Determination of paper whiteness by spectrophotometer

Color difference has always been one of the common problems of printing product quality. Whether it is the same batch of products or between different batches of products, the appearance of color difference will greatly reduce the quality of products. In the process of printing, it is often because the paper is not enough or it is directly replaced by the printing paper whose whiteness seems to be consistent, but the final printing effect is not ideal, which directly affects the visual effect of the printed matter. Therefore, in the printing process, we use a more accurate instrument than the chromatic aberration meter – the spectrophotometer to measure and understand the whiteness of the paper, and then use it after checking the consistency. At present, there are many ways to express the brightness of paper, mainly ISO brightness and CIE brightness.

ISO brightness is defined in ISO2470:1999 Paper, Board and Pulp – Determination of Diffuse Reflection Factor (ISO Brightness): the internal reflection factor measured by an instrument equipped with a filter or with corresponding functions to make the effective wavelength of the spectral characteristics 457nm and the half wave width 44nm, and by adjusting the UV content of the light source illuminating the sample to be consistent with CIE illuminant C.

Another way to express whiteness is CIE whiteness, which is widely used internationally. ISO/TC6 has formulated corresponding test method standards, and some domestic enterprises also adopt this whiteness expression method. CIE whiteness is divided into two lighting conditions: C light source and CLEDs (all band balanced Led light source) light source.

How to measure the whiteness of paper?
According to the national standard GB8940.1-88, based on the blue light of D65 illuminant and d/8 illumination observation conditions, the paper whiteness is a measure of the diffuse reflectance of 457nm blue light, and the whiteness reflects the brightness characteristics. Therefore, by detecting the brightness of the paper, you can obtain the whiteness of the paper. The brightness and whiteness are the same.

The research shows that the dual optical path array sensor has a better response sensitivity to 457 nm blue light. After A/D conversion, the color image acquired by the dual optical path array sensor is stored point by point in the computer in the unit of pixel. The amount of information stored at each point includes the component value whose reflectivity is proportional to the received radiant energy.

Problems to be noted when testing the whiteness of paper:
1. When cutting the sample, it shall be evenly cut on the paper banner, and the total thickness of the test piece shall reach the extent that the reflection factor will no longer increase with the increase of the number of layers.
2. All test pieces shall be stacked on both sides, vertically and horizontally.
3. General instruments need multi angle conversion when measuring, and the conversion must be in place.
4. For the paper containing fluorescent whitening agent, an instrument containing ultraviolet light source radiation is required,
5. In general, when the samples are tested continuously, after several samples are tested, the instrument needs to be recalibrated to eliminate the influence of instrument drift.

LISUN lauched Portable Spectrophotometer HSCD-800 is dual optical path design improves repeatability accuracy dE*ab≤0.04. The dual optical path design monitors the fluctuation of light source energy while measuring the sample signal, reduces interference during measurement, obtains higher measurement stability, and improves the measurement repeatability index of the instrument to dE*ab≤0.04. It guarantees the high standard of measurement speed, accuracy, stability and inter-station difference consistency of the instrument.

Lisun Instruments Limited was found by LISUN GROUP in 2003. LISUN quality system has been strictly certified by ISO9001:2015. As a CIE Membership, LISUN products are designed based on CIE, IEC and other international or national standards. All products passed CE certificate and authenticated by the third party lab.

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Tags：HSCD-860