Whiteness and Yellowness Indices in a SpectroEye

Yellowness

Yellowness is defined as a measure of the degree to which the color of a surface is shifted from preferred white (or colorless) towards yellow.

Yellowness, as defined by ASTM E 313, has been applied successfully to a variety of white or near-white materials, including paints, plastics, and textiles. In terms of colorimeter readings, it was YI=100(1-B/G) where B and G are respectively amber blue (B) and green (G) colorimeter readings. Its derivation assumed that, because of the limitation of the concept to yellow (or blue) colors, it was necessary to take account of variations in the amber or red colorimeter readings.

Yellowness according to ASTM E 313 (D 1925) was developed specifically for determining the yellowness of homogeneous, non-fluorescent, nearly colorless, transparent, nearly white translucent, or opaque plastics, as viewed under daylight lighting conditions. It can also be applied to materials other than plastic fitting this description. The indices can be calculated, rounded, and adjusted in the last retained significant digit to minimize the residual error in the white point values. The equation is: YI=100(CxX-CzZ)/Y, where Cx and Cy standard coefficients described in the standard and correspond with observer angle and color temperature.

Whiteness

Whiteness is defined as a measure of how closely a surface matches the properties of a perfect reflecting diffuser, i.e. an ideal reflecting surface that neither absorbs nor transmits light, but reflects it at equal intensities in all directions. For the purposes of this standard, the color of such a surface is known as preferred white.

ASTM E313 – measuring procedure and settings are described in the same standard (ASTM E313: whiteness and yellowness of paper) like the Yellowness indices. This method is based on the use of colorimeter readings B and G. The idea was that chromaticity factor G-B required three times the weighting of the lightness factor G of the lightness. The equation is: WI=G-4(G-B)=4B-3G

Berger – this whiteness index is specified for illuminant C and 2-degree observer functions only. However, the equation is commonly used with other illuminants and observer functions, therefore the value shown will depend on the primary illuminant and the observer function you have chosen. The formula is: WI=Y+0.3018Z-3.831X.

Stensby – based on Hunter Lab color coordinates. Values greater than 100 indicate a bluish white, while values less then 100 indicate a yellowish white. The formula is: WI=L-3b+3a

Brightness ISO R457 (ISO 2469) – ISO brightness, which is often referred to as TAPPI brightness, indicates how much light is reflected in a very specific area of the spectrum. It entails a filter with a maximum transmission curve of 457-nm wavelength (lambda 457) and a specific half peak width.

Standard ISO 2470 specifies a half peak width of 44 nm. The entire filter values are given in the TAPPI and CPPA Standards. The ISO brightness value indicates the blue reflection of the paper. In the case of non lightened samples, standard illuminant C, i.e. light without a UV component, can be used for measurement. Standard illuminant D65 must be used for measurement of optically lightened papers. The optical lightener is effective only due to the UV radiation. The effect occurs at approximately 360 nm; maximum radiation is at approximately 460 nm. It should be clear that the distribution of relative spectral radiation of the light on the sample during measurement plays an important role. The greater the illumination intensity in the UV-range, the more the greater the detection of the optical lightener. However, this also intensifies ISO brightness. Comparable values between two instruments can be obtained only when both are measured with the same light.

CIE Whiteness – Some disadvantages of the previously-mentioned indices is that a whiteness that is calculated with this formula does not differ if the measured sample has a color drift or is just less white. To make the white weighting more informative, the CIE recommended in 1981 a formula that is today known as “CIE Whiteness.” These indices specified by the CIE for the D65 and illuminant C in combination with either 2° or 10° observer function. However, the equation is commonly used with other illuminants; therefore the value shown will depend on the primary illuminant you have chosen.

WI=Y+(WI, x)(xn-x)+(WI,y)(yn-y), where Y, x, y – the luminance factor and the chromaticity coordinates of the specimen; xn and yn – the chromaticity coordinates for CIE standard illuminant and source used; WI,x and WI,y – numerical coefficients.

Tint CIE - Two quantifying parameters are based on the white evaluation by Ganz/Griesser. It is about a simplified version of the Ganz/Griesser-formula. The difference is, that the Ganz/Griesser-formula contents variable parameters, which are calibrated to a white standard, while the CIE-formula is based on fixed factors.

What is mainly annoying with the one-dimensional white evaluation is, that in the same measurement value there is a quality valuation (How white is the sample?) combined with a describing criteria (color drift), the intention of the CIE-white evaluation is to separate this two criteria. Because with papers less white is mostly identical with yellowish and bright white with bluish, therefore the CIE Whiteness is more or less according to a blue/yellow axis, by a yellowish tint it decreases the value and a bluish tint increases the value. It is even proved, that the CIE Whiteness correlates really well with the CIELab value b*, which represents the blue/yellow axis in this system. The color deviation number Tw shows additionally, if there is a red hue (negative value of Tw) or a green hue (positive value of Tw).

Tw=T,x(xn-x)-T,y(yn-y), where the symbols have meanings analogous to previous.

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