A generalization of photon sphere based on escape/capture cone Masaya Amo, Keisuke Izumi, Hirotaka Yoshino, Yoshimune Tomikawa and Tetsuya Shiromizu Eur. Phys. J. C, 84 6 (2024) 638 Published online: 25 June 2024 DOI: 10.1140/epjc/s10052-024-12954-9