The generalized terahertz (THz) absorption spectra of biomolecules and the remarkably non-thermal THz bioeffects easily cause doubts whether there exists THz information in the biosystem and the molecules or cells communicate via the THz fields. Herein, by constructing a cluster model of the retinal together with its surrounding residues, we precisely calculate the photon radiation after the cis-to-trans isomerization of retinal triggered by visible or UV light. Considering the energy consumed to drive the necessary proton deprotonation of the all-trans retinal, the left THz photons are used to trigger the conformational change and activation of the rhodopsin.