Horiuchi S, Tokunaga F, Yoshizawa T
Biochim Biophys Acta. 1978 Aug 8;503(2):402-4. doi: 10.1016/0005-2728(78)90198-6.
Hypsorhodopsin was formed in frog retina by irradiation at liquid helium temperature and converted into bathorhodopsin above about 29K.