It may be laid down that there is no really non-actinic illumination in the true sense of the word; that is to say, that there is no light, whatever may be its spectral distribution and however weak in intensity it may be, which will not fog a photographic emulsion if allowed to act for long enough.

—L.P. Clerc, “Photography—Theory and Practice”, 2nd edition, Pitmann Press, London, 1937, p. 166.