Overview
In the paper quality evaluation system, whiteness is one of the key indicators to measure its optical performance. It directly affects the visual appearance, printability, and performance of the final product. While traditional whiteness assessment relies on subjective visual comparison, modern paper whiteness analyzers provide objective, quantifiable measurement data by simulating standard lighting and observation conditions. Among them, CIE whiteness calculated based on the recommended formula of the International Council on Illumination (CIE) has become a widely recognized and adopted standard evaluation method in the industry. This method comprehensively considers the perceptual characteristics of white color by human vision, and provides a unified and scientific benchmark for paper produced by different materials and processes.
CIE whiteness measurement principle
CIE whiteness is calculated based on the spectral reflectance data of paper under a specific light source, usually a D65 standard light source that simulates average daylight. The instrument measures the reflection spectrum of the sample in the visible light wavelength range (usually 360nm to 740nm) through the built-in optical system, and calculates it according to the weighting calculation according to the colorimetric function specified by the CIE, and finally obtains a value representing the whiteness. The core idea is that an ideal fully reflected diffuser (with a reflectivity of 100% in the visible spectrum) is defined as 100, while the whiteness value of the actual sample may be higher or lower than this baseline, depending on its spectral reflectance properties.
Commonly used CIE whiteness formulas (such as the CIE 1982 whiteness formula) not only give the whiteness value W, but also include a pale tone factor T to characterize the degree of slightly colored white (bluish or yellowish). Its basic form is as follows:
W = Y + 800(xn - x) + 1700(yn - y)
T = 900(xn - x) - 650(yn - y)
where Y is the luminance factor of the sample; x, y are the color coordinates of the sample; xn, ynIt is the coordinate of the color of the fully reflected diffuser under the same illuminator. This formula is suitable for the case where the whiteness W value of the sample is between 40 and 5W-150 and the light tone coefficient T is between -3 and +3 under the D65 light source and 10° standard observer conditions.
Measuring instruments
Modern paper whiteness meters are mostly benchtop spectrophotometers or specialized whiteness meters. Its design should strictly follow the requirements of relevant standards for geometric optical conditions, light source spectral power distribution, and detector spectral sensitivity to ensure the accuracy and reproducibility of measurement results. The main technical parameters of the instrument directly affect the reliability of the measurement data.
| Measure geometric conditions | Typically diffuse/8° (including specular reflection) or 45°/0° |
| Standard lighting body | Mainly D65, some instruments support other light sources such as C |
| Standard Observer | 10° field of view or 2° field of view |
| Measure the aperture | According to the sample size selection, Φ25mm, Φ20mm, etc. are common |
| Measure the wavelength range | Typically covers 400nm to 700nm |
| Whiteness ruler | It supports CIE whiteness and often provides whiteness indexes for other industries at the same time |
Measurement process
To ensure consistency and comparability of measurement results, operations need to follow a standardized process. The main steps include: instrument warm-up and calibration, sample preparation and placement, parameter setting, measurement execution, and data logging and analysis.
| Instrument calibration | Use the included standard whiteboard and blackboard for zero and range calibration |
| Sample preparation | Samples should be flat, clean, and wrinkle-free, with multiple layers superimposed to be opaque |
| Measure position | Multiple measurements were taken at different parts of the sample to take an average |
| environmental conditions | Avoid direct light and control the ambient temperature and humidity in a stable range |
| Data logging | The CIE whiteness W value and the light tone coefficient T value were recorded |
Special attention should be paid to the measurement process: the standard whiteboard for calibration needs to be measured or replaced regularly to avoid pollution and scratches; For paper treated with fluorescent brighteners, the instrument should be equipped with a light source containing ultraviolet radiation components to accurately evaluate its whitening effect; The measurement results should clearly indicate the standard, illumination body, observer angle and whiteness formula version on which it is based.
Application:
CIE whiteness measurement has a wide range of application values in paper production, processing and use. In the production process, it is a key parameter to control the degree of slurry bleaching, the amount of filler added and the coating process, which helps stabilize product quality and reduce production costs. In the trade link, it provides an objective quality acceptance basis for buyers and sellers, reducing disputes caused by subjective judgment. For the printing and packaging industry, paper whiteness is an important reference for predicting the color contrast, vibrancy, and overall visual effect of printing. In addition, whiteness changes are often used as one of the indicators to evaluate the durability of materials in archival preservation and paper aging studies.
The CIE whiteness is accurately measured by the paper whiteness tester, and the scientific quantification of the "white" attribute of paper is realized. This is not only an effective tool for production process control and quality assurance, but also provides solid data support for product development, technical improvement, and market circulation. With the continuous improvement of the standard system and the continuous advancement of instrument technology, CIE whiteness measurement will play a more important role in improving the quality of paper products and promoting industry standardization.
References
International Illumination Commission. CIE 15:2004 Colorimetry.
International Organization for Standardization. ISO 11475:2017 Paper and cardboard — Determination of CIE whiteness, D65/10° (outdoor daylight).
General Administration of Quality Supervision, Inspection and Quarantine of the People's Republic of China. GB/T 7974-2013 Paper, cardboard and pulp - Determination of blue light diffuse reflectance coefficient D65 brightness.