Optical-wireless digital-sound transmission system with spherical solar cells

Spherical Si solar cells (diameter: 1 mm) with a reflector cup (diameter: 2.2 mm) were used with 1-bit SigmaDelta-modulated illumination from white LEDs to provide optical wireless digital-sound transmission. The solar cells have a high directivity with regard to incident light, thus largely eliminating the environmental noise from fluorescent lighting. In the experiment, the transmitter produced a 1-kHz analog signal at an illumination of 70 lx that traveled over a distance of 50 cm. The measured output spectrum of the receiver showed that it recreated a sinusoidal waveform with little distortion. Even under fluorescent lighting with an illumination of 200 1x, the measured spectrum showed that a sinusoidal waveform with a signal-to-peak-noise ratio of 25 dB was obtained.