Posted by
John T. Sharp on
Jul 10, 2006; 3:03pm
URL: http://imagej.273.s1.nabble.com/grayscale-displays-and-human-vision-tp3702214p3702224.html
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.
JTS
-------------- Original message ----------------------
From: Jeff Brandenburg <
[hidden email]>
> The human eye can perceive far more than 256 levels of gray, especially
> if you consider accommodation (the eye's mechanisms for adapting to
> very dark or very bright environments). However, 256 gray levels is a
> good match for the dynamic range of common scenes. It's also very
> convenient for computers. :-)
>
> Radiologists look at uncommon scenes, and have been trained to have
> uncommon perceptual abilities. For them, it can make sense to pay a
> high premium for a little extra detail. For the rest of us, it almost
> never does.
>
> My group works with small-animal volume images (CT, MRI) with up to
> 15-bit dynamic range. To take advantage of that range, we usually use
> window-level adjustments to stretch contrast in areas of interest,
> pegging uninteresting areas to 0 or 255. In other words, we stretch
> contrast so that a small portion of the data histogram is spread across
> the display's complete dynamic range. In my experience, trained
> radiologists also do the same thing when working with digital images --
> they can quickly "steer" the window-level settings to emphasize the
> detail they want to see. Even for them, it's easier to see
> low-contrast detail if you first stretch its contrast.
>
> If you're interested in human visual perception and how best to support
> it, I highly recommend Colin Ware's excellent _Information
> Visualization: Perception for Design_, ISBN 1558608192, now in its
> second edition.
>
> On Jul 6, 2006, at 2:01 PM, John Oreopoulos wrote:
>
> > Hello,
> >
> > I am new to Image J and I have a very general question about how
> > grayscale images are displayed on a computer monitor. I use a 12-bit
> > monochrome CCD camera to capture fluorescent microscope images and
> > save as .tiff files. When I open my images in ImageJ, the image is
> > displayed as an 8-bit (0-255) on the monitor. When I hover over a
> > pixel in the image with the mouse pointer, the 12-bit value (0-4095)
> > that was captured by the camera is listed in the ImageJ toolbar. I
> > looked at the ImageJ documentation and read up a little bit on digital
> > displays, and it seems that all standard computer monitors will
> > display grayscale images in 8-bit only. Why is this so? Is it
> > because of hardware limitations and costs? Is it because the human
> > eye can only detect 256 discrete shades of gray? If this is not the
> > case, then is there not some loss of information in the visual image
> > when it gets displayed at 8-bit? Am I losing some of the "true"
> > contrast when I look at my 12-bit images in ImageJ?
> > I did a google search on "12-bit grayscale displays" and found some
> > sites that sell special X-ray and MRI monitors with 12-bit or even
> > 16-bit grayscale resolution. If these kinds of monitors exist, then
> > this means the human eye can infact detect more than 256 shades of
> > gray, correct?
> >
> > Thank you in advance for any replies!
> >
> > John O
> >
> --
> -jeffB (Jeff Brandenburg, Duke Center for In-Vivo Microscopy)