###### Abstract  
The optimization problem of the number of mirrors under energy efficiency (EE) maximization for vehicular visible light communication (VVLC) via mirror array-based intelligent reflecting surface (IRS) is investigated. Under considering that the formulated optimization problem is subject to the real and non-negative of the transmitted signal, the maximum power consumption satisfied luminous ability and eye safety, the minimum achievable rate, and the required bit error ratio (BER), EE is proved to be a unimodal function of the number of mirrors. Then, the binary search-conditional iteration (BSCI) algorithm is proposed for quickly finding the optimal number of mirrors with maximum EE. Numerical results demonstrate that fewer mirrors can obtain the maximum EE, and the computational complexity of the BSCI algorithm is reduced by \(10^{5}\) orders of magnitude, compared with the Bubble Sort method. v +
Footnote †: journal: _Photomics_  
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Footnote 0: Copyright** © 2022 by the authors. Liconse MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ([[https://creativecommons.org/licenses/py/4.0/](https://creativecommons.org/licenses/py/4.0/)]([https://creativecommons.org/licenses/py/4.0/](https://creativecommons.org/licenses/py/4.0/))). **Citation:** [PERSON]; [PERSON]; [PERSON]; [PERSON] An Optimal Scheme for the Number of Mirrors in Vehicular Visible Light Communication via Mirror Array-Based Intelligent Reflecting Surfaces. _Photomics_**2022**, 9, 129. [[https://doi.org/10.3390/photonics9030129](https://doi.org/10.3390/photonics9030129)]([https://doi.org/10.3390/photonics9030129](https://doi.org/10.3390/photonics9030129))  
Received: 25 January 2022  
Accepted: 23 February 2022  
Published: 24 February 2022  
**Published:** maps and institutional affiliations. **Published:** **Copyright** © 2022 by the authors. Liconse MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ([[https://creativecommons.org/licenses/py/4.0/](https://creativecommons.org/licenses/py/4.0/)]([https://creativecommons.org/licenses/py/4.0/](https://creativecommons.org/licenses/py/4.0/))). **Published:** **Notice**: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. reach the receiver directly through the line-of-sight (LOS) link [17; 18; 19; 20]. For VLC with the non-line-of-sight (NLOS) link, the road surface can be used as the reflector [21]. The light emitted by the headlamp reaches the road surface firstly and then reaches the receiver through the reflection of the road surface. In this case, the receiver is in the front and the transmitter is in the back, and a certain distance should be maintained to ensure that the reflected light is within the field of view (FOV) of the receiver. When the headlamps or taillights are used as transmitters, the light emitted by the transmitters cannot reach the receivers which are installed on other vehicles for parallel. According to the propagation characteristics of optics, it also cannot be reached by road reflection. The auxiliary means need to be considered to realize VLC between parallel vehicles. The intelligent reflecting surface (IRS) [22; 23] is a tunable metasurface composed of many low-cost passive reflective elements, which can manipulate the wavelength, polarization, and phase of the incident wave [24; 25]. In the RF-based vehicular networks, the metasurfaces can revise the Snell's law that redirecting the radio waves in the desired direction, which solves the problem that the communication is obstructed by strong obstacles and extends coverage in the highly dynamic vehicular environment [26], realizing keyless, secure transmission [27].