###### Abstract
In the optical wireless communication, beam steering [28], beam shaping [29], and improving the service level of the link [30] for coherent light using metasurface-based IRS have been studied. For incoherent light (such as visible light), [PERSON] et al. [31] install IRS on the wall which reflects the incident light to the receiver by intelligently controlling the phase gradient of each metasurface and the orientation of each mirror in the indoor environment and the results proved that the performance of the mirror array is better than that of the metasurface. For the VLC between parallel vehicles, the mirror array-based IRS can be installed on the transportation infrastructure, and the light from the transmitter is reflected into the receiver by controlling the rotation angle of each mirror, which solves the problem of realizing VLC for parallel vehicles. Compared with the hybrid VLC-WiFi [32], the hardware implementation is simple, and the disadvantages of RF communication are solved. In wireless communications, EE is defined as the ratio of transmitted bits to energy consumption. It is usually expressed in bits per Joule (bits/J) [33; 34; 35]. The higher the EE, the less energy the system expends for the same communication performance. It mainly contains two elements that are achievable rate and power consumption. * To ensure the effectiveness of the communication system, the achievable rate needs to reach a certain value. Since the transmitted signal is non-negative, real, and limited amplitude, the classical Shannon capacity formula is not suitable for VLC. Researchers have been studied the lower bound of the achievable capacity of the VLC system [36; 37; 38], and the achievable rate is proportional to the signal-to-noise ratio (SNR) [39]. Each mirror in the IRS is independently controlled, and the light reaches the receiver through their reflection. The total channel gain equals the sum of channel gain corresponding to each mirror, and the SNR becomes larger with the number of mirrors increasingSo, the achievable rate is not only related to the channel gain corresponding to each mirror, but also to the number of mirrors. * In the IRS-aided VLC system, the power consumption of the system is mainly included that of the transmitter, receiver, and IRS. The power consumption of the transmitter and receiver mainly includes signal power, DC offset, and the hardware static power consumption [40; 41]. The power consumption of the IRS equals the sum of that for each mirror rotating. Therefore, the total power consumption changes depending on the number of mirrors. Because the achievable rate and power consumption are related to the number of mirrors, the EE is also affected by the number of mirrors in the VLC system via mirror array-based IRS. To get the maximum EE, it is necessary to optimize the number of mirrors. Although the time allocation, power control, and phase matrix are analyzed for EE optimization [42], the influence of the number of mirrors in IRS on EE has not been analyzed, as far as the authors know. The main contributions of this paper are as follows. * The VLC system via mirror array-based IRS for parallel vehicles is designed, which provides convenience for parallel vehicles to realize VLC. The right headlamp of the right vehicle is used as the transmitter, the receiver is installed between the two headlamps of the left vehicle, and the IRS is installed on the street light pole. The channel model of the system is analyzed, and the channel gain is calculated. * The calculation methods of the achievable rate and power consumption are given. According to the system model, the calculation formulas of the SNR and the instantaneous achievable rate are given. Based on reference [40], the total power consumption of the system and the power consumption of each mirror are analyzed. Both the achievable rate and the total power consumption are functions of the number of mirrors \(N\), and thus EE is also a function of \(N\). * The number of mirrors optimization problem under the EE maximization is formulated.