Re: Ooops - D65 != D65 ???
Re: Ooops - D65 != D65 ???
- Subject: Re: Ooops - D65 != D65 ???
- From: Robin Myers <email@hidden>
- Date: Wed, 20 Jun 2007 09:37:51 -0700
On Jun 19, 2007, at 21:18 , Uli Zappe wrote:
<snipped>
When I enter 6500 Kelvin in the "user-defined
whitepoint" [translated from the German GUI] dialog panel, the
panel displays an xy of 0.314 / 0.324. Robin Myers said at the
beginning of this thread that D65 is defined as 0.3127 / 0.329,
which is also what Eye-One Match reports for 6500 Kelvin in its
corresponding dialog panel. Now where does that inconsistency come
from?
But, anyway - now that I have a chance to actually measure it, I
can report what the various devices regard as D65 = 0.314 / 0.324
or 0.3127 / 0.329 (of course the Spyder is consistent to itself, so
the results below don't tell which device is "objectively" right,
but rather the difference *between* the devices)
Eye-One Display: 0.315 0.341 6316 K
Eye-One Pro: 0.314 0.333 6407 K
huey: 0.307 0.328 6836 K
Spyder2Pro: 0.313 0.325 6510 K
(0.311 0.321 6657 K with ICC v4; is the
chromatic adaptation transformation lossy?)
(Kelvin calculated by entering the xy results into Spyder's "user-
defined whitepoint" dialog panel)
So, according to the Spyder ;-) the Spyder comes close, both Eye-
Ones have too much y to varying degrees, and the huey is just
faaaar off with x.
Robin Myers said at the beginning of this thread:
If you are getting some variability in the third decimal place
between instruments then it might be alright, depending on the
amount. If you are only getting repeatability to the second
decimal place, then something is terribly wrong with the
instruments, the display, or both.
As the above shows, there's not even repeatability to the second
decimal place, so something *is* terribly wrong - that's why I
started this thread ;-) (But note that there *is* repeatability
+-0.001 for each device in itself!)
You are confusing "repeatability" with "inter-instrument agreement".
"Repeatability" is the same instrument giving you the same
measurement. "Inter-instrument agreement" is having different
instruments give you the same measurement result. The above results
table is showing the variability of the readings between instruments,
or the "inter-instrument agreement", or lack thereof.
Looking at the error in your measurements we have:
x y dx dy
D65 Target 0.313 0.329
i1 Display 0.315 0.341 +0.002 +0.012
i1 Pro 0.314 0.333 +0.001 +0.004
Huey 0.307 0.328 -0.006 -0.001
Spyder2Pro 0.313 0.325 0.000 -0.004
The dx and dy values are the differences in the x and y coordinates
from the reference D65 values.
As you can see, the i1 Pro and the Spyder2Pro show almost the same
error and they were the best of the test. The questions raised by
these measurements is how they were sampled? Were they single
measurements? Were they multiple measurements and averaged? If
averaged, how many measurements? What was the ambient temperature?
The warm-up time? Were the measurements made in exactly the same
place on the screen? Was the instrument in the exact same orientation
(LCD screens emit polarized light which effects the measurements
differently for each type of measuring device as shown in the
following report:
http://www.rmimaging.com/information/lcd_spectro.html
As you are discovering, measuring emissive displays in ways that
remove the variables and allow true comparisons between instruments
is a very difficult task.
I also performed tests with a "native whitepoint" setting (apart
from the huey, which does not have such a setting), which at that
time Spyder's Info window reported to be 0.311 / 0.338
Eye-One Display: 0.309 0.337
Eye-One Pro: 0.310 0.337
Spyder2Pro: 0.310 0.337
So here the results are basically consistent. I still don't know
enough about all this, but I *assume* that a "native whitepoint"
setting basically just means *omitting* a calibration step, and if
the issue happens during this step, it's clear why this setting
will work. :-)
Since the instruments are agreeing with each other with respect to
the native white point measurement, other places to look for the
error in your D65 settings are in the various software packages, the
video card's ability to achieve a particular white point and the
monitor's ability to produce the requested white point.
Robin Myers
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