Spherical Si solar cells (diameter of 1 mm) with a reflector cup (diameter of 2.2 mm) were used with 1-bit ΔΣ-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. An optical wireless transmission system consists of a transmitter and a receiver. The transmitter contains a battery-operated lamp with 50 white LEDs and a 1-bit ΔΣ modulator that controls the current to the lamp. The receiver consists of just a spherical-solar-cell module and an earphone. In the experiment, the transmitter converted a 1-kHz analog signal into digital pulses at a sampling frequency of 1 MHz and sent the digital data over a distance of 50 cm at an illumination of 70 lx. 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 lx, the measured spectrum showed that a sinusoidal waveform with a signal-to-peak-noise ratio of 25 dB was obtained.
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Kitamura et al. (2010) studied this question.
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