An "electric eye" alarm detects a break in the beam of light not with a simple "electric" cell but with this type of cell
... 
Other switch closures from beam break gadgets, vibration sensors, door ... A 
simple CdS cell placed in the focal plane of a lens makes a telescopic 'electric 
eye'... [DOC] [TLE] Patent US4840248 - Safety switch light fence - Google Patents [PAR] Jun 20, 1989 ... One or more light beams are directed across the exit opening.