the first signal emitted in proximity to the power beam; a second emitter arranged to emit a second signal having a second characteristic, the second characteristic different from the first characteristic, the second signal emitted in proximity to the first signal; and a detector module arranged to distinguish between the first characteristic and the second characteristic including: a first detector coupled to several filters and being arranged to respond to the first and second signals; and a second detector coupled to several filters and being arranged to respond to the first and second signals, characterized in that the plurality of emitters and the plurality of detectors have wide fields of view creating a light curtain having a dense weave of separately identifiable and distinguishable paths from each emitter to multiple detectors and vice versa, wherein the detector module is arranged to detect blocking of a line-of-sight transmission path between any emitter and any detector by distinguishing between the first characteristic and the second characteristic, wherein the blocking is determined to be an obstacle (120A, 120B) near or impeding the power beam. a power beam transmission circuit; a power beam reception circuit arranged to receive a power beam from the power beam transmission circuit; an emitter module, the emitter module including: a first emitter arranged to emit a first signal having a first characteristic, the first signal emitted in proximity to the power beam; a second emitter arranged to emit a second signal having a second characteristic, the second characteristic different from the first characteristic, the second signal emitted in proximity to the first signal; and a first emitter arranged to emit a first signal having a first characteristic, the first signal emitted in proximity to the power beam; a second emitter arranged to emit a second signal having a second characteristic, the second characteristic different from the first characteristic, the second signal emitted in proximity to the first signal; and a detector module arranged to distinguish between the first characteristic and the second characteristic including: a first detector coupled to several filters and being arranged to respond to the first and second signals; and a second detector coupled to several filters and being arranged to respond to the first and second signals, a first detector coupled to several filters and being arranged to respond to the first and second signals; and a second detector coupled to several filters and being arranged to respond to the first and second signals, characterized in that the plurality of emitters and the plurality of detectors have wide fields of view creating a light curtain having a dense weave of separately identifiable and distinguishable paths from each emitter to multiple detectors and vice versa, wherein the detector module is arranged to detect blocking of a line-of-sight transmission path between any emitter and any detector by distinguishing between the first characteristic and the second characteristic, wherein the blocking is determined to be an obstacle (120A, 120B) near or impeding the power beam.