Calibration and Use of Paper Whiteness Tester

How it works:

A paper whiteness meter is an instrument based on the principle of optical reflection to evaluate the whiteness of the surface of paper and similar materials. Its core mechanism is to irradiate the sample surface with a specific light source and measure the diffuse reflectance of the sample to the blue light band (typically the main wavelength is about 457 nm) under standard geometric conditions. The instrument compares the measured reflectance with a standard whiteboard with a known reflectivity and calculates the whiteness value through a built-in algorithm. This value is usually expressed as a percentage or a specific index, with higher values representing higher visual whiteness of the sample. This process follows the internationally accepted whiteness evaluation formula, ensuring that the measurement results are comparable across instruments and laboratories.

Calibration process

To ensure the accuracy and reliability of the measurement data, regular and standardized calibration is essential. Calibration should be carried out in accordance with relevant national or international standards, such as ISO 2469 and GB/T 7973. The core of the calibration lies in the use of a set of pass-standard whiteboards and work-standard whiteboards.

First, calibrate the instrument zero point: use a black tube or the instrument's own zero calibration device to adjust the instrument's reflection reading to the theoretical zero point. Then the range calibration is carried out: a high-reflectivity working standard whiteboard (the measurement value must be traceable to the national metrology standard) is placed at the measuring port, and the instrument readings are adjusted to the calibration value of the standard whiteboard. For instruments with multi-wavelength or fluorescence whiteness measurements, specific calibration is also required using fluorescent standards to correct for the effect of UV light content on samples containing fluorescent brighteners. Calibration intervals are recommended based on frequency of use and environmental stability, usually no more than 12 months, or immediately after instrument service and move.

Procedure:

Standardized operation is the prerequisite for obtaining effective data. The usage process can be divided into three stages: preparation, measurement and maintenance.

Before measurement, the instrument and the sample should be balanced under constant temperature and humidity conditions (usually 23±1°C, 50±2% RH) for at least 4 hours. Prepare multiple samples to ensure that their surfaces are flat, clean, and free of wrinkles and blemishes. After each start-up, the instrument should be preheated to the specified time and the above calibration steps should be performed.

When measuring, the sample is stacked in layers until it is opaque, and it is placed flat under the measuring hole. Ensure consistent sample coverage for each measurement and avoid direct finger contact with the measurement area. Each sample should be measured at least three times at different sites, taking the arithmetic average as the final result. For paper containing fluorescent brighteners, the UV content conditions of the instrument light source used (e.g., D65 light source, UV content) should be indicated in the report.

After use, use special cleaning tools to clean the measuring port and standard whiteboard to avoid contamination. Store a standard whiteboard in a dark, dry environment.

Influencing factors

Understanding and controlling the following variables is critical to ensuring measurement consistency.

Data processing

The calculation of the whiteness value is usually done in-house by the instrument, but the user needs to understand its physical implications. Common blue light whiteness formulas such as ISO brightness can be expressed as:

W = R₄₅₇

where W represents the whiteness value, R₄₅₇Represents the reflection factor measured at 457 nm. For more comprehensive evaluations, formulas such as Hunter whiteness, CIE whiteness, etc. may be involved, which incorporate a wider range of chromaticity parameters. When reporting results, the mean value, standard deviation, number of measurements, and the whiteness formula standard and instrument conditions used should be clarified.

FAQs

The long-term stable operation of the instrument depends on daily maintenance. If poor repeatability of measurements is found, sample preparation, environmental conditions, and recalibration should be checked. If the calibration fails, it may involve the aging of the light source or the contamination of the optical components, and it needs to be inspected by professionals. Vibration and impact should be avoided on a daily basis, and the instrument performance should be checked regularly using the manufacturer's recommended methods. Establishing a complete record file of instrument use, calibration and maintenance is a basic requirement of the quality management system.

References

GB/T 7973 Paper, cardboard and pulp - Determination of diffuse reflectance factor (diffuse/vertical method).

ISO 2469:2014 Paper, paperboard and pulp - Determination of diffuse reflectance coefficients.

ISO 11475:2017 Paper and cardboard - Determination of CIE whiteness, D65/10° (outdoor daylight).