Dynamic range of the human eye

Discussion in 'Eye-Care' started by Joerg Sczepek, Apr 11, 2005.

  1. Hi ng,

    I read a lot of articles and postings regarding the dynamic range (or
    lightness range) the human eye can deal with, but I´m not quite sure
    if I got things right. So maybe someone can help me answer some
    questions regarding this.

    Does scientific data prove that the eye can generally handle intensity
    levels on the order of 10 raised to the power of 4 in scotopic vision
    and 10 raised to the power of 6 in photopic vison ? And, as I´m
    interested in photography, what would those values mean on the
    logarithmic scale of f/stops ?

    As far as I understood things the visual system doesn´t operate over
    such a wide dynamic range simultaneosly, but accomplishes the task by
    changing it´s overall sensitivity (brightness adaptation). So is my
    imagination right that, when the eye roams through an ordinary seeing
    situation, it constantly brings it´s adaptation level in line with the
    average brightness of the scene that dominates the field of view ? And
    : How large is the resulting dynamic range for any of those levels ?
    - I think this value must be defined by the light absorption
    characteristics of the Rhodopsin in the rods and cones.

    In short : What the visual system does then, is sampling together a
    patchwork of single images, produced through the ever moving eye, of
    which each is made with the producing machinery adjusted to the
    specialized sensitivity.
     
    Joerg Sczepek, Apr 11, 2005
    #1
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  2. Joerg Sczepek

    otisbrown Guest

    Dear Joerg,
    The primate eye is truly a marvel of engineering.

    I think that the aperture accounts for intensity
    control of about 100.

    Retinal adaptation takes care of the rest.

    You have made more a statement -- rather than
    asking a question.

    Best,

    Otis
     
    otisbrown, Apr 11, 2005
    #2
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  3. Joerg Sczepek

    retinula Guest

    what units of intensity are your referring to? lumens? foot-candles?
    light adaption makes the eye functional over about 10 orders of
    magnitude of light intensity
    basically, yes

    well, not entirely. adaption occurs at many other levels within the
    photoreceptor cell, as well as the brain.

    Please see the following references for further reading on this topic:
    http://www.ncbi.nlm.nih.gov/entrez/...ve&db=pubmed&list_uids=10448166&dopt=Abstract
    http://www.jgp.org/cgi/content/full/116/6/791
     
    retinula, Apr 11, 2005
    #3

  4. Well, yes it´s a statement with two minor questions and the major one
    if the general picture is correct.
    About the intensity levels : I dont´t knwo myself to what units they
    refer but they are the only values I found when reading through a
    bunch of websites.
     
    Joerg Sczepek, Apr 11, 2005
    #4
  5. (Joerg Sczepek) wrote in
    Actually, to some extent, but its much more related to the connection
    architecture used in the eye. Because of the "lateral inhibition" that
    takes place at the retina, the dynamic range of vision is many times
    greater than one would expect from the behavior of any photoreceptor in
    isolation. Usually, people think of lateral inhibition in retina as the
    mechanism by which contrast is enhanced at the retinal level, and it does
    this exquisitely, but it also has great manifestation upon the dynamic
    range.

    There are many modeling and physiology studies about how the eye moves to
    sample a scene and how the brain takes this information and processes it,
    but I don't think any such researchers have asserted any relationship to
    dynamic range.

    Scott
     
    Scott Seidman, Apr 11, 2005
    #5

  6. Dear Scott,

    Lateral inhibition as a way to expand the dynamic range - that´s
    interesting !
    I read something about multiplicative lateral inhibition (shunting
    inhibition). In this model a proportion of the output signals of each
    cell and it´s neighbors is subtracted from the signal in each channel.
    As I do not understand how that could expand the dynamic range and you
    seem to be quite knowledgeable about the stuff could you please
    explain the concept a little further ?

    Thanks !
    Jörg
     
    Joerg Sczepek, Apr 13, 2005
    #6
  7. (Joerg Sczepek) wrote in
    It's a little something that Terry Sejnowski taught me in a class almost 20
    years ago.

    If every cell on retina is inhibited by its neighbors, then each cell will
    fire (well, photoreceptors don't have action potentials, but the idea is
    the same) at some rate based on the log of its own illumination, minus the
    inhibitory effects from its neighbors. Keep in mind that if the neighbors
    are illuminated at the same level, their base rate is the same as that of
    the original cell. (BTW, this does seem similar to your "multiplicative
    lateral inhibition, but I haven't seen it termed like that before. A quick
    search for "lateral inhibition retina" should yield you some gold with
    respect to the basic neural substrate physiology)

    If you think about this, if the inhibition is anything less than zero,
    then the receptor must be firing slower (still making believe that
    photoreceptors fire, and are excited, rather than hyperpolarized, by
    illumination, but it makes no difference for this derivation) than it would
    for the same level of illumination shining on that receptor alone, if that
    receptor were isolated from the net, and it would take a brighter light to
    make it fire the same rate while in the network. Wallah-- increased
    dynamic range. The more neurons connected together in this way, the
    greater the effect. This way, we can have exquisitely sensitive
    photoreceptors, while still maintaining a useful dynamic range. This still
    isn't wide enough, though, which is why we still have two systems-- rods
    and cones -- of different levels of sensitivity that work best under very
    different levels of illumination.

    Note that this same network, through disinhibition, has the effect of
    highlighting sudden changes in contrast--a form of edge detection, right at
    early levels of retina.

    The latter is how most people think of the functionality of lateral
    inhibition in retina, but the dynamic range issue is just as important.

    That same year, DA Robinson showed us that exactly the same network, taking
    advantage of membrane time constants, serves as a very stable neural
    integrator in the brainstem (this was work by his student, Steve Cannon).
    I don't know if anyone has shown that the network works this way in retina
    or not, but if the response to a light pulse were perpetuated, I wouldn't
    be surprised.

    Thus, lateral inhibition brings us
    1- improved dynamic range
    2- spatial differentiation (i.e., edge detection)
    3- neural integration

    It's my favorite morphology, and I try to get that across to my students.

    Scott
     
    Scott Seidman, Apr 13, 2005
    #7
  8. Joerg Sczepek

    g.gatti Guest

    Do you think that imperfect eyesight corrected by eyeglasses has the
    same magnitude of light intensity?

    I'm very much interested in your answer.
     
    g.gatti, Apr 13, 2005
    #8
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