指纹紫外光谱特性研究及变化规律分析

指纹的唯一性和终身不变性使得指纹可以验证一个人的身份信息，在生物识别领域具有广泛的应用。汗潜指纹对紫外线有着特殊的反射、散射以及荧光特性，所以可以利用紫外波段对汗潜指纹进行提取，并且不污染现场和目标样本。目前对紫外波段的指纹提取已经有着广泛的研究，但对指纹随时间变化的研究较少，一般都是通过化学方法测量汗潜指纹成份含量的变化。研究发现，指纹各个组成成份的紫外光谱特性不同，且这些成份随时间的挥发程度也不一致，利用多通道紫外成像系统对汗潜指纹进行凝视成像，发现各通道成像DN值随时间推移的变化程度并不一样，可通过研究各通道DN值的变化对指纹进行时间分析。首先，利用紫外光谱仪和氙灯，对汗液、酒精、食用油等几种手指容易接触的物质的反射光谱进行研究，得到了这些物质的反射光谱特性。然后，针对这几种类型的指纹，研制了一套多通道紫外成像设备，分别在240~280 nm（通道1）、280~315 nm（通道2）以及315~340 nm（通道3）三个紫外通道对其进行凝视成像，得到清晰的指纹图像，将指纹图像嵴线上码值最高的10个点的平均值进行比较，得到不同通道DN值与时间变化的关系。实验结果发现，汗潜指纹在紫外波段具有良好的成像特性，其成像DN值随时间的推移逐渐降低，其中240~280 nm的通道1中三个指纹在第七天的成像DN值分别降为第一天的0.62, 0.60和0.59；320~340 nm的通道2中三个指纹在第七天的成像DN值分别降为第一天的0.57, 0.61和0.60；340~420 nm的通道3中三个指纹在第七天的成像DN值分别降为第一天的0.56, 0.63和0.58。实验结果表明，不同类型的指纹在紫外波段的光谱特性并不一致，成像DN并不相同，且随时间的变化规律也不一样，但指纹成份的挥发具有一定的规律，成像DN值从第一天到第七天降低至60%左右，可以一定程度上反映指纹的挥发性质。结合紫外多通道成像系统，可以很好的研究指纹的变化规律，为刑侦探测中指纹研究提供了一个重要的手段。

Ultraviolet Multi-Channel Imaging of Sweat Latent Fingerprints and Analysis of Its Characteristics Over Time

The uniqueness and lifetime invariance of fingerprints enable fingerprints to verify a person’s identity information, which has a wide range of applications in the field of biometrics. The sweat latent fingerprint has special reflection, scattering and fluorescence characteristics for ultraviolet light, so the sweat latent fingerprint can be extracted by the ultraviolet band, and the scene and target samples are not polluted. At present, there are extensive researches on fingerprint extraction in the ultraviolet band, but there are few studies on the change of fingerprint with time. Generally, the changes of the content of sweat latent fingerprint components are generally measured by chemical methods. The study found that the UV spectral characteristics of each component of the fingerprint are different, and the volatilization degree of these components is also inconsistent with time. The multi-channel UV imaging system is used to perform gaze imaging on the sweat latent fingerprint, and the DN value of each channel is found to change with time. The degree is not the same, and the fingerprint can be timely analyzed by studying the change of the DN value of each channel. First, the reflectance spectra of substances that are easily contacted by fingers such as sweat, alcohol, and edible oil were studied by ultraviolet spectrometer and xenon lamp, and the reflection spectrum characteristics of these materials were obtained. Then, for these types of fingerprints, a multi-channel UV imaging device was developed, which has three UV channel pairs at 240～280 nm (channel 1), 280～315 nm (channel 2) and 315～340 nm (channel 3). The gaze imaging was performed to obtain a clear fingerprint image, and the average values of the 10 points with the highest code value on the fingerprint image were compared, and the relationship between the DN value and the time change of different channels was obtained. The experimental results show that the sweat latent fingerprint has good imaging characteristics in the ultraviolet band, and its imaging DN value gradually decreases with time. The imaging DN values of the three fingerprints in channel 1 of 240～280 nm were reduced to the first day. The imaged DN values of the three fingerprints in channel 2 of 320～340 nm on day 7 decreased to 0.57, 0.61, and 0.60 on the first day, respectively; the three fingerprints in channel 3 of 340～420 nm were respectively reduced by 0.62, 0.60, and 0.59. The imaging DN values on the seventh day were reduced to 0.56, 0.63, and 0.58 on the first day, respectively. The experimental results show that the spectral characteristics of different types of fingerprints in the ultraviolet band are not consistent, the imaging DN is not the same, and the law of change with time is different, but the volatilization of fingerprint components has a certain law, imaging DN value from the first day By the seventh day, it will be reduced to about 60%, which can reflect the volatile nature of the fingerprint to some extent. Combined with the ultraviolet multi-channel imaging system, the variation law of fingerprints can be well studied, which provides an important means for fingerprint research in criminal investigation.