An optical element conveying human-discernible information to a user has its brightness modulated to also convey machine-discernible information to an optical detector. A frame section in which a bit of data can be conveyed begins at a falling edge of a preceding frame section corresponding to the brightness of the optical element decreasing to a low level. At a first predetermined point within the frame section, if the bit of data is logic one, the brightness is increased to a high level, corresponding to a rising edge. At a later, second predetermined point within the frame section, if the bit of data is logic zero, the brightness is increased to the high level, corresponding to the rising edge. At the end of the frame section, the brightness is decreased to the low level, corresponding to a falling edge of the frame section.