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diff --git a/doc/WideGamutColmters.html b/doc/WideGamutColmters.html new file mode 100644 index 0000000..67a5817 --- /dev/null +++ b/doc/WideGamutColmters.html @@ -0,0 +1,120 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> +<html> + <head> + <title>Wide Gamut Displays & Colorimeters</title> + <meta http-equiv="content-type" content="text/html; + charset=ISO-8859-1"> + </head> + <body> + <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> + </body> +</html> |