颜料粉末的高光谱成像无损表征技术

Characterization of pigments using non-destructive hyperspectral imaging

通过高光谱无损表征技术在壁画颜料粉末鉴定中的应用效果评估,为高光谱颜料粉末数据库的建立打下基础。对壁画中常见的蓝铜矿和孔雀石颜料粉末进行XRD成分检测,并将颜料粉置于30 mm×20 mm×A mm的凹槽中,其中A分别为0.2,0.4,0.6,0.8,1.0,1.2,1.4,1.6 mm。用高光谱成像仪得到不同粉层厚度的蓝铜矿和孔雀石的光谱反射率。进一步将颜料筛分为0~50μm,50~74μm,74~100μm和100~150μm四种粒度范围,在相同测量条件下进行高光谱成像。在此基础上,建立小型的壁画颜料粉末高光谱数据库并开展壁画颜料的鉴定工作。结果表明:在0.2~1.6 mm厚度范围内,不同厚度的同种颜料,其光谱反射率变化幅度在2%以内,且光谱反射率的曲线形状与关键光谱信息几乎没有变化。随颜料粉末粒度减小,在400~600 nm和800~1 000 nm范围内的蓝铜矿,与在400~700 nm和900~1 000 nm范围内的孔雀石,其光谱反射率增大,但光谱反射率曲线的形状等没有显著变化。高光谱能够准确鉴定出壁画中所用颜料的成分。颜料层的厚度和粒度对颜料反射率光谱曲线的影响不大,粒度在一定波段内会造成颜料的光谱反射率增大,但不影响光谱曲线形状等关键信息,即在0~150μm范围内的颜料均可用于标准数据库的建立。

The hyperspectral reflectance database of the pigments was established by assessing the application of non-destructive hyperspectral imaging system for identifying the ingredients of pigments that are commonly found in murals. Two representative pigments used in murals, azurite and malachite, were characterized by XRD. Then, they were placed in a 30 mm×20 mm×A mm groove, where A is 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, and 1.6 mm, respectively. The spectral reflectance of azurite and malachite powder layer with different thickness was systemically measured by hyperspectral imaging system. The azurite and malachite powders were sieved to obtain powder with four kinds of particle size range： 0-50μm, 50-74μtm, 74-100μm, and 100-150μm. The dependence of the spectral reflectance of azufite and malachite on the particle size was further investigated. Then, the hyperspectral reflectance of thirteen pigments that are commonly used in murals was measured and a small hyperspectral database of pigments was established. The results show that, in a layer thickness ranging from 0.2-1.6 mm, the change of the spectral reflectance of the same pigment layer with different thickness is within 2%, and the shape of the spectral reflectance curve which is the key information for revealing the ingredient of pigments is almost unchanged. For azurite particle with size in a range of 400-600 nm and 800-1 000 nm and malachite particle with size in a range of 400-700 nm and 900-1 000 nm, the smaller the pigment particle size is, the larger the spectral reflectance is. However, the size of pigment particle does not change the shape of spectral reflectance curves. It is shown that the ingredient of pigments in the murals can be accurately identified by fitting the hyperspectral curves. The influence of the thickness of pigment layer and particle size of pigment powder on the spectral reflectance is not obvious. Although the particle size results in the increase of the spectral reflectance of the pigments, it does not change the shape of the spectral curve. It is concluded that pigment particle with 0-150μm size can be used to establish the standard hyperspectral database. Hyperspectral imaging is a powerful tool to identify the ingredient of pigments used on the mural.