How many grey levels can the human eye distinguish?

Discussion in 'Off Topic' started by Mark R, Dec 11, 2005.

  1. Mark R

    Mark R Diamond Member

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    Answers, preferably with a reference please?

    Am worrying about an exam tomorrow, and this is the sort of thing they might ask.

    My text book says 35, but this seems a rather unlikely value.
    (This is also a text that says an 8 MHz monitor bandwidth is 'high resolution' - so I guess we differ in out definitions).
     
  2. arcenite

    arcenite Lifer

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    If your text book says 35, my guess is 35.
     
  3. randumb

    randumb Platinum Member

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  4. Anubis

    Anubis No Lifer

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    2.6 How many gray levels can the eye distinguish?

    Several studies have converged on the same answer, roughly 450. Eight bit color (256) will produce banding artifacts if mapped over a wide brightness range. The situation is complicated by the inequality of step sizes across brightness levels. At the low end steps should be very small while the high end needs larger steps (Weber?s Law). It is unlikely that 8 bits really supply 256 gray levels, let alone 450. In general, the steps at the low end are too large, so each gray level jumps more than one JND while at the high end, the steps are too small, so successive gray levels appear identical.

    But there is good news. CRT monitors have a defect, nonlinear gamma, that partly corrects this error by automatically decreasing step size at the low end and increasing the number of discriminable gray levels. Unfortunately, there is also bad news. Both the size of human brightness JND?s and the screen gamma change with viewing conditions, so the number of steps that is really available also changes and is somewhat unpredictable.

    2.7 How can I get more than 256 gray levels out of my computer by using color? Even a 24 bit system only supplies a maximum 256 levels of gray since it uses 3 eight bit numbers to control intensity. However, there is a simple trick which can improve gray level resolution.

    The relative intensity of the colors green-red-blue form a rough ratio of 4-2-1. The exact ratio will vary from monitor to monitor, but the order is always the same - green is most intense, then red, the blue. Now suppose that you have a gray at (red=50,green=50,blue=50). Normally, the next step would be (51,51,51). But you can create 6 intermediate steps between these two levels:

    2.8 How many saturation levels can the eye distinguish?

    Starting at white and going out to a pure spectral color, there are gradations in apparent saturation. However, some hues appear inherently more saturated than others. The most saturated yellow still appears pale compared to saturated red, saturated green and especially saturated blue. As a result, the number of distinguishable saturation levels is smaller in yellow than in the rest of the spectrum. One study concluded that there are only 10 saturation steps around yellow with the number gradually rising as wavelength increased or decreased. The lowest wavelengths, blue-violet had about 60 steps while the red reached about 50. Viewers will find small differences in blue, violet and red saturation highly discriminable while small differences in yellow saturation will be hard to detect. Green and orange are in the middle
     
  5. Baked

    Baked Lifer

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    It depends. Some people can't even tell the difference between red or green light.
     
  6. darkxshade

    darkxshade Lifer

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    I'd imagine you can distinguish more grey levels colorblind
     
  7. myusername

    myusername Diamond Member

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    It really does depend. Mostly on what subject the exam is in. If your textbook says 35, it is referring to the ability to distinguish non-adjacent gray patches, and would be a suitable answer for a bio or psych exam.
    If this is a computer course, the right answer is probably 256, though as anubis notes, this too is not the "right" answer
     
  8. BDawg

    BDawg Lifer

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    I would think that the answer is infinate. The eyes aren't a digital device, and can recognize an infinate number of variations.
     
  9. spidey07

    spidey07 No Lifer

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    AFAIK, eyes are a digital device. Either the rod/cone fires or not.
     
  10. Mark R

    Mark R Diamond Member

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    But the signal is in the rate of firing. Hence analogue.