The whiteness of paper refers to the brightness of the paper, which affects the printing effect of the paper. There are many ways to express whiteness, mainly ISO brightness, D65 brightness and CIE whiteness. The following are several common methods for measuring paper whiteness.
1. Comparison method
The comparison method is one of the simplest and most commonly used paper whiteness test methods. This method mainly evaluates the whiteness of paper by comparing the whiteness difference between the paper sample and the standard white board with the human eye. In the specific operation, place the standard white board (such as Ganz white board or Stuft-Whitten white board) and the paper sample to be tested side by side, and observe the whiteness difference between the two with the naked eye. This method is easy to operate, but due to the influence of ambient light and human visual differences, the results are prone to errors, so it is not accurate enough.
2. Reflection method
Reflection method is a relatively accurate paper whiteness test method. This method uses the intensity of reflected light to evaluate the whiteness of paper. In specific operation, a reflectance photometer (such as a brightness meter or a colorimeter) is used to measure the intensity of reflected light on the surface of the paper, and then the measured value is compared with the intensity of reflected light from a standard white board to calculate the whiteness value of the paper.
In my country, the measurement of whiteness is usually carried out in accordance with the GB/T7974-2002 standard method. This method stipulates that a parallel light beam is irradiated onto the surface of the paper at an angle of 45°, and the reflectivity in the normal direction of the paper is measured. A color filter is placed in the incident light to filter out infrared light, and a color filter is also placed in the reflected light to provide 457nm blue light as the measurement light. The XT-48BN Brightness Meter and the YT-ACM Automatic Colorimeter are designed based on this principle.
The specific steps for measuring paper whiteness using a reflectance photometer are as follows:
1. Turn on the power supply to preheat. After the power supply is stable, calibrate the instrument with a standard black tube and a working standard white board.
2. Place the sample on the test hole and measure the whiteness value, which is usually expressed as a percentage (100%).
It should be noted that during the test, the cutting and stacking of the sample should be ensured to meet the requirements to avoid affecting the accuracy of the test results. At the same time, for paper containing fluorescent whitening agents, the ultraviolet radiation of the instrument light source should be adjusted to ensure the accuracy of the test results.
3. Spectrophotometry
Spectrophotometry is a highly accurate paper whiteness test method. This method measures the reflectivity of paper under light of different wavelengths to obtain the spectral curve of the paper, and then calculates the whiteness value of the paper. In specific operation, a spectrophotometer is used to measure the reflectance spectrum of the paper, and then the whiteness value of the paper is calculated based on the spectral data.
Spectral spectrophotometers can provide rich spectral information and can accurately measure the whiteness of paper. However, due to the high price of the equipment and high technical requirements for operation, they are rarely used in practical applications.
In summary, there are many methods for measuring paper whiteness, including comparison method, reflection method and spectroscopy method. In practical applications, you can choose a suitable test method as needed, and combine multiple methods for comprehensive evaluation to obtain more accurate test results.
