Fluorescent Whitener Additive Compensation (FWA Compensation)
Introduction
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, or Optical Brightening Agents -
OBA) 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".
For more technical treatment of this topic, please refer to this
excellent paper: <http://www.axiphos.com/BrightnessReview.pdf>
Fluorescence
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.
Reflection Models and Spectro-colorimetry
The way a spectrometer measures the effect of ink on paper, depends
on a model of how an illuminant 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 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 assumed viewing illumination at
each wavelength (typically D50 for standard ICC profiles) is then
multiplied by the reflectance at each wavelength, and the overall
spectral reflectance is in this way converted into CIE tri-stimulus
values using an observer model.
So while the instrument measures with one type of light (type A, or
a white LED), it returns a measurement as if it had been measured
under a different kind of light (D50) by making use of a simple
model of light reflection off the media.
Notice that a key assumption of this simple model 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. So the color measurements do
not accurately portray the appearance of the media when FWA is
present. A more complicated bi-spectral measurement (2 dimensional
spectral reflectance) is actually needed to fully characterize
fluorescent materials.
What Argyll's FWA compensation does
The FWA compensation function in Argyll improve on 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.
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),
or use an illumination source such as a "white" LED that doesn't
emit any U.V.
Such UV excluded instruments are not suitable for use with FWA
compensation.
The effects of FWA are modeled spectrally, so a spectral reading
instrument is also required.
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.
Ideally the level of FWA would be measured by comparing the paper
spectrum with and without U.V. present in the instruments
illumination. Because not all instruments allow these two
measurements to be done without some sort of manual intervention,
Argyll avoids the need for an FWA inactive (UV cut) or extra UV (UV
LED) 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, unless the instrument has the capability of
measuring with two different levels of UV.
Note that typically in Argyll, if a viewing illuminant is specified,
then it is used for computing the appearance under that illumination
(CIE XYZ values), and if FWA compensation is used, then that same
illuminant will be assumed for the simulated measurement illuminant.
This results in measurements that better reflects the appearance as
the media as if it was being viewed under that illuminant, FWA
effects and all.
It is possible to also simulate the measurement of a media
under one illuminant, while then computing the tristimulus values as
if being viewed under a different illuminant, but this scenario is
only really useful for reproducing standardized measurement
conditions such as ISO 13655:2009 M0, M1 and M2, and is less useful
than the normal FWA compensation scenario in modelling real world
situations.
[The Argyll FWA compensation algorithm is described in the paper: A
Practical Approach to Measuring and Modelling Paper
Fluorescense for Improved Colorimetric Characterisation of
Printing Processes", Graeme W. Gill, Proc. IS&T/SID
11th Color Imaging Conference,
Scottsdale, Arizona; November 2003; p. 248-254, and was first published on December 2, 2002
in the argyllu_2002_12_02 source code. ]
Using FWA Compensation with proofing
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.
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, modeled
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.
[ 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. ]
Using FWA compensation for single, general use profiles
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 the 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 (ISO 13655:2009 M1). 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 (ie. most instruments are actually
returning ISO 13655:2009 M0 measurements).
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 D50M2 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.
Measuring the illuminant
For FWA compensation to work well, it is necessary to know what the
spectral shape of the illuminant used for viewing is. While many
instruments provide an illuminant measurement capability over the
visible spectrum, for FWA compensation it is desirable to know the
Ultra Violet (UV) component of the illuminant. Few color instruments
are capable of reading to such short wavelengths though (the JETI specbos 1211 is an
exception). Argyll provides an indirect way of estimating the UV
component of an illuminant using its illumread
utility. Using illumread in combination with FWA compensation is the
recommended approach to modelling real world appearance of paper
containing FWA.
FWA myths
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 simply 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 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, if the viewing environment is very poor in
UV, then 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.
Instrument UV filters
Note that to be able to measure the FWA in the paper, the instrument
has to be able to trigger Fluorescence, which it cannot do if it is
fitted with a UV filter, or uses a light source that emits no UV
(e.g. a white LED). So UV excluded instruments are not suitable for
use with FWA compensation.