Ñānadassana-visuddhi: 'purification of knowledge and vision', is the last of the 7 purifications and a name for path-knowledge maggañāna i.e. the penetrating realization of the path of Stream-winning, Once-returning, Non-returning or Arahantship. Vis.M XXII furnishes a detailed explanation of it see: visuddhi.
In A. IV, 41 ñānadassana apparently means the divine eye dibbacakkhu abhiññā being produced through concentrating the mind on light.
Maha Thera Nyanatiloka. Manual of Buddhist Terms and Doctrines, Buddhist Publication Society, first edition 1952.