Bṛhadāraṇyaka Upaniṣad 2.4.55. The one without a fixed shape, on the other hand, consists of breath and the space within the body; it is immortal and in motion; and it is Tyam. The person within the right eye is the essence of the one that is without a fixed shape, that is immortal and in motion, and that is Tyam—for he is the essence of Tyam.
6. Now, the visible appearance of this person is like a golden cloth, or white wool, or a red bug, or a flame, or a white lotus, or a sudden flash of lightning. And when a man knows this, his splendour unfolds like a sudden flash of lightning. Here, then, is the rule of substitution: "not—, not—," for there is nothing beyond this "not." And this is the name—"the real behind the real," for the real consists of the vital functions, and he is the real behind the vital functions
The "person" referred to hear is "nirguna Brahman" or "Brahman without attributes", the ultimate Being beyond all words. It seems the author here has taken the nimittas experienced in deep meditation for Brahman.