By considering the threefold roles of indoor visible light communications (VLC), i.e., illumination, secure information transmission, and light power transfer, the tradeoff between the secrecy capacity (SC) and the harvested energy (HE) is investigated in this paper. An indoor VLC system consisting of a transmitter, a legitimate receiver, and an eavesdropper is established. The legitimate receiver can perform secure information transmission and light power transfer simultaneously, while the eavesdropper is just an information eavesdropping receiver. Under the non-negativity and the dimmable average optical intensity constraint, a lower bound of the average SC is derived. By employing the dynamic power splitting (DPS) for the legitimate receiver, the closed-form expression of the average HE is obtained. After that, the SC-HE region for the DPS receiver is constructed. As the variants of the DPS receiver, the time switching (TS), static power splitting (SPS) and on-off power splitting (OPS) receivers are provided, and the corresponding SC-HE regions are also obtained. As a benchmark, the performance upper bound of the SC-HE region for any practical receiver is derived. The simulation results show that the performance of the OPS receiver always outperforms that of the other receivers. The DPS and SPS receivers achieve the same performance. The TS receiver is the worst receiver when HE is small, but it is the best receiver when HE is large. Moreover, the impacts of the dimming target, the estimated error, signal processing noise, and the nominal optical intensity on system performance are discussed.
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