
How is paper whiteness tested? How should the standard be defined?
Release Time: 2025-01-04 00:00:00.000
What is whiteness?
There are many whites in nature, such as snow, lime, cotton, etc. White has the characteristics of high spectral reflectance and low chroma Generally, it is considered that when the reflectance of an object's surface to all wavelengths of the visible spectrum is above 80%, the surface of the substance can be considered white. Whiteness is the "degree of apparent whiteness", which represents the degree of whiteness or near-whiteness of a sample, The larger the value, the whiter it is, generally using WI to represent 。
There are many ways to express whiteness, and different industries use different methods. The more commonly used methods are blue light whiteness and CIE whiteness. In addition, there are Ganz whiteness, Hunter whiteness, and Berger whiteness methods. The printing industry mainly uses Tappi brightness (a kind of blue light whiteness) and CIE whiteness to judge the degree of whiteness of paper or white ink.
Blue light whiteness is a single-band whiteness formula, which measures the diffuse reflectance factor of blue light at a short wavelength of 457 nm, represented by R457. It is mainly divided into Tappi brightness (also known as GE brightness), ISO brightness, and D65 brightness.
Whiteness standards
Below are the standards for different blue light whiteness, and a comparison of optical geometry and light source conditions:
International Standard | Optical Geometry | Light Source |
Tappi452 | 45°/0° | CIE C |
ISO Brightness (ISO2470-1) | Diffuse reflection (d/0°) | CIE C |
D65 Brightness (ISO2470-2) | Diffuse reflection (d/0°) | CIE D65 |
Next, we will introduce another commonly used whiteness standard in the printing and papermaking industry: CIE whiteness:
CIE whiteness is consistent with the whiteness formula after ASTM E313-95, which is applicable to D65 light source and C light source. ISO11475 and ISO11476 are the CIE whiteness determination of paper and paperboard under outdoor light (D65/10°) and indoor light (C/2°) conditions, respectively. When using CIE whiteness, attention should also be paid to the required light source viewing angle conditions, and the instrument structure type should also be noted.
The CIE whiteness formula is as follows:
• T-Light color tone index
• Y-Y value in the tristimulus values of the sample in the XYZ colorimetric system
• x, y-x and y values in the trichromatic coordinates of the sample in the XYZ colorimetric system
• x0, y0-x0 and y0 values in the trichromatic coordinates of the perfect diffuser in the XYZ colorimetric system under the corresponding light source viewing angle
The paper parameter function of X-Rite Spectrophotometer eXact can also measure CIE whiteness. eXact has three ASTM E313 whiteness formulas: E313-SE, E313-98, E313-05. Among them, E313-SE is the same whiteness formula as the old spectrophotometer SpectroEye, which is E313-1973; E313-05 is equivalent to CIE whiteness in measurement values. Figure 3 shows the results of three different E313 formulas for measuring the same type of paper.
Let's take a certain type of printing paper as an example to measure the CIE whiteness, as shown in Figure 4. The first line is ,The second line is
. Figure 5 shows the corresponding CIE whiteness measurement results: CIE whiteness is 80.94; CIE hue is -0.34. When CIE whiteness is greater than 100, it indicates that the sample is blue-toned white, and less than 100 is yellow-toned white, so this paper sample is less than 100, which is yellow-toned white; when the hue index T is greater than 0, it indicates that the sample is biased towards green-toned white, and less than 0 is biased towards red-toned white. This paper sample is negative, which is biased towards red-toned white, so overall, this paper sample is biased towards warm colors.
Then we will introduce other whiteness parameters that eXact can measure, Berger whiteness and Stensby whiteness, as shown in Figure 6:
Berger whiteness formula in the papermaking and textile industries:
The measurement conditions of the formula are also C/2°. If the Stensby whiteness is greater than 100, it is blue-toned white, and less than 100 is yellow-toned white. Taking a certain paper measurement sample as an example, as shown in Figure 7:
From the data, it can be concluded that this paper sample is blue-toned white.
With the increasingly detailed and specific requirements for whiteness from printing brand manufacturers and third-party testing institutions, it is hoped that printing users can fully understand various different whiteness standards and correctly apply whiteness parameters to daily production work to control material quality and meet customer requirements for paper whiteness parameters.
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