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<h2><u>Fluorescent Whitener Additive Compensation (FWA Compensation)</u></h2>
<br>
<h3>Introduction</h3>
To make paper look "whiter" without increasing the cost of production,
paper manufactures often employ a couple of different techniques. One
technique is to add "shading agents" to the paper, that absorb a little
of the middle wavelengths, thereby changing the color of the paper to
be a little less green. By far the most powerful way of making the
paper appear more white is to add Fluorescent Whitener Additive (FWA)
to the paper. This is basically a fluorescent material that absorbs
light at Ultra Violet (U.V.) wavelengths, and re-emits it at a slightly
longer blue wavelengths. Subjectively something that appears more blue,
is regarded as being "whiter".<br>
<br>
For more technical treatment of this topic, please refer to this
excellent paper: &lt;<a
 href="http://www.axiphos.com/BrightnessReview.pdf">http://www.axiphos.com/BrightnessReview.pdf</a>&gt;<br>
<br>
<h3>Fluorescence</h3>
Fluorescent materials absorb light radiation at one wavelength, and
then almost instantaneously re-emit some of that energy at a longer
wavelength. Typical FWA absorbs wavelengths in the U.V. between about
300 and 400 nm, and re-emit it between 400 and 460nm. The visual effect
of FWA depends on the amount of it present in the paper, and the amount
of U.V. in the illumination compared to the level of normal, visible
light. Generally better quality papers have lower levels of whitening
agents, and cheaper papers more.<br>
<br>
<h3>Reflection Models and Spectro-colorimetry</h3>
The way a spectrometer measures the effect of ink
on paper, depends on a model of how an illumination is
reflected by the ink and the paper. Typically a spectrometer instrument
illuminates the sample with a known illumination, often a incandescent
tungsten lamp having a color temperature of&nbsp; 2800 degrees Kelvin.
It measures the amount of light reflected by the sample at each
wavelength, and then converts that to spectral reflectance value
between 0 and 100% by dividing by it's measurement illuminant's
intensity at each
wavelength. When it comes time to use that measurement to create an ICC
profile,
the intensity of the illumination that is assumed is going to be used
to view the sample at each wavelength (typically D50 for standard ICC
profiles) is then multiplied by the reflectance
at each wavelength, and the overall spectral levels are in this way
converted
into tri-stimulus values using an observer model.<br>
<br>
Notice that a key assumption in all of this is that the light that
impinges on the sample at a given wavelength is reflected back at
exactly the same wavelength at a diminished intensity. Notice also
that any sort of fluorescent material (such as FWA) breaks this model,
since fluorescent materials emit light a different wavelengths to which
they absorb it.<br>
<br>
<h3>What Argyll's FWA compensation does</h3>
The FWA compensation function in Argyll, improve this simple model of
spectral reflection by taking into account the action of FWA. To do
this, it needs to measure the amount and nature of the FWA in the
media, and then have enough information about the viewing environment
to model how that FWA will behave.<br>
<br>
To be able to measure the level of FWA in the media, the instrument
needs to be able to "see" the FWA in action, so the instrument needs to
be illuminating the samples with some level of U.V. Typically all
instruments do this, unless they have been fitted with a filter that
filters out any U.V. illumination (so called "UV cut" instruments), so
such UV filtered instruments are not suitable for use with FWA
compensation.<br>
The effects of FWA are modelled spectrally, so a spectral reading
instrument is also required.<br>
<br>
Argyll can compute a model for the effects of FWA given the media's
spectral characteristics, and the illuminations spectral
characteristic, which must include the levels of U.V. in the
illuminant. Given these two things, Argyll can calculate how much
effect the FWA will have on the light being reflected and emitted by
the media under the intended illumination.<br>
<br>
Ideally the level of FWA would be measured by comparing the paper
spectrum with and without U.V. present in the instruments illumination.
Because few if any instruments allow these two measurements to be done
without some sort of manual intervention, Argyll avoids the need for an
FWA inactive (UV cut) measurement by employing a heuristic to estimate
the FWA inactive spectrum from the spectrum of the paper with FWA
active. Being a heuristic, it can sometimes be fooled by certain paper
colors into estimating more or less FWA content than is actual present.
The heuristic works best with high quality papers with an essentially
flat non-FWA enhanced spectrum. Papers with colored tints or
particularly off white appearance may not work well with FWA
compensation.<br>
<br>
<img alt="Graph showing FWA effect on UV vs. UV cut measurement."
 src="FWA_measure.jpg" style="width: 387px; height: 284px;"><br>
<br>
Note that typically in Argyll, if a viewing illuminant is specified,
then it is used for computing the appearance under that illumination,
and if FWA compensation is used, then that same illuminant will be
assumed for the measurement illuminant. This results in measurements
that reflect the appearance as the media as if it was being viewed
under that illuminant, FWA effects and all. Currently there is no way
of simulating the measurement of a media under one illuminant, while
then computing the tristimulus values as if being viewed under a
different illuminant, since this mismatch is the very problem that FWA
compensation is designed to avoid.<br>
<h3>Using FWA Compensation with proofing</h3>
The most common situation for employing FWA compensation, is in
proofing. This is when you have one printing device, the target (say a
printing press), and wish to emulate the behaviour of it with a
different device, the proofer (say an inkjet printer). The aim is to be
able to put both prints next to each other in a viewing booth, and have
them look identical. Typically the printing process, the inks, and the
media will be different between the target device and the proofer. The
aim of applying color profiling is to compensate for these differences.
Since the printing process can only darken a white media, the selection
of the proofing stock is critical. Ideally it should be exactly the
same color as the target, or if not possible, lighter, so that the
proofer can tint the proofing media to match the target. If the two
media had identical levels and types of FWA in them, then there would
be no need to use FWA compensation, since the appearance of the media
would match under any viewing condition. Typically though, the levels
and types of FWA are different between the target paper and the
proofing paper. A limitation imposed by tri-stimulus colorimetry is
that the differences between the two media, inks and FWA can only be
compensated for perfectly, under a fixed and known illuminant.<br>
<br>
By allowing Argyll to model the effects of FWA for both the source
profile (the target device), and the destination profile (the proofing
device), the effects can be accounted for, modelled accurately, and
incorporated in the profiles, so that a subsequent transformation from
source to destination device spaces using absolute colorimetric intent,
achieves a (hopefully) perfect colorimetric reproduction. Since this is
a closed system, where both the source and destination profiles are
made for each other, non-standard parameters such as illuminant and
observer models can be used, as long as they are the same for both
profiles. For proofing, FWA should be applied identically to both
profiles, by specifying the same illuminant, and (optionally) the
same observer model.<br>
<br>
<h3>Using FWA compensation for single, general use profiles</h3>
For creating ICC profiles that will be interchanged with other unknown
ICC profiles, or used with non-print source or destination profiles,
there is less flexibility, since ICC profiles by convention assume that
all media is being viewed under D50 illumination. The implication of
this is that to be fully interchangeable, it's not really possible to
make the profile for your actual viewing environment. Note that the D50
values that are calculated without FWA compensation do not actually
reflect the appearance of a media under real D50, because they fail to
take
into account the different levels of FWA activity between the
illumination using by the instrument to measure the media, and real
D50. To
allow for this and actually meet letter of the ICC specifications, FWA
compensation should ideally be used when building a interchangeable ICC
profile, by selecting the D50 illuminant, and the 1931_2 observer
model. Note however that this is likely to make profiles less
interchangeable rather than more, since few if any other profiles will
represent the appearance under real D50, since few if any instruments
use a real D50 illuminant that will trigger the correct level of FWA
response, and few if any other packages will compensate for the
differences in FWA activity between the instrument illuminant used and
real D50.<br>
<br>
Similarly, the effects of viewing the media in an environment with a UV
filter fitted over the D50 illuminant can be simulated by using FWA
compensation with the provided ref/D50_0.0.sp illuminant, and the
1931_2
observer, thereby simulating the results one would get if the media had
been measured with a "UV cut" type instrument, although such profiles
are not technically ICC compatible.<br>
<br>
In practice it is possible to compensate for the color shift that
results in viewing the media under non-D50 illumination or using a non
1931_2 observer, or allowing
for FWA effects without severe incompatibility because all
rendering intents except absolute rendering, normalize to the media
color, rendering the media white as white, even though the absolute
values are not measured using a D50 illuminant.<br>
<h3>FWA myths</h3>
Amongst the user (and to some degree) vendor community, there is a
widely held belief that the solution to fluorescent whitener affecting
color profiles is to use a UV filter fitted instrument. Exactly what
the origin of the legend is, is hard to tell. Possibly it is a
misinterpretation of the&nbsp; ANSI CGATS.5-1993 Annex B
recommendations for measuring the impact of fluorescent effects, a
translation of some of paper whiteness measurement standards into the
color profiling world, or possibly in some common situations, the
viewing environment is very poor in UV, and adding a UV filter to the
tungsten instrument illuminant makes for a better instrument/viewing
illuminant match. There seems to be no scientific or practical basis
for believing that a UV filter fitted instrument magically makes all
FWA induced problems go away.<br>
<br>
<br>
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