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>