Posted by
Jeff Brandenburg on
URL: http://imagej.273.s1.nabble.com/grayscale-displays-and-human-vision-tp3702214p3702226.html
On Jul 10, 2006, at 10:03 AM, John T. Sharp wrote:
> I'm not an authority on human vision but I read somewhere that the
> human eye could distingusih approximately 16 different gray levels.
> This is far less than 256 of 8 bit. In fact it is only 3 bit.
>
> I tested this in a small demonstration with a group of colleagues who
> spend a lot of time reading x-ray films of the hands and feet and 16
> levels appeared to be about what the best could do.
>
> Try it your self. You can compose images with gray scales of anything
> between 0 and 256. Code them, mix them up in random fasion and ask
> your colleagues to tell you which is whiter (or blacker ) comparing A
> to B, A to C, A to D etc.
Well, sort of. Being able to identify unique shades *in isolation*, or
in non-contiguous comparisons, is a very different problem from
distinguishing contrast within a scene.
In reality, I'd say that 16 is way too high, because our perception of
gray levels is so tied up with foreground illumination, background
illumination, and so forth. We've all probably seen the optical
illusions where a 50% gray square against a white background looks
darker than a 50% gray square against a black background.
As Joachim Wesner pointed out before I could finish this message :-),
gray-level resolution is tested most severely in areas of smooth
gradation. There's even a well-known term, "posterization", for the
visual effect that you get when you reduce the number of steps too far.
If you're curious, load your favorite grayscale image into ImageJ, and
try Process:Math:AND... with a value of 11110000 to reduce the image to
16 levels of gray. (The boats.gif sample image is a good one.) It's
*very* easy to see the difference!
--
-jeffB (Jeff Brandenburg, Duke Center for In-Vivo Microscopy)