Given a set of n≥1 unit disk robots in the Euclidean plane, we consider the fundamental problem of providing mutual visibility to them: the robots must reposition themselves to reach a configuration where they all see each other. This problem arises under obstructed visibility, where a robot cannot see another robot if there is a third robot on the straight line segment between them. This problem was solved by Sharma et al. G. Sharma, R. Alsaedi, C. Busch, and S. Mukhopadhyay. The complete visibility problem for fat robots with lights. In Proceedings of the 19th International Conference on Distributed Computing and Networking, pages 1-4, 2018. in the luminous robots model, where each robot is equipped with an externally visible light that can assume colors from a fixed set of colors, using 9 colors and O(n) rounds. In this work, we present an algorithm that requires only 2 colors and O(n) rounds. The number of colors is optimal since at least two colors are required for point robots G.A. Di Luna, P. Flocchini, S.G. Chaudhuri, F. Poloni, N. Santoro, and G. Viglietta. Mutual visibility by luminous robots without collisions. Information and Computation, 254:392-418, 2017..
Alsaedi et al. (2025) studied this question.