This work suggests how a global trade-off between energy consumption and encryption strength allows for optimal security mode selection in optical wireless, with respect to energy consumption. An adaptive security scheme is proposed, that involves the minimum energy needed to achieve a desired security level. The subsequent security approach is twofold. First, encryption strength is adjusted according to the severity of the requested service. This helps save energy, while preserving the encryption strength. Second, for battery-powered devices, data can be encrypted according to a specified threshold, or even based on the battery level itself. The reliability function, which is the backbone of the proposed adaptive security scheme, serves as a quality factor that describes all encryption parameters and their impact on energy consumption and therefore as a global indicator of the overall security with respect to the energy consumption.