266 lines
15 KiB
HTML
Executable File
266 lines
15 KiB
HTML
Executable File
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<title>Fluorescent Whitener Additive Compensation</title>
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<meta http-equiv="content-type" content="text/html;
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charset=windows-1252">
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<body>
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<h2><u>Fluorescent Whitener Additive Compensation (FWA Compensation)</u></h2>
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<br>
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<h3>Introduction</h3>
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To make paper look "whiter" without increasing the cost of
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production, paper manufactures often employ a couple of different
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techniques. One technique is to add "shading agents" to the paper,
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that absorb a little of the middle wavelengths, thereby changing the
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color of the paper to be a little less green. By far the most
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powerful way of making the paper appear more white is to add
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Fluorescent Whitener Additive (FWA, or Optical Brightening Agents -
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OBA) to the paper. This is basically a fluorescent material that
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absorbs light at Ultra Violet (U.V.) wavelengths, and re-emits it at
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a slightly longer blue wavelengths. Subjectively something that
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appears more blue, is regarded as being "whiter".<br>
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<br>
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For more technical treatment of this topic, please refer to this
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excellent paper: <<a
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href="http://www.axiphos.com/BrightnessReview.pdf">http://www.axiphos.com/BrightnessReview.pdf</a>><br>
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<br>
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<h3>Fluorescence</h3>
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Fluorescent materials absorb light radiation at one wavelength, and
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then almost instantaneously re-emit some of that energy at a longer
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wavelength. Typical FWA absorbs wavelengths in the U.V. between
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about 300 and 400 nm, and re-emit it between 400 and 460nm. The
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visual effect of FWA depends on the amount of it present in the
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paper, and the amount of U.V. in the illumination compared to the
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level of normal, visible light. Generally better quality papers have
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lower levels of whitening agents, and cheaper papers more. <br>
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<br>
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<h3>Reflection Models and Spectro-colorimetry</h3>
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The way a spectrometer measures the effect of ink on paper, depends
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on a model of how an illuminant is reflected by the ink and the
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paper. Typically a spectrometer instrument illuminates the sample
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with a known illumination, often a incandescent tungsten lamp having
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a color temperature of 2800 degrees Kelvin. It measures the
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amount of light reflected by the sample at each wavelength, and then
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converts that to spectral reflectance value between 0 and 100% by
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dividing by it's measurement illuminant's intensity at each
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wavelength. When it comes time to use that measurement to create an
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ICC profile, the intensity of the assumed viewing illumination at
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each wavelength (typically D50 for standard ICC profiles) is then
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multiplied by the reflectance at each wavelength, and the overall
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spectral reflectance is in this way converted into CIE tri-stimulus
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values using an observer model.<br>
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<br>
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So while the instrument measures with one type of light (type A, or
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a white LED), it returns a measurement as if it had been measured
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under a different kind of light (D50) by making use of a simple
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model of light reflection off the media.<br>
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<br>
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Notice that a key assumption of this simple model is that the light
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that impinges on the sample at a given wavelength is reflected back
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at exactly the same wavelength at a diminished intensity. Notice
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also that any sort of fluorescent material (such as FWA) breaks this
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model, since fluorescent materials emit light a different
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wavelengths to which they absorb it. So the color measurements do
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not accurately portray the appearance of the media when FWA is
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present. A more complicated bi-spectral measurement (2 dimensional
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spectral reflectance) is actually needed to fully characterize
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fluorescent materials.<br>
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<br>
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<h3>What Argyll's FWA compensation does</h3>
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The FWA compensation function in Argyll improve on this simple model
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of spectral reflection by taking into account the action of FWA. To
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do this, it needs to measure the amount and nature of the FWA in the
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media, and then have enough information about the viewing
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environment to model how that FWA will behave.<br>
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<br>
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To be able to measure the level of FWA in the media, the instrument
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needs to be able to "see" the FWA in action, so the instrument needs
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to be illuminating the samples with some level of U.V. Typically all
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instruments do this, unless they have been fitted with a filter that
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filters out any U.V. illumination (so called "UV cut" instruments),
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or use an illumination source such as a "white" LED that doesn't
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emit any U.V.<br>
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Such UV excluded instruments are not suitable for use with FWA
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compensation.<br>
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The effects of FWA are modeled spectrally, so a spectral reading
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instrument is also required.<br>
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<br>
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Argyll can compute a model for the effects of FWA given the media's
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spectral characteristics, and the illuminations spectral
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characteristic, which must include the levels of U.V. in the
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illuminant. Given these two things, Argyll can calculate how much
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effect the FWA will have on the light being reflected and emitted by
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the media under the intended illumination.<br>
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<br>
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Ideally the level of FWA would be measured by comparing the paper
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spectrum with and without U.V. present in the instruments
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illumination. Because not all instruments allow these two
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measurements to be done without some sort of manual intervention,
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Argyll avoids the need for an FWA inactive (UV cut) or extra UV (UV
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LED) measurement by employing a heuristic to estimate the FWA
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inactive spectrum from the spectrum of the paper with FWA active.
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Being a heuristic, it can sometimes be fooled by certain paper
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colors into estimating more or less FWA content than is actual
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present. The heuristic works best with high quality papers with an
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essentially flat non-FWA enhanced spectrum. Papers with colored
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tints or particularly off white appearance may not work well with
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FWA compensation, unless the instrument has the capability of
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measuring with two different levels of UV.<br>
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<br>
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<img alt="Graph showing FWA effect on UV vs. UV cut measurement."
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src="FWA_measure.jpg" style="width: 387px; height: 284px;"><br>
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<br>
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Note that typically in Argyll, if a viewing illuminant is specified,
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then it is used for computing the appearance under that illumination
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(CIE XYZ values), and if FWA compensation is used, then that same
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illuminant will be assumed for the simulated measurement illuminant.
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This results in measurements that better reflects the appearance as
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the media as if it was being viewed under that illuminant, FWA
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effects and all.<br>
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<br>
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It is possible to also simulate the measurement of a media
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under one illuminant, while then computing the tristimulus values as
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if being viewed under a different illuminant, but this scenario is
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only really useful for reproducing standardized measurement
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conditions such as ISO 13655:2009 M0, M1 and M2, and is less useful
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than the normal FWA compensation scenario in modelling real world
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situations.<br>
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<br>
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[The Argyll FWA compensation algorithm is described in the paper: <font
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color="#000000"><font face="Times, serif"><font style="font-size:
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8pt;" size="1"><a
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href="http://www.imaging.org/IST/store/epub.cfm?abstrid=22190">A
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Practical Approach to Measuring and Modelling Paper
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Fluorescense for Improved Colorimetric Characterisation of
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Printing Processes", <i>Graeme W. Gill, Proc. IS&T/SID
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11th Color Imaging Conference</i></a><span
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style="font-style: normal;"><a
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href="http://www.imaging.org/IST/store/epub.cfm?abstrid=22190">,
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Scottsdale, Arizona; November 2003; p. 248-254</a><font
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size="1">, and <font size="1">w<font size="1">as f<font
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size="1">irst publi<font size="1">shed <font
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size="1">on December 2, 2002</font></font></font></font></font></font></span></font></font></font>
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in the argyllu_2002_12_02 source code. ]<br>
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<br>
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<h3>Using FWA Compensation with proofing</h3>
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The most common situation for employing FWA compensation, is in
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proofing. This is when you have one printing device, the target (say
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a printing press), and wish to emulate the behaviour of it with a
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different device, the proofer (say an inkjet printer). The aim is to
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be able to put both prints next to each other in a viewing booth,
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and have them look identical. Typically the printing process, the
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inks, and the media will be different between the target device and
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the proofer. The aim of applying color profiling is to compensate
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for these differences. Since the printing process can only darken a
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white media, the selection of the proofing stock is critical.
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Ideally it should be exactly the same color as the target, or if not
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possible, lighter, so that the proofer can tint the proofing media
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to match the target. If the two media had identical levels and types
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of FWA in them, then there would be no need to use FWA compensation,
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since the appearance of the media would match under any viewing
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condition. Typically though, the levels and types of FWA are
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different between the target paper and the proofing paper. A
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limitation imposed by tri-stimulus colorimetry is that the
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differences between the two media, inks and FWA can only be
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compensated for perfectly, under a fixed and known illuminant.<br>
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<br>
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By allowing Argyll to model the effects of FWA for both the source
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profile (the target device), and the destination profile (the
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proofing device), the effects can be accounted for, modeled
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accurately, and incorporated in the profiles, so that a subsequent
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transformation from source to destination device spaces using
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absolute colorimetric intent, achieves a (hopefully) perfect
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colorimetric reproduction. Since this is a closed system, where both
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the source and destination profiles are made for each other,
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non-standard parameters such as illuminant and observer models can
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be used, as long as they are the same for both profiles. For
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proofing, FWA should be applied identically to both profiles, by
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specifying the same illuminant, and (optionally) the same observer
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model.<br>
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<br>
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[ In practice it is possible to compensate for the color shift that
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results in viewing the media under non-D50 illumination or using a
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non 1931_2 observer, or allowing for FWA effects without severe
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incompatibility because all rendering intents except absolute
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rendering normalize to the media color, rendering the media white as
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white, even though the absolute values are not measured using a D50
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illuminant. ]<br>
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<h3>Using FWA compensation for single, general use profiles</h3>
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For creating ICC profiles that will be interchanged with other
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unknown ICC profiles, or used with non-print source or destination
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profiles, there is less flexibility, since ICC profiles by
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convention assume that all media is being viewed under D50
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illumination. The implication of this is that to be fully
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interchangeable, it's not really possible to make the profile for
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your actual viewing environment. Note that the D50 values that are
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calculated without FWA compensation do not actually reflect the
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appearance of a media under real D50, because they fail to take into
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account the different levels of FWA activity between the
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illumination using by the instrument to measure the media, and real
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D50. To allow for this and actually meet the letter of the ICC
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specifications, FWA compensation should ideally be used when
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building a interchangeable ICC profile, by selecting the D50
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illuminant, and the 1931_2 observer model (ISO 13655:2009 M1). Note
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however that unless you are interchanging with profiles made using
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M1 measurement mode data, that this is likely to make profiles <b><span
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style="text-decoration: underline;">less</span></b>
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interchangeable rather than more, since few if any profiles made
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with older instruments will represent the appearance under real D50,
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since few if any of those instruments use a real D50 illuminant that
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will trigger the correct level of FWA response, and few if any other
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packages will compensate for the differences in FWA activity between
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the instrument illuminant used and real D50 (ie. most instruments
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return M0 measurements by default, and older instruments are
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generally not capable of M1 measurement).<br>
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<br>
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Similarly, the effects of viewing the media in an environment with a
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UV filter fitted over the D50 illuminant can be simulated by using
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FWA compensation with the D50M2 illuminant, and the 1931_2 observer,
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thereby simulating the results one would get if the media had been
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measured with a "UV cut" type instrument, although such profiles are
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not technically ICC compatible.<br>
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<br>
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<h3>Measuring the illuminant</h3>
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For FWA compensation to work well, it is necessary to know what the
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spectral shape of the illuminant used for viewing is. While many
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instruments provide an illuminant measurement capability over the
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visible spectrum, for FWA compensation it is desirable to know the
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Ultra Violet (UV) component of the illuminant. Few color instruments
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are capable of reading to such short wavelengths though (the <a
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href="instruments.html#specbos">JETI specbos 1211</a> is an
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exception). Argyll provides an indirect way of estimating the UV
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component of an illuminant using its <a href="illumread.html">illumread</a>
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utility. Using illumread in combination with FWA compensation is the
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recommended approach to modelling real world appearance of paper
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containing FWA.<br>
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<br>
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<h3>FWA myths</h3>
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Amongst the user (and to some degree) vendor community, there has
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been a widely held belief that the solution to fluorescent whitener
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affecting color profiles is to simply use a UV filter fitted
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instrument. Exactly what the origin of the legend is, is hard to
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tell. Possibly it is a misinterpretation of the ANSI
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CGATS.5-1993 Annex B recommendations for measuring the impact of
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fluorescent effects, a translation of some of paper whiteness
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measurement standards into the color profiling world, or possibly in
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some common situations, if the viewing environment is very poor in
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UV, then adding a UV filter to the tungsten instrument illuminant
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makes for a better instrument/viewing illuminant match. There seems
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to be no scientific or practical basis for believing that a UV
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filter fitted instrument magically makes all FWA induced problems go
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away.<br>
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<br>
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<h3>Instrument UV filters</h3>
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Note that to be able to measure the FWA in the paper, the instrument
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has to be able to trigger Fluorescence, which it cannot do if it is
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fitted with a UV filter, or uses a light source that emits no UV
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(e.g. a normal white LED). So UV excluded instruments are not
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suitable for use with FWA compensation.<br>
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<br>
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<br>
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<br>
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<br>
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</body>
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</html>
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