Definition
A pulp whiteness meter is a specialized optical instrument used to measure the whiteness or brightness value of pulp, paper and related materials. The measurement results are usually expressed in the internationally accepted whiteness index, which reflects the material's ability to reflect blue light under specific light source conditions, and is one of the key parameters for evaluating the quality and appearance of pulp.
Principle
The measurement principle of the pulp whiteness meter is based on diffuse reflectance spectroscopy. The internal light source of the instrument illuminates the sample at a specific angle, and the detector receives the light flux diffused from the surface of the sample. The sensitive response of the human eye to the shortwave blue light region under standard light sources (such as D65 light sources) is simulated by filters, and the spectral reflectance at the main wavelength around 457 nanometers is measured. According to standards such as ISO 2470 or TAPPI T452, the whiteness value R can be calculated using the following formula:
R = k ∫ S(λ) ρ(λ) V(λ) dλ
where k is the normalization coefficient, S(λ) represents the relative spectral power distribution of the light source, ρ(λ) is the spectral reflectance of the sample, and V(λ) represents the visual function of the instrument. Modern instruments mostly use digital signal processing technology to directly output whiteness values that meet the standards specified in the standard.
Measurement method
Standard measurement processes are subject to corresponding specifications. First, a uniform specimen with a thickness that meets the opaque requirements is prepared, usually using multi-layer superposition or special sample press. Once the instrument is warmed up and stabilized, calibrate using a standard whiteboard. Place the specimen flat in the measuring hole to ensure complete coverage of the spot area. After triggering the measurement, the instrument automatically collects the reflected light data and calculates the whiteness value. Each sample should be measured at least three times at different locations, taking the arithmetic average as the final result. When measuring, attention should be paid to the control of ambient temperature and humidity to avoid stray light interference.
Influencing factors
The accuracy of the measurement results is influenced by multiple factors. In terms of sample preparation, the uniformity, flatness and thickness of the specimen will cause changes in the reflection characteristics. In optical systems, light source aging, detector sensitivity drift, and filter performance degradation may introduce system errors. In environmental conditions, ambient light leakage, instrument vibration, and temperature fluctuations can also affect the measurement stability. In addition, the fluorescent brightener in the pulp produces blue light emission under UV excitation, so the calibration state of the UV component of the instrument has a significant impact on the measurement of fluorescent materials.
Application:
This instrument plays a fundamental role in the quality control of the paper industry. It is used to monitor the bleaching process and optimize the amount of chemicals added during the production process. The quality inspection department grades the pulp according to the whiteness data to ensure that the product meets the contract specifications. R&D institutions use whiteness meters to evaluate the efficacy of new bleaches, fillers and optical brighteners. In the field of recycled paper, whiteness measurement helps determine the parameters of the deinking process. Relevant standards regard whiteness as an important technical indicator for printing paper, writing paper, sanitary paper and other products.
Selection
When choosing an instrument, technical parameters and usage needs should be comprehensively considered. The measurement geometry must comply with the d/0° or 45°/0° structure specified in ISO 2469. The spectroscopic system should meet the requirements of relevant standards for light source spectrum, filter function and detector matching. For the measurement of fluorescent materials, the instrument needs to have UV adjustment function. The operation interface should be intuitive and easy to use, and the data storage and transmission functions should be adapted to the laboratory information management system. Calibration traceability requires a certificated standard plate and verifiable calibration procedures. Maintenance costs, measurement repeatability, and long-term stability are also factors to evaluate.