Some laboratories and institutions use bare LR-115 polymer (cellulose nitrate + polyester) as a nuclear track detector for radon measurements. They detect alpha particles from radon and its progeny products in the cellulose nitrate layer, whether emitted from the so-called effective volume located in front of the cellulose nitrate layer. However, concentric rings were recorded on the etched LR-115 detector when the possibility of alpha registration was canceled, and the polyester was exposed to natural radiation in different configurations. Concentric rings were formed, apparently, due to the thermally induced self-defocusing effect. The outdoor causative agent can heat and change the refractive index of polyester, generating latent concentric rings. In this work, different configurations have been studied to find the outdoor causative agents and to clarify the possible physical causes. Furthermore, results show reproducibility and suggest that the ultraviolet radiation can cause concentric rings on the cellulose nitrate layer.