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Brightness: originally used for assessing the bleaching level of a pulp. It is the integrated value from weighted reflectance according to a bell shaped curve with maximum at 460 nm; values of the curve are subjected to international norms. Regardless of the nature of brightness definition (Tappi, ISO, daylight, etc.) it has no relationship with perceived whiteness, on the contrary fluorescence from FWA introduces a false element into its definition and reduces it to a senseless number.
Daylight: strictly speaking corresponds to the spectral distribution of D65 as given by the CIE (see illuminants) and correspond to the radiation of a black body at 6500 K. As such it cannot be built as a light source in the laboratory, instruments try to approach the spectral distribution of D65 by filtering light from a Xenon light source.
Fluorescence elimination: in many cases it is desirable to eliminate fluorescence from the substrate; especially when recycled paper is used in the papermaking process it is unavoidable to introduce FWA in the production process. Fluorescence elimination can be achieved by two different methods: by adding a quencher (see below) or by adding a mild oxidative bleaching step. A clear distinction must be made on the nature of these methods, only the second one eliminates fluorescence through chemical removal of FWA molecules as such. The former method eliminates fluorescence through a physical method, thus hiding FWA molecules.
FWA: Fluorescent Whitening Agent, also known as Fluorescent Brightening Agent (FBA), Optical Brightening Agent (OBA), Optical Bleacher, etc. They are fluorescent molecules specifically tailored for compensating the yellowness of a substrate through an additive color mixing process; by doing this the perceived whiteness of the substrate is effectively increased.
Greening effect: agglomeration of FWA leads to a reduction of intensity (quenching) and shift of the maximum value of fluorescence towards higher wavelengths, resulting in greenish appearance. Agglomeration results through overload of FWA or application under wrong conditions (pH, Temperature, etc.)
Illuminant: spectral distribution of (virtual) lighting subject to given convention; according to CIE only D65 (daylight) and A are the recommended illuminants to be used for color assessment.
Kashas rule: in general fluorecence occurs only from the lowest excited singlet of lowest excited triplet state. According to this rule fluorescence will appear always at higher wavelengths (i.e. lower energies) than the exciting wavelength.
Kasha-Valivov rule: quantum yield of emission is independent of exciting wavelength (see quantum yield below).
Light fastness: light fastness properties of white samples include light fastness values of each of three contributions to perceived whiteness and should be measured and assessed by themselves. Unfortunately there is no norm establishing a procedure for assessing light fastness of fluorescent whitened samples.
Light source: actual spectral distribution of lighting used during assessment of samples or just for general observation. Its distribution may or may not correspond to one given by convention (see illuminants). Certain light sources are accompanied by certified or specified values given by manufacturers; in general spectral intensities of any light source can be measured and be used thus as a calibrated light source, fluorescence values and by the same token, whiteness values are attached to this particular spectral distribution.
Metamerism: since colored substrates are always compared in pairs, they do not in general show the same appearance under all illuminating conditions. The phenomenom of different color perception of pairs depending on illumination conditions is known as metamerism and is a key property to consider during color matching process (see also Whiteness metamerism below).
Near-white: colors having luminous reflectance over 65% and Munsell chroma not greater than 0.5 for B hues, 0.8 for Y hues and 0.3 for all other hues.
Numerical UV control: new generation instruments (see Minolta) for measuring whiteness use two lamps with different UV content to quantify the level of fluorescence. The procedure involves a numerical technique rather than reposition of moving parts.
Measurement with filters: due to the fact that whiteness is the result of three contributions, fluorescence being the more influential, it is important to separate and quantify each of the contributions by themselves. Fluorescence contribution can be estimated by measuring samples with light filtered by different cut-off filters; in general filters with cut-off wavelengths at 400, 420 and 460 nm are used for this purpose, fluorescence contribution can thus be determined by applying mathematical techniques within proper software.
Opacity: this property is related to the ability of a material to prevent the transmission of light, it is also denoted as hiding power i.e. the ability to hide a surface behind and in contact with it.
material backing: the ratio is built from reflectances of a piece of material with known thickness over a black surface and that from the material with infinite thickness
white backing: the ratio is built from reflectances of a piece of material with known thickness over a black surface and that from the same piece of material over a known white surface (usually having a reflectance factor of 0.89)
The presence of fluorescent whitening agents influences the measured opacity to a very small extent; on the other hand the value of opacity, specially of
those objects with low opacity, can influence the value of whiteness to a large extent, since it expresses the penetration depth of the flux i.e. the optical path, and as such the amount of molecules excited to
fluoresce.
Depending of the application area, the importance of opacity is understood in different ways, for paints the emphasis lies on the hiding power of the overlying layer, while in the printing area it is
important not to see waht has been printed on the other side of the paper sheet, something that has to do with the diffusion of the pinting ink into the paper sheet.
The amount of light coming from the back of
the sample and its spectral distribution is crucial for proper assessment of whiteness of a materials, this is point that must be considered and taken into account during visual and instrumental assessment.
Photobleaching: certain molecules have the property to effectively activate Oxygen from the atmosphere under the action of light, these are known as photobleaching agents. They are deep-blue molecules that are added to detergents to be used in the so-called sun-belt countries, where garments are dried in the exterior under high levels of sunlight. Photobleaching agents are quite efficient provided they act in the presence of water and sunlight; they degrade chemically during the photoactivation. For the latter reason they are also used as temporary shading agents in those regions where sunlight levels are not very high, the property of photodegradation prevents an accumulation of the material and thus the danger of stain.
Quantum yield: measure of the efficiency of the emission process (photons out versus photons in). It must be considered that all excited states produce certain amount of emission, but in practice quantum yields below 10-4 are very difficult to measure; quantum yields of fluorescent colors and FWAs lie in the range of about 0.8-0.9
Quencher (also Fluorescence Quencher): in many cases it is desirable to eliminate, partially or totally the fluorescence of substrates; to acomplish this goal a material is added that has the property to quench the fluorescence from the FWA. There are two mechanism to achieve quenching:
Competition for the UV light: any molecule absorbing in the UV region will compete with the FWA and will lower the amount of fluorescence produced by the FWA; this method is effective only at high levels of fluorescence and will not eliminate it completely. The efficiency of the process will depend on the concentration of the quencher
Molecular hindering: certain molecules have the property to deactivate the excited state of the FWA and prevent the production of fluorescence at all; this is achieved by a process of molecular association with the FWA. The process is quite efficient and fluorescence can be eliminated almost completely.
Sphere error: the amount of fluorescence produced by a particular sample depends on the total amount of light i.e. spectral distribution inciding onto the sample; while using an instrument equiped with an Ulbricht sphere (to achieve diffuse illumination), part of the fluorescence emitted by the sample is reflected back onto the sample by the sphere wall and thus able to excite additional fluorescence, leading to an error known as sphere error. This additional amount of fluorescence does not exist with directional geometries, because there is no back-reflection from the instrumental geometry. Since the wavelength of fluorescence is lower than that needed to excite molecules (see Kashas rule above) this effect appears only with those molecules showing considerable overlapping between absorption and fluorescence; studies have shown that for FWAs the deviation of color determination due to sphere error is of the order of 2%, while with color fluorescent materials can reach up to 40%
Transmittance: defined as the ratio of transmitted flux to incident flux; geometrical and spectral conditions must be specified. This property is related to amount of energy that passes through a material. Observe that for fluorescent samples the factor is falsed since the material is a self-emitter and the amount of light produced within the sample is not accounted for. Following concepts are related:
diffuse transmittance: the transmitted flux is measured at all forward angles except the regular transmission angle
internal transmittance: ratio of flux reaching the exit surface of a specimen to the flux that penetrates the entry surface
regular transmittance: transmittance without diffusion total transmittance: the transmitted flux is measured at all forward angles to the incidence flux
Triplet effect: when the light source of the instrument has a too high intensity, certain FWA can show an additional absorption in the 520 nm region. This absorption originates by molecules that have been promoted into a ground triplet state and correspond to a triplet-triplet absorption. This is an instrumental error in whiteness assessment, because under normal light intensity levels are not sufficient to populate the triplet state to show the effect. Modern instruments are equipped with a soft-flash, where intensity is well below the threshold for triplet population.
White: color presented by objects that have the property of reflecting inciding light in high extent and without any loss. This color can be found along the axis defined by the achromatic point in the color solid of the CIE and is characterized by high luminosity values and zero saturation.
UV calibration: since the amount of UV is directly responsible for the level of fluorescence and thus for the perceived whiteness, it must be calibrated and maintained in order to assure repeatibility and accuracy of measured whiteness values over time. This is done using a set of whiteness standards selected for the required application; it must be emphasized that the UV calibration is made on basis of visual assessment and not on level or spectral distribution of fluorescence.
Whiteness: color perceived as white by the human eye. It is expressed as two numbers: whiteness and shade (or tint); it must be described at least by two numbers otherwise characterization is not complete. In general whiteness consists of three different contributions:
Base white: whiteness of the substrate itself
Shaded white: whiteness contributed from addition of a shading dye (subtractive color mixing)
Fluorescent white: whiteness contribution from FWA (additive color mixing)
Whiteness calibration: perceived whiteness will depend on the amount of present fluorescence, that in turn depends on the level of UV in the light source (see UV calibration). Adjustment of the UV amount of the light source is carried out using a set of samples
(see whiteness standards) with known whiteness values based on visual assessment. It is
important to note that calibration is made on the basis of perceived whiteness and not on amount or distribution of UV amount.
Whiteness metamerism: the phenomenon of metamerism is shown by pairs of samples appearing equal under certain light source but different when the light source is changed. In the case of white samples this phenomenon is more pronounced due to the different contributions to perceived whiteness (see Whiteness above). Two white samples will show no metamerism only when all three contributions to whiteness are the same under all illumination conditions.
Whiteness scale: formula transforming color coordinates into numbers that correlate with perceived whiteness. Modern methods express whiteness in Ganz or CIE whiteness numbers (whiteness and tint; see Whiteness), older formulas try to express whiteness by just one number (and are in this sense
incomplete); still today whiteness numbers expressed according to Berger, Taube or Stensby are encountered in daily business.
Whiteness standards: set of fluorescent samples with known whiteness values. Their values can be traced back to reflectance values determined using a two-monochromator method and certified by a National Institute. Depending on the apllication different materials are offered, as for example paper, cotton and polyester.
Yellowness: attribute by which an object is judged to depart from a preferred white toward yellow.
Yellowness index: near white or off-white, non-fluorescent samples with dominant wavelength between 570 and 580 nm can be characterized by its degree of yellowness as given by some formula as yellowness index. This type of samples do not show whiteness as such, for this reason it is preferably to characterize their appearance by one of the many yellowness indices. In the plasteics area the formula ASTM D-1925 or DIN-6167 is quite popular, while in textiles the modern formula ASTM E-313 founds more acceptance.
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