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<title>Wide Gamut Displays & Colorimeters</title>
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<h2 style="text-decoration: underline; font-weight: bold;">Wide
Gamut Displays and Colorimeters<br>
</h2>
With the introduction of more wide color gamut displays, many people
are finding that their Colorimeter instruments don't work so well on
them. Why is this, and what can be done about it ?<br>
<h3>What's the difference between a Colorimeter and a Spectrometer ?</h3>
Colorimeters and Spectrometers both have the same aim: to measure
tri-stimulus color values, but they go about this in two quite
different ways.<br>
<br>
A spectrometer breaks the captured light up into a narrow series of
wavelengths, measures the response at each of the wavelengths, and
then weights and sums each wavelength response by the Standard
Observer weighting curves, to arrive at the CIE XYZ tri-stimulus
values. Because a Spectrometer computes the Standard Observer
weightings in software, the accuracy of the curves is nearly
perfect, the primary errors being due to wavelength calibration
errors, spectrum calibration errors, and the quantised nature of the
discrete wavelength bands.<br>
<br>
A Colorimeter uses physical filters that approximate the Standard
Observer weighting curves to filter the captured light onto three
sensors, the sensor values then<br>
being measured, and then multiplied by a 3x3 calibration matrix to
arrive at the CIE XYZ tri-stimulus values. The main advantage of a
Colorimeter is its simplicity, which results in a lower cost
instrument. In theory it is also possible to make a Colorimeter that
cheaply captures more light by using larger sensors, but this
possibility is rarely exploited by low cost instruments. Also due to
cost constraints, the physical filters used in these instruments may
not be a very good match to the CIE Standard Observer weightings,
and if nothing were done about it, this would result in large
measurement errors. Because such Display Colorimeters are typically
used with additive, 3 colorant displays, it is possible to calibrate
these errors out for any particular display, and this is the purpose
of the 3x3 calibration matrix that is used by the instrument and/or
instrument drivers. Since the calibration depends on the spectral
characteristics of the display primaries, no single calibration
matrix will be perfect for all display technologies, and typically
the instruments will come with two matrices, one for "typical" CRT
(Cathode Ray Tube) type displays, and one for "typical" LCD (Liquid
Crystal) type displays. Each individual Colorimeter may have
slightly different filters to others of the same model, due to batch
variations in the filter material. If each Colorimeter is calibrated
against a reference instrument, then this source of error can also
be minimised.<br>
<h3>Why don't Colorimeters work so well on Wide Gamut displays ?</h3>
As explained above, due to the imperfect match between the
Colorimeter filters and CIE Standard Observer weighting curves,
Colorimeters have calibration matrices that are created for
"typical" CRT or LCD displays. A Wide Gamut display by its very
nature has primaries that have narrower spectral characteristics
than typical displays, and this spectral difference exacerbates the
approximations and errors in the Colorimeter filters.<br>
<br>
Since Spectrometers have mathematically computed weighting curves,
they are less sensitive to the spectral characteristics of the
display primary colors, and generally work better on Wide Gamut
displays.<br>
<h3>What can be done about this ?</h3>
There are three approaches to addressing this problem:<br>
<br>
One is to use a Spectrometer to measure Wide Gamut displays. Since
lower cost Spectrometers are now available (e.g. Color Munki
Design/Photo), this may be the best general solution, since a
Spectrometer offers a good deal more flexibility and display
technology independence than a Colorimeter. Spectrometers are more
expensive than colorimeters though, and typical low cost instruments
are not well compensated for temperature changes (making reliable
black measurement somewhat tricky), and may take longer, or be less
accurate at measuring low light levels than the best colorimeters.<br>
<br>
The second approach is to correct the Colorimeter for the specific
type of Wide Gamut Display. Often this is what has been done when a
Colorimeter ("Puck") is supplied with a Wide Gamut display :- the
3x3 calibration matrix inside the Colorimeter will have been "tuned"
to match the display, or the Colorimeter driver or color management
software will include an additional 3x3 correction matrix for that
Colorimeter/Display combination.<br>
<br>
The third approach is to make a colorimeter that has filters that
are closer to the standard observer curves, reducing the calibration
needed for the instrument, and making it less dependent on the exact
type of display technology. The X-Rite i1 DisplayPro, Pantone
ColorMunki Display and possibly the Spyder 4 may have such an
improvement. <br>
<br>
Argyll V1.3.0 has a facility to create and apply a <a
href="File_Formats.html#.ccmx">correct matrix</a> to Colorimeter
measurements. To create the correction matrix, the display, the
Colorimeter and a reference Spectrometer are needed. (see <a
href="ccxxmake.html">ccxxmake</a>). The correction matrix can then
be used with the usual display measurement utilities (see <a
href="dispcal.html#X">dispcal</a>, <a href="dispread.html#X">dispread</a>
and <a href="spotread.html#X">spotread</a> -X option).<br>
<br>
Some recent colorimeters take a slightly different approach to
calibration, and rather than using pre-defined 3x3 calibration
matricies, they instead contain the spectral sensitivity curves for
each particular colorimeter (e.g. i1 DisplayPro and ColorMunki
Display, Spyder 4). It's then possible to create 3x3 calibration
matricies automatically for any display for which the spectral
characteristics are known. This makes it easy to tailor the
colorimeters measurements to a particular type of display without
having to cater for each colorimeter & display combination. <a
href="ccxxmake.html">ccxxmake</a> also allows creation of these <a
href="File_Formats.html#.ccss">Colorimeter Calibration Spectral
Sample</a> files.<br>
<br>
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