Sensitivity of human eye to luminance I heard once that the human eye has a logarithmic scale for luminance, e.g. to "feel" that a surface is three times as luminous compared to another, the former emits a light 8 times more powerful than the latter. Definitions are indeed vague as my memory doesn't serve me well.

Is this based on anything scientific?
From Stevens &amp; Galanter (1957)


  Although an extensive investigation of the subjective scale of
  brightness is still in progress in this laboratory, enough has been
  learned to show that, for patches of white light viewed in a dark
  room, subjective brightness is a power function of luminance.
  Moreover, the exponent is of the order of one-third which is in
  reasonable agreement with results obtained by Hanes.





Stevens, S. S., &amp; Galanter, E. H. (1957). Ratio scales and category scales for a dozen perceptual continua. Journal of Experimental Psychology, 54(6), 377.


and the related study that is mentioned:


Hanes, R. M. (1949). The construction of subjective brightness scales from fractionation data: a validation. Journal of experimental psychology, 39(5), 719.