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This paper introduces a novel dimming method for Light Fidelity (LiFi) based on a time-domain sample index modulation (TIM), called indexed dimming (iDim). The aim is to provide a wide brightness range, while maintaining a high signal-to-noise ratio (SNR) performance and a high transmission rate. Direct current optical orthogonal frequency division multiplexing (DCO-OFDM) is used and novel line codes are presented. The system performance is experimentally validated by an implementation on a NI PXIe-1085 and NI-7966R Field Programmable Gate Array (FPGA). Implementation results show that the proposed iDim jointly offers multiple benefits compared to existing dimming methods such as amplitude modulation (AM) dimming. Specially, the lowest optical power of the iDim method is measured at <inline-formula> <tex-math notation="LaTeX">$20~\mu \text{W}$ </tex-math></inline-formula>, which is 10 times lower than the measured limit of AM dimming. The iDim system provides a SNR of 22.5 dB for all brightness levels, while the AM-dimming method dramatically reduces its SNR when light is dimmed. The transmission rate of iDim is also higher than DCO-OFDM. This results in up to 40&#x0025; reduced optical power per bit when compared to sDim, a pulse-width-modulation (PWM)-inspired dimming method. Therefore, iDim is a promising dimming method for applications targeting extremely low illumination levels.

iDim: Practical Implementation of Index Modulation for LiFi Dimming