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:19940928, PubMed:21267060, PubMed:23716695, PubMed:26817847, PubMed:27626383, PubMed:27693674, PubMed:28233792, PubMed:31042040, PubMed:30782831). NPQ includes dissipating excess light energy to heat (qE) and the reversible coupling of LHCII to photosystems (state transitions or qT), which are considered separate NPQ mechanisms (PubMed:28233792). Is responsible for most of the excess light energy to heat dissipation (qE), also known as energy-dependent chlorophyll fluorescence quenching activity of chlorophyll excited states (PubMed:21267060, PubMed:28233792). Involved in a de-coupling and re-coupling of energy transfer to photosystem II (PSII) during qT (PubMed:28233792). Binds chlorophyll a and b (PubMed:21267060, PubMed:27150505). Is able to sense luminal acidification of the thylakoid membranes, which occurs along with elevated electron flow caused by excess light (PubMed:21267060, PubMed:26817847, PubMed:31042040). Establishes interactions with photosystem II (PSII) antenna components upon lumen acidification, and protonation of lumen-exposed, negatively charged residues both in LHCSR3 and in PSII antenna components (PubMed:26817847). 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:30782831). Activates PSI-dependent fluorescence quenching in addition to dissipating excitation energy in LHCII to avoid photooxidative stress under excess light (PubMed:30782831). Contributes with PGRL1 to the regulation of electron flow upstream of photosystem I (PSI), and limits the accumulation of electrons on the PSI acceptor side, thus avoiding PSI photoinhibition (PubMed:27693674). Interacts with the photosystem II-light-harvesting complex II (PSII-LHCII) supercomplex to form PSII-LHCII-LHCSR3 supercomplex. Induced by sulfur deprivation (PubMed:15470261). Induced by high light stress (at protein level) (PubMed:19940928, PubMed:21267060, PubMed:24850838, PubMed:27358399, PubMed:27693674). Impaired in non-photochemical quenching (NPQ) under high light conditions. Belongs to the light-harvesting chlorophyll a/b-binding (LHC) protein family.