SCAN-G 5:03 Revised 2003
Pulp, paper and board
Basic equations for optical properties
0 Introduction 1 Scope This SCAN-test guideline is a revision of SCAN-G 5:94. This SCAN-test guideline provides a summary of the The major changes are: equations used for determining the optical properties of pulp, paper and board. It should be used in conjunction • Consideration has been given to the fact that many with the particular Methods describing the determina- SCAN-test methods have been withdrawn in favour tions of the properties desired. of ISO Standards. This guideline provides the information necessary for • The clause Definitions has been extended and the those engaged in development of software for definitions are placed in a special annex, Annex A. computation of optical properties in accordance with • Some of the symbols have been replaced in order to current ISO standards and SCAN-test methods. avoid the use of the same symbol for different quantities. One exception is k which is used in 2 References different meanings in Eqs. [2]–[5] and [12]–[13], and in Eqs. [22] and [24]. The symbol k is retained since The following standards are normative in the application it is established in both situations. Another exception of this guideline: is W which has different meanings in Eqs. [6]–[7] and in Eqs. [40]–[47]. The symbol w is used for ISO/CIE 10526:1999 CIE standard illuminants for grammage in Eqs. [17], [21] and [23] although it colorimetry might be confused with the suffix w meaning white in the symbols Tw , Rgw and Rw . ISO/CIE 10527:1991 CIE Standard colorimetric • For clarity, the use of suffixes has been extended. observers • It is emphasised that reflectance factors, although they are often reported as percentages, are quantities ASTM E308:99 Standard method for computing the on a scale from zero to unity for the perfect reflecting colors of objects by using the CIE system diffuser. The scale from zero to unity is also emphasised for opacity in Eqs. [14]–[15]. CIE Technical Report 15.2 Colorimetry, 1986 • The section on the calculation of tristimulus values now includes the calculation from reflectance factor This guideline is intended for use together with the ISO values measured at 10 nm and 20 nm intervals. standards and SCAN-test methods listed in clause 16. • Weighting factors for the calculation of ISO brightness are now tabled also for Δλ = 20 nm. Note that within these ISO standards, ISO 2469 is • The section on dominant wavelength and excitation considered to be the primary standard to which all the purity has been rearranged in order to improve the others are related. clarity. • The equations for the approximate calculations of dominant wavelength and excitation purity for fillers and pigments have been considered obsolete and have been excluded. SCAN-G 5:03 Page 2
3 Definitions The tristimulus values X , Y , Z for an object, which is reflective and may also be luminescent and which is Important definitions are included in the clause where they illuminated by a chosen illuminant, S()λ , are defined as: belong. A comprehensive list of definitions that apply for the purpose of this guideline are found in Annex A. The definitions given are those published in the ISO standards XkR=⋅z bgλλλλ ⋅ S bg ⋅ x bg ⋅d or other documents indicated. YkR=⋅z bgλλλλ ⋅ S bg ⋅ y bg ⋅d [2] 4 Colour appearance, tristimulus values ZkR=⋅z bgλλλλ ⋅ S bg ⋅ z bg ⋅d It is important to note that colour is a feature of human perception and cannot therefore be measured. Colour where can however be described in terms of different attributes 100 (lightness, redness, saturation etc). Psychometric scales k = [3] and systems of alphanumerical notations have been Sybgλλλ⋅⋅ bgd developed purely on the basis of perceptual judgements z to provide a means of locating colours in a three- and dimensional colour space. R()λ is the spectral reflectance factor (or Physically, it is only possible to measure the intensity spectral radiance factor) of the material of the radiation which is the stimulus giving rise to the (on a scale from zero to unity for the perception of colour. For pulp, paper and board, it is perfect reflecting diffuser); assumed that the sample is illuminated diffusely and that S()λ is the relative spectral power of the the viewing direction is normal to the surface of the chosen illuminant; sample. The stimulus, i.e. the spectral radiation flux x()λ , y()λ , z()λ are the three colour matching functions F ()l , reflected from the sample in the normal refl of the CIE 1931 Standard Colorimetric direction is proportional to the product of the incident Observer. spectral radiation flux F ()l falling upon the surface in and the spectral reflectance factor R()λ of the surface. Note that the factor 100 in Eq. [3] is not a conversion to percentage units. It defines the scale for the tristimulus rR [1] FFreflbglll=◊ bg ◊ in bg values. If the material fluoresces and any type of where fluorescence can be expected, the relative spectral power r is the ratio of the solid angle of the detection of the light source of the instrument and S()λ have to be cone to the solid angle of the hemisphere (2p the same. However, in special cases, reliable measure- steradians). ments can be made although this is only approximately true. One such case is when the only fluorescent In order to establish some connection with the visual component is a Fluorescent Whitening Agent (FWA). experience, it is essential to realise that any colour percept Here the most important property of the light source is may be produced by the mixing of three reference colour the relation between its energy in the excitation band stimuli in a generalised colour matching experiment. A (mainly UV radiation) and the emission band (blue end requirement is that the colour of none of the reference of the visible spectrum) of the FWA. stimuli can be produced by mixing the other two. The Since S(λ) and the colour matching functions x()λ , values to which the fluxes of the reference stimuli have to y()λ , z()λ are defined for only a limited number of be adjusted to make a perfect colour match of a test wavelengths λi , the tristimulus values are obtained by stimulus are described by the tristimulus values for the test summation instead of integration. Equidistant λi -values stimulus. In colorimetry according to CIE (Commission are assumed. The wavelength pitch is denoted Δλ . For Internationale d'Eclairage), the test stimuli are called X, Y the CIE 1931 Standard Observer, Eq. [2] modifies to: and Z or X10, Y10 and Z10. These are adapted to colour matching experiments where the fields of view were 2° Xk=⋅∑ Rbgbgbgλλλiii ⋅ S ⋅ x ⋅Δλ and 10° respectively. The tristimulus values for a specific i test stimulus are evaluated for the CIE 1931 Standard Yk=⋅ Rλλλ ⋅ S ⋅ y ⋅Δλ [4] Colorimetric Observer for the case of the 2° field of view ∑ bgbgbgiii and the tristimulus values are denoted X , Y , Z. i Correspondingly, the CIE 1964 Supplementary Standard Zk=⋅∑ Rbgbgbgλλλii ⋅ S ⋅ zi ⋅Δλ Colorimetric Observer is used in the case of the 10° field i of view and the tristimulus values are denoted X10, Y10 , Z10 . SCAN-G 5:03 Page 3
where where WX , WY, WZ and W10, X , W10,Y , W10,Z are called tristimulus weighting factors, and the values tabled in 100 k = [5] ASTM E308:99 for different combinations of standard ∑ Sybgbgλλii⋅⋅Δλ observers and standard illuminants shall be used. Note that i Rbgλi is on a scale from zero to unity for the perfect reflecting diffuser. In the corresponding calculation of X10 , Y10 , Z10 As an alternative to the determination from spectro- for the CIE 1964 Supplementary Standard Observer (10° photometric data, the tristimulus values can be determined observer), the colour matching functions x()λi , y()λi from measurements with a three-filter instrument such as and z()λi in Eqs. [4]–[5] are replaced by x10 ()λi , the Zeiss Elrepho, where the combination of the spectral y10 ()λi and z10 ()λi . properties of the lamps, the optics, the filters and the The bandwidth of the spectrophotometer is the detector are chosen by the manufacturer to give wavelength interval surrounding the nominal wavelength reflectance factors which are weighted averages over three of the monochromator within which the sensitivity of the broad wavelength bands. The measured values are instrument exceeds half the sensitivity at the nominal designated Rx, Ry and Rz and are on a scale from zero to wavelength. In the ISO standards, the bandwidth is unity for the perfect reflecting diffuser, although they are considered to be equal to the wavelength pitch Δλ and usually reported as percentages. Thereafter, X, Y and Z are the sensitivity is considered to be a triangular function of calculated as follows: wavelength. For Δλ -values up to 5 nm, the CIE tables for x()λ , C-illuminant, 2° observer y()λ , z()λ and x10 ()λ , y10 ()λ , z10 ()λ given in X = 78,321Rx + 19,753 Rz [8] ISO/CIE 10527 shall be used, together with the tables YRy= 100 for the appropriate illuminant given in ISO 10526 or CIE ZRz= 118, 232 publ. 15.2. The tables are provided for Δλ -values of 1 nm and 5 nm. For other Δλ -values below 5 nm a proper table of interpolated values has to be calculated. D65-illuminant, 10° observer X10 = 76,841Rx + 17,970 Rz
Note – Tables for the Δλ -value of 1 nm is available YRy10 = 100 [9] from CIE on disk (CIE Disk D001 and CIE Disk ZRz10 = 107,304 D002). The disks can be obtained from CIE Central The inverse relationships are Bureau, Kegelgasse 27, A-1030 Wien, Austria. C-illuminant, 2° observer For Δλ -values exceeding 5 nm, the bandwidth of the Rx=− X0,16707 Z 78,321 spectrophotometer affects the shape of the R()λ curve ( ) such that steep slopes cannot be rendered with sufficient Ry= Y 100 [10] precision. To neutralise this influence to some extent, Rz= Z 118, 232 ASTM has prepared tables of special weighting functions. To calculate the tristimulus values X , Y , Z D65-illuminant, 10° observer and X10, Y10 , Z10 for Δλ -values of 10 nm and 20 nm, Rx=−( X0,16747 Z ) 76,841 the following equations shall be used: 10 10 Ry= Y10 100 [11] Rz= Z 107,304 XRW=◊ bglliiX bg 10 i YRW=◊ bglliiY bg [6] 5 Reflectance factor, Y-value, opacity, i transmittance ZRW=◊ll  bgiiZ bg 5.1 Reflectance factor, R – Ratio of the radiation i reflected by a body to that reflected by the perfect and reflecting diffuser under the same conditions of illumination and detection (ISO 2469, ISO 5631). XRW10=◊ bgllii 10,X bg i Note 1 – The reflectance factor is expressed as a percentage. YRW10=◊ bgllii 10,Y bg [7] i Note 2 – The reflectance factor is influenced by the ZRW10=◊ bgllii 10,Z bg i backing if the body is translucent.
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In ISO 2469 and all other standards or methods to SC bgli is the energy distribution of the C-illuminant; which this guideline is related, it is specified that the V λ is the visual efficiency function which is illumination shall be diffuse and that the direction of bgi identical to the CIE 1931 colour matching detection shall be perpendicular to the surface of the function y λ for the Y tristimulus value. specimen. There is no or considerably reduced irradiation bgi from directions close to the direction of detection, since a When the measurement is made with a single sheet of so-called gloss-trap is also specified. the material over a black cavity, the luminance factor is It is a primary requirement of all optical measurements designated R . The intrinsic luminous reflectance factor relating to pulp, paper and board that the reflectance factor v,0 is designated R . When it can be unambiguously measurements be traceable to the perfect reflecting v,∞ deduced from the context, R may be designated R . diffuser, which is the absolute standard. ISO 2469 specifies v,∞ ∞ that a calibration procedure shall be adopted in which an 5.4 Y-value (C/2°) – Tristimulus value Y in the IR3 reference standard supplied by an ISO/TC6 CIEXYZ-system of a layer of material of such a Authorized Laboratory (cf. ISO 4094) is used. The values thickness that there is no change in Y when the assigned to this standard shall be traceable to the perfect thickness is increased. reflecting diffuser via an IR2 reference standard supplied This shall be interpreted as being the tristimulus by a Standardizing Laboratory. value Y of a layer of material of such a thickness that Although Note 1 above explicitly mentions the there is no change in the value when the thickness is custom of reporting reflectance factors as percentages, it doubled. CIE illuminant C is used in the determination must be emphasised that a reflectance factor of e.g. 80 % of the Y -value. means that R = 080, . The Y -value was in the withdrawn SCAN-P 8 defined
as a quantity to be reported per se. Now, reference is made 5.2 Intrinsic reflectance factor, R – Reflectance ∞ to ISO 5631. The Y -value is related to the intrinsic factor of a layer or pad of material thick enough to be luminous reflectance factor as YR= 100 . opaque, i.e. such that increasing the thickness of the pad v,∞ by doubling the number of sheets leads to no change in 5.5 Opacity, Ω – Ratio of the single-sheet luminous the measured reflectance factor (ISO 2469). reflectance factor, R , to the intrinsic luminous Reflectivity is synonymous with intrinsic reflectance v,0 reflectance factor, R , of the same sample (ISO 2471). factor. v,∞ The single-sheet reflectance factor is defined as the Values of the intrinsic reflectance factor are often reflectance factor of a single sheet of paper with a black expressed as percentages. cavity as backing.
The opacity is calculated as: 5.3 Luminous reflectance factor, Rv – Reflectance factor defined with reference to the CIE illuminant C Rv,0 and the CIE 1931 colour matching function y λ , and Ω= [14] ( ) R corresponding to the attribute of visual perception of the v,∞ reflecting surface (ISO 2471). However, opacity is usually expressed as a percentage, This term is analogous to the CIE term "luminance i.e.: factor", except that CIE does not define the illuminant. The CIE definition refers to the V ()λ function which Rv,0 corresponds to the spectral sensitivity to light of the Ω= ×100 % [15] R normal eye. This function is mathematically the same as v,∞ the y λ function for the CIE 1931 (2º) observer. To () Note that the use of R and R means that the two provide a value which takes into account the appearance v, 0 v, ∞ reflectance factors involved are weighted with respect to of the material in daylight, the reflectance is thus weighted the C-illuminant and the y λ function for the CIE 1931 with respect to the energy distribution of the C-illuminant ( ) (2º) observer. If the sample contains a fluorescent whiten- and the spectral sensitivity to light of the normal eye. ing agent (FWA), the measurements are here recom- mended to be made under the UV(C) condition as de- RkR S V [12] vC=◊◊◊ bgllliii bgbgDl scribed in Annex B. i If luminance factor data are available for a given grammage, w , it is possible to predict the opacity of a where 1 sheet of the same furnish and structure at another 1 k = [13] arbitrary grammage, w 2 , by application of the Kubelka-  SVC bgllii◊◊ bgDl Munk theory. The equation is: i
2 Ww2 =-()/()AAR1 -v,• [16] SCAN-G 5:03 Page 5
where The symbol RB may be used instead of R457. If the sample is non-fluorescent, the C-illuminant requirement ww21/ can be relaxed. AR=−⎡⎤1 R R R − R [17] ⎣⎦v,∞∞∞()() v,0,ww11 v, v, v,0, Table 1. Weighting factors for the calculation of ISO and the reflectance factors are on a scale from zero to brightness. In ISO 2470, values of Fbgλi for unity for the perfect reflecting diffuser. Δ λ = 5 nm are also given. It may be necessary to make this correction if it is Wavelength λ F λ for F λ for desired to calculate the mean opacity from measure- i bgi bgi nm Δλ =10 nm, Δλ =20 nm, ments on a number of sheets having slightly different arbitrary units arbitrary units grammages. 380 0,0 0,0 5.6 Transmittance, τ, – Ratio of the transmitted to the 390 0,0 incident flux. 400 1,0 1,0 Transmittance from reflectance factor measurement, 410 6,7 T – Transmittance calculated from reflectance factor data 420 18,2 18,2 determined under the conditions of illumination and 430 34,5 detection specified in ISO 2469 (ISO 22891 in prepara- 440 57,6 57,6 tion). 450 82,5 460 100,0 100,0 The transmittance is calculated from R0 and R∞ or from R , R and R using one of the equations: 470 88,7 0 w 480 53,1 53,1 490 20,3 F 1 I 500 5,6 5,6 T =−G RR00Ja ∞ − Rf [18] H R∞ K 510 0,3 520 0,0 0,0 Sum 468,5 235,5 F 1 I T =-G RRR00Jbg- [19] 6.2 D65 diffuse blue reflectance factor (D65- H Rw K brightness) R457,D65 – Diffuse blue radiance factor measured when the UV-content of the irradiation has where R is the reflectance factor of a single sheet over a been adjusted as specified in ISO 11475 to correspond to white backing with the reflectance factor Rw . All reflec- the CIE standard illuminant D65. tance factors are on a scale from zero to unity for the For the determination of R457,D 65, the instrument perfect reflecting diffuser. must be adjusted so that the UV-content of the illumina- tion matches the D65-illuminant. Thereafter the spectral 6 Diffuse blue reflectance factor (ISO brightness) radiance factor is measured and the D65-brightness is calculated using Eq. [20]. 6.1 Diffuse blue reflectance factor (ISO brightness), R457 – Intrinsic radiance factor measured with a reflectometer having the characteristics described in ISO 7 Light-scattering and light-absorption coeffi- 2469, equipped with a filter or corresponding function cients having an effective wavelength of 457 nm and a width at 7.1 Light-scattering coefficient, s – The fraction of half height of 44 nm, and adjusted so that the UV-content the diffuse light flux that is reflected on its passage of the illumination incident upon the test piece through an infinitesimally thin layer of material. This corresponds to that of the CIE illuminant C (ISO may be related to a finite layer using the Kubelka-Munk 2470:1999). theory which takes into account the grammage of the The diffuse blue reflectance factor (ISO brightness) material, so that s has the units m2/kg (ISO 9416). shall be calculated from reflectance factor data according s is one of the two parameters of the Kubelka-Munk to a special set of weighting factors, Table 1. The equations. When s is determined as a spectral quantity, calculated brightness is usually designated R457: it is denoted s()λ . It is determined according to ISO 9416 as a weighted ∑ RF()λλii⋅ () average for the C illuminant and the colour matching i R457 = [20] function y (λ) for the CIE 1931 (2°) observer, and is ∑ F()λi denoted sv . If the information in the suffix can be i unambiguously deduced from the context, the suffix may be omitted.
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7.2 Light-absorption coefficient, k – The fraction of 7.4 Calculations of s and k − In the Kubelka-Munk the diffuse light flux that is absorbed on its passage theory, s and k are calculated from reflectance data. through an infinitesimally thin layer of material. This However, in common practice the reflectance factor is may be related to a finite layer using the Kubelka-Munk used instead of the reflectance. This is often an acceptable theory which takes into account the grammage of the approximation. material, so that k has the units m2/kg (ISO 9416). In Eqs. [21]-[26] all reflectance factors are on a scale k is one of the two parameters of the Kubelka-Munk from zero to unity for the perfect reflecting diffuser. equations. When k is determined as a spectral quantity, The spectral light-scattering coefficient is calculated it is denoted k()λ . as: It is determined according to ISO 9416 as a weighted average for the C illuminant and the colour matching R λ RRR∞∞bgλλλ1− 0 bg bg s λ = ∞ bg ln [21] function y ()λ for the CIE 1931 (2°) observer, and is bg 2 w 1− R∞ bgλ RR∞ bgλλ− 0 bg denoted kv. If the information in the suffix can be unambiguously deduced from the context, the suffix may be omitted. where s()λ is the light-scattering coefficient, in square 7.3 Conditions for the Kubelka-Munk theory − The metre per kilogram; Kubelka-Munk theory is based on a number of assump- w is the grammage of the conditioned sheet, in tions. These may be summarised as: kilogram per square metre (Caution: Grammage is usually reported in gram per 1 The sample is a turbid medium forming a plane layer square metre); (perpendicular to the z-axis) of constant, generally R0 ()λ is the spectral reflectance factor (black backing); finite, thickness and large enough for edge effects to R∞ ()λ is the intrinsic spectral reflectance factor. be negligible 2 The sample is homogeneous in the respect that the The spectral light-absorption coefficient is calculated optical inhomogenities (causing the scattering of the as: radiation) are incomparably smaller than the thickness of the layer and are uniformly distributed in the 2 ksλλ=−12 R λ/ R λ [22] material bg bg∞∞ bg bg 3 The medium of the material is the same as the medium from which the radiation enters the material where (generally air) k()λ is the light-absorption coefficient, in square 4 One side of the layer is irradiated (the side having its metre per kilogram. normal in the +z -direction) 5 The reflected or transmitted radiation is not again The light-scattering and light-absorption coefficients reflected to the specimen unless a backing, reflecting are, as indicated in these equations, dependent on the the transmitted radiation in a defined manner, is used wavelength. When non-spectral values are desired, the 6 Both irradiation and detection are monochromatic calculations are usually based on the luminance factors and of the same wavelength Rv,0 and Rv,• , which means that the data are averaged 7 The irradiation is perfectly diffuse or parallel having over the visible spectrum using a product of a daylight an angle of incidence of 60° from the normal irradiation spectrum (the C-illuminant) and the colour 8 The scattering in the material is isotropic, i.e. it is matching function y (λ) for the CIE 1931 (2°) observer independent of the angle between the incident and as weighting function. The light-scattering and light- scattering directions. absorption coefficients are calculated as:
It is required that these assumptions are not violated in Rv,• RRRvvv,,,••di1- 0 such a way that the results are significantly affected. One sv = ln [23] 2 RR- example is that the irradiation need not be monochromatic w ej1- Rv,• vv,,• 0 provided that the sample is non-fluorescent. If the material 2 contains a fluorescent whitening agent (FWA), it is a ksv=− v()12 R v,∞ R v,∞ [24] requirement of ISO 9416 that a 420 nm cut-off filter be placed in the light beam to ensure that the fluorescence If only a single sheet of paper is available, no value effect is eliminated. for R∞ is available in Eqs. [21]-[24]. However, a value Two other requirements are: (i) the difference for R∞ can be determined if measurements are made between the reflectance factors of the sample measured over two different backings, preferably black and white. under different conditions, which are used in the R∞ is then calculated according to the equation: equations below, must be sufficiently large, and (ii) the 2 12 eflectance factor of the sample must be sufficiently high. Raa∞ =−di −1 [25] SCAN-G 5:03 Page 7
where In Figure 1 the loci of the sample stimulus and an achromatic stimulus have been plotted in the CIE xy 1 diRRgw-+ gsbgbg11 RRRRRR w s --+ w sdi gw gs chromaticity diagram. Since additive colour mixings are a = represented as straight lines in this diagram, the 2 RR- R R sgw wgs definition means that the locus of the dominant [26] wavelength is found if a straight line from the achromatic locus (,xynn ) through the stimulus locus and (,xyss ) is extended to the spectral locus (,xyDD ). The R is the reflectance factor of the black backing; gs wavelength having the coordinates (,xyDD ) is l D. Rgw is the reflectance factor of the white backing; Stimuli with a purplish appearance have no dominant Rs is the reflectance factor of a single sheet against wavelength. For these samples the complementary the backing R ; gs wavelength lC may be used. lC is defined by the point
Rw is the reflectance factor of a single sheet against of intersection of the spectral locus and the line used to
the backing Rgw . determine the dominant wavelength but extended in the opposite direction. In order to obtain accurate values of l D (and lC ) the chromaticity coordinates of the stimulus Rgs , Rgw , Rs and Rw may be spectral reflectance factors or luminance factors. must differ significantly from the chromaticity coordinates In Eqs. [21]-[26], the difference between the of the achromatic stimulus. reflectance factors measured under different conditions must be sufficiently large, depending on the accuracy 9.2 Excitation purity, pe – Ratio of the distance from required and on the noise level of the instrument. A the point representing the achromatic stimulus ( xnn, y ) to satisfactory accuracy is usually obtained when the the colour stimulus considered ( xss, y ) to the distance opacity of the sheet does not exceed 95 %. To lower the from the point representing the achromatic stimulus opacity of a dark pulp, the sheets must be made to a ( xnn, y ) to the point on the spectrum locus representing lower grammage than is usual. Very thin sheets are, the dominant wavelength ( xDD, y ) (cf. CIE publ. 17.4, however, sometimes too inhomogeneous, and in addition 845-03-48). the requirement of the Kubelka-Munk theory that there pe is thus defined in terms of figure 1 as shall be multiple reflections within the material may not pdde = 12, where d1 is the distance between (,xynn ) be met. For mechanical pulp, the lower limit of and (,xyss ) and d2 is the distance between (,xynn ) grammage is about 30 g/m2, and for bleached chemical and (,xyDD ). The excitation purity is usually reported pulps, the lower limit is about 50 g/m2. as a percentage.
1 8 Chromaticity coordinates
The chromaticity coordinates x and y for the CIE 1931 520 nm
Standard Observer are defined as: 0.8 540 nm xXXYZ=++ a f spectral locus [27] 560 nm yYXYZ=++af 0.6 (x , y ) 500 nm D D
The corresponding coordinate z is superfluous since y 580 nm z ≡ 1 - x - y . d2 λD 0.4 Correspondingly the chromaticity coordinates x10 (x , y ) 600 nm d S S and y10 for the CIE 1964 Supplementary Standard 1 Observer are defined as: (xn , yn) 0.2 xXXYZ10=++ 10a 10 10 10 f λ [28] C purple line yYXYZ10=++ 10af 10 10 10 0 0 0.2 0.4 0.6 0.8 1 9 Dominant wavelength, excitation purity x
9.1 Dominant wavelength, l D – Wavelength of the Figure 1. The CIE xy chromaticity diagram. In the monochromatic stimulus which can be additively mixed diagram are shown the loci of CIE illuminant C with a chosen achromatic stimulus so that the mixed (,xynn ), a sample (,xyss ), the dominant stimulus matches the stimulus from the sample (cf. CIE wavelength l D of the sample at the coordinates publ. 17.4, 845-03-44). (,xyDD )and the complementary wavelength lC .
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9.3 Calculation of l D and pe – Within SCAN-test Note – The excitation purity is here given on a and ISO/TC 6 there is no standardized procedure for how scale from zero to one for the pure spectral colour. to calculate l D and pe for an arbitrary l D within the If it is desired to express it as a percentage, the visible spectrum. Section 9.4 presents approximations used value given by Eq. [31] must be multiplied by 100. for newsprint shade. These approximations have been developed in order to adapt to the limited computer 10 CIELAB colour space coordinates capacity of older instruments. In the future this special case will be replaced by a general calculation procedure. 10.1 CIELAB colour (C/2°), ()Lab*, *, * – L*, a* Newer instruments usually calculate l D and pe for an and b* values of the sample according to the CIELAB arbitrary l D. 1976 system, evaluated according to the CIE 1931 Standard Observer and the CIE illuminant C. 9.4 Approximation used for newsprint shade – If the sample is fluorescent, the UV condition of the CIELAB colour (C/2°) coordinates (see clause 10) are instrument must be adjusted to correspond to the CIE recommended for the reporting of newsprint shade, but illuminant C, as described in ISO 2470. the (,Ypl De , )-values of the sample may also be used. L* is a measure of lightness. In the CIELAB The following equation was recommended in the now representation, a* is an approximate measure of the withdrawn SCAN-P 71:95 for dominant wavelengths in component on the red-green axis (a* > 0 is reddish and the narrow range from 570 nm to 580 nm. Other l D- a* < 0 is greenish) and b* is an approximate measure of values must be calculated according to the definition the component on the yellow-blue axis (b* > 0 is above. yellowish and b* < 0 is bluish).
23 l D =+ABMCMDM + + [29] 10.2 CIELAB colour (D65/10º), ()Lab*, *, * – L*, a* and b* values of the sample according to the where CIELAB 1976 system, evaluated according to the CIE 1964 Supplementary Standard Observer and the CIE standard illuminant D65. Myyxx=-bgbgsn sn - [30] If the sample is fluorescent, the UV condition of the instrument must be adjusted to correspond to the CIE and illuminant D65, as described in ISO 11475. x and y are the C/2° chromaticity coordinates of the s s newsprint sample; 10.3 Calculations – In the CIELAB system, the x and y are the C/2° chromaticity coordinates of the n n coordinate system is chosen so that a* = 0 and b* = 0 perfect reflecting diffuser. represent achromatic stimuli. If at the same time Numerically x and y are: n n L* = 100 , then the stimulus corresponds to the perfect
reflecting diffuser under the illuminant chosen. To xy==0,31006; 0,31615 nn achieve this, the tristimulus values for the perfect
reflecting diffuser under the chosen illuminant have to In order to obtain l in nm, the following constants D be incorporated into the equations. These values are are used: denoted X , Y , Z and X , Y , Z for the A = 601,075 C = 14,4225 n n n n,10 n,10 n,10 respective standard observers. The equations below are B = -35,4710 D = -2,41788 given for the 2° observer. The corresponding equations
for the 10° observer are obtained by replacing X , Y , Z, Eq. [30] is not valid if x = x . The condition that sn X , Y and Z by X , Y , Z , X , Y and these constants shall only be used within the range of n n n 10 10 10 n,10 n,10 Z . l from 570 nm to 580 nm means that n,10 D The CIE lightness L * is calculated as: 1,7803≥≥M 0,84199 . In the same situation, the recommended equation for 13 excitation purity is: |R116◊-bgYYnn 16 YY > 0, 008856 L* = S [32] T|903,, 3◊£bgYYnn YY 0 008856 22 ()()xxsn−+− yy sn p = [31] e 23 EF+λ+λ+λDD G H D If l D is given in nm, the constants in this equation when the dominant wavelength is in the relatively narrow region from 570 nm to 580 nm are:
E = 40,4558 G = 8,95164×10-5 F = -0,130132 H = 2,65050×10-8 SCAN-G 5:03 Page 9
The a * and b * values of the CIELAB system are reported. The chosen values for φ mean that hab has its calculated as: zero value at the a *-axis and that hab increases in the counter-clockwise direction. Mathematically hab is undefined when C *0= . It is however recommended to afXXfYY* =◊500 bgbgnn - ab [33] report hab as undefined when C *ab is so small that the bfYYfZZ* =◊200 bgbgnn - stimulus cannot be perceptually distinguished from an achromatic stimulus having the same L *-value. where 11 Colour differences in CIELAB R ξξ13 > 0, 008856 f bgξ = S [34] 11.1 Introduction – Several colour-difference equations 7,, 787⋅+ξξ 16 116 ≤ 0 008856 T exist. However, none of these are standardized in any SCAN-test method or ISO standard within the pulp and where ξ is XXn , YYn or ZZn . The required values paper sector. Here the most important equation is of X n , Yn, Zn and X n,10, Yn,10, Zn,10 , the tristimulus mentioned, but other colour difference measures such as values for the perfect reflecting diffuser, for different the CIE 1994 (*,*,ΔL ΔC abΔH *) ab colour-difference illuminants are: model, ΔE * , may be considered. 94
A-illuminant: X n = 109,850 X n,10 = 111,144 11.2 CIELAB colour difference, ΔE * – Distance in Y = 100 Y = 100 ab n n,10 the CIELAB colour space between two colour stimuli. Zn = 35,585 Zn,10 = 35,200 The CIELAB colour systems is constructed to be C-illuminant: X n = 98,074 X n,10 = 97,285 approximately uniform with respect to colour differences. However, it has to be noted that it is an Yn = 100 Yn,10 = 100 approximation and allows neither for large colour Zn = 118,232 Zn,10 = 116,145 differences nor exact comparisons of colour differences in different parts of the colour space. ΔE * is used for D65-illuminant: X n = 95,047 X n,10 = 94,811 ab applications in which the CIELAB system can be Y 100 Y = 100 n = n,10 considered to be sufficiently uniform. The colour Z = 108,883 Z = 107,304 n n,10 difference ΔE *ab between two stimuli bgLab*,111 *, * and bgLab*,222 *, * is defined as The chromatic information in the a * and b * coordinates may also be expressed in polar coordinates 222 ΔΔΔΔELab****ab =++afafaf [38] C *ab and hab where the CIE 1976 a, b chroma C *ab is a correlate of chromaticness and is analogous to the where excitation purity, and the CIE 1976 a, b hue-angle hab is a correlate of hue and is analogous to the dominant ΔLL**= 21− L * wavelength. C *ab and hab are calculated as: Δaa**=−21 a * [39] 22 Δbb**=−21 b * Cab***ab =+()() [35]
b * 180° RarctanF I ⋅ +≠φ a*0 | H a *K π 12 CIE-whiteness | 90°=>ab*0 *0 hab = S [36] 12.1 CIE-whiteness, W – Measure of whiteness derived | 270°= the context, the observer and illuminant shall be specified A positive Tw -value indicates a greenish tint and a as a suffix, e.g. WD65 10. If the sample is fluorescent, the negative value a reddish tint. The primary purpose of the UV condition of the instrument must be correctly adjusted. tint value calculation is not to provide a colorimetric WD65 10 with a UV content in the light source parameter but to provide a limit for the applicability of corresponding to illuminant D65 is recommended as the whiteness equations. The material is considered to be outdoor daylight conditions (ISO 11475). WC2 with a UV white only if the tint value lies within the limits given content in the light source corresponding to illuminant C is by: recommended as indoor daylight conditions (ISO 11476). For more details concerning the UV-conditions, see -<33Tw < [46] Annex B. The CIE-whiteness value W is defined as: Table 2. Data for x , y , x and y for two WY10=+ 10800didi xnn,, 10 - x 10 + 1700 y 10 - y 10 [40] n n n,10 n,10 illuminants. for the 10° observer and For the C- xn = 0,31006 yn = 0,31615 illuminant: xn,10 = 0,31039 yn,10 = 0,31905 WY=+800bg xnn - x + 1700 bg y - y [41] For D65- xn = 0,31273 yn = 0,32902 for the 2° observer, where xn,10, yn,10, xn and yn are illuminant: xn,10 = 0,31382 yn,10 = 0,33100 the chromaticity coordinates for the perfect reflecting diffuser under the illumination considered and for the 10° observer and the 2° observer respectively. Values 13 Fluorescence component of CIE-whiteness are given in Table 2. 13.1 Fluorescence component (of CIE-whiteness), WF The material is considered to be white provided that – Measure of the extent to which the whiteness of the W lies within the limits given by: material is affected by excitation of the added fluorescent whitening agent (FWA) under the conditions 40< 12.2 Green/red tint, CIE tint, Tw – Measure of deviation from whiteness towards the green or red region, and expressed as tint units (ISO 11475, ISO WWWF =-0 [47] 11476). Under proper viewing conditions, a correlate to the where perceived deviation from whiteness of the material W is the whiteness value under D65 or C conditions; towards the greenish or reddish regions. W0 is the whiteness value when the fluorescent Two tint values exist, analogous to the two whiteness effect has been fully eliminated. values for D65/10° and C/2° conditions. If it cannot be unambiguously deduced from the context, the observer It is not sufficient merely to eliminate the UV- and illuminant should be specified as a suffix, e.g. component of the illumination since the added Tw,D65 10. Fluorescent Whitening Agent is to some extent excited The CIE green/red tint value is calculated for the CIE by visible light. The instrument shall be equipped with a 1964 and 1931 Standard Observers according to: sharp cut-off, UV-absorbing filter having a transmittance not exceeding 5,0 % at and below a wavelength of 410 nm and not exceeding 50 % at a wavelength of 420 Txxyywn,,10=---900chch 10 10 650 n , 10 10 [44] nm. The cut-off filter shall have characteristics such that a reliable reflectance factor value is obtained at 420 nm. or The reflectance factor value obtained at 420 nm shall then be considered for computational purposes to be the Txxyywn=−−−1000() 650 ( n ) [45] value which applies at all lower wavelengths at which it is not possible to make any measurements. where the values for xn,10, yn,10, xn and yn are given in Table 2. SCAN-G 5:03 Page 11 14 Yellowness index If, in the primary condition, the DE *ab,I-value is not exactly zero, a correction can be made to take into 14.1 Yellowness index, J – A measure of the extent to account this deviation. The corrected metamerism index which a nearly achromatic stimulus deviates in is then given by: yellowness from an achromatic stimulus having the same Y-value. 22 The yellowness index is calculated (cf. DIN 6167) MLLaaII I=-+-ebgbgDD** II I DD ** II I as: [52] 2 12 +-bgDDbb**II I j J =−100()RRRxz / y [48] where ΔL *, Δa * and Δb *are defined in Eq. [39]. For the C/2° illuminant-observer condition, the Eq. [52] may only be used to correct for small variations yellowness index is therefore calculated as: in samples which are basically metameric pairs. 11059,277X − , Z J = 100 [49] Y 16 Literature 16.1 CIE Publication 17.4, CIE International Lighting and for the D65/10° illuminant-observer condition the Vocabulary, 4th Ed Geneva 1987 yellowness index is calculated as: 16.2 ISO 2469 Paper, board and pulps – Measurement 1,, 301X − 1149 Z J = 100 [50] of diffuse reflectance factor Y 16.3 ISO 2470 Paper, board and pulps – Measurement of diffuse blue reflectance factor (ISO brightness) 15 Metamerism index 16.4 ISO 2471 Paper and board – Determination of 15.1 Metamerism – Phenomenon that object colours opacity (paper backing) – Diffuse reflectance appear the same although the stimuli are spectrally different (cf. CIE publ. 17.4, 845-03-05). 16.5 ISO 5631 Paper and board – Determination of co- Two such stimuli are called a metameric pair or lour (C/2°) – Diffuse reflectance method metamers. A problem related to the phenomenon of metamerism is that two objects having the same 16.6 ISO 9416 Paper – Determination of light perceived colour in one illumination may have different scattering and absorption coefficients (using Kubelka- perceived colours in another illumination. Munk theory) Colorimetrically metameric colour stimuli are defined as colour stimuli having the same tristimulus 16.7 ISO 11475 Paper and board – CIE whiteness, values but different spectral reflectance factor D65/10° (outdoor daylight) distributions. This definition also implies that the CIELAB colour difference ΔE *ab between these stimuli 16.8 ISO 11476 Paper and board – Determination of is zero. CIE whiteness, C/2° (indoor illumination) 15.2 Metamerism index, M ΙΙΙ/ – Colour difference 16.9 ISO 22891 In preparation ΔE *ab between two samples, which are metamers under a reference illuminant/observer condition, determined 16.10 ISO 4094 Paper, board and pulps – International under a different set of illuminant/observer conditions. calibration of testing apparatus – Nomination and The metamerism index is the difference in colour acceptance of standardizing and authorized laboratories between two samples with regard to the secondary illuminant/observer conditions II when they are the same 16.11 SCAN-P 72 Papers and boards − Colour in the primary illuminant/observer conditions I. If the (D65/10°) ΔE *ab-value of the two samples is exactly zero in condition I then the metamerism index is given by: 16.12 DIN 6167:1980-1 Beschreibung der Vergilbung von nahezu weißen oder nahezu farblosen Materialen MEII/, I=D *ab II [51] 16.13 Wyscecki, G. and W.S. Stiles, Color Science – where DE *ab,II for the two samples is calculated using Concepts and methods, quantitative data and formulae. Eq. [38] for condition II. 2nd ed. 1982, New York: John Wiley & Sons. 950 p SCAN-G 5:03 Page 12 Annex A – Definitions For the purpose of this guideline the following consideration all radiation not merely reflected definitions are relevant: radiation. For non-fluorescent samples, the radiance factor is also the reflectance factor. For A.1 Reflectance − Ratio of the reflected radiant or fluorescent samples, the radiance factor is different luminous flux to the incident flux. from the reflectance factor. Note 1 – Unless otherwise specified, the reflected Note 2 – The radiance factor is often denoted β flux in all possible reflection directions is meant, instead of R . which gives the total reflectance. A.6 Spectral luminous efficiency function, V()λ – Note 2 – It is sometimes convenient to divide the Function describing the sensitivity to light of the human total reflectance into the sum of the specular and eye at different wavelengths. It is mathematically the diffuse reflectance. defined as: ratio of the radiant flux at wavelength l m to that at wavelength λ , when the two fluxes produce the A.2 Reflectance factor, R − Ratio of the radiation same photopic sensation under specified photometric reflected by a body to that reflected by the perfect conditions, l m being chosen so that the maximum value reflecting diffuser under the same conditions of of this ratio is unity (CIE publ. 17.4, 845-01-22). illumination and detection (ISO 2469, ISO 5631). Note 1 – The data of V()λ apply to the CIE 1924 Note 1 – In the context of ISO 2469 and related Standard Photometric Observer for Photopic Vision. standards, the irradiation is diffuse and the direction of detection is perpendicular to the Note 2 – λm = 555 nm . surface of the specimen. Note 3 – Photopic vision is light-adapted vision, i.e. Note 2 – A gloss trap ensures that there is no or the eye response is the response of the cones in the considerably reduced irradiation from directions retina. close to the direction of detection. Note 4 – For computational purposes, the V (λ) A.3 Spectral reflectance factor, R()λ or R()λi − function is identical with the y ()λ function for the Reflectance factor expressed as a function of CIE 1931 (2°) standard observer. wavelength. A.7 Luminous reflectance factor, Luminance Note 1 – In this guideline, a wavelength-specific factor, Rv – Reflectance factor defined with reference to reflectance factor is indicated as R(λ). In the CIE illuminant C and the CIE 1931 colour matching computational contexts, the reflectance factor function y (λ) , and corresponding to the attribute of variable is denoted R(λi). visual perception of the reflecting surface (ISO 2471). A.4 Intrinsic reflectance factor, R∞ – Reflectance Note 1 – The CIE term "luminance factor" is a factor of a layer or pad of material thick enough to be wider term since it does not define the illuminant. opaque, i.e. such that increasing the thickness of the pad by doubling the number of sheets leads to no change in A.8 Intrinsic luminous reflectance factor, Intrinsic the measured reflectance factor (ISO 2469). luminance factor, Rv,• – Luminous reflectance factor of a layer or pad of material thick enough to be opaque, i.e. Note 1 – Reflectivity is synonymous with intrinsic such that increasing the thickness of the pad by doubling reflectance factor. the number of sheets results in no change in the measured reflectance factor. A.5 Radiance factor, R, – Ratio of the radiant flux reflected and emitted by a body to that reflected by the A.9 Opacity, Ω – Ratio of the single-sheet luminous perfect reflecting diffuser under the same conditions of reflectance factor, Rv,0 , to the intrinsic luminous illumination and detection (cf. ISO 11475, ISO 11476). reflectance factor, Rv,∞ , of the same sample (ISO 2471). Note 1 – The radiance factor is analogous to the reflectance factor except that it takes into SCAN-G 5:03 Page 13 The single-sheet reflectance factor is defined as the A.19 Complementary wavelength, lC – Wavelength of reflectance factor of a single sheet of paper with a black the monochromatic stimulus which can be additively cavity as backing. mixed with the stimulus from the sample so that the A.10 Transmittance, τ – Ratio of the transmitted to the mixed stimulus matches a chosen achromatic stimulus. incident flux. Note 1 – Not all stimuli have a corresponding A.11 Transmittance from reflectance factor complementary wavelength, e.g. stimuli with measurement, T – Transmittance calculated from l D ª 520 nm. reflectance factor data determined under the conditions of illumination and detection specified in ISO 2469 A.20 Excitation purity, pe – Ratio of the distance from (ISO 22891 in preparation). the point representing the achromatic stimulus ( xnn, y ) to the colour stimulus considered ( xss, y ) to the distance A.12 Tristimulus values – A triplet of numerical values from the point representing the achromatic stimulus describing the strengths of three reference colour stimuli ( xnn, y ) to the point on the spectrum locus representing when these match the colour of a test stimulus in a well the dominant wavelength (xD, yD) (cf. CIE publ. 17.4, specified colour matching situation. 845-03-48). A.13 CIE tristimulus values, X,,Y Z – Tristimulus A.21 Diffuse blue reflectance factor (ISO brightness), values for a colour stimulus evaluated for the CIE 1931 R457 – Intrinsic radiance factor measured with a Standard Colorimetric Observer (2° observer). reflectometer having the characteristics described in ISO 2469, equipped with a filter or corresponding A.14 CIE tristimulus values, X10,,Y 10Z 10 – function having an effective wavelength of 457 nm and a Tristimulus values for a colour stimulus evaluated for width at half height of 44 nm, and adjusted so that the the CIE 1964 Standard Colorimetric Observer (10° UV-content of the illumination incident upon the test observer). piece corresponds to that of the CIE illuminant C (ISO 2470:1999). A.15 CIE colour matching functions, xyz(),(),()λ λ λ – Functions describing the tristimulus values X,,Y Z for Note 1 – The symbol RB may be used instead of monochromatic colour stimuli of equal radiance and R457. where the wavelength λ is a variable. A.22 D65 diffuse blue reflectance factor (D65- A.16 CIE colour matching functions, xy10(),λ 10 (),λ brightness) R457,D65 – Diffuse blue radiance factor z10 ()λ – Functions describing the tristimulus values measured when the UV-content of the irradiation has X10,,Y 10Z 10 for monochromatic colour stimuli of equal been adjusted as specified in ISO 11475 to correspond to radiance and where the wavelength λ is a variable. the CIE standard illuminant D65. A.17 Y-value (C/2°) – Tristimulus value Y in the A.23 Light-scattering coefficient, s – The fraction of CIEXYZ-system of a layer of material of such a the diffuse light flux that is reflected on its passage thickness that there is no change in Y when the through an infinitesimally thin layer of material. This thickness is increased. may be related to a finite layer using the Kubelka-Munk theory which takes into account the grammage of the Note 1 – This shall be interpreted as the tristimulus material, so that s has the units m2/kg (ISO 9416). value Y of a layer of material of such a thickness that there is no change in the value when the Note 1 – When s is determined as a spectral thickness is doubled, see A.4. quantity, it is denoted s()λ . Note 2 – The irradiation is here CIE illuminant C. Note 2 – When it is determined according to ISO 9416 as a weighted average for the C-illumi- A.18 Dominant wavelength, l D – Wavelength of the nant and the photopic visual efficiency function, it monochromatic stimulus which can be additively mixed is denoted sv. If the information in the suffix can with a chosen achromatic stimulus so that the mixed unambiguously be deduced from the context, the stimulus matches the stimulus from the sample (cf. CIE suffix may be omitted. publ. 17.4, 845-03-44). A.24 Light-absorption coefficient, k – The fraction of Note 1 – Not all stimuli have a corresponding domi- the diffuse light flux that is absorbed on its passage nant wavelength, e.g. stimuli with a purplish through an infinitesimally thin layer of material. This appearance. may be related to a finite layer using the Kubelka-Munk SCAN-G 5:03 Page 14 theory which takes into account the grammage of the A.30 Fluorescence component (of CIE-whiteness), WF material, so that k has the units m2/kg (ISO 9416). – Measure of the extent to which the whiteness of the Note 1 – When k is determined as a spectral material is affected by excitation of the added quantity, it is denoted k()λ . fluorescent whitening agent (FWA) under the conditions specified in the relevant International Standard Note 2 – When it is determined according to (ISO 11475, ISO 11476). ISO 9416 as a weighted average for the C-illuminant and the photopic visual efficiency function, it is Note 1 – The UV-radiation from the source of light denoted kv. If the information in the suffix can is excluded by a UV-absorbing filter having a cut- unambiguously be deduced from the context, the off wavelength of 420 nm. suffix may be omitted. A.31 Fluorescence component of diffuse blue A.25 CIELAB colour (C/2°), (*,*,*)Lab – L *, a * reflectance factor (fluorescence component of ISO and b * values of the sample according to the CIELAB brightness), R457, F – Difference between the ISO 1976 system, evaluated according to the CIE 1931 brightness measured using a source of light having a Standard Observer and the CIE illuminant C. UV-content corresponding to the C-illuminant and the ISO brightness measured with a source without radiation A.26 CIELAB colour (D65/10°), (*,*,*)Lab – L *, a * in the excitation band (cf. A.30). and b * values of the sample according to the CIELAB 1976 system, evaluated according to the CIE 1964 Note 1 – R457,F is used as a measure of the extent Supplementary Standard Observer and the CIE standard to which the ISO brightness of the material is illuminant D65. affected by emission from added fluorescent whitening agent (FWA) when the light source A.27 CIELAB colour difference, ΔE *ab – Distance in emits UV radiation. the CIELAB colour space between two colour stimuli. Note 2 – The of UV-radiation from the source of A.28 CIE-whiteness, W – Measure of whiteness derived light is excluded by a UV-absorbing filter having a from the CIE tristimulus values determined under cut-off wavelength of 420 nm. specified conditions, and expressed as whiteness units (ISO 11475, ISO 11476). Note 3 – The symbol RBF, may be used instead of R457,F . Note 1 – If it cannot be unambiguously deduced from the context, the observer and illuminant shall be A.32 Yellowness index, J – A measure of the extent to specified as a suffix, e.g. WD6510. which a nearly achromatic stimulus deviates in yellowness from an achromatic stimulus having the same Note 2 – If the sample is fluorescent, the UV Y-value. content in the radiation falling on the sample shall correspond to UV(D65) or UV(C) conditions A.33 Metamerism – Phenomenon that object colours respectively to meet the requirements of ISO 11475 appear the same although the stimuli are spectrally or ISO 11476. different (cf. CIE publ. 17.4, 845-03-05). Note 3 – For paper and board and for outdoor Note 1 – Two such stimuli are called a metameric daylight conditions, WD6510 is used, and for indoor pair or metamers. illumination conditions, WC2, i.e. C/2°, is used. A.34 Metamerism index, MII I – Colour difference A.29 Green/red tint, CIE tint, Tw – Measure of ΔE *ab between two samples, which are metamers under deviation from whiteness towards the green or red a reference illuminant/observer condition, determined region, and expressed as tint units (ISO 11475, under a different set of illuminant/observer conditions. ISO 11476). Note 1 – If it cannot be unambiguously deduced from the context, the observer and illuminant should be specified as a suffix, e.g. Tw, D65/10 . SCAN-G 5:03 Page 15 Annex B – UV conditions for whiteness and brightness B.1 Illuminant and observer conditions for whiteness • UV(D65) – The UV content corresponds to the UV content of illuminant D65. Note however that the Illuminants used in the paper industry are: light source in the instrument is not spectrally • CIE illuminant C identical to illuminant D65. • CIE standard illuminant D65. • UV(C) – The UV content corresponds to the UV content of illuminant C. Note however that the light If a colorimetric value is reported as having been source in the instrument is not spectrally identical to calculated according to a certain illuminant, this does not illuminant C. automatically mean that the spectrum of the actual light source used in the measurement resembles the spectrum If both filters are used at the same time, the UV condi- of the illuminant. In many cases, the spectrum of the tion is strictly the product of the two, e.g. UV(C) × light source used is unimportant. UVex(420), but this is usually considered to be CIE has defined two standard observers: UVex(420). The UV conditions may also be obtained by a • CIE 1931 standard observer – the 2° observer combination of separate measurements under e.g. the • CIE 1964 standard observer – the 10° observer. UV(full), UVex(395) and UVex(420) conditions. The paper industry uses the following combinations of illuminant and observer function for whiteness: B.4 Samples containing optical brighteners • C/2° indoor whiteness (ISO 11476:2000) When measurements are made on papers containing • D65/10° outdoor whiteness (ISO 11475:1999). optical brightening agents (OBA, FWA), the UV condition must match the measured quantities according to the following table. B.2 Brightness calculations Weighting UV(C) UV(D65) UVex(420) The diffuse blue reflectance factor (ISO brightness), R457, is calculated from radiance factor data using the weighting function D65/10° – Outdoor Whiteness function F()l according to clause 6 on page 5. The C- whiteness, without illuminant is specified, but it is only important in the case W fluorescence, of fluorescent samples (ISO 2470:1999). D65 10 WD65 10, 0 C/2° Indoor white- – Whiteness B.3 UV conditions of the instrument ness, WC2 without All modern spectrophotometers use one or more xenon fluorescence, flash lamps as light source. Two UV-absorption filters WC20, with cut-off wavelengths of 395 nm and 420 nm adjust Fbgλ ISO bright- D65 bright- Brightness the UV-content of the lamp. If either of the filters is fully ness, R457 ness, without inserted in the light beam before the light enters the R457,D 65 fluorescence, integrating sphere, we have the UV-excluded conditions: R457, 0 • UVex (395) The fluorescent components of the quantities • UVex(420). WD65 10, WC2 and R457 are calculated as In most instruments, the 395 nm filter can be partly in- WWW=− serted into the beam making it possible to adjust the UV D65 10,F D65 10 D65 10, 0 content of the source. The filter is sometimes called the UV WWWC2,F=− C2 C2,0 [B.1] trim filter or the UV adjustment filter. Different positions RRR=− of the UV trim filter give the following UV conditions: 457,F 457 457,0 There is no ISO standard for D65 brightness. • UV(full) – The filter is not in the beam. The UV content is the maximum UV content of the In the measurement of non-fluorescent samples, the instrument and is not defined. It is recommended that UV condition is not important except that UVex(420) is this setting should never be used except when it is not acceptable. If the sample is sensitive to UV known that there is no fluorescence present. radiation, UVex(395) may be recommended. SCAN-G 5:03 Page 16 SCAN-test Methods are issued and recommended by KCL, PFI and STFI-Packforsk for the pulp, paper and board industries in Finland, Norway and Sweden. Distribution: Secretariat, Scandinavian Pulp, Paper and Board Testing Committee, Box 5604, SE-114 86 Stockholm, Sweden.