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authorJörg Frings-Fürst <debian@jff-webhosting.net>2014-09-01 15:43:52 +0200
committerJörg Frings-Fürst <debian@jff-webhosting.net>2014-09-01 15:43:52 +0200
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+<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
+<html>
+ <head>
+ <title>The i1pro Hi Res. Mode</title>
+ <meta http-equiv="content-type" content="text/html;
+ charset=ISO-8859-1">
+ <meta content="Graeme Gill" name="author">
+ </head>
+ <body>
+ <h2 style="text-decoration: underline; font-weight: bold;">Does the
+ i1pro High Resolution mode improve accuracy ?<br>
+ </h2>
+ A question that has been asked is : "<span style="font-weight:
+ bold;">You've extended the Eye-One Pro with a high resolution
+ spectral mode, giving readings at 3.3nm spacing rather than the
+ default 10nm. Does this mode improve accuracy ?</span>"<br>
+ <br>
+ This is a quite reasonable question. The following attempts to
+ answer it.<br>
+ <h4 style="text-decoration: underline;">Why would a higher
+ resolution spectral mode improve accuracy ?<br>
+ </h4>
+ A spectrometer computes CIE tri-stimulus values by measuring
+ spectral values and then weighing those values by the observer
+ curves before summing the weigted values. The accuracy depends on
+ the correct weighting being applied at each wavelength. If the color
+ is composed of very narrow spectra peaks, as is sometimes the case
+ for certain light sources and many display devices, then the exact
+ positioning of one of the peaks on the observer curves may be
+ influencial in the final color value, and too coarse a quanization
+ of the spectral readings may lead to tri-stimulus errors. So in
+ theory increasing the spectral reading resolution to 3.3 nm should
+ lead to improved color accuracy with narrow spectra color sources. <br>
+ <h4 style="text-decoration: underline;">Why may this not work in
+ practice ?</h4>
+ <p>The instrument spectral resolving power is set by a number of
+ factors, and a critical one is the entrance slit width. By
+ measuring a very narrow band source such a as a laser, using the
+ default 10nm resolution indicates a FWHM (<a
+ href="http://en.wikipedia.org/wiki/Full_width_at_half_maximum">Full
+
+
+
+
+ width at half maximum</a>) of about 25nm. Doing a measurement at
+ 3.3nm resolution reveals that the optical limit seems to be about
+ 15nm, so there is some hope of improvement from that perspective.</p>
+ <p>Another factor is that the calibration data for the instrument is
+ only given at 10nm intervals. So to produce calibrated readings at
+ 3.3nm intervals, it is necessary to up-sample the calibration data
+ with sufficient accuracy. If the calibration data is sufficiently
+ smooth (indicating that the underlying device characteristics are
+ also smooth), or any slight inaccuracy will get calibrated out
+ (which is typically the case for reflective measurements) then
+ this may not be a limitation either. In the case of the i1pro2,
+ which seems to have a diffraction grating/light sensor with a less
+ smooth spectral efficiency curve than the Rev A - D models, the
+ task of up-sampling the emissive calibration data with sufficient
+ accuracy is a more difficult.<br>
+ </p>
+ <h4 style="text-decoration: underline;">The verification experiment<br>
+ </h4>
+ To give some indication of whether ArgyllCMS's high resolution
+ spectral mode is capable of improving color measurement accuracy, or
+ at least to indicate that it doesn't noticeably worsen it, the
+ following fairly simple, real world experiment was performed:<br>
+ <br>
+ A measurement target consisting of white + primary + secondary
+ colors (White, Red, Green, Blue, Cyan, Magenta, Yellow) repeated 10
+ times was used. This target was displayed on a conventional LCD
+ screen with a CCFL backlight (MacBook display), and measured using
+ using ArgyllCMS V1.6.0 <a href="dispread.html">dispread</a>:<br>
+ <br>
+ 1) Using a <a
+href="http://www.jeti.com/cms/index.php/instruments-55/radiometer/specbos-1211">JETI
+
+
+
+
+ specbos 1211</a> reference Tele-Spectro-Radiometer.<br>
+ <br>
+ 2) Using an i1pro2 in standard 10nm mode.<br>
+ <br>
+ 3) Using an i1pro2 in ArgyllCMS 3.3nm mode.<br>
+ <br>
+ The resulting readings were then analyzed using <a
+ href="colverify.html">colverify</a>.<br>
+ <br>
+ The results were analyzed two ways, first in absolute value error
+ terms, and secondly in brightness (Y) normalized terms, the latter
+ corresponding to the typical way such readings are used for display
+ calibration and profiling. <br>
+ <br>
+ A second, similar experiment was run on a CRT type display.<br>
+ <h4 style="text-decoration: underline;">Results:</h4>
+ <p><br>
+ LCD display:<br>
+ </p>
+ <p>Absolute errors of i1pro2 10nm mode to specbos 1211:<br>
+ </p>
+ &nbsp; Total errors (CIEDE2000):&nbsp;&nbsp;&nbsp;&nbsp; peak =
+ 3.070420, avg = 2.204137<br>
+ <br>
+ Absolute errors of i1pro2 3.3nm mode to specbos 1211:<br>
+ <br>
+ &nbsp; Total errors (CIEDE2000):&nbsp;&nbsp;&nbsp;&nbsp; peak =
+ 2.108411, avg = 1.568577<br>
+ <br>
+ <br>
+ White Y normalised errors of i1pro2 10nm mode to specbos 1211:<br>
+ <br>
+ &nbsp; Total errors (CIEDE2000):&nbsp;&nbsp;&nbsp;&nbsp; peak =
+ 2.419800, avg = 0.747926<br>
+ <br>
+ White Y normalised errors of i1pro2 3.3nm mode to specbos 1211:<br>
+ <br>
+ &nbsp; Total errors (CIEDE2000):&nbsp;&nbsp;&nbsp;&nbsp; peak =
+ 1.595033, avg = 0.578270<br>
+ <br>
+ <br>
+ So in this particular situation, hi-res mode improves accuracy by
+ somewhere between 0.2 and 0.6 DeltaE 2K.<br>
+ <br>
+ <br>
+ Example of white spectrum for the three measurements (red: 10nm
+ i1pro2, green: 3.3nm i1pro2, black: specbos):<br>
+ <img alt="specbos 1211 (Black), i1pro2 10nm (Red), i1pro2 3.3nm
+ (Green)" src="i1proHiRes.jpg" height="335" width="667"><br>
+ <br>
+ <p><br>
+ CRT display:<br>
+ </p>
+ <p>Absolute errors of i1pro2 10nm mode to specbos 1211:<br>
+ </p>
+ &nbsp; Total errors (CIEDE2000):&nbsp;&nbsp;&nbsp;&nbsp; peak =
+ 1.516886, avg = 0.965740<br>
+ <br>
+ Absolute errors of i1pro2 3.3nm mode to specbos 1211:<br>
+ <br>
+ &nbsp; Total errors (CIEDE2000):&nbsp;&nbsp;&nbsp;&nbsp; peak =
+ 1.751776, avg = 0.887878<br>
+ <br>
+ <br>
+ White Y normalised errors of i1pro2 10nm mode to specbos 1211:<br>
+ <br>
+ &nbsp; Total errors (CIEDE2000):&nbsp;&nbsp;&nbsp;&nbsp; peak =
+ 1.509129, avg = 0.654752<br>
+ <br>
+ White Y normalised errors of i1pro2 3.3nm mode to specbos 1211:<br>
+ <br>
+ &nbsp; Total errors (CIEDE2000):&nbsp;&nbsp;&nbsp;&nbsp; peak =
+ 1.284044, avg = 0.622501<br>
+ <br>
+ <h4 style="text-decoration: underline;">Conclusions:</h4>
+ The results for the conditions of this particular experiment
+ indicate that ArgyllCMS High Resolution mode can very slightly
+ improve colorimetric measurement accuracy of display devices.
+ Accuracy may conceivably be improved a little more than indicated by
+ this experiment for i1pro rev A-D instruments which have a smoother
+ diffraction grating/light sensor characteristic, or it is also
+ conceivable that an unfortunate combination of display spectra and
+ the i1pro2 may result in reduced accuracy. The High Resolution mode
+ is primarily useful for showing more spectral detail, and should
+ probably not be used for colorimetric measurement when the highest
+ possible robustness and reliability is desired. The potential for
+ improved accuracy may be of benefit in other situations though. <br>
+ <h4 style="text-decoration: underline;">Raw Data:</h4>
+ The raw measurement data is available in this <a
+ href="i1proHiRes.zip">.ti3 archive</a>.<br>
+ <br>
+ <br>
+ <br>
+ <br>
+ <br>
+ <br>
+ </body>
+</html>