Posted by
Jeff Brandenburg on
Jul 07, 2006; 8:58pm
URL: http://imagej.273.s1.nabble.com/grayscale-displays-and-human-vision-tp3702214p3702222.html
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)