Rationale
Whiteness is an important visual attribute to evaluate the appearance of coatings, which characterizes the psychological perception of the material's ability to reflect light. In the coating industry, whiteness not only affects the aesthetics of the final product, but is also often indirectly related to the evaluation of the product's coverage, cleanliness, and the dosage of titanium dioxide and other materials in the formulation. By simulating the human eye's perception of white, the whiteness meter uses specific optical geometric conditions and mathematical formulas to convert the reflected light signal into a quantifiable whiteness value.
Its core physics is based on the diffuse reflection of visible light on the surface of an object. The instrument's internal light source illuminates the sample at a specific angle (typically 45°/0° or d/8°), and the detector receives diffuse reflected light from the sample surface. By measuring the reflectance of the sample at multiple wavelengths, especially the blue light band, such as around 457nm, and referring to the data from a standard whiteboard, the instrument calculates the whiteness value of the sample. Commonly used whiteness formulas are designed to consolidate spectral reflectance data into a single numerical value to match the order observed by the human eye.
Workflow
A typical whiteness meter consists of the following core components: a stabilized light source system, an optical system for splitting or filtering, a photodetector, a data processor, and a display unit. The workflow follows a standardized step: first calibrate the instrument and set the reference point for reflectivity using a built-in or matching standard whiteboard and blackboard; Subsequently, the paint sample to be tested was prepared into a uniform, flat, and thick coating film and completely dried. Finally, the sample is placed at the measuring port, and the instrument automatically completes the irradiation, detection, calculation and display of the results.
In order to ensure the accuracy and comparability of measurement results, international and domestic standards have strictly stipulated the measurement conditions. For example, the geometry of illumination and observation, the type of standard light source (e.g., D65 light source simulating daylight), the spectral response function, and the calculation formulas that need to be reported are specified. These specifications are the basis for data comparison between different laboratories.
Whiteness evaluation
Whiteness is a psychophysical quantity, so physical measurements need to be linked to human eye perception through mathematical models. At present, most of the widely used whiteness formulas are recommended by the International Illumination Commission (CIE) or national standards bodies. These formulas are usually calculated based on the reflectance of the sample at a specific wavelength or the triple stimulus value Y (luminosity) and the color coordinates.
A classic Ganz whiteness formula takes the form of:
W = Y + a(x₀ - x) + b(y₀ - y)
where W represents the whiteness value; Y is the brightness value of the sample; x, y are the color coordinates of the sample; x₀, y₀ are the color coordinates of the completely diffuse reflector under the light source used; a, b are coefficients that depend on the light source and observer conditions. The formula takes into account both the luminosity contribution and the impact of chromaticity shift (bluish or yellowish) on whiteness perception. For bluish-whitened samples, the tonal angle or pale tonal index is also used for auxiliary evaluation.
Specific applications:
In the research and development, production and quality control of coating products, whiteness meters play an indispensable role. Its application runs through multiple links.
In the R&D and formulation design stages, technicians scientifically quantify the whiteness of samples with different formulations and raw materials (such as titanium dioxide varieties and dosages, fluorescent brighteners, ultramarine and other toning pigments), so as to optimize the formula and strike a balance between cost and performance.
In the production quality control process, the whiteness meter is used to quickly inspect the whiteness of each batch of products, ensure the consistency of product color, and prevent product color differences caused by raw material fluctuations or process deviations. This is crucial for brands to maintain their product image.
In addition, whiteness measurement is also used to test the weathering and yellowing properties of coatings. By comparing the change of whiteness (ΔW) of the sample before and after the accelerated aging test, the stability and durability of the coating system can be objectively evaluated.
Measurement factors
To obtain reliable, reproducible whiteness data, measurement conditions and sample conditions must be tightly controlled. The main influencing factors and control points are shown in the table below:
| Sample preparation | The coating film should be uniform, untextured, fully covered, and infinitely thick. Substrate color may affect the results. |
| Surface state | The flatness, gloss, and roughness of the sample surface will change the light reflection characteristics and need to be prepared according to the standard. |
| Instrument calibration | Standard whiteboards and blackboards must be calibrated regularly to ensure that the instrument is in stable condition. |
| Measurement conditions | It is necessary to clarify and record the light source, observer field of view, and whiteness formula used to ensure data comparability. |
| Fluorescent substances | For samples containing fluorescent brighteners, the instrument light source must contain the ultraviolet component, otherwise the measured value is low. |
During operation, the principle of "same instrument, same method, same operator" should be followed for comparison testing to reduce system errors. For high-gloss samples, it is sometimes necessary to measure in different modes including specular reflection (SPIN) or exclusion specular reflection (SPEX), and select the appropriate mode according to the product standard.
Relevant standards
The measurement of paint whiteness is subject to a series of international, national and industry standards that ensure the uniformity of measurement methods and the reliability of the data. The main relevant standards relate to the specification of instrument performance, sample preparation, measurement steps and calculation formulas.
| International standards | ISO 7724-3, ISO 11475 (paper and cardboard, methods can be referenced) |
| Chinese national standards | GB/T 5950 (Method for measuring whiteness of building materials and non-metallic mineral products) |
| Industry Applications | Sample preparation and measurement are often carried out with reference to relevant standards such as coating coverage and color determination. |
In actual work, the test report should clearly indicate the standard number on which it is based, the name and code of the whiteness formula used, the instrument model and measurement conditions, which are the basic requirements for data validity and traceability.
Epilogue
The whiteness meter provides an effective tool for the coating industry to objectively and quantitatively evaluate the whiteness of products. A deep understanding of its measurement principles based on light reflection, following standard operating procedures, and taking into account the various influences of the sample and the environment are key to obtaining accurate and reliable measurement data. With the continuous improvement of coating product quality requirements and the continuous advancement of testing technology, whiteness measurement will continue to play an important role in product quality control, new product development and process improvement.
References
1. International Illumination Council (CIE). CIE Publication No. 15: Colorimetry.
2. National Color Standardization Technical Committee. GB/T 3979-2008 Measurement method for object color.
3. Compilation of national standards related to coating testing methods.