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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <head>
    <title>Wide Gamut Displays &amp; Colorimeters</title>
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      charset=ISO-8859-1">
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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&nbsp; 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 &amp; 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>
  </body>
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