Wide Gamut
Displays and Colorimeters
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 ?
What's the difference between a Colorimeter and a Spectrometer ?
Colorimeters and Spectrometers both have the same aim: to measure
tri-stimulus color values, but they go about this in two quite
different ways.
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.
A Colorimeter uses physical filters that approximate the Standard
Observer weighting curves to filter the captured light onto three
sensors, the sensor values then
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.
Why don't Colorimeters work so well on Wide Gamut displays ?
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.
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.
What can be done about this ?
There are two approaches to addressing this problem:
One is to use a Spectrometer to measure Wide Gamut displays. Since
lower cost Spectrometers are now available, this is the best solution,
since a Spectrometer offers a good deal more flexibility and display
technology independence than a Colorimeter.
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.
Argyll V1.3.0 has a facility to create and apply such a correct matrix
to Colorimeter measurements. To create the correction matrix, the
display, the Colorimeter and a reference Spectrometer are needed. (see
ccxxmake). The correction matrix can then
be used with the usual
display measurement utilities (see dispcal,
dispread and spotread
-X
option).
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. 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. ccxxmake also allows creation of these Colorimeter Calibration Spectral Sample
files.