Re: Two Displays, One Graphics Card, Two Profiles, Which Profile wins?
Re: Two Displays, One Graphics Card, Two Profiles, Which Profile wins?
- Subject: Re: Two Displays, One Graphics Card, Two Profiles, Which Profile wins?
- From: Peter Karp <email@hidden>
- Date: Fri, 4 Feb 2005 13:18:09 +0100
Hi Andrew,
> I have an 20" Apple Cinema Display (DVI) and a 17" Apple Studio
> Display (ADC) plugged into the Radeon 9600 on my G5 2.5DP.
> After profiling, the two displays still look different. I have a the
> same photo as a background on both displays, and the colors are
> slightly different.
That's normal. You'll not achieve a perfect match between different
display types.
Measuring the color values of a display accurate is not trivial.
Especially for older flat panel displays with a narrow viewing angle
it's hard to measure meaningful tristimulus values.
Lower-priced colorimeters and spectrophotometers (I mean all the usual
devices like the DTP-series, the EyeOne series, the Spyder...) use a
not too sophisticated "filter system" which can not truly describe the
CIE 1931 standard observer color matching functions. Those devices are
calibrated to a "standard display", but the calibration can not take
all the different display types into account. So that's another
reason why you won't get an absolute match between different displays
(especially with different panel tpyes or LCD vs. CRT...). Last not
least even with a "perfect" measurement device (maybe the LMT
colorimeters are closest to that goal) you'll still have the observer
metamerism -- in this case between you as an observer and the
"standard observer".
Maybe Tom Lianza can to comment in more detail here?
> From the Gretag Macbeth website:
>> Please note, the monitor profiling process in i1Match supports two
>> steps: the first step is the monitor calibration, which stores an
>> individual gamma table in the graphic card. The second step is the
>> profile generation, which stores an individual ICC monitor profile as
>> current system profile in the control panel "Display". If you have a
>> dual monitor card with one graphic ship and two output signals, it is
>> not possible to store an individual gamma table for each monitor (both
>> monitors will be controlled by the same gamma table). But it is
>> possible to set an individual ICC profile for each monitor in the
>> system's control panel "Display". On dual monitor cards with one
>> graphic chips you need to decide, which of the two monitors should be
>> used for color critical work. However, we recommend to use a dual
>> graphic card with two graphic chips.
> Is this the cause of the color difference?
No, this statement is true for the Windows OS. On a Mac the operating
system (OS 9 or X) can use different profiles for a dual head graphics
card with a single graphic chip. You don't have to worry about that
when using a Mac. BTW, on a PC you can use a second graphics card to
overcome this limitation of the operating systems colormanagement
options. I'm pretty confident that this will be improved in
Longhorn...
> Do I need to use the same profile for both displays?
No, you'll want to calibrate each display indiviudally (to the same
target settings of course).
> If not, how do I make sure that I am using the gamma table from the
> profile for the primary display (the 20" cinema display).
To check which calibration curves are active on the video card LUT,
you can use the program "Gammas" which is found as an example
application of the Mac OS X. You need to install the fourth CD with the
developer tools. If you like to, I can upload the tiny file to my
website, so you don't need to install some GB on your hard drive to
get a 10kB zipped program ;-)
> As a side note, both displays are supposed to have a native white point
> of 6500. The 17" was measured as 5900 and the 20" was measured at
> 7100.
That's in the usual range of variations. The preset values are often
not very accurate.
Best regards
Peter Karp
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