首楞嚴義疏注經 9.26

9.26

吸取、任持無量淨光，以表言詮，名為教體。此光表詮離諸麁重，故云發化清淨。隨機開示無不明了，故云應用無盡。以二禪界地無前五識，但用光明以為表詮，以光為音，名光音天。

Drawing in and holding limitless pure light to express verbal explanation is called "the substance of teaching." This light expressing verbalization is free from all coarseness and heaviness, hence it is said, "emitting pure transformations." Manifesting instructions according with capacities with complete clarity, it is said, "its responsive applications are inexhaustible." Because the stage of the second dhyāna lacks the first five consciousnesses, they merely use radiance for verbal expression; using light as sound, it is called the Heaven of Radiant Sound.