Paper whiteness refers to the ability of paper to be completely reflected after being exposed to light, expressed as a percentage. From a technical point of view, the whiteness of paper and cardboard includes various complex factors, which are functions of the total spectral reflectance of the paper, the distribution of illumination energy, the viewing conditions, and the characteristics of the observer. In GB / T17749-1999 "Whiteness Representation", whiteness is defined as the degree of characterizing the whiteness of an object. The larger the whiteness value, the greater the whiteness. At present, there are many methods for expressing whiteness in various industries, about 100 kinds. No matter what method is used to express whiteness, the evaluation result of whiteness should be consistent with human visual experience.
1. Whiteness Standard
At present, the relevant state departments have entrusted the China Pulp and Paper Research Institute to draft a whiteness standard for paper products. This standard will stipulate the maximum limit for the whiteness (brightness) of paper and paperboard. Any product that exceeds this limit is not qualified.
Jiang Rongqi, the executive secretary general of Shanghai Papermaking Society, said, "The whiteness of a piece of ordinary printing paper is about 100%, but from the perspective of eye comfort, a whiteness of about 85% is enough." And the concept of citizen consumption exists Deviation, thinking that the whiter the paper is, the better, but in fact, manufacturers have to add fluorescent bleach to increase the whiteness of the paper, which wastes resources and causes pollution. At the same time, the paper with too high whiteness will also harm the human vision. The recycled paper with low whiteness was difficult to promote because of the wrong consumption concept of the citizens. In response to the above problems, in order to help the citizens establish a correct consumption concept, it is imperative to formulate whiteness standards.
2. The expression of whiteness
There are currently many ways to express whiteness, mainly ISO brightness, D65 brightness and CIE whiteness.
ISO 2470: 1999 "Paper, Paperboard and Pulp-Determination of Diffuse Reflectance Factor (ISO Brightness)" defines the ISO brightness: equipped with filters or with corresponding functions to make the spectral characteristics effective wavelength is 457 nm, half The wave width is 44nm, and the internal reflection factor measured by the instrument whose UV content of the light source irradiated to the sample is consistent with the CIE illuminant C is adjusted.
The brightness defined in China's national standard GB / T 7974-2002 "Determination of Brightness (Whiteness) of Paper, Paperboard and Pulp (Diffuse Vertical Method)" is: Analog D65 of the reflection photometer specified in GB / T7973 Under the condition of light source, the internal reflection factor of the sample to the main wavelength (457 ± 0.5) nm blue light. This definition is what we call whiteness in daily use and is called D65 brightness in the ISO 2470: 1999 standard.
Another method of expressing whiteness is CIE whiteness, which is widely used in the world. ISO / TC6 has developed corresponding test method standards. Some domestic enterprises also adopt this method of whiteness. CIE whiteness is divided into C light source and D65 light source. The corresponding ISO standards are ISO 11475 "Paper and cardboard-Determination of CIE whiteness, D65 / 10o (outdoor light)" and ISO 11476 "Paper and cardboard- CIE whiteness measurement, C / 2o (indoor light).
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