Acts as a blue-light photoreceptor and photo-protectant. Essential for inhibiting damaged induced by excess blue-green light via a process known as non-photochemical quenching (NPQ). Binding carotenoids improves OCP's intrinsic photoprotectant activity by broadening its absorption spectrum and facilitating the dissipation of absorbed energy (PubMed:15751975). In the dark or dim light the stable inactive form (OCP-O) is orange, upon illumination with blue-green light it converts to a metastable active red form (OCP-R), inducing energy dissipation, quenching cellular fluorescence via NPQ (By similarity). Binds 1 3'-hydroxyechinenone molecule per subunit. Also shows other maxima at 465 nm and 275 nm. The red form (RCP) shows a broad peak at 500 nm. Homodimer. Associated with the phycobilisome. Proteolytically cleaved into a red 16.7 kDa form named red carotenoid-binding protein (RCP) which lacks 15 residues from the N-terminus and approximately 150 residues from the C-terminus. Belongs to the orange carotenoid-binding protein family.