This article describes in detail the method for determining the diffuse reflectance factor of pulp, paper and cardboard, including the definition of the reflectance factor, the standardization process and the calibration conditions. Diffuse reflection factor measurement involves the optical properties of pulp, paper, and cardboard, such as the blue light diffuse reflection factor, the opacity of paper, and the light scattering and light absorption coefficients. The article explains the use of primary, secondary, and tertiary reference standards, describes the geometric conditions and spectral characteristics requirements of the reference instrument, and provides detailed calibration methods and maintenance of working standards. The standardization of the test procedure ensures the accuracy and consistency of the measurement results.
definition
Reflection Factor (R)Refers to the ratio of the radiant flux reflected by an object to the radiant flux of a completely reflected diffuser under the same conditions, usually expressed as a percentage. This metric is used to measure the ability of an object's surface to reflect light.
Internal reflection factorIt refers to the reflectance factor value that no longer changes with the thickness of the sample layer when the thickness of the sample layer increases. This value reflects the stability of the optical properties of the material at a certain thickness.
Level 1 Reference StandardsRefers to a standard with reliable uniform and fully reflective diffuser characteristics, the reflectance of which is equal to 1 in the full spectral range. It is implemented by a standardized laboratory with an absolute diffuse reflection factor Measurement Instrument and is used to accurately calibrate other reference standards.
Secondary reference standardsIt is a transfer standard for measurement and calibration based on a primary reference standard. These standards are used by authorized laboratories for the calibration of reference instruments to guarantee their accuracy and consistency.
Three-level reference standardIt is a standard calibrated by a reference instrument calibrated by a secondary reference standard. They are used for the calibration of instruments and working standards in working laboratories to ensure the reliability of daily measurements.
The basic conditions for the transfer of standard values
Reference instruments
Geometric conditions – the reference instrument needs to use an integrating sphere to diffuse illumination of the specimen, the diameter of the integrating sphere is not less than 100mm, and the pore area in the sphere does not exceed 10% of the sphere area. The diameter of the specimen hole should not be less than 20mm. The observation angle requirements include the angle between the normal of the specimen surface and the axis of the observation beam of 8 degrees, and the maximum angle between the observation axis and the observation ray is 2.5° ±0.2°.
Spectral Characteristics – The instrument should calibrate the spectral characteristics of the light source by means of a fluorescent standard plate with a UV cut-off filter with a cut-off range of less than 400 nm. The spectral characteristics must meet the optical measurement standards, and the linearity error of the instrument should not exceed 0.3%.
Secondary reference standards
Preparation of barium sulfate or magnesium oxide tablets – The preparation of high-purity barium sulfate or magnesium oxide powder tablets requires the use of specific equipment and materials, such as powder presses, porcelain spoons, synthetic fiber soft brushes, etc. The preparation process includes cleaning and drying equipment, checking the surface quality of the tablet presses, ensuring the uniformity and stability of the reflectance factor, and keeping the storage conditions dry and free of contamination.
Fluorescent White Metric Calibration – White metric calibration is required using standards with and without fluorescent brighteners.
Three-level reference standard
These include paper patterns with different reflectance factors, raw ceramic plates, whiteboards with and without optical brighteners, and swatches of different colors. The standard should maintain the stability and consistency of the reflectance factor under careful protection and ensure that the surface is straight and free of matte or blemishes.
Standards of work
Rough ceramic plates and opalescent glass plates – for routine calibration of laboratory instruments.
Blackbody with an absolute reflectance factor of no more than 0.1% – used to calibrate the zero point of the instrument to ensure an accurate reference.
Calibration methods for working laboratory instruments and working standards
Calibration of working standard plates
Calibration with secondary standards: The working standard board needs to be calibrated with a secondary reference standard to ensure the accuracy of its reflectance factor. The calibration procedure involves applying the spectral characteristics and light transmittance of the secondary standard to the working standard plate to ensure that it matches the reflectance factor of the primary reference standard.
Calibration procedure and reflectance factor accuracy requirements: The calibration procedure should be executed accurately and operated according to the instrument manual to ensure that the accuracy of the reflectance factor reaches 0.1%. The difference between the control standard of the reflectance factor and the calibration value of the working standard should not exceed 0.3%.
Cleaning and maintenance of working standards
Working standards should be cleaned with distilled water and synthetic detergents, and detergents containing fluorescent components should be avoided to prevent contamination.
When cleaning the working standard board, it is first cleaned with distilled water and a soft synthetic fiber brush, then thoroughly rinsed with ethanol and dried. According to the actual situation of the laboratory, it should be cleaned regularly every week to ensure that the surface of the standard board is free of dust and dirt.
The working standard board should be calibrated with the secondary standard every month, and the tertiary standard should be obtained from the authorized laboratory every two months for inspection. If a change in the reflectance factor is found, it should be cleaned and recalibrated according to the procedure.
If the reflectance factor of the operating standard changes, it needs to be recalibrated. If there are still changes after cleaning, a three-level reference standard should be applied to calibrate again to ensure measurement accuracy and consistency.
Test procedure
Select a representative sample of pulp, paper or cardboard in accordance with the relevant standards and test requirements. Ensure that the sample is handled in accordance with the specifications of the test to obtain reliable data.
Diffuse reflectance factor measurements are performed using calibrated instruments. The operation process should be carried out in strict accordance with the instrument instructions and standard test methods to ensure the accuracy and repeatability of the data.
The diffuse reflectance factor is calculated based on the test results, and the data processing involved should be in accordance with the method of the measurement standard. Ensure that the calculation process is accurate and that the necessary data corrections are made according to the requirements of the instrument.
Organize test results and prepare a report that includes basic information about the test sample, test conditions, calculation results, and any factors that may affect the measurement. The report should be clear, accurate, and provide a comprehensive picture of the test process and results.
summary
Accurate measurement of the diffuse reflectance factor of pulp, paper and cardboard is essential to evaluate their optical properties. High accuracy and reliability of measurement results can be ensured through the use of first, second, and tertiary reference standards, and by following strict calibration and maintenance procedures. The standardized methods and instrumentation requirements described here provide systematic guidance to laboratories to ensure the consistency and accuracy of test results, thereby supporting research and applications in related fields.
