121 lines
6.8 KiB
HTML
Executable File
121 lines
6.8 KiB
HTML
Executable File
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<html>
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<head>
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<title>Wide Gamut Displays & Colorimeters</title>
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<meta http-equiv="content-type" content="text/html;
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charset=ISO-8859-1">
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</head>
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<body>
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<h2 style="text-decoration: underline; font-weight: bold;">Wide
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Gamut Displays and Colorimeters<br>
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</h2>
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With the introduction of more wide color gamut displays, many people
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are finding that their Colorimeter instruments don't work so well on
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them. Why is this, and what can be done about it ?<br>
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<h3>What's the difference between a Colorimeter and a Spectrometer ?</h3>
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Colorimeters and Spectrometers both have the same aim: to measure
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tri-stimulus color values, but they go about this in two quite
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different ways.<br>
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<br>
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A spectrometer breaks the captured light up into a narrow series of
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wavelengths, measures the response at each of the wavelengths, and
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then weights and sums each wavelength response by the Standard
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Observer weighting curves, to arrive at the CIE XYZ tri-stimulus
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values. Because a Spectrometer computes the Standard Observer
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weightings in software, the accuracy of the curves is nearly
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perfect, the primary errors being due to wavelength calibration
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errors, spectrum calibration errors, and the quantised nature of the
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discrete wavelength bands.<br>
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<br>
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A Colorimeter uses physical filters that approximate the Standard
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Observer weighting curves to filter the captured light onto three
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sensors, the sensor values then<br>
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being measured, and then multiplied by a 3x3 calibration matrix to
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arrive at the CIE XYZ tri-stimulus values. The main advantage of a
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Colorimeter is its simplicity, which results in a lower cost
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instrument. In theory it is also possible to make a Colorimeter that
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cheaply captures more light by using larger sensors, but this
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possibility is rarely exploited by low cost instruments. Also due to
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cost constraints, the physical filters used in these instruments may
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not be a very good match to the CIE Standard Observer weightings,
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and if nothing were done about it, this would result in large
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measurement errors. Because such Display Colorimeters are typically
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used with additive, 3 colorant displays, it is possible to calibrate
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these errors out for any particular display, and this is the purpose
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of the 3x3 calibration matrix that is used by the instrument and/or
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instrument drivers. Since the calibration depends on the spectral
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characteristics of the display primaries, no single calibration
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matrix will be perfect for all display technologies, and typically
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the instruments will come with two matrices, one for "typical" CRT
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(Cathode Ray Tube) type displays, and one for "typical" LCD (Liquid
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Crystal) type displays. Each individual Colorimeter may have
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slightly different filters to others of the same model, due to batch
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variations in the filter material. If each Colorimeter is calibrated
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against a reference instrument, then this source of error can also
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be minimised.<br>
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<h3>Why don't Colorimeters work so well on Wide Gamut displays ?</h3>
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As explained above, due to the imperfect match between the
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Colorimeter filters and CIE Standard Observer weighting curves,
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Colorimeters have calibration matrices that are created for
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"typical" CRT or LCD displays. A Wide Gamut display by its very
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nature has primaries that have narrower spectral characteristics
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than typical displays, and this spectral difference exacerbates the
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approximations and errors in the Colorimeter filters.<br>
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<br>
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Since Spectrometers have mathematically computed weighting curves,
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they are less sensitive to the spectral characteristics of the
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display primary colors, and generally work better on Wide Gamut
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displays.<br>
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<h3>What can be done about this ?</h3>
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There are three approaches to addressing this problem:<br>
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<br>
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One is to use a Spectrometer to measure Wide Gamut displays. Since
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lower cost Spectrometers are now available (e.g. Color Munki
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Design/Photo), this may be the best general solution, since a
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Spectrometer offers a good deal more flexibility and display
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technology independence than a Colorimeter. Spectrometers are more
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expensive than colorimeters though, and typical low cost instruments
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are not well compensated for temperature changes (making reliable
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black measurement somewhat tricky), and may take longer, or be less
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accurate at measuring low light levels than the best colorimeters.<br>
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<br>
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The second approach is to correct the Colorimeter for the specific
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type of Wide Gamut Display. Often this is what has been done when a
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Colorimeter ("Puck") is supplied with a Wide Gamut display :- the
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3x3 calibration matrix inside the Colorimeter will have been "tuned"
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to match the display, or the Colorimeter driver or color management
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software will include an additional 3x3 correction matrix for that
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Colorimeter/Display combination.<br>
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<br>
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The third approach is to make a colorimeter that has filters that
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are closer to the standard observer curves, reducing the calibration
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needed for the instrument, and making it less dependent on the exact
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type of display technology. The X-Rite i1 DisplayPro, Pantone
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ColorMunki Display and possibly the Spyder 4 may have such an
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improvement. <br>
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<br>
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Argyll V1.3.0 has a facility to create and apply a <a
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href="File_Formats.html#.ccmx">correct matrix</a> to Colorimeter
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measurements. To create the correction matrix, the display, the
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Colorimeter and a reference Spectrometer are needed. (see <a
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href="ccxxmake.html">ccxxmake</a>). The correction matrix can then
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be used with the usual display measurement utilities (see <a
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href="dispcal.html#X">dispcal</a>, <a href="dispread.html#X">dispread</a>
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and <a href="spotread.html#X">spotread</a> -X option).<br>
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<br>
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Some recent colorimeters take a slightly different approach to
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calibration, and rather than using pre-defined 3x3 calibration
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matricies, they instead contain the spectral sensitivity curves for
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each particular colorimeter (e.g. i1 DisplayPro and ColorMunki
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Display, Spyder 4). It's then possible to create 3x3 calibration
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matricies automatically for any display for which the spectral
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characteristics are known. This makes it easy to tailor the
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colorimeters measurements to a particular type of display without
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having to cater for each colorimeter & display combination. <a
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href="ccxxmake.html">ccxxmake</a> also allows creation of these <a
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href="File_Formats.html#.ccss">Colorimeter Calibration Spectral
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Sample</a> files.<br>
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<br>
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</body>
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</html>
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