Āloka-saññā: Perception of light. The recurring canonical passage reads: Here the Bhikkhu contemplates the perception of light. He fixes his-mind to the experience of the daylight; as at day-time so at night, and as at night, so in the day. In this way, with a clear and unclouded mind, he develops the stage of mind that is full of brightness. It is one of the methods of overcoming drowsiness, recommended by the Buddha to Mahā-Moggallāna A. VII, 58. According to D. 33, it is conducive to the development of 'knowledge and vision.'
Maha Thera Nyanatiloka. Manual of Buddhist Terms and Doctrines, Buddhist Publication Society, first edition 1952.