RE: gamma on Mac vs. Windows PC
RE: gamma on Mac vs. Windows PC
- Subject: RE: gamma on Mac vs. Windows PC
- From: "Andy Tisdale" <email@hidden>
- Date: Wed, 18 Jul 2001 18:50:05 -0400
Hi Folks
The gamma component of the display we see is dependant on the Display/ Video
card combo
The gamma of any display monitor before it is attached to a computer is a
fixed constant and is determined when the electron gun of the monitor is
sealed in to the CRT Vacum. It is the measured relationship between the
video voltage in vs the amount of light the CRT Outputs and in TV theory is
on average of x rasied to a power of 2.5 where x is equal to the voltage
applied to the CRT or what we know and the signal comming out of the
computer. This is refered to in video engineering as the 5/2 power law and
is taken into account when a TV signal is braodcast. In TV land (where this
issues was frist hashed out by the SMPTE a TV Picture tube has a gamma of
2.5 so rather then apply a gamma coreection in each TV set the video signal
(which is nominally linear and would produce a very dark and contrasty
output) a gamma correction applied at the camera or broadcast source to it
so that the resulting TV picture you receive is properly balanced for a
dimmed surround viewing environment.
Now most computer monitor CRT's today have a nominal gamma of between 2.2
and 2.4. This is the fixed physical charateristic curve of the CRT with a
liner voltage applied to the input. Thus a video card sending and
uncorrected gamma signall to a monitor is sending linear data or a gamma of
unity or 1.0 1.0(Video Card gamma) X 2.20 (CRT Characteristic curve yields
a dispalyed image with a gamma of 2.2 A PC Video Card has a default setting
which produces a linear output at start up. NT /2000 have no controls to
adjust the LUT value and thus the display always displays a gamma of 2.2.
A Mac on the other hand has a default setting whereby a LUT value other then
Unity is applied at start up. The default quick draw driver sets the video
card LUT value to .82 .82x the 2.2 monitor characterstice curve yields a
displayed gamma of 1.8. In each case the monitor has the same gamma response
2.2 it is the video signal which has changed gray scale we see on screen.
If you were to get extremely nit picky no two displays have identical gamma
response and in fact within each CRT the individual RGB guns vary in their
gamma responses so that apply a singly LUT correction will actually yield
three different response gamma's though the differences are so minor that
they rarely affect the perceived color on screen.
If you actually wanted to see linear gamma on screen you would have to apply
a gamma correction at the Video Card LUT that was the inverse to that CRT's
natural gamma characteristic curve. or 1/2.2
or .45 this is in fact a typical televison camera gamma corection. TV's are
actually set to display blacker then black and the dimmed surround is approx
a gamma of 2.2 so you TV looks good a night.
On a calibrated monitor this would not work everything would be to bright.
Hope this helps,
Best Regards,
Andy Tisdale
President/Color Scientist
A to Z Color Consulting
829 E. Rollingwood Rd.
Aiken, SC. 29801
Phone: (803)649-4679
Cell Phone:(803)439-4190
Fax: (803)6444-5112
email@hidden
http://www.atozcolor.com
"We know color from A to Z"
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