'perception of light'.
The recurring canonical passage reads:
"Here the monk contemplates the perception of light. He fixes his-mind to the perception of the day; as at day-time so at night, and as at night, so in the day. In this way, with a mind clear and unclouded, he develops a 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' (s. visuddhi), and it is said to be helpful to the attainment of the 'divine eye' (s. abhiññā).