Visible Light Communication (VLC) is expected to enable a wide range of applications in the next generation wireless networks. These applications are recognized as sensitive and prone to dangerous threats. So far, VLC research activities have been more focused on developing high data rate solutions and more robust systems for both indoor and outdoor applications, with a reduced focus on the security aspects. This is mostly due to the fact that VLC systems are based on short range and occurs in line of sight and it is then considered inherently secure by design. The specific characteristics of VLC systems make the traditional countermeasures adopted in radio-frequency-based systems not applicable, with a concrete need to conceive ad hoc solutions. Basing on these premises, in this work we consider a physical layer perspective by analyzing the intrinsic properties of visible light signals so as to develop a secure by design VLC system to be used in downlink. By exploiting the light bias, we guarantee an improved security level in respect of eavesdropper attack by granting also a good illumination level. A key aspect of this work is that the proposed solution does not rely on external devices or extra hardware.