Contrast ratio is an important optical performance index to evaluate the masking ability of coating materials, and is widely used in quality control in coatings, inks, paper, and various surface treatment industries. The measurement results are directly related to the appearance and use performance of the product. In this measurement process, black and white test paper is used as the basic carrier, and its standardization and stability play an indispensable role in ensuring data accuracy and comparability.
Basic principles of ratio determination
Contrast ratio is defined as the ratio of the reflectance of the coating material on the black substrate to the reflectance coated on the white substrate under the same specified thickness or mass per unit area, usually expressed as a percentage. Its calculation formula can be expressed as:
CR = (RBlack / RWhite) × 100%
where CR stands for Contrast Ratio, RBlackRepresents the reflectivity of the coating on a black background, RWhiteRepresents the reflectivity of the coating on a white background. The closer the ratio is to 100%, the stronger the covering ability of the coating. The determination is usually done using a reflectometer such as a colorimeter or spectrophotometer.
As a standard substrate for load-bearing coatings, the performance of black and white test paper must meet strict technical specifications to ensure that the results are not interfered with by substrate variables. The main technical requirements focus on optical and physical properties.
In terms of optical characteristics, the white test paper should have high and stable reflectivity, and its color should be close to neutral white to avoid hue deviation affecting the reflectance reading. Black test paper requires very low reflectivity and its surface color should be neutral black. The gloss of both surfaces needs to be consistent and low to eliminate specular interference with diffuse reflection measurements.
In terms of physical properties, the test paper needs to have a uniform surface texture, consistent absorbency or hydrophobicity, and good dimensional stability and mechanical strength. The flatness, thickness uniformity, and absence of fluorescent brighteners are also prerequisites for ensuring that the coating forms a uniform film layer and obtains reliable reflection data.
The effect of test strip standardization on the assay results
Using test strips that are not standardized or have inconsistent performance can introduce significant measurement errors. For example, a white backer that is insufficiently reflective or colored will result in RWhiteThe value is low, so the contrast ratio calculated is inflated. If the black backing paper has a high reflectivity, it will lead to RBlackLarge values will also result in inaccurate contrast ratios. The difference in surface gloss and liquid absorption of the substrate will affect the leveling and film formation of the coating, and then affect its true masking performance.
Therefore, following the provisions of domestic and foreign standards (such as GB/T, ISO, ASTM related standards) for test papers is the basis for data comparison and product quality grading between laboratories. Standards usually clearly specify the reflectivity range, color tolerance and preparation method of black and white test paper.
Operational points
In actual measurements, in addition to ensuring the use of test papers that meet the standards, the operating procedure also needs to be standardized. The sample to be tested should be evenly coated on a black and white test paper placed side by side in the prescribed method. After coating, it is dried or cured under standard conditions. When measuring, ensure that the measuring aperture of the instrument completely covers the coating area and avoid measuring close to the junction of black and white paper. It is recommended to coat and measure the same sample multiple times, with the average as the final result to reduce accidental errors.
Summary of the provisions for black and white test papers in common standards
| Standard system | Briefly describe the core requirements for black and white test papers |
| GB/T 23981 series | It is stipulated that the reflectivity of white substrates is not less than 80%, and the reflectivity of black substrates is not higher than 1%. The substrate should be a matte neutral color. |
| ISO 6504 series | The whiteboard reflectivity is required to be about 84%, and the blackboard reflectivity is less than 2%. It is emphasized that the surface of the substrate should be a uniform diffuse reflector. |
| ASTM D2805 | The black and white coverage test paper with a specific reflectivity is explicitly used, and the flatness and non-fluorescence characteristics of the paper are specified. |
Conclusion
The black and white test paper is not a simple background material, but a reference element in the ratio determination system. The standardization of its optical and physical properties is the fundamental guarantee for obtaining accurate, reliable and reproducible contrast ratio data. In laboratory quality control, strict screening, standardized use, and regular verification of black and white test papers are as important as maintaining testing instruments. An in-depth understanding of the technical requirements of test strips and their influence mechanisms on measurement results can help improve the reliability of test data and the efficiency of technical communication within the industry.
References
1. National standard GB/T 23981.1-2019, Determination of covering power of colored paints and varnishes.
2. International standard ISO 6504-1:2019, Paints and varnishes — Determination of hiding power.
3. ASTM D2805-96a, Standard Test Method for Hiding Power of Paints.
4. Technical Handbook of Coating Technology, Chemical Industry Press.