The whiteness of white pigments is a key optical indicator for evaluating their quality, reflecting the material's comprehensive ability to diffuse reflection of visible light. As a specialized optoelectronic instrument, the whiteness meter provides objective and quantitative whiteness values by simulating the human eye's perception of white. Establishing standardized measurement methods is fundamental to ensuring consistency and comparability of data between different laboratories and guiding production and application. This method aims to standardize the instrument operation, sample preparation and data processing process.
Principles and formulas
The evaluation of whiteness is based on the principle of colorimetry. At present, the whiteness formula recommended by the CIE (International Illumination Commission) is widely used in the world, and its calculation is based on the CIE Lab color space or XYZ chromaticity system. The commonly used CIE whiteness formula (for D65 standard light source and 10° standard observer conditions) is as follows:
W = Y + 800(xn - x) + 1700(yn - y)
W10 = Y + 800(0.3138 - x) + 1700(0.3310 - y)
Among them, W or W10Indicates the whiteness value; Y represents the luminance factor of the sample; x, y are the color coordinates of the sample; xn, ynis the color coordinate of the fully diffuse reflector under the specified illumination body. The latter two items in the formula introduce hue compensation, and include reddish or bluish tones in the whiteness evaluation system. When determining, the relevant algorithm is usually integrated inside the whitemeter to read the calculation results directly.
Instruments and materials
The core instrument is a spectrophotometric or photoelectric integral whiteness meter that meets the measurement requirements. The instrument should be able to provide data under D65 standard illuminator and 10° standard observer conditions, and be calibrated regularly using a standard whiteboard and a working whiteboard. The required materials include: white pigment powder to be tested, sample press or film making device, standard whiteboard, working whiteboard, constant temperature and humidity environment control equipment. All tests should be performed in an environment with a temperature of 23±2°C and a relative humidity of 50±10% to reduce the impact of environmental fluctuations on optical measurements.
Assay steps
The first step is instrument calibration. After the whiteness meter is preheated and stabilized, calibrate it with a standard whiteboard and a working whiteboard in order to ensure that the instrument is in a standard state. The second step is sample preparation. A sufficient amount of representative pigment powder is taken and pressed under constant pressure using a sample press to form a smooth specimen with a flat, texture-free, and blemish-free surface. It can also be made into uniform, opaque paint film or plastic sheet according to relevant product standards. The preparation process should avoid contamination and scratches. The third step is the measurement operation. The prepared specimen is completely covered with the measuring hole, ensuring that the measurement area is uniform and gap-free. At least three different points on the same specimen surface are selected for measurement, and the whiteness value and hue angle of each point are recorded. The specimen can be moved slightly before each measurement to obtain new regional data. The fourth step is data processing. The arithmetic mean of the whiteness values at multiple measurement points of the same specimen was calculated as the final result, and the standard deviation was calculated to assess uniformity.
The accuracy of the measurement results is affected by multiple factors and needs to be controlled during the standardization process.
| Sample surface state | The flatness, texture, and roughness should be consistent to avoid specular reflection interference. |
| Specimen thickness and opacity | It is necessary to ensure that the specimen is infinitely thick and that the light transmission does not affect the reading. |
| Instrument calibration status | Regularly trace the source to national benchmarks, and check the whiteboard for daily use. |
| Measuring aperture selection | Selected according to the uniformity of the sample, large pore size can be selected for poor uniformity. |
| environmental conditions | Stable temperature and humidity, avoid direct light on instruments and samples. |
The results show
The measurement report should be clear and complete, including at least the following information: sample description and identification, standard method number used, instrument model and number, standard whiteboard data, calibration status, test environment temperature and humidity, sample preparation method, whiteness average and standard deviation, and the name of the whiteness formula selected (e.g., CIE W).10), any deviation from the standard steps. The whiteness value in the report should be kept to one decimal place.
This standardized method is suitable for the determination of whiteness of various inorganic and organic white pigments, such as titanium dioxide, zinc oxide, Lide powder, etc. in powder form, slurry or in the form of a uniform opaque film. Not suitable for use with fluorescent whitening materials or pigments with obvious color. It should be noted that different industries or product standards may have specific requirements for sample preparation (such as tablet pressure, dispersion media), and their regulations should be followed first. In the event of a dispute, the standardized method agreed upon by the relevant parties shall prevail.
References
GB/T 5950 Method for determining the whiteness of building materials and non-metallic mineral products
ISO 787-24 General Test Methods for Pigments and Constitution Pigments Part 24: Determination of Relative Coloring Power of Coloring Pigments and Relative Scattering Power of White Pigments Photometric Method
CIE 15:2004 Chromatics
JIS K 5107 Pigment Test Method