Re: Ooops - D65 != D65 ???
Re: Ooops - D65 != D65 ???
- Subject: Re: Ooops - D65 != D65 ???
- From: Robin Myers <email@hidden>
- Date: Mon, 18 Jun 2007 14:20:15 -0700
On Jun 15, 2007, at 18:37 , Uli Zappe wrote:
Hi everybody,
back to this weird thing called "color management" after quite some
time ...
Currently, I have several colorimeter/spectrophotometer packages at
home for evaluation and comparison, among them the X-Rite/GM
product range (Eye-One Display, Eye-One Pro, huey Pro).
Soon after starting to create test profiles, I realized that even
if I set the 3 different X-Rite products all to exactly the same
whitepoint/gamma settings, e.g. D65 G1.8, they produced profiles
with *very* different whitepoints.
Now, I'm aware that (speaking in terms of an xyY chromaticity
chart) there's many different possible (x,y) combinations to
produce a 6500 K whitepoint, but somehow I would have thought that
a D65 whitepoint is an unambiguous standard that defines one of the
possible combinations as a standard.
D65 is standardized and unambiguous. Tristimulus values of X 95.04, Y
100.00, Z 108.89, chromaticity coordinates of x 0.3127, y 0.3290.
Also, I'm aware of the argument that the whitepoint setting isn't
too important, anyway.
It can be very important, depending on the task and how well you
believe the color adaptation transforms (CAT) are working to convert
colors relative to one white point to another. This is a big subject
so I will not go further into it here.
Still, I would think it should be reasonable to expect some kind of
consistency at least in the product range of *one single
manufacturer* ... What's the point of having a standardized D65
setting if it means something different to each measurement device?
That doesn't exactly build confidence with regard to color management.
You are probably encountering measurement error between the devices.
Although the i1 is a very good and useful color measuring device,
there is an uncertainty to its measurements. These uncertainties can
be due to variations within the manufacturing tolerances, ambient
temperature, the time between measurements, etc. The manufacturer
specifies an average inter-instrument agreement of 0.4 dE94 with a
peak agreement error of 1.0 dE94. That inter-instrument difference
range will result in different chromaticity values. Even for the same
instrument the short term repeatability for emissive display
measurement is xy +- 0.002 when measuring the monitor white. Other
colors may have more variation. For reflective measurements the short
term repeatability is listed as an average of 0.1 dE94 for 10
measurements taken every 3 seconds. If you notice, the standard
deviation of these samplings is not listed and would be helpful to
see the instrument's variability.
If you are measuring fluorescent illuminated LCD displays, there are
more variables that can influence the measurement. For instance, the
amount of time warming up the display, whether the LCD is in a laptop
or a desktop display, the ambient temperature, room illumination
levels and color, etc.
Because of all these variables, and more, if you are getting monitor
chromaticity value repeatability with the same instrument to the
third decimal place, then you are doing very well. 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.
I'm very puzzled by this experience, and would appreciate comments
from the experts in this forum. Do I expect something unreasonable?
Do others experience consistent D65 results and I'm doing something
wrong (no idea what that could be, though)?
One additional remark: I was amazed that even in the expensive Eye-
One Pro package, the software is nothing more than a black box that
somehow produces a profile. There's no way to access the and work
with the physical measurement data at all. Is this generally the
case with this kind of products (well, AFAIK there aren't many
alternatives to an Eye-One Pro XT package, anyway), a kind of
"people in the graphics industry are no scientists" syndrom?
(That's really a difference to the music industry, BTW. When you
buy a package to calibrate a loudspeaker, you'll have access to the
basic physics of the process as much as you want.) Is there third
party software to fill this (expensive) void? So far, I stumbled
across SpectraShop and BabelColors, but hadn't time yet to look if
they really do this kind of thing.
As for modifying the physics of the spectrometer, very few
instruments allow this. A spectrometer is a precision aligned device
and too fault intolerant to allow dynamic adjustment by the user.
There are some instruments that allow for selecting the type of
diffraction grating and slit when the product is ordered, and some
allow for adjusting the slit opening at run time. Some devices allow
for switching the measuring head; transmissive, reflective, emissive,
45/0 geometry or integrating sphere, etc. But the basic wavelength
tuning and light sensing parts are locked down.
Speaking for SpectraShop, it can take in data from an i1 and let you
work with it in many ways. As far as I know it is the only software
with a built-in spectral calculator that lets you combine spectra in
many different ways, including; addition, subtraction,
multiplication, division, averaging, scaling, extrapolation and
normalization. If it does not have the type of operations you need,
you may export the data into a text file for spreadsheet processing.
There are lots of built-in calculations for things like spectral
searches, metamerism indices, color inconstancy, CRI, etc. Please
contact me offline if you need more information.
Robin Myers
email@hidden
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