Case study of Hetian jade spectral measurement

1. What is the hyperspectral spectrum

Imaging spectroscopy technology was developed from the spectrometer. It is a new technology, also known as hyperspectral imaging technology. The traditional spectral analysis technology can only do local average spectral analysis, while the hyperspectral technology can do the spectral analysis of each point of the whole picture. The imaging spectrum has gaze imaging type, push broom type and pendulum sweep type. It can generate an image and obtain the spectral information of each pixel of the image, realizing "spectral integration". The spectral information obtained by the hyperspectral can include the spectrum of any pixel in the image, while the ordinary ground spectrometer can only obtain the average spectrum of the test object, so the data acquired by the high spectral object can be accurately analyzed and refined. Its appearance marks that optical remote sensing has entered the stage of hyperspectral remote sensing. Using the ground reflection spectral features converted from hyperspectral data, it can study the classification of earth surface objects, material composition, content, existence state, spatial distribution and dynamic change.

1. 1 Spectral detection method

With the development of the society and the improvement of peoples living standards, the pursuit of beauty also improves. Baoyu jade is more and more favored by people for its unique aesthetic value. Baoyu cultural relics not only have a strong artistic color, but also have an inestimable historical value. In some unearthed cultural relics, there will be difficult to accurately distinguish the categories of precious jade materials, so it is particularly important to quickly and accurately identify the type.

The traditional identification of precious jade material mainly includes the direct observation method of gem magnifying glass and gem microscope observation. This method is not only professionally demanding, but also the identification results obtained by experience rarely have reliable technical support and lack of persuasion. In addition, the commonly used analytical instruments include Raman spectrum analyzer, infrared spectrum analyzer and X-ray diffraction analyzer, which can analyze the composition of unknown minerals according to the standard map of minerals. However, because the three analytical instruments are large and not portable, the samples can only be used in the laboratory for analysis. This not only affects the timeliness of the material identification of precious jade cultural relics, but also affects the integrity of cultural relics. Hyperspectral technology is the combination of remote sensing technology and spectroscopic technology, which can obtain the spectral data of continuous narrow band. In the ground experiments, it can be divided into portable ground imaging spectrometer and imaging spectrometer. By using portable ground object spectrometers, narrow, complete, nearly continuous spectral reflectivity data can be obtained in the band range of 350-2500nm (visible-near-infrared-short-wave infrared band), which is often the basis of imaging spectroscopic data analysis. Portable ground object spectrometer because of its small size, easy to work in the field, and fast measurement speed, easy to operate, so improve the measurementThe rate of the data ensures the quality of the measured data. In addition, the use of the spectrometer requires low energy, the analysis time only takes a few seconds, without any chemical reagents, will not cause harm to the human body. The hyperspectral band can accurately reveal the frequency and frequency absorption information of the vibration of the group molecules in Baoyu, and analyze the complex structural information such as the vibration characteristics of chemical bonding, which has great potential for the analysis of Baoyu. At present, portable spectrometer has been used in the spectral feature analysis of blue and white materials in different ages and the composition analysis of Chinese pigments, but the spectral feature analysis of different kinds of jade materials is still insufficient.

2. Experimental content

The spectral reflectivity test of Hetian jade sample was conducted by using portable hyperspectra.

Experimental instrument: iSpecHyper-V S 1 00 0 portable hyperspectral imaging system (Figure 2-1), with spectral range 400-1000nm, 300 spectral channels, 400 spatial channels (4 images), CMOS, field angle 22.6°@f= 24 mm and 99% standard reflectivity plate. One set of indoor analog light sources. Figure Figure 2-2 shows a schematic representation of the experiment. Figure 2 – 3 shows the experimental field plots.

 Figure 2-1 iSpecHyper-V S 1 0 00 portable hyperspectral spectrum

 Figure Figure 2-2- -Schematic diagram of the experiment 

 Figure 2-3 Experimental field plots 

2.1 Data processing

In the dark chamber environment, under the condition of constant light path, the camera scans the spectral data within the location range of the sample and measures the 400-1000nm spectrum. Figures 2 – 4 show the experimental Fig.

 Figures 2 – 4 Experimental Fig 

Spectrograms after the calculation of the ENVI software

Figure 2 – 5 relative white reference spectra

Relative reflectance spectrum at position 1 Relative reflectance spectrum at position 2                      

Relative reflectance spectrum at position 3 The relative reflectance spectrum at position 4                         

Relative reflectivity spectrum of position 5 relative reflectivity spectrogram of the center of the sample                       

3. Experimental conclusion

As can be seen from the comparison of the test data from the five positions of the sample, the reflection spectrum of the overall surface of the sample is different. The visible light camera captures the brighter position. The relative reflectivity in the 450-1000nm band averaged about 50%, while the visible light camera captures the darker position, and the relative reflectance in the 450-1000nm band averaged about 40%. The overall reflectance in the visible and posterior outer bands is relatively stable with no characteristic peak. According to the literature query, the spectral characteristics of Hetian jade are from short infrared to far infrared, so to measure the spectral characteristics of Hetian jade more accurately, i S pec F ield-NIR / WNIR can be used for the reflectivity test. Figure 3-1 Portable ground object spectrometer i S pec F ield-NIR / WNIR.

Figure 3-1 Portable ground object spectrometer iSpecField-NIR / WNIR