隐性字迹的快速光谱显现与高光谱分类技术研究

Study on Rapid Spectral Reappearing and Hyperspectral Classification of Invisible Writing

擦除、密写、掩盖等隐性字迹的快速、无损显现与检验是法庭科学文件检验领域的研究难点。当前多采用切换多波段光源与滤光片的方法对隐性字迹进行显现,但对隐性字迹显现的光谱学机理分析较少,因此隐性字迹的显现效率与检验成功率均不高。为提高擦除、密写、掩盖三类隐性字迹的显现效率与检验精度,通过测量字迹的激发与荧光光谱、反射与透过光谱、微观形貌,对其显现机理与快速显现方法进行深入研究。并且基于液晶可调谐滤光器(LCTF)的高光谱成像技术与支持向量机(SVM)分类算法,提出一种对隐性字迹同时显现与分类的快速检验方法。晨光与百乐可擦笔字、荧光密写笔、柠檬汁均可在365 nm长波紫外光激发下发出较强荧光,其中可擦笔和柠檬汁的荧光波长为716 nm左右,荧光密写笔荧光波长为447 nm。此外,采用254或365 nm波长对柠檬汁隐性字迹进行紫外反射成像也可有效显现柠檬汁字迹。掩盖字迹的研究中发现,在700~2 500 nm红外波段,走珠笔、记号笔、可擦笔字迹透过率在60%以上,而中性笔字迹透过率在20%以下。因此,采用近红外波段850 nm成像有效显现了百乐走珠笔所覆盖的晨光中性笔字迹。同时,采用LCTF高光谱相机对三类隐性字迹在400~720 nm范围进行步长为5 nm的高光谱成像,并通过SVM分类算法对图像中不同笔迹成分进行同时显现和分类,分类总精度达99.284 4%, Kappa系数达0.959 1。以365 nm波长光源作为激发光进行光致发光成像可有效显现擦除与密写字迹。由于不同墨迹在近红外波段反射率差异较大,近红外成像可以有效显现掩盖字迹。基于LCTF高光谱成像的SVM分类技术可实现不同类别隐性字迹的同时显现与分类,并且有较高的显现效率与分类精度。

Rapid and non-destructive detection of invisible handwriting, such as erasure, steganography and covering, is a research difficulty in the field of forensic scientific document inspection. In the current research, the method of switching multi-band light source and filter is mostly used to visualise the invisible handwriting, but the spectroscopy mechanism of invisible handwriting is less analyzed. Therefore, the efficiency of invisible handwriting and the success rate of testing are both not high. In order to improve the efficiency and accuracy of erasure, steganography and covering in document examination, the mechanism and rapid visualization method of the three kinds of invisible handwriting were studied by measuring the excitation and fluorescence spectra, reflection and transmission spectra, and micro-topography. Based on the hyperspectral imaging technology of liquid crystal tunable filter(LCTF) and support vector machine(SVM) classification algorithm, a rapid test method for simultaneous display and classification of invisible handwriting is proposed. Chenguang and Pilot erasable pen, fluorescent writing pen and lemon juice emit strong fluorescence under the excitation of 365 nm long-wave ultraviolet light. The fluorescence wavelength of erasable pen and lemon juice is about 716 nm, and the fluorescence wavelength of fluorescent writing pen is 447 nm. Besides the lemon juice invisible writing can also be effectively visualized by using 254 or 365 nm ultraviolet reflection imaging. In the study of covering handwriting, it is found that the transmittance of a ballpoint pen, marker pen and erasable pen is more than 60% in the infrared band of 700~2 500 nm, and the transmittance of a gel pen is less than 20%. Therefore, the near-infrared band of 850 nm imaging is used to effectively visualize the Chenguang gel pen covered by a Pilot ballpoint pen. The LCTF hyperspectral camera was used to image the three kinds of invisible handwriting in the range of 400~720 nm with a step of 5 nm, and the different handwriting in the image were visualized classified simultaneously by SVM classification algorithm, the total classification accuracy was 99.284 4%, and the Kappa coefficient was 0.959 1. Photoluminescence imaging using a 365 nm light source as an excitation light can effectively visualize erasure and steganography handwriting. Because the reflectivity of different inks in the near-infrared band is quite different, near-infrared imaging can effectively visualize the covering handwriting. SVM classification technology based on LCTF hyperspectral imaging can realize the simultaneous display and classification of different types of invisible handwriting and has high visualization efficiency and classification accuracy.