A digitally controlled electronic ballast, on command, optically transmits its identification signature or other data by CW modulation of the luminosity of one or more lamps connected to the ballast. The data is transmitted by momentarily interrupting the lamp current to mark the beginning and the end...http://www.google.de/patents/US6333605?utm_source=gb-gplus-sharePatent US6333605 - Light modulating electronic ballast
