Problems to be noticed when measuring paper whiteness with spectrophotometer

Color difference has always been one of the common problems in the quality of printed products. 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 the printing paper with the same whiteness is directly used to replace it, but the final printing effect is not ideal, which directly affects the visual effect of the printed matter. So in the printing process, use a more accurate instrument than the colorimeter, 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 whiteness of paper, mainly ISO brightness and CIE brightness.

ISO brightness is defined in ISO2470:1999 “Paper, paperboard and pulp – Determination of diffuse reflectance 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 spectral characteristics 457nm, half-wave width 44nm, and by adjusting the UV content of the light source irradiated to the sample to be consistent with CIE illuminant C.

Another expression method of whiteness is CIE whiteness, which is widely used internationally. ISO/TC6 has developed corresponding test method standards, and some enterprises in China also use this whiteness expression method. CIE whiteness is divided into two lighting conditions: C light source and CLEDs (full band balanced Led light source) light source.

So 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, the brightness of the paper can be obtained, and the brightness and whiteness are the same.

White paper

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

Problems to pay attention to when testing the whiteness of paper:
① When cutting the sample, it should be evenly cut on the paper banner, and the total thickness of the test piece should reach the extent that the reflection factor will not increase with the increase of the number of layers.
② All test pieces shall be uniformly stacked in front and back, vertical and horizontal directions.
③ General instruments need multi-angle conversion during measurement, and the conversion must be in place.
④ For paper containing fluorescent brightener, an instrument containing ultraviolet light source radiation is required,
⑤ Generally, when the sample is tested continuously, after several samples are tested, the instrument needs to be recalibrated to eliminate the impact of instrument drift.

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HSCD-860 Portable Spectrophotometer

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