1991, COSPAR. Plenary Meeting, Jean-Louis Fellous, COSPAR., Global change and relevant space observations: proceedings of Symposium 1 of the COSPAR twenty-eighth Plenary Meeting held in The Hague, the Netherlands, 25 June-6 July 1990, Pergamon For higher degrees, the spatial integration tends to produce a cancellation and a rapid loss of visibility, although less rapidly for velocity measurement than for luminosity measurement, on account of the directivity of the velocity vector.
Source: wiktionary