( ( In luminescence ) ( , ( ( ( ( ( ( only ( a ( small fraction ) ) ) ( of atoms ) ) , ) ( called ( emission ( centers ( or ( luminescence centers ) ) ) ) ) ) , ) ( ( emit light ) . ) ) ) )
(ROOT (S (PP (IN In) (NP (NN luminescence))) (, ,) (NP (NP (RB only) (DT a) (JJ small) (NN fraction)) (PP (IN of) (NP (NNS atoms))) (, ,) (VP (VBN called) (NP (NN emission) (NNS centers) (CC or) (NN luminescence) (NNS centers))) (, ,)) (VP (VBP emit) (ADJP (JJ light))) (. .)))
In luminescence, only a small fraction of atoms, called emission centers or luminescence centers, emit light.
(ROOT (S (NP (NP (NNP Producing) (NN light)) (PP (IN without) (NP (DT a) (JJ high) (NN temperature)))) (VP (VBZ is) (VP (VBN called) (S (ADJP (JJ luminescence))))) (. .)))