Required for non-photochemical quenching (NPQ), a mechanism that converts and dissipates the harmful excess absorbed light energy into heat and protect the photosynthetic apparatus from photo-oxidative damage (PubMed:27335457, PubMed:29555769, PubMed:30782831). Is able to sense luminal acidification of the thylakoid membranes, which occurs along with elevated electron flow caused by excess light, and to induce a large, fast, and reversible pH-dependent quenching in LHCII-containing membranes (PubMed:27335457). Mediates excitation energy transfer from light-harvesting complex II (LHCII) to photosystem I (PSI), rather than photosystem II (PSII), at low pH, which mimics the acidified lumen of the thylakoid membranes in high light-exposed chloroplasts (PubMed:29555769, PubMed:30782831). Activates PSI-dependent fluorescence quenching in addition to dissipating excitation energy in LHCII to avoid photooxidative stress under excess light (PubMed:29555769, PubMed:30782831). Transiently induced by transition from dark to light (at protein level) (PubMed:8980495, PubMed:10794530). Induced by high light stress (at protein level) (PubMed:19940928, PubMed:24850838). Belongs to the light-harvesting chlorophyll a/b-binding (LHC) protein family.