eye sensitivity

Note on the Eye Response

          The figure to the right is just a schematic reproduction of the standard eye sensitivity curves given by the CIE. Schematic means I crudely drew the curves by hand trying to match a printed figure: qualitatively they are OK if you accept that the glitches are meaningless. I've not yet found a source that tells me exactly what PHOTOPIC and SCOTOPIC conditions are: they probably correspond to average day-like and average night-like conditions. You will note that eyes are much more sensitive in the dark, but, of course, there is a lot less light to see there. Also under dark conditions color vision is much reduced.

          Actually humans can see in the INFRARED out to at least 900 nm if the source is intense enough. But maybe it is not safe to look at such sources. Alas, seeing infrared is no revelation---not a unspeakable new color---infrared just looks deep red. Reference: Pedrotti, L.S., & Pedrotti, F. L. 1998, ``Optics and Vision'' [Upper Saddle River, New Jersey, Prentice Hall], p. 19.

          One should be aware that the curves measure PSYCHO-PHYSICAL response and therefore are strongly dependent on exactly the conditions of measurement. The GSU HyperPhysics's page on vision perceived brightness gives a more details than yours truly. It also gives an accurate figure and the tabulated CIE curves.

          It seems that eye response varies APPROXIMATELY LOGARITHMICALLY with intensity in the photopic an scotopic ranges, but with a different constant. Again what one is measuring is psycho-physical response, and so the response depends on how the measurement is done to a great extent.

          The whole question of eye response is complex and misstatements by the unwary (i.e., yours truly) are easily made. The web is no great help since the pages either give the brief qualitative answers without references or wend off into complicated eye response jargon.

          Below are some URLs that seem a bit more clear than most. You'll note that one page asserts eye response goes as a power-law with intensity which I've heard before. But no references are given and maybe that URL is talking about different conditions. I'm cutting the Gordian knot by finishing this disquisition without knowing exactly how the bits fit together.

    eye is power law sensitive with  power of .4,
    not logarithmic

    says approximately logarithmic.
    It was Norman R. Pogson in 1856
   another paper that says approximately logarithmic

    Another vote for logarithmic response.
    a recent Russian paper says logarithmic

    Says log and has a plot and updated 1999.
    There is even a reference:  Gonzalez and Woods, 1992.