Re: Quoting accuracy of color measurement instruments
Re: Quoting accuracy of color measurement instruments
- Subject: Re: Quoting accuracy of color measurement instruments
- From: David Scharf <email@hidden>
- Date: Wed, 31 Oct 2007 01:27:05 -0700
iOne Pro: Wow, 35 or 36 bands in 10 nm steps, but only measuring 3.5 nm
for each 10 nm band? (According to the on-line brochure). That's a 350
nm spectral range but only 122.5 nm of that range is actually measured
(the rest interpolated, I assume). Do I understand this correctly?
My SpectroCam has 72 5nm bands. That's twice the resolution! Perhaps I
won't get a new spectro after all. A serial to USB adapter will work. I
just wonder how weak a link the old ProfilerPro package is, since none
of the new profiling packages support it. Or can the newer profiling
packages import the measurement table output by the SpectroCam?
Any insights or commments?
Regards all,
David
*DAVID SCHARF PHOTOGRAPHY*
Scanning Electron Microscopy
Phone 323-666-8657
Los Angeles, CA 90039
http://www.scharfphoto.com
http://www.electronmicro.com
tom lianza wrote:
Hi to all,
The subject of quoting accuracy often comes up and people ask why we
don't quote accuracy numbers even more often. The simple answer to
that : we really don't know how to quote accuracy of a spectral
device in a meaningful way.
Roger wrote the following
What I'm particularely after is the "Accuracy" in x,y for a white point
(presumably 6500K, on a choice of Reference Monitor) and, if possible, as
that would be the cherry on the icing, "Accuracy" in x,y for R,G,B.
My research into other instrument's performance typically turned out
that,
on a white point, Accuracy is often quoted in the order of ±0.002
while it
is often in the order of ±0.004 or (worse) ±0.006 for R,G,B.
Here is a quote from the NIST laboratory about how they calibrate a
standard instrument:
from the link:
http://ts.nist.gov/MeasurementServices/Calibrations/photometric.cfm
NIST will calibrate submitted colorimeters or spectroradiometers for
chromaticity coordinates (x,y) of several colors of a display. The
instrument under test will be calibrated against a NIST reference
spectroradiometer, using a standard CRT display or and LCD, or any
other display submitted by the customer. The calibration will be
performed for the primary colors (red, green, and blue) plus white of
the display, and a correction matrix for the Four-Color Method will be
provided, which can be used to correct the errors for any other colors
of the display measured with the test instrument. The expanded
uncertainty (/k/=2) of the calibration is typically 0.001 to 0.002 in
(x,y) depending on the color and display type.
Note two points: they mention an "expanded uncertainty" and they note
that it is dependent upon display type. The method, quoted above is
used to calibrate i1Displays relative to a population of reference
i1Pro Devices. The expanded uncertainty is larger than NIST's
standard because we are one step further from the source and we
include a population average of the i1Pro's in the mix. A spectral
measurement instrument is very difficult to specify because of the
bandwidth of the device. If we calibrated an i1Pro and a Minolta
CS1000 to a tungsten source, they would agree nearly exactly when
looking at that source, but they would have very large differences
when looking at a complex spectra such as CRT or CCFL LCD. These
differences could be as large as .01 xy.; huge. When an i1Pro is
calibrated, it is subjected to a number of calibrated sources with
structure and without structure. Part of this process of calibration
includes a "deconvolution" step which is used to minimize the effects
of bandwidth, but there is no way that we can determine the effective
performance on every source, so we can't quote it. My own internal
studies with my i1Pro and the CS1000 in our lab show numbers on the
order +/- .002 on white on an LCD with CCFL (not wide gamut), and
differences on the order .003-.004 on the primaries with an
uncertainty in measurement setup of .0015 (k=2). Things start to
depart very rapidly when looking at wide gamut ccfl or led backlights,
particularly in the green, because of the shift of the primary
wavelength from 550 towards 520. But PLEASE, don't quote me.
Regards,
Tom
_______________________________________________
Do not post admin requests to the list. They will be ignored.
Colorsync-users mailing list (email@hidden)
Help/Unsubscribe/Update your Subscription:
This email sent to email@hidden
_______________________________________________
Do not post admin requests to the list. They will be ignored.
Colorsync-users mailing list (email@hidden)
Help/Unsubscribe/Update your Subscription:
This email sent to email@hidden