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diff --git a/doc/FWA.html b/doc/FWA.html new file mode 100644 index 0000000..c1d4c6d --- /dev/null +++ b/doc/FWA.html @@ -0,0 +1,262 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> +<html> + <head> + <title>Fluorescent Whitener Additive Compensation</title> + <meta http-equiv="content-type" content="text/html; + charset=ISO-8859-1"> + </head> + <body> + <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, 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".<br> + <br> + For more technical treatment of this topic, please refer to this + excellent paper: <<a + href="http://www.axiphos.com/BrightnessReview.pdf">http://www.axiphos.com/BrightnessReview.pdf</a>><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 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.<br> + <br> + 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.<br> + <br> + 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.<br> + <br> + <h3>What Argyll's FWA compensation does</h3> + 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.<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), + or use an illumination source such as a "white" LED that doesn't + emit any U.V.<br> + Such UV excluded instruments are not suitable for use with FWA + compensation.<br> + The effects of FWA are modeled 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 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.<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 + (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.<br> + <br> + 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.<br> + <br> + [The Argyll FWA compensation algorithm is described in the paper: <font + color="#000000"><font face="Times, serif"><font style="font-size: + 8pt;" size="1"><a + href="http://www.imaging.org/IST/store/epub.cfm?abstrid=22190">A + Practical Approach to Measuring and Modelling Paper + Fluorescense for Improved Colorimetric Characterisation of + Printing Processes", <i>Graeme W. Gill, Proc. IS&T/SID + 11th Color Imaging Conference</i></a><span + style="font-style: normal;"><a + href="http://www.imaging.org/IST/store/epub.cfm?abstrid=22190">, + Scottsdale, Arizona; November 2003; p. 248-254</a><font + size="1">, and <font size="1">w<font size="1">as f<font + size="1">irst publi<font size="1">shed <font + size="1">on December 2, 2002</font></font></font></font></font></font></span></font></font></font> + in the argyllu_2002_12_02 source code. ]<br> + <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, 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.<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>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 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 <b><span + style="text-decoration: underline;">less</span></b> + 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).<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 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.<br> + <br> + <h3>Measuring the illuminant</h3> + 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. Argyll + provides an indirect way of estimating the UV component of an + illuminant using its <a href="illumread.html">illumread</a> + utility. Using illumread in combination with FWA compensation is the + recommended approach to modelling real world appearance of paper + containing FWA.<br> + <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 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.<br> + <br> + <h3>Instrument UV filters</h3> + 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.<br> + <br> + <br> + <br> + <br> + </body> +</html> |