What is the whiteness index?

The whiteness index is a value extensively tested by the industry in a multitude of materials and supports.

A fundamental characteristic in the measurement and characterization of paper supports, as is the case of paper and cardboard, is the whiteness index. This index, together with the gloss and the tone, allows the classification of the supports in reference to the optical properties.

However, despite the fact that in the graphic industry The whiteness index is a widely used metric, and it's not the only sector that uses it. Whiteness is commonly measured in textiles, pharmaceuticals, ceramic materials, paint, and plastics, among many other materials.

Table of contents

What is the whiteness index?

Whiteness is the measured amount of light reflected by a support through the visible light spectrum. This value is obtained by calculating the amount of light White -that is, the sum of reflected wavelengths of the spectrum- that the surface manifests. Thus, the value is useful in determining how white is a support to the human eye. The measurement of whiteness is expressed as a percentage, on a scale of 1-100%, with 100% being the value that should correspond to a perfect white.

To carry out the measurement of whiteness, the CIE sets the standard for lighting D65, which roughly corresponds to the incidence of the midday sun in Western and Northern Europe.

The whiteness index and the gloss are different parameters, although their unit of measurement is percentage. Thus, while the first considers the reflection of white light and is measured through color coordinates by a spectrophotometer; the second, more difficult to detect, is measured with a glossmeter.

Thus, we can define brightness as the measure of the amount of blue light reflected by a surface or material. Therefore, we could say that brightness is the volume of reflected light and represents a narrower measure of light reflectance than whiteness.

For its part, whiteness is the measure of the amount of light reflected on a material or support. We could say that it is qualityof light, according to white lightreference. So different levels of whiteness make us perceive materials differently. Thus, with a lower level of whiteness, the materials are perceived as neutral, greyish or more creamy(yellowish); while with a higher level of whiteness we perceive them with bluish tones. Thus, in a perfect sample of reflective and non-fluorescent white , the whiteness index (CIE) would be 100.

How to achieve greater whiteness in the paper industry?

Sometimes, to achieve greater whiteness, the paper industry makes use of optical brightening agents (OBA). However, OBAs can alter the way whiteness works under different lighting conditions. For example, if the paper is a white base sheet without OBA, it might compare well indoors, while it would appear opaque outdoors when compared to sheets with OBA added.

Therefore, it is always a good idea to compare paper samples under different lighting conditions. Visually detecting whether the material incorporates OBA in its manufacture is simple using a visual comparison camera incorporating an ultraviolet lamp.

For cases in which OBAs have been incorporated into the paper support, it is important to point out that, when making a measurement, it must be considered that, although the maximum whiteness range is 100, when incorporating OBAs, the materials reflect light UV as visible light, which will result in a measurement greater than 100. This occurs because, by reflecting UV light, the mount reflects more visible light than is emitted by the actual light source. Similarly, fluorescent OBAs will cause whiteness values ​​to rise above 100.

Concepts

When determining the whiteness index of a support, a series of concepts must be taken into account, such as the shade or yellowing.

“Shade” part of the whiteness. Characterization of supports

The shadow (shade o shading) is basically a subcategory of whiteness. While the percentage rating can sometimes be used by the manufacturer to define whiteness, other companies will identify it by a name that describes the kind of white.

There are three basic categories for shadow. Now, since there is no standard, these categories are given different names:

1. Usually it is used the White blue, since the human being perceives it as whiter. Blue-white papers absorb yellow and warmer wavelengths and reflect blue or cooler wavelengths.
2. On the other hand, the cream papers cream (Cream White) have a yellowish tint, due to the fact that they absorb blues and cooler colors, and reflect yellows and warmer colors. Cream white can also be called warm white.
3. Finally, supports that reflect all wavelengths equally are called TrueWhite. These types of media, under the correct lighting conditions, will appear absolutely white. The TrueWhite, true white, also known as balanced white.

At present, the bluish white is the most common in printing and art media, since a greater number of designers use media with this hue. However, previously, the most common tonality used to be TrueWhite. The trend has changed and today a fresh and bright white is chosen more than a warmer and more natural.

Supports with a bluish-white hue have a greater color range, reproduce more colors and shades, brighter shades and allow, from the manufacturing point of view, to create lower cost papers, which provide a perception of higher quality through of whitening.

Whiteness and yellowing

As explained above, whiteness could be said to be an attribute by which the color of the perceived material is judged to be closest to the preferred white o white set.

For its part, yellowness, or yellowness index, is determined by means of colorimetric or spectrophotometric data and indicates the degree of departure of a color object from colorless or from an established white, towards yellow.

In this regard, when visually assessing whiteness, glossy coatings should be taken into account, as the appearance of the paper is altered by the additional, reflective surface.

Applicable regulations

When determining the whiteness index in paper or cardboard, as well as in other types of materials, there are regulations that must be taken into account. Specifically, we will talk about the rules ASTM-E313 and ISO 11475:2017.

ASTM-E313

The American Society for Testing and Materials (ASTM:) is an international organization that develops standards for producers, users and consumers.

The legislation ASTM-E 313 contemplates the standard practice for the calculation of yellowness and whiteness index with instrumental measurement of colorimetric coordinates.

The procedure uses the model CIE X,Y,Z tristimulus coordinates.

For whiteness (WI) and tint (T), it establishes two recommended calculation formulas:

1. The first of these, based on CIE D65 illuminant and observer 1931.
2. The second one, based on CIE C and D50 illuminant, and observer 1931 (for sectors textile, chemical and for colorists)

Where:

• Y,x,y: Luminance factor and chromaticity coordinates of the sample
• x_n  y_n: Chromaticity coordinates for CIE standard illuminant and the source used
• (WI,x) (WI,y): Numerical coefficients

Where:

• (T,x) (T,y): numerical coefficients

Now, these equations should be used only to show that they satisfy that:

• The application of the equations is restricted to samples commercially called White, similar in color and fluorescence and measured with the same instruments and at the same time.
• The higher the WI value, the higher the whiteness.
• The higher T, the greener the sample. Negative T indicates reddish sample.
• For a perfect reflective diffuser: WI will be 100 and T will be 0.

As far as the yellowing index is concerned, the calculation formula used has been derived from the Hunter equation of 1942.

This equation was later modified to improve rounding. Finally, the formula recommended in the procedure E313, to minimize the residual error in the values ​​of the white points, is the following:

Where:

• (C_x) (C_z): Numerical coefficients

ISO 11475: 2017

International standard ISO 11475:2017 includes the procedure for determining the CIE Whiteness of supports in measurement condition D65/10 (daylight). In Spain, the current norm is the UNE-ISO 11475: 2010.

This standard is applicable not only to paper and cardboard, but also to paints, plastics and other surface colorants.

The values ​​obtained correspond to the visual appearance of white paper and cardboard, with or without fluorescent whitening agents, when viewed under illuminant D65 (daylight). It is based on reflectance data obtained over the full range of the visible spectrum (VIS), in contrast to the ISO whiteness measurement which is limited to the blue region of the VIS.

The standard specifies a method to adjust the UV content corresponding to illuminant D65 (daylight), insofar as the results obtained when fluorescent optical brighteners are present depend on the UV content of the radiation incident on the sample. Thus, this method is specific for the measurement of fluorescence in the blue region of the spectrum. However, it is not applicable to colored papers containing fluorescent dyes.

This standard should be read in conjunction with the ISO 2469. Paper, cardboard and pastes. Measurement of diffuse radiance factor (diffuse reflectance factor)

CIE 15

Regarding the CIE 15 standard, it provides the recommendations of the International Commission on Illumination (CIE) on colorimetry.

This standard recommends that formulas for whiteness (W or W₁₀) and the dye (T_w or T_w,10) below are used to compare the whiteness of samples evaluated for the CIE standard illuminant D65.

Where:

• Y is the tristimulus value Y of the sample for the CIE 1931 Standard Colorimetric Observer
• x and y are the x,y chromaticity coordinates of the sample for the CIE 1931 Standard Colorimetric Observer
• x_n, and_n are the chromaticity coordinates of the perfect diffuser for the CIE 1931 Standard Colorimetric Observer
• Y₁₀,x₁₀, and₁₀,x_n.10 and y_n.10 are similar values ​​for the CIE 1964 standard colorimetric observer.

According to the standard, these formulas must be applied only to samples that are commercially called 'white', that do not differ much in color and fluorescence, and that are measured in the same instrument at almost the same time.

Laboratorios Eyco, experts in optical testing

Our laboratory is technically equipped to determine and evaluate properties related to color adjusted to the guidelines of different standardized norms.
Among the properties for which Eyco offers a testing service are whiteness and yellowness indices, according to standards ASTM-E 313 y UNE-ISO 11475: 2010

Furthermore, Eyco has the technical capacity to carry out tests adjusted to the standard of other types of properties such as luminance factor or gloss.

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