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Re: Display on a 12bit monochrome monitor

Posted by Michael Schmid on Jan 26, 2010; 6:53pm
URL: http://imagej.273.s1.nabble.com/Display-on-a-12bit-monochrome-monitor-tp3689586p3689590.html

Hi everyone,

a few further remarks on this never-ending topic:

(1) The transfer curve is nonlinear, roughly quadratic (it would be  
quadratic at gamma=2, see http://en.wikipedia.org/wiki/ 
Gamma_correction). With a quadratic curve, the difference between  
gray levels 254 and 255 is roughly 0.8%, no problem. The difference  
between 127 and 128 is 1.6%, which is noticeable, and it becomes  
worse at lower levels.
That's also why sRGB uses a lower effective gamma at lower light levels.

(2) LCD monitors connected by analog (VGA) signals cause additional  
problems due to D/A and A/D conversion. If you create a grayscale map  
(new image, 8-bit, 512*512 pixels, fill with ramp) you can easily see  
artifacts on almost all monitors.

Even without the additional D/A+A/D conversion there are often  
artifacts. If I try on my iMac with built-in LCD screen, apart form  
the weak steps at low intensities (almost invisible above approx.  
gray level 130) I can clearly see vertical stripes on such a zoomed  
ramp image, especially when zooming in and panning the image (space  
bar down). Careful Monitor calibration (Control panel  
'Displays'>Color, expert mode) leads to only a slight improvement.

So in my view, the best one can do (apart from ultraexpensive  
radiology systems) is avoiding analog (VGA) connections and checking  
a ramp image before selecting a graphics card and monitor.


Michael
________________________________________________________________

On 26 Jan 2010, at 13:51, Gluender wrote:

> Hello,
>
> it appears as if you're approaching the facts...
>
>> Even if it isn't strictly subject related
>>
>> Sure that human vision can distinguish more than 8 bit grayscale  
>> images!!!.
>> That what human vision can't do is to distinguish more than  
>> 128/256 gray
>> levels simultaneously (i.e. brain has much more narrower dynamic  
>> range that
>> eyes who must adapt them to the average luminance of the field of  
>> view).
>
> According to the classic measurements of Steinhardt (1936), humans  
> show a threshold for light density (luminance) discrimination (just  
> noticeable difference) of at best 1% (Weber fraction).
> "At best" here implies a mean density of around 10^4 Troland and a  
> test area of larger than 25 degrees of visual angle.
> For test fields of 0.5 degrees of visual angle the Weber fraction  
> approaches 10%.
>
> With respect to the earlier post of Marcel:
>
> "Print products (e.g. the good old analogue BW photography paper)  
> do have a
> lot wider gray scale ranges than monitors and you are able to see  
> that."
>
> ...I should like to add that in principle b&w-photographic material  
> shows essentially a binary behaviour at least on the microscopic  
> level. (Facts to think about...)
>
> Best
> --
>
>                   Herbie
>
>          ------------------------
>          <http://www.gluender.de>